COSTILLA ENERGY INC
424B1, 1996-10-03
CRUDE PETROLEUM & NATURAL GAS
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<PAGE>
PROSPECTUS
 
                                  $100,000,000
 
                                                               [LOGO]
                             COSTILLA ENERGY, INC.
 
                         10 1/4% SENIOR NOTES DUE 2006
                             ---------------------
 
    The  10 1/4%  Senior Notes  due 2006  (the "Notes")  are being  offered (the
"Notes Offering") by Costilla Energy,  Inc., a Delaware corporation  ("Costilla"
or the "Company"). The net proceeds of the Notes Offering, together with the net
proceeds of the other financing described herein, will be used by the Company to
refinance  existing indebtedness,  to pay certain  costs in  connection with the
Corporate Reorganization (as defined herein) and for general corporate purposes.
 
    The Notes mature on October 1, 2006, unless previously redeemed. Interest on
the Notes is payable semiannually on April 1 and October 1, commencing April  1,
1997.  The Notes will be redeemable at the option of the Company, in whole or in
part, on or after October  1, 2001, at the  redemption prices set forth  herein,
plus   accrued  and   unpaid  interest,   if  any,   to  the   redemption  date.
Notwithstanding the  foregoing,  at any  time  on  or before  October  1,  1999,
Costilla  may redeem up to 30% of the original aggregate principal amount of the
Notes with the  net proceeds  of an  Equity Offering  (as defined  herein) at  a
redemption  price equal to 110.25% of the principal amount thereof, plus accrued
and unpaid interest thereon, if  any, to the redemption  date. Upon a Change  of
Control  (as defined herein), the  Company will be required  to make an offer to
repurchase all  outstanding Notes  at  101% of  the aggregate  principal  amount
thereof plus accrued and unpaid interest, if any, to the date of repurchase. See
"Description of Notes."
 
    Concurrently  with  the Notes  Offering, the  Company is  offering 4,800,000
shares (5,520,000 shares if the underwriters' over-allotment option is exercised
in full) of its Common Stock (the "Common Stock Offering" and together with  the
Notes  Offering, the "Offerings")  pursuant to an  underwritten public offering.
The Notes Offering  and the Common  Stock Offering are  each conditioned on  the
consummation of the other.
 
    The  Notes will be  general unsecured senior obligations  of the Company and
will rank equally  in right of  payment with all  other Senior Indebtedness  (as
defined  herein) of the Company, which  will include borrowings under the Credit
Facility (as  defined  herein), but  will  be effectively  subordinated  to  any
existing  or future secured Senior Indebtedness. The Credit Facility is expected
to be secured by substantially all of the assets of the Company. As of June  30,
1996,  on a pro forma basis after giving effect to the Corporate Reorganization,
the Offerings and the application of the proceeds therefrom, as described  under
"Use  of Proceeds," the  Company would have  had $0.4 million  of secured Senior
Indebtedness. No indebtedness of  the Company is  expressly subordinated to  the
Notes.  Initially, both  the Notes and  the Credit Facility  will be effectively
subordinated to liabilities of the Company's subsidiaries. On a pro forma basis,
the total liabilities of  the Company's subsidiaries were  $6.5 million at  June
30,  1996,  all  of  which  were  operating  liabilities.  See  "Risk  Factors,"
"Capitalization" and "Description of Notes."
 
    The Notes will be represented by a Global Certificate registered in the name
of the nominee of The Depository Trust Company, which will act as the Depositary
(the "Depositary"). Beneficial interests in the Global Certificate will be shown
on, and transfers thereof will be  effected only through, records maintained  by
the  Depositary  and  its participants.  Except  as described  herein,  Notes in
definitive form  will not  be issued.  See "Description  of Notes--  Book-Entry,
Delivery and Form."
 
    The  Company does not  intend to list  the Notes on  any national securities
exchange. See "Risk Factors--  Absence of Public Market."  The Common Stock  has
been  approved for listing on The Nasdaq Stock Market's National Market ("Nasdaq
National Market") under the symbol COSE.
 
    SEE "RISK FACTORS"  BEGINNING ON PAGE  10 FOR A  DISCUSSION OF FACTORS  THAT
SHOULD BE CONSIDERED IN EVALUATING AN INVESTMENT IN THE NOTES.
                            ------------------------
THESE  SECURITIES HAVE  NOT BEEN APPROVED  OR DISAPPROVED BY  THE SECURITIES AND
EXCHANGE  COMMISSION  OR  ANY  STATE  SECURITIES  COMMISSION  NOR  HAS   THE
    SECURITIES AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION
       PASSED UPON THE ACCURACY OR ADEQUACY OF THE PROSPECTUS. ANY
                REPRESENTATION  TO  THE  CONTRARY  IS  A CRIMINAL
                                    OFFENSE.
<TABLE>
<S>                                                  <C>               <C>               <C>
- ---------------------------------------------------------------------------------------------------------
 
<CAPTION>
                                                         Price to        Underwriting      Proceeds to
                                                        Public (1)      Discounts (2)     Company (1)(3)
<S>                                                  <C>               <C>               <C>
- ---------------------------------------------------------------------------------------------------------
Per Note...........................................        100%             3.25%             96.75%
Total..............................................    $100,000,000       $3,250,000       $96,750,000
- ---------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Plus accrued interest, if any, from October 8, 1996.
(2) The Company  has agreed to  indemnify the Underwriters  (as defined  herein)
    against  certain liabilities, including liabilities under the Securities Act
    of 1933, as amended. See "Underwriting."
(3) Before deducting expenses payable by the Company, estimated at $600,000.
 
    The Notes are  being offered,  subject to  prior sale,  by the  Underwriters
when,  as and  if issued  to and  accepted by  the Underwriters,  and subject to
various prior conditions. The Underwriters reserve the right to withdraw, cancel
or modify such offer and  to reject orders in whole  or in part. It is  expected
that delivery of the Global Certificate will be made on or about October 8, 1996
in  book-entry form  through the facilities  of the  Depositary, against payment
therefor.
 
NATIONSBANC CAPITAL MARKETS, INC.  PRUDENTIAL SECURITIES INCORPORATED
 
                 The date of this Prospectus is October 2, 1996
<PAGE>
                            PRIMARY OPERATING AREAS
 
              [A GEOGRAPHICAL MAP INDICATING WHERE THE COMPANY HAS
                      OIL AND GAS PROPERTIES AND OFFICES]
 
    IN CONNECTION WITH THIS NOTES  OFFERING, THE UNDERWRITERS MAY OVER-ALLOT  OR
EFFECT TRANSACTIONS WHICH STABILIZE OR MAINTAIN THE MARKET PRICE OF THE NOTES AT
A  LEVEL  ABOVE THAT  WHICH MIGHT  OTHERWISE  PREVAIL IN  THE OPEN  MARKET. SUCH
TRANSACTIONS  MAY  BE   EFFECTED  IN   THE  OPEN  MARKET   OR  OTHERWISE.   SUCH
STABILIZATION, IF COMMENCED, MAY BE DISCONTINUED AT ANY TIME.
<PAGE>
                               PROSPECTUS SUMMARY
 
    THE  FOLLOWING  SUMMARY  IS  QUALIFIED  IN  ITS  ENTIRETY  BY  THE  DETAILED
INFORMATION, FINANCIAL STATEMENTS  AND OTHER  DATA APPEARING  ELSEWHERE IN  THIS
PROSPECTUS. THE PRO FORMA INFORMATION GIVES EFFECT TO THE CONVERSION OF COSTILLA
FROM A LIMITED LIABILITY COMPANY TO A CORPORATION, CERTAIN MATERIAL ACQUISITIONS
AND  THE OFFERINGS AND THE APPLICATION  OF THE ESTIMATED NET PROCEEDS THEREFROM.
SEE "-- SIGNIFICANT  ACQUISITIONS," "THE COMPANY  -- CORPORATE  REORGANIZATION,"
AND  "USE OF PROCEEDS." AS USED HEREIN, REFERENCES TO THE COMPANY OR TO COSTILLA
ARE TO COSTILLA ENERGY, INC.  AND ITS SUBSIDIARIES. UNLESS OTHERWISE  INDICATED,
THE  INFORMATION  IN THIS  PROSPECTUS  ASSUMES THE  UNDERWRITERS' OVER-ALLOTMENT
OPTION WITH RESPECT TO THE COMMON STOCK OFFERING WILL NOT BE EXERCISED.  CERTAIN
OIL AND GAS TERMS USED IN THIS PROSPECTUS ARE DEFINED IN THE "GLOSSARY" INCLUDED
HEREIN.  CERTAIN  TERMS USED  IN  CONNECTION WITH  THE  NOTES ARE  DEFINED UNDER
"DESCRIPTION OF  NOTES  --  CERTAIN DEFINITIONS."  "ADJUSTED  EBITDA,"  AS  USED
HEREIN,  MEANS  NET INCOME  (LOSS), PLUS  INTEREST, INCOME  TAXES, DEPRECIATION,
DEPLETION AND AMORTIZATION, EXPLORATION AND ABANDONMENT COSTS AND  EXTRAORDINARY
LOSS FROM EXTINGUISHMENT OF DEBT.
 
                                  THE COMPANY
 
    Costilla  is  an  independent  energy company  engaged  in  the exploration,
acquisition and development  of oil  and gas properties.  The Company's  primary
operations are in the Permian Basin area of Texas and New Mexico, the Gulf Coast
and  the Rocky  Mountain regions. The  Company's strategy  focuses on increasing
reserves through  a  targeted  exploration  program,  the  exploitation  of  its
existing  properties  and  selective  property  acquisitions.  In  addition, the
Company recently acquired an  interest in an entity  which has a concession  for
the  development of  mineral interests  in the  Republic of  Moldova, in Eastern
Europe. The Company also has minor  interests in the domestic gas gathering  and
transmission business.
 
    The  Company's  predecessor began  operating in  1988  with the  strategy of
acquiring and exploiting undervalued oil and gas properties, and at December 31,
1992 had net proved reserves  of 4.7 MMBOE. Since  January 1, 1993, the  Company
has  successfully closed seven  transactions for an  aggregate purchase price of
approximately $101 million. As of April 1, 1996, the Company had total estimated
net proved reserves of 16.5 Mmbbls of oil and 112.9 Bcf of gas, aggregating 35.3
MMBOE, with a  PV-10 Value of  approximately $179.5 million,  assuming the  1996
Acquisition  (as defined below) had occurred at  April 1, 1996. The Company also
has a  substantial  undeveloped acreage  position  consisting of  180,704  gross
(165,166  net) acres at June  30, 1996. The Company  has identified in excess of
185 drilling locations of which 64 are included in its proved reserves.
 
    Costilla  has  in-house  exploration   expertise  which  uses  3-D   seismic
technology  as  a  primary  tool  to  identify  drilling  opportunities  and has
experienced high rates of  success in each  of its first  two major 3-D  seismic
drilling  programs. Since 1994, the Company has  drilled 37 wells based on these
3-D surveys,  31  of  which  have been  productive.  The  Company  has  recently
completed  two additional 3-D surveys and intends to commence drilling on one of
these acreage blocks in the second half of 1996. The Company currently plans  to
drill 63 wells through 1997 based on its 3-D surveys.
 
    Since  1993,  Costilla  has  generated significant  growth  in  reserves and
production. The Company increased its  estimated proved reserves from 6.0  MMBOE
at  December 31, 1993  to 35.3 MMBOE  at April 1,  1996 (pro forma  for the 1996
Acquisition), representing a compound annual  growth rate of 114%. This  reserve
growth has been achieved at an average all-in finding cost of $3.60 per BOE over
such period, a level which the Company believes is lower than industry averages.
Concurrently,  the Company increased  its average net  daily production from 827
BOE for the  year ended December  31, 1993 to  10,231 BOE for  the three  months
ended  March  31, 1996  (pro  forma for  the  1996 Acquisition),  representing a
compound annual growth rate of 190%.
 
                                       3
<PAGE>
                               BUSINESS STRATEGY
 
    The Company's strategy is to increase  its oil and gas reserves,  production
and  cash flow from operations through  a two-pronged approach which combines an
active exploration program  using 3-D seismic  and other technological  advances
with the acquisition and exploitation of producing properties. The Company seeks
to  reduce its operating and commodity risks by holding a geographically diverse
portfolio of properties,  the reserves attributable  to which are  approximately
balanced  between oil and gas. The Company  also seeks to manage the elements of
its business strategy  through the  operation of  a significant  portion of  its
properties,  the use of a  rate of return analysis  and the direct marketing and
hedging of its oil  and gas production. The  elements of the Company's  strategy
may be further described as follows:
 
- -   EXPLORATION   EFFORTS.     The  Company  uses   extensive  geological  and
    geophysical analysis  to carefully  focus its  3-D seismic  surveys.  This
    focus  allows the Company to successfully direct the size and scope of its
    exploration program in order  to improve the  likelihood of success  while
    managing  overall exploration costs. The Company's exploration efforts are
    concentrated currently on  known producing regions.  The Company plans  to
    drill 24 exploratory wells during the last half of 1996 and 36 exploratory
    wells in 1997. Capital budgeted for exploration activities is $8.1 million
    for the last six months of 1996 and $10.8 million for 1997.
 
- -   EXPLOITATION  ACTIVITIES.    The  Company  is  actively  pursuing numerous
    exploitation opportunities within its existing properties, including areas
    where no proved reserves  are currently assigned. Exploitation  activities
    currently  in  progress  include a  carbon  dioxide  flood, recompletions,
    workovers,  infill  and  horizontal  drilling  and  a  secondary  recovery
    project.  The Company's capital budget for such activities is $8.4 million
    for the last six months of 1996 and $9.2 million for 1997, which  includes
    the  drilling of 12 development wells in  1996 and 13 development wells in
    1997.
 
- -   PROPERTY ACQUISITIONS.  The Company seeks to acquire producing  properties
    where  it has identified opportunities to increase production and reserves
    through both  exploitation and  exploration  activities. The  Company  has
    increased  the  value of  its  acquisitions by  aggressively  managing the
    operations of existing proved  properties and by successfully  identifying
    and  developing  previously  unproved reserves  on  acquired  acreage. The
    Company seeks to acquire reserves  which will fit its existing  portfolio,
    are  generally not  being actively  marketed and  where a  negotiated sale
    would be the method of  purchase. The Company does  not rely on major  oil
    company divestitures or property auctions.
 
- -   PROPERTY  DIVERSIFICATION.  The  Company holds a portfolio  of oil and gas
    properties located in  the Permian  Basin, the  Gulf Coast  and the  Rocky
    Mountain  regions. The Company believes  that by conducting its activities
    in distinct  regions  it is  able  to  reduce commodity  price  and  other
    operational risks. The Company's Moldovan interest is an extension of this
    strategy  and  can  be  characterized by  low  initial  costs, significant
    reserve potential  and the  availability  of technical  data that  may  be
    further developed by the Company.
 
- -   CONTROL  OF  OPERATIONS.   The  Company  prefers  to operate  and  own the
    majority working  interest  in its  properties.  This allows  the  Company
    greater  control over future development, drilling, completing and lifting
    costs and marketing of production. At April 1, 1996, the Company  operated
    wells  constituting approximately 72% of its  total PV-10 Value (pro forma
    for the 1996 Acquisition).
 
                                       4
<PAGE>
                            SIGNIFICANT ACQUISITIONS
 
    1995 ACQUISITION.  In  a $46.6 million acquisition  completed in June  1995,
the  Company acquired a group  of oil and gas  properties located in the Permian
Basin, Gulf Coast and  Rocky Mountain regions. At  the date of acquisition,  the
net  proved reserves included 7.1 Mmbbls of oil and 44.1 Bcf of gas, aggregating
14.4 MMBOE.  From the  date of  acquisition until  March 31,  1996, the  Company
produced  1.5  MMBOE from  the acquired  properties  and sold  a portion  of the
acquired properties for approximately  $3.6 million. At April  1, 1996, the  net
proved  reserves  of  the remaining  properties  were 13.4  MMBOE.  The acquired
properties also included 103,010 gross (93,787 net) undeveloped acres.
 
    1996 ACQUISITION.  In June 1996, the Company acquired a group of oil and gas
properties located primarily  in the Permian  Basin and Gulf  Coast regions  for
approximately  $42.5  million.  This acquisition  included  properties  with net
proved reserves at  April 1,  1996 of 5.0  Mmbbls of  oil and 33.5  Bcf of  gas,
aggregating  10.6  MMBOE. The  acquired  properties also  included  42,855 gross
(16,646 net) undeveloped acres and a pipeline located in Pennsylvania which  had
an allocated purchase price of $3.5 million.
 
                              DRILLING ACTIVITIES
 
    Exploration  efforts since  January 1,  1996 include  the drilling  of eight
wells located on  the Company's  Edwards/McElroy Ranch Prospect  in the  Permian
Basin.  While one of such wells is being drilled, the remaining seven wells have
been completed as  producers. Three  of the  productive wells  have resulted  in
separate   field  discoveries   which  have  confirmed   the  Company's  seismic
interpretation of  a  significant  trend. Since  beginning  operations  on  this
prospect,  the Company  has drilled 11  producing wells  and one dry  hole. As a
result, the Company has identified up to 68 additional drilling locations,  none
of which are included in the Company's proved reserves.
 
    The  Company  has also  continued drilling  in  the McGyver-Green  Acres 3-D
Prospect. Since commencing activity  in this prospect in  July 1994 the  Company
has  drilled 17 wells, of  which 14 have been  successful. During June 1996, the
average  daily  capacity  from  the   producing  wells  in  this  prospect   was
approximately 83 BOE per well. The Company has identified 34 additional drilling
locations in the McGyver-Green Acres Prospect, nine of which are included in the
Company's proved reserves.
 
    The  Company has also  drilled and completed seven  development wells in the
Permian Basin and Gulf Coast regions since the beginning of 1996. Currently, the
Company's principal exploitation  activities include a  carbon dioxide flood  in
the  East Goldsmith  Unit, infill  drilling primarily  in the  Permian Basin and
horizontal drilling in the Susan Peak Field.
 
                                       5
<PAGE>
                                  THE OFFERING
 
<TABLE>
<S>                                           <C>
Securities Offered..........................  $100,000,000  aggregate  principal  amount  of
                                              10  1/4% Senior Notes due  2006 of the Company
                                              (the "Notes").
Maturity Date...............................  October 1, 2006.
Interest Payment Dates......................  April 1  and October  1, commencing  April  1,
                                              1997.
Optional Redemption.........................  On  or after October 1,  2001, the Company may
                                              redeem the Notes, in whole or in part, at  the
                                              redemption   prices  set  forth  herein,  plus
                                              accrued and unpaid  interest, if  any, to  the
                                              date   of   redemption.   Notwithstanding  the
                                              foregoing, at any time on or before October 1,
                                              1999, the Company may redeem up to 30% of  the
                                              original  aggregate  principal  amount  of the
                                              Notes with  the  net  proceeds  of  an  Equity
                                              Offering  (as defined herein)  at a redemption
                                              price equal to 110.25% of the principal amount
                                              thereof, plus accrued and unpaid interest,  if
                                              any,  to the date of redemption, provided that
                                              at  least  70%   of  the  original   aggregate
                                              principal   amount   of   the   Notes   remain
                                              outstanding immediately after such redemption.
                                              See  "Description   of   Notes   --   Optional
                                              Redemption."
Mandatory Redemption........................  None, except at maturity on October 1, 2006.
Ranking.....................................  The  Notes  will be  general  unsecured senior
                                              obligations of  the  Company,  and  will  rank
                                              equally  in  right of  payment with  all other
                                              Senior Indebtedness of the Company  (including
                                              the  Credit Facility)  and senior  in right of
                                              payment   to   all    existing   and    future
                                              Subordinated   Indebtedness  of  the  Company.
                                              Borrowings  under  the  Credit  Facility   are
                                              expected to be secured by substantially all of
                                              the  assets of the  Company and any subsidiary
                                              of   the   Company   that   guarantees    such
                                              borrowings.  Initially, none  of the Company's
                                              subsidiaries will guarantee the obligations of
                                              the Company under the Credit Facility. To  the
                                              extent of pledged collateral, such
                                              Indebtedness   will  have  priority  over  the
                                              Notes. At June 30, 1996, on a pro forma  basis
                                              after   giving   effect   to   the   Corporate
                                              Reorganization,   the   Offerings   and    the
                                              application of the net proceeds therefrom, the
                                              Company would have had $0.4 million of secured
                                              Senior  Indebtedness  and  no  other unsecured
                                              Senior Indebtedness.  The Notes  also will  be
                                              effectively  subordinated to  all indebtedness
                                              and  other   liabilities  of   the   Company's
                                              subsidiaries   until  such  subsidiaries  (the
                                              'Subsidiaries Guarantors") deliver  Subsidiary
                                              Guarantees   to   fully   and  unconditionally
                                              guarantee the  Notes on  a senior  basis  (the
                                              "Subsidiary  Guarantees").  At June  30, 1996,
                                              the  subsidiaries'   total  liabilities   were
</TABLE>
 
                                       6
<PAGE>
 
<TABLE>
<S>                                           <C>
                                              $6.5  million,  all  of  which  were operating
                                              liabilities. The Indenture  pursuant to  which
                                              the  Notes  will be  issued  (the "Indenture")
                                              requires  each   Subsidiary   to   deliver   a
                                              Subsidiary  Guarantee  as a  condition  to its
                                              incurrence   of   Indebtedness   (other   than
                                              intercompany Indebtedness). At the date of the
                                              Indenture,   there   will  be   no  Subsidiary
                                              Guarantees.  See  "Description  of  Notes   --
                                              Ranking" and "-- Subsidiary Guarantees."
Change of Control...........................  Upon  a Change of Control (as defined herein),
                                              the Company will be required to make an  offer
                                              to repurchase all outstanding Notes at 101% of
                                              the  principal amount thereof plus accrued and
                                              unpaid interest thereon, if  any, to the  date
                                              of  repurchase. See  "Description of  Notes --
                                              Repurchase at the Option of Holders --  Change
                                              of Control."
Covenants...................................  The   Indenture  will  restrict,  among  other
                                              things,  the   Company's  ability   to   incur
                                              additional indebtedness, pay dividends or make
                                              certain   other  restricted   payments,  incur
                                              liens,  engage  in  any  sale  and   leaseback
                                              transaction, sell stock of subsidiaries, apply
                                              net  proceeds from certain  asset sales, merge
                                              or consolidate  with any  other person,  sell,
                                              assign,  transfer, lease,  convey or otherwise
                                              dispose of substantially all of the assets  of
                                              the    Company,   or    enter   into   certain
                                              transactions with affiliates.
Common Stock Offering.......................  Concurrently  with  the  Notes  Offering,  the
                                              Company is offering 4,800,000 shares of Common
                                              Stock   to  the  public.   See  "Common  Stock
                                              Offering." The closings of the Notes  Offering
                                              and   the  Common  Stock   Offering  are  each
                                              conditioned  upon  the  consummation  of   the
                                              other.
Use of Proceeds.............................  The Company intends to use the net proceeds of
                                              the  Notes  Offering,  together  with  the net
                                              proceeds of the Common  Stock Offering (i)  to
                                              repay   existing  indebtedness,  (ii)  to  pay
                                              certain costs incurred in connection with  the
                                              Corporate  Reorganization (as defined herein),
                                              including   redeeming    certain    membership
                                              interests  of  the  Company's  predecessor and
                                              (iii) for general corporate purposes. See "Use
                                              of Proceeds."
</TABLE>
 
                                  RISK FACTORS
 
    See "Risk  Factors" for  a  discussion of  certain  factors that  should  be
considered in evaluating an investment in the Notes.
 
                                       7
<PAGE>
                         SUMMARY FINANCIAL INFORMATION
 
    The  following table  sets forth  certain summary  historical and  pro forma
financial data of  the Company.  The historical  information should  be read  in
conjunction  with the  Consolidated Financial  Statements and  the notes thereto
included  elsewhere  in  this  Prospectus.  The  Company  acquired   significant
producing  oil  and gas  properties in  certain of  the periods  presented which
affect the comparability of the historical financial and operating data for  the
periods  presented. The pro forma information should be read in conjunction with
the  Pro  Forma  Condensed  Financial  Statements  and  notes  thereto  included
elsewhere  in this Prospectus. Neither the  historical results nor the pro forma
results are  necessarily  indicative  of  the  Company's  future  operations  or
financial results.
 
<TABLE>
<CAPTION>
                                                                 YEAR ENDED DECEMBER 31,               SIX MONTHS ENDED JUNE 30,
                                                        ------------------------------------------  -------------------------------
                                                                  HISTORICAL             PRO FORMA       HISTORICAL       PRO FORMA
                                                        -------------------------------  ---------  --------------------  ---------
                                                          1993       1994       1995      1995(1)     1995       1996      1996(1)
                                                        ---------  ---------  ---------  ---------  ---------  ---------  ---------
                                                                     (IN THOUSANDS, EXCEPT RATIOS AND PER SHARE DATA)
<S>                                                     <C>        <C>        <C>        <C>        <C>        <C>        <C>
STATEMENT OF OPERATIONS DATA:
  Revenues............................................  $   4,397  $   7,836  $  21,816  $  52,637  $   5,573  $  19,525  $  28,748
  Expenses:
    Oil and gas production............................      1,688      2,351     10,355     26,937      2,413      8,278     13,295
    General and administrative........................        952      1,184      3,571      4,850      1,008      2,809      3,010
    Compensation related to option settlement.........         --         --        656        656        656         --         --
    Exploration and abandonments......................        218        793      1,650      2,761      1,007        308        555
    Depreciation, depletion and amortization..........        884      1,847      5,958     14,176      1,367      4,620      6,981
    Interest..........................................        605      1,458      4,591     10,635      1,046      4,156      5,317
    Other.............................................         --         --          2          2         --         --         --
  Net income (loss) before income taxes and
   extraordinary item.................................         50        203     (4,967)    (7,380)    (1,924)      (646)      (410)
  Net income (loss)...................................         73        163     (4,970)    (7,383)    (1,924)    (2,286)      (410)
  Pro forma earnings (loss) per common share..........         --         --         --      (0.75)        --         --      (0.04)
  Pro forma weighted average common shares
   outstanding........................................         --         --         --      9,861         --         --     10,000
STATEMENT OF CASH FLOWS DATA:
  Net cash provided by (used in):
    Operating activities..............................  $     322  $   1,527  $   6,366         --  $  (3,040) $    (122)        --
    Investing activities..............................     (6,731)   (12,146)   (62,467)        --    (57,773)   (49,723)        --
    Financing activities..............................      6,315     10,618     58,830         --     62,094     48,143         --
OTHER FINANCIAL DATA:
  Capital expenditures................................  $   6,862  $  11,868  $  62,220         --  $  57,773  $  49,723         --
  Adjusted EBITDA (2).................................      1,757      4,301      7,232  $  20,192      1,496      8,438  $  12,443
  Adjusted EBITDA/interest expense (2)................        2.9x       2.9x       1.6x       1.9x       1.4x       2.0x       2.3x
  Ratio of earnings to fixed charges (3)..............        1.0        1.1         --         --         --         --         --
BALANCE SHEET DATA (AS OF PERIOD END):
  Working capital.....................................  $   1,612  $   1,081  $   2,496         --         --  $   4,266  $  13,757
  Total assets........................................     13,290     24,904     87,367         --         --    135,047    146,434
  Total debt..........................................     12,034     23,613     71,494         --         --    122,365    100,365
  Redeemable members' capital.........................         --         --     11,576         --         --     13,171         --
  Members' capital....................................         51       (747)    (7,445)        --         --    (11,326)        --
  Pro forma stockholders' equity......................         --         --         --         --         --         --     35,232
  ACNTA (4)...........................................                                                                      200,923
  Ratio of ACNTA to total debt........................         --         --         --         --         --         --        2.0x
</TABLE>
 
- ------------------------------
(1)  Assumes  that  the 1995  Acquisition, the  1996 Acquisition,  the Corporate
     Reorganization (as defined in "The Company-- Corporate Reorganization") and
     the Offerings and the application of proceeds therefrom had taken place  on
     June  30, 1996 for  purposes of the  Balance Sheet Data  (to the extent not
     already reflected) and as of January  1, 1995 for purposes of Statement  of
     Operations Data and Other Financial Data.
(2)  Adjusted  EBITDA and the  ratio of Adjusted EBITDA  to interest expense are
     presented because of  their wide  acceptance as financial  indicators of  a
     company's  ability  to  service or  incur  debt. Adjusted  EBITDA  (as used
     herein) is  calculated  by  adding interest,  income  taxes,  depreciation,
     depletion   and  amortization,   exploration  and   abandonment  costs  and
     extraordinary loss  resulting from  extinguishment of  debt to  net  income
     (loss).  The ratio of Adjusted EBITDA  to interest expense is calculated by
     dividing Adjusted EBITDA  by interest. Interest  includes interest  expense
     accrued  and amortization of deferred  financing costs. Adjusted EBITDA and
     the ratio of Adjusted EBITDA to  interest expense should not be  considered
     as  alternatives  to  earnings  (loss), or  operating  earnings  (loss), as
     defined by generally accepted accounting  principles, as indicators of  the
     Company's financial performance or to cash flow as a measure of liquidity.
(3)  For  purposes  of  calculating  the ratio  of  earnings  to  fixed charges,
     "earnings" are net income (loss)  before extraordinary loss resulting  from
     extinguishment  of debt, plus income taxes and fixed charges. Fixed charges
     are  comprised  of  interest  on  indebtedness,  amortization  of  deferred
     financing  costs,  and that  portion of  operating  lease expense  which is
     deemed  to  be  representative  of   an  interest  factor.  Earnings   were
     insufficient  to cover fixed charges by $4,967,000, $1,924,000 and $646,000
     for the historical periods ended December 31, 1995, June 30, 1995 and  June
     30,  1996,  respectively, and  $7,380,000 and  $410,000  for the  pro forma
     periods ended December 31, 1995 and June 30, 1996, respectively.
(4)  ACNTA means Adjusted  Consolidated Net  Tangible Assets as  defined in  the
     Indenture. See "Description of Notes-- Certain Definitions."
 
                                       8
<PAGE>
                              SUMMARY RESERVE DATA
 
<TABLE>
<CAPTION>
                                                                 AS OF DECEMBER 31,           AS OF APRIL 1, 1996
                                                           -------------------------------  ------------------------
                                                             1993       1994       1995      ACTUAL    PRO FORMA(1)
                                                           ---------  ---------  ---------  ---------  -------------
<S>                                                        <C>        <C>        <C>        <C>        <C>
ESTIMATED PROVED RESERVES (2):
  Oil (MBbls)............................................      2,365      4,009     10,788     11,479        16,477
  Gas (Mmcf).............................................     21,619     27,512     78,152     79,420       112,921
  MBOE...................................................      5,968      8,594     23,813     24,716        35,297
  Percent of proved developed reserves...................       67.0%      62.3%      76.1%      73.9%         78.2%
  Present value of estimated future net cash flow, before
   income taxes, discounted at 10% (in thousands)........  $  26,377  $  36,779  $ 113,296  $ 129,091   $   179,527
  Reserve life index (in years) (3)......................       19.8       14.4       13.6         --            --
RESERVE REPLACEMENT DATA:
  Production replacement ratio (4).......................        513%       540%       969%        --            --
  All-in finding costs per BOE (5).......................  $    4.31  $    3.67  $    3.43  $    2.84   $      3.72
</TABLE>
 
- ------------------------------
 
(1)  Gives effect to the 1996 Acquisition as if such transaction had occurred as
    of April 1, 1996.
 
(2) Estimates of net proved oil and gas  reserves at April 1, 1996 are based  on
    reports  prepared by Williamson  Petroleum Consultants, Inc. ("Williamson"),
    independent petroleum engineers. The 1995 reserve estimates were prepared by
    the Company and such estimates of gross reserves with respect to certain  of
    the  Company's  producing properties  were subject  to  a limited  review by
    Williamson. Prior reserve estimates are based on information compiled by the
    Company. See "Risk Factors  -- Uncertainty of  Estimates of Proved  Reserves
    and  Future  Net  Revenues" and  "Business  and  Properties --  Oil  and Gas
    Reserves."
 
(3) Calculated by dividing year-end proved reserves by annual production for the
    most recent year.
 
(4) Calculated by dividing reserve  additions through acquisitions of  reserves,
    extensions  and discoveries and revisions during  the year by production for
    such year.
 
(5) The average  all-in finding costs  over the period  January 1, 1993  through
    March 31, 1996 (pro forma for the 1996 Acquisition) was $3.60 per BOE.
 
                             SUMMARY OPERATING DATA
 
<TABLE>
<CAPTION>
                                                             YEAR ENDED DECEMBER 31,
                                                  ----------------------------------------------
                                                                                                  SIX MONTHS ENDED JUNE 30,
                                                            HISTORICAL             PRO FORMA(1)              1996
                                                  -------------------------------  -------------  --------------------------
                                                    1993       1994       1995         1995        ACTUAL     PRO FORMA(1)
                                                  ---------  ---------  ---------  -------------  ---------  ---------------
<S>                                               <C>        <C>        <C>        <C>            <C>        <C>
PRODUCTION DATA:
  Oil (MBbls)...................................        158        330        950        2,085          709           990
  Gas (Mmcf)....................................        865      1,600      4,806       11,984        3,504         5,345
  MBOE..........................................        302        597      1,751        4,083        1,293         1,881
AVERAGE SALES PRICE PER UNIT:
  Oil (per Bbl).................................  $   16.93  $   15.25  $   15.53    $   15.75    $   18.93     $   18.26
  Gas (per Mcf).................................       1.82       1.63       1.45         1.59         1.91          1.87
COSTS PER BOE:
  Production costs, including severance taxes
   (2)..........................................  $    5.59  $    3.94  $    5.91    $    6.60    $    6.40     $    7.07
  Depreciation, depletion and amortization......       2.93       3.09       3.40         3.47         3.57          3.71
</TABLE>
 
- ------------------------------
 
(1)  Gives effect to  the 1995 Acquisition  and the 1996  Acquisition as if such
    transactions had occurred as of January 1, 1995.
 
(2) Production costs per BOE in 1995 and for the six months ended June 30,  1996
    were  unusually high as  a result of relatively  high workover expenses with
    respect to properties acquired in the 1995 Acquisition which did not produce
    related production improvements until subsequent periods. Additionally,  the
    Company's 1995 production costs were adversely affected by expenses incurred
    in  connection  with plugging  wells  to comply  with  applicable regulatory
    requirements.
 
                                       9
<PAGE>
                                  RISK FACTORS
 
    PRIOR  TO  MAKING  AN  INVESTMENT  DECISION,  PROSPECTIVE  INVESTORS  SHOULD
CONSIDER  FULLY,  TOGETHER  WITH  THE   OTHER  INFORMATION  CONTAINED  IN   THIS
PROSPECTUS, THE FOLLOWING FACTORS.
 
SIGNIFICANT LEVERAGE AND DEBT SERVICE
 
    As  of  June 30,  1996, as  adjusted for  the Corporate  Reorganization, the
Offerings and the application of the net proceeds therefrom, the Company's total
debt and stockholders' equity would have been $100.4 million and $35.2  million,
respectively. See "Capitalization." In addition, the Company may currently incur
additional indebtedness under its Credit Facility (as defined under "Description
of   Other  Indebtedness").  Immediately  following   the  consummation  of  the
Offerings, the  Credit  Facility  will  afford  the  Company  $50.0  million  of
available  borrowing  capacity, none  of which  is expected  to be  necessary to
finance the Company's existing business  plan. See "Management's Discussion  and
Analysis  of  Financial Condition  and Results  of  Operations --  Liquidity and
Capital Resources" and "Description of Other Indebtedness."
 
    The Company's level of indebtedness  will have several important effects  on
its future operations, including (i) a substantial portion of the Company's cash
flow  from  operations must  be  dedicated to  the  payment of  interest  on its
indebtedness and  will  not be  available  for other  purposes,  (ii)  covenants
contained  in the  Credit Facility  and the  Indenture governing  the Notes will
require the Company to meet certain financial tests, and other restrictions  may
limit  its ability to  borrow additional funds  or to dispose  of assets and may
affect the Company's flexibility  in planning for, and  reacting to, changes  in
its  business, including possible acquisition activities and (iii) the Company's
ability to  obtain  additional financing  in  the future  for  working  capital,
capital expenditures, acquisitions, general corporate purposes or other purposes
may  be  impaired.  See  "Management's  Discussion  and  Analysis  of  Financial
Condition and Results of Operations -- Liquidity and Capital Resources."
 
    The Company's ability to meet its debt service obligations and to reduce its
total indebtedness  will be  dependent upon  the Company's  future  performance,
which  will be subject to general economic conditions and to financial, business
and other factors  affecting the operations  of the Company,  many of which  are
beyond  its control. Earnings  were insufficient to cover  fixed charges by $5.0
million and $0.6 million  for the year  ended December 31,  1995 and six  months
ended June 30, 1996, respectively, and $7.4 million and $0.4 million for the pro
forma  year  ended  December  31,  1995 and  six  months  ended  June  30, 1996,
respectively. Based upon the  current and anticipated  level of operations,  the
Company  believes, however,  that its cash  flow from  operations, together with
amounts available under its Credit Facility and its other sources of  liquidity,
will  be adequate to meet its anticipated requirements in the foreseeable future
for working  capital,  capital  expenditures, interest  payments  and  scheduled
principal  payments.  There can  be no  assurance,  however, that  the Company's
business will continue to generate cash flow at or above current levels. If  the
Company is unable to generate sufficient cash flow from operations in the future
to  service its debt,  it may be required  to refinance all or  a portion of its
existing debt, including the Notes, or to obtain additional financing. There can
be no  assurance  that  any such  refinancing  would  be possible  or  that  any
additional  financing  could be  obtained.  The inability  to  obtain additional
financing could have a  material adverse effect on  the Company. For example,  a
default  by  the Company  under the  terms of  the Indenture  could result  in a
default under the terms of the Credit Facility.
 
EFFECTIVE SUBORDINATION OF NOTES
 
    The obligations  of the  Company  and any  subsidiary  of the  Company  that
guarantees the obligations of the Company under the Credit Facility are expected
to  be  secured by  substantially  all of  the assets  of  the Company  and such
subsidiary. Holders of secured Indebtedness of the Company and its subsidiaries,
including the lenders under the Credit Facility, have claims with respect to the
assets constituting  collateral for  such  Indebtedness that  are prior  to  the
claims  of holders of the Notes. In the event of a default on such Indebtedness,
or  a  bankruptcy,  liquidation  or  reorganization  of  the  Company  and   its
subsidiaries,  such assets will be available to satisfy obligations with respect
to the
 
                                       10
<PAGE>
Indebtedness secured thereby before any payment  therefrom could be made on  the
Notes.  Accordingly, the  Notes will  be effectively  subordinated to  claims of
secured creditors of  the Company  and its subsidiaries  to the  extent of  such
pledged  collateral. As of June  30, 1996, after giving  pro forma effect to the
Corporate Reorganization, the Offerings and the application of the net  proceeds
therefrom,   the  Company  would  have  had   $0.4  million  of  secured  Senior
Indebtedness outstanding and will have $50.0 million available under the  Credit
Facility. See "Description of Notes -- Ranking."
 
    The  Indenture does not require the Subsidiaries to guarantee the payment of
the Notes unless  the Subsidiaries incur  Indebtedness (other than  intercompany
Indebtedness).  The  Indenture prohibits  Subsidiaries  that are  not Subsidiary
Guarantors  from  incurring   Indebtedness.  The  Notes   will  be   effectively
subordinated to claims of creditors (other than the Company) of the Subsidiaries
that   are  not   Subsidiary  Guarantors,  including   trade  creditors,  taxing
authorities and tort claimants. Such  creditors generally will have priority  as
to  the assets of such Subsidiaries over  the claims and equity interests of the
Company and, thereby  indirectly, the  holders of indebtedness  of the  Company,
including  the Notes.  At June  30, 1996,  on a  pro forma  basis, the Company's
Subsidiaries had  $6.5  million of  liabilities,  all of  which  were  operating
liabilities. Any Subsidiary Guarantees will be effectively subordinated in right
of  payment  to  all existing  and  future  secured Senior  Indebtedness  of the
Subsidiary Guarantors. On the date of the Indenture there will be no  Subsidiary
Guarantees.  See "Description of  Notes -- Ranking,"  "-- Subsidiary Guarantees"
and "-- Incurrence of Indebtedness and Preferred Stock."
 
SUBSIDIARY GUARANTEES MAY TERMINATE; FRAUDULENT CONVEYANCE CONSIDERATIONS
RELATING TO SUBSIDIARY GUARANTEES
 
    The Indenture does not require any Subsidiary to guarantee the Notes  unless
such  Subsidiary incurs Indebtedness (other  than intercompany Indebtedness). On
the date  of the  Indenture  there will  be  no Subsidiary  Guarantees.  Various
fraudulent conveyance laws have been enacted for the protection of creditors and
may  be used by  a court of  competent jurisdiction to  subordinate or avoid any
Subsidiary Guarantee that may be delivered. To  the extent that a court were  to
find  that (x) a  Subsidiary Guarantee was  incurred with the  intent to hinder,
delay or  defraud  any  present  or future  creditor  or  that  such  Subsidiary
Guarantor  contemplated insolvency with a design  to favor one or more creditors
to the exclusion in whole or in part of others or (y) a Subsidiary Guarantor did
not receive fair consideration  or reasonably equivalent  value for issuing  its
Subsidiary  Guarantee and, at the time  it issued the Subsidiary Guarantee, such
Subsidiary Guarantor (i) was  insolvent or rendered insolvent  by reason of  the
issuance  of the Subsidiary Guarantee, (ii) was  engaged or about to engage in a
business or  transaction  for which  the  remaining assets  of  such  Subsidiary
Guarantor  constituted unreasonably small capital or (iii) intended to incur, or
believed that it would incur, debts beyond its ability to pay such debts as they
matured, a court could avoid or subordinate the Subsidiary Guarantee in favor of
such Subsidiary  Guarantor's  other  creditors.  Among  other  things,  a  legal
challenge  of the  Subsidiary Guarantee issued  by such  Subsidiary Guarantor on
fraudulent conveyance grounds  may focus on  the benefits, if  any, realized  by
such  Subsidiary Guarantor  as a result  of the  issuance by the  Company of the
Notes. To  the extent  the  Subsidiary Guarantee  was  avoided as  a  fraudulent
conveyance  or held unenforceable for any other reason, the holders of the Notes
would cease to  have any claim  against such Subsidiary  Guarantor and would  be
creditors  solely of the Company and  any Subsidiary Guarantors whose Subsidiary
Guarantees were not avoided or held unenforceable. In such event, the claims  of
the  holders of the Notes against the  issuer of an invalid Subsidiary Guarantee
would be subject  to the  prior payment of  all liabilities  of such  Subsidiary
Guarantor. There can be no assurance that, after providing for all prior claims,
there  would be sufficient  assets to satisfy  the claims of  the holders of the
Notes relating to any avoided portions of any of the Subsidiary Guarantees.
 
    The measure of insolvency for purposes of the foregoing considerations  will
vary  depending upon the law applied in any such proceeding. Generally, however,
a Subsidiary Guarantor  may be  considered insolvent if  the sum  of its  debts,
including  contingent liabilities, was greater than the fair market value of all
of its assets  at a  fair valuation,  if the present  fair market  value of  its
assets was less than
 
                                       11
<PAGE>
the  amount that would be required to pay its probable liability on its existing
debts, including contingent liabilities, as they become absolute and mature,  or
if it had insufficient capital to carry on its business.
 
POTENTIAL INABILITY TO FUND A CHANGE OF CONTROL OFFER AND ASSET SALE OFFER
    The  Company must offer to purchase the Notes upon the occurrence of certain
events. The Indenture provides that upon the occurrence of a Change of  Control,
the  Company is required  to offer to  repurchase any or  all of the outstanding
Notes at  a price  equal to  101%  of the  aggregate principal  amount  thereof,
together  with accrued  and unpaid  interest, if any,  to the  date of purchase.
Generally, a "Change of Control" includes any person or group other than  Cadell
S. Liedtke, Michael J. Grella and Henry G. Musselman, the Chairman of the Board,
President  and Executive Vice President  of the Company, respectively, acquiring
50% or more of the voting securities  of the Company, and certain other  events.
In  the event of certain asset dispositions,  the Company will be required under
certain circumstances  to use  the  Excess Proceeds  (as  defined) to  offer  to
purchase  the Notes at  100% of the  principal amount thereof,  plus accrued and
unpaid interest to the date of purchase. See "Description of Notes -- Repurchase
at the Option of Holders."
 
    The  Credit  Facility  prohibits  the  Company  from  prepaying  the  Notes,
including  prepayments pursuant to a  Change of Control or  Asset Sale. Prior to
commencing such an offer to purchase, the  Company may be required to (i)  repay
in  full all indebtedness of  the Company that would  prohibit the repurchase of
the Notes including that under the Credit Facility, or (ii) obtain any requisite
consent to permit the repurchase. If the Company is unable to repay all of  such
indebtedness  or is  unable to obtain  the necessary consents,  then the Company
will be unable to offer to purchase the Notes, and such failure will  constitute
an  Event of Default under the Indenture.  It is unlikely that the Company would
have sufficient internally generated funds available  at the time of any  Change
of Control or Asset Sale Offer to satisfy all of its debt obligations (including
repurchases  of the  Notes and  payment of  the Credit  Facility) simultaneously
without refinancing the indebtedness.
 
    The events that  constitute a  Change of Control  or require  an Asset  Sale
Offer  under  the Indenture  may  also be  events  of default  under  the Credit
Facility or other senior indebtedness of the Company and the Subsidiaries.  Such
events may permit the lenders under such debt instruments to accelerate the debt
and, if the debt is not paid, to enforce security interests on substantially all
the  assets of the Company and  the Subsidiaries, thereby limiting the Company's
ability to  raise cash  to  repurchase the  Notes,  and reducing  the  practical
benefit  of the  offer to purchase  provisions to  the holders of  the Notes. In
addition, the Change of Control covenant in the Indenture could have the  effect
of  discouraging a takeover of the Company by making such an attempt potentially
more expensive.
 
UNCERTAINTY OF ESTIMATES OF PROVED RESERVES AND FUTURE NET CASH FLOWS
 
    There are numerous uncertainties in estimating quantities of proved reserves
and in  projecting future  rates of  production and  the timing  of  development
expenditures,  including many  factors beyond  the control  of the  Company. The
reserve data  set forth  in this  Prospectus are  estimates only.  Although  the
Company  believes  such  estimates  to  be  reasonable,  reserve  estimates  are
imprecise and should  be expected  to change as  additional information  becomes
available.  Furthermore, estimates  of oil and  gas reserves,  of necessity, are
projections based on engineering data,  and there are uncertainties inherent  in
the  interpretation of such  data as well  as the projection  of future rates of
production and the timing of development expenditures. Reserve engineering is  a
subjective  process of estimating underground accumulations  of oil and gas that
cannot be  exactly measured,  and the  accuracy  of any  reserve estimate  is  a
function  of the  quality of  available data  and of  engineering and geological
interpretation  and  judgment.  Accordingly,   estimates  of  the   economically
recoverable  quantities of oil  and gas attributable to  any particular group of
properties, classifications  of such  reserves based  on risk  of recovery,  and
estimates  of the future net cash flows expected therefrom prepared by different
engineers or by the  same engineers at different  times may vary  substantially.
Moreover,  there can  be no  assurance that the  reserves set  forth herein will
ultimately be produced or that the proved undeveloped reserves will be developed
within the periods  anticipated. Variances from  the estimates contained  herein
could be material. In addition, the estimates of future net revenues from proved
 
                                       12
<PAGE>
reserves  of the Company  and the present  value thereof are  based upon certain
assumptions about production levels, prices and costs, which may not be correct.
The Company emphasizes with respect to such estimates that the discounted future
net cash flows  should not  be construed as  representative of  the fair  market
value  of the proved  oil and gas  properties belonging to  the Company, because
discounted future net cash flows are based upon projected cash flows that do not
provide for changes  in oil and  gas prices  or for escalation  of expenses  and
capital costs. The meaningfulness of such estimates is highly dependent upon the
accuracy  of  the assumptions  upon which  they were  based. Actual  results may
differ materially from  the results estimated.  Prospective purchasers of  Notes
are  cautioned not to place undue reliance  on the reserve data included in this
Prospectus.
 
ACQUISITION RISKS
    The Company's rapid growth  in recent years has  been largely the result  of
acquisitions  of  producing  properties.  The  Company  expects  to  continue to
evaluate and  pursue acquisition  opportunities  available on  terms  management
considers  favorable  to the  Company. The  successful acquisition  of producing
properties requires an assessment  of recoverable reserves,  future oil and  gas
prices, operating costs, potential environmental and other liabilities and other
factors  beyond the Company's control. Such an assessment is necessarily inexact
and its accuracy is inherently uncertain. In connection with such an assessment,
the Company  performs a  review of  the  subject properties  it believes  to  be
generally  consistent with industry practices. Such  a review, however, will not
reveal all existing or potential problems, nor will it permit a buyer to  become
sufficiently familiar with the properties fully to assess their deficiencies and
capabilities. Inspections may not be performed on every well, and structural and
environmental problems are not necessarily observable even when an inspection is
undertaken. The Company is generally not entitled to contractual indemnification
for  preclosing liabilities, including  environmental liabilities, and generally
acquires interests in the properties on an "as is" basis.
 
VOLATILITY OF OIL AND GAS PRICES
    The Company's financial results and,  therefore, its ability to service  its
debt,  including the Notes, are significantly affected by the price received for
the Company's oil and gas production. Historically, the markets for oil and  gas
have  been volatile and may continue to be volatile in the future. Prices of oil
and gas are  subject to  wide fluctuations  in response  to market  uncertainty,
changes  in supply and demand and a  variety of additional factors, all of which
are beyond  the control  of  the Company.  These  factors include  domestic  and
foreign  political conditions, the overall level of supply of and demand for oil
and gas, the price of  imported oil and gas,  weather conditions, the price  and
availability of alternative fuels and overall economic conditions. The Company's
future financial condition and results of operations will be dependent, in part,
upon  the prices received for  the Company's oil and  gas production, as well as
the costs of acquiring,  finding, developing and  producing reserves. To  reduce
its  exposure to price risks in the sale  of its oil and gas, the Company enters
into hedging  arrangements from  time to  time. Although  the Company  hedges  a
significant  portion of its  production, any substantial  or extended decline in
the price of oil and gas would  have a material adverse effect on the  Company's
financial  condition and results of operations, as  well as reduce the amount of
the Company's oil and gas that could be produced economically. Moreover, if  oil
and  gas prices fall materially below  their current levels, the availability of
funds and the Company's  ability to repay outstanding  amounts under its  Credit
Facility and the Notes could be materially adversely affected. See "Management's
Discussion and Analysis of Financial Condition and Results of Operations."
 
CONFLICTS OF INTEREST
    The Company has a continuing relationship with A&P Meter Sales and Services,
Inc.  ("A&P"), a corporation in which  Messrs. Liedtke, Grella and Musselman own
60.0% of the outstanding common stock. A&P owes the Company $437,000  (including
accrued  interest through December 31, 1995) pursuant to a promissory note under
which the Company is  not entitled to any  principal or interest payments  until
December 31, 2004. A&P also owes the Company $247,000, which is represented by a
promissory note payable upon demand. See "Certain Transactions."
 
                                       13
<PAGE>
    Under the Company's current credit arrangements, Messrs. Liedtke, Grella and
Musselman  are each  liable for  a portion of  the Company's  existing debt (see
"Description of Other  Indebtedness") pursuant to  limited guaranties.  However,
these  individuals will not be  liable for, or guarantee  amounts due under, the
Credit Facility or the indebtedness represented by the Notes.
 
DEPENDENCE ON KEY PERSONNEL
    The Company depends to  a large extent on  the services of Messrs.  Liedtke,
Grella and Musselman. The loss of the services of any of Messrs. Liedtke, Grella
or  Musselman could have a material  adverse effect on the Company's operations.
Pursuant  to  employment  agreements  which   are  to  be  effective  upon   the
consummation of the Offerings, Messrs. Liedtke, Grella and Musselman have agreed
not  to compete with the  Company for a one-year  period should they voluntarily
leave the  Company's employment  or should  their employment  be terminated  for
cause  within  the initial  three-year term  of  each employment  agreement. The
Company believes that its success is also dependent upon its ability to continue
to employ and retain skilled technical personnel. See "Management."
 
CONTROL OF THE COMPANY
    If the Offerings are completed,  Messrs. Liedtke, Grella and Musselman  will
own  directly and indirectly, in the  aggregate, 42.6% of the outstanding Common
Stock (or 39.8% if the underwriters'  over-allotment option in the Common  Stock
Offering  is  exercised  in  full).  Accordingly,  Messrs.  Liedtke,  Grella and
Musselman may be  able to exercise  significant influence over  the election  of
directors of the Company and the control of the Company's management, operations
and   affairs.  See  "Security  Ownership   of  Certain  Beneficial  Owners  and
Management."
 
FOREIGN INVESTMENT
    The Company's investment in Moldova involves risks typically associated with
investments in  emerging  markets  such as  foreign  exchange  restrictions  and
currency  fluctuations, foreign taxation, changing political conditions, foreign
and  domestic  monetary  and   tax  policies,  expropriation,   nationalization,
nullification,  modification  or  renegotiation  of  contracts,  war  and  civil
disturbances and other risks that may limit or disrupt markets. In addition,  if
a  dispute arises in its Moldovan operations,  the Company may be subject to the
exclusive jurisdiction of foreign courts or may not be successful in  subjecting
foreign  persons to the jurisdiction of  the United States. The Company attempts
to conduct its business and financial affairs so as to protect against political
and economic risks  applicable to  operations in Moldova,  but there  can be  no
assurance the Company will be successful in so protecting itself.
 
DRILLING RISKS
    Drilling  involves numerous risks,  including the risk  that no commercially
productive oil or gas will be encountered. The cost of drilling, completing  and
operating  wells is often  uncertain, and drilling  operations may be curtailed,
delayed or cancelled as a result  of a variety of factors, including  unexpected
drilling   conditions,  pressure  or  irregularities  in  formations,  equipment
failures or accidents, adverse weather conditions and shortages or delays in the
delivery of  equipment. The  Company's  future drilling  activities may  not  be
successful and, if unsuccessful, such failure may have a material adverse effect
on the Company's future results of operations and financial condition.
 
OPERATING HAZARD AND UNINSURED RISKS
    The  Company's operations are  subject to hazards and  risks inherent in the
drilling for and production and transportation of oil and gas, including  fires,
natural  disasters, explosions, encountering formations with abnormal pressures,
blowouts, cratering, pipeline ruptures, and spills,  any of which can result  in
loss  of hydrocarbons, environmental pollution, personal injury or loss of life,
severe damage to and  destruction of properties of  the Company and others,  and
suspension of operations. Although the Company maintains insurance coverage that
it  considers to  be adequate  and customary  in the  industry, it  is not fully
insured against certain  of these risks,  either because such  insurance is  not
available  or because  of high  premium costs.  The occurrence  of a significant
event not fully covered by insurance could have a material adverse effect on the
Company's financial condition and results of operations.
 
                                       14
<PAGE>
COMPETITION
    The Company  encounters  substantial competition  in  acquiring  properties,
marketing  oil and  gas and  securing trained  personnel. Many  competitors have
substantially larger financial resources,  staffs and facilities. See  "Business
and Properties -- Competition and Markets."
 
GOVERNMENT LAWS AND REGULATIONS
    The  Company's operations are affected from  time to time in varying degrees
by political developments and federal, state and local laws and regulations.  In
particular,  oil and gas  production, operations and economics  are or have been
significantly affected by price controls, taxes  and other laws relating to  the
oil  and gas industry, by changes in  such laws and by changes in administrative
regulations. The Company cannot predict how existing laws and regulations may be
interpreted by enforcement  agencies or court  rulings, whether additional  laws
and  regulations will  be adopted, or  the effect  such changes may  have on its
business, financial  condition  or  results of  operations.  See  "Business  and
Properties -- Regulation."
 
ENVIRONMENTAL REGULATIONS
 
    The  Company's  operations are  subject to  complex and  constantly changing
environmental  laws  and  regulations  adopted  by  federal,  state  and   local
governmental  authorities. The Company  believes that compliance  with such laws
has had no  material adverse effect  upon the Company's  operations to date  and
that  the  cost of  such  compliance has  not  been material.  Nevertheless, the
discharge of oil, gas or other pollutants  into the air, soil or water may  give
rise to significant liabilities on the part of the Company to the government and
third  parties  and  may  require  the Company  to  incur  substantial  costs of
remediation. Moreover, the Company has agreed to indemnify sellers of  producing
properties   from  whom  the  Company  has  acquired  reserves  against  certain
liabilities for  environmental  claims  associated  with  the  properties  being
purchased by the Company, including, without limitation, in connection with both
the  1995 Acquisition and the  1996 Acquisition. No assurance  can be given that
existing  environmental  laws  or  regulations,  as  currently  interpreted   or
reinterpreted  in the future, or future laws or regulations, will not materially
adversely affect the Company's results of operations and financial condition  or
that  material indemnity claims will not  arise against the Company with respect
to  properties  acquired  by  the  Company.  See  "Business  and  Properties  --
Environmental Matters."
 
ABSENCE OF PUBLIC MARKET
 
    There  is no existing public  market for the Notes  and the Company does not
intend to  list the  Notes on  any national  securities exchange.  Although  the
Underwriters  have  advised the  Company that  they currently  intend to  make a
market in  the Notes,  the  Underwriters are  not obligated  to  do so  and  may
discontinue  such  market-making  at  any time.  Accordingly,  there  can  be no
assurance that  an active  market will  develop upon  completion of  this  Notes
Offering  or,  if developed,  that such  market will  be sustained.  The initial
offering price of the Notes will be determined through negotiations between  the
Company  and the Underwriters, and may bear  no relationship to the market price
of the Notes  after the Notes  Offering. Factors such  as quarterly or  cyclical
variations  in  the Company's  financial  condition and  results  of operations,
variations in interest rates, future announcements concerning the Company or its
competitors, government regulation,  general economic and  other conditions  and
developments  affecting the oil and gas industry could cause the market price of
the Notes to fluctuate substantially.
 
FORWARD-LOOKING STATEMENTS
 
    Certain  statements  contained   in  this   Prospectus,  including   without
limitation,   statements   containing  the   words   "believes,"  "anticipates,"
"intends," "expects," and words  of similar import, constitute  "forward-looking
statements".  Such forward-looking  statements involve known  and unknown risks,
uncertainties and other factors that  may cause the actual results,  performance
or  achievements of the Company or industry  to be materially different from any
future results,  performance  or  achievements  expressed  or  implied  by  such
forward-looking  statements.  Certain of  these  factors are  discussed  in more
detail  elsewhere  in  this  Prospectus,  including  without  limitation   under
 
                                       15
<PAGE>
"Prospectus  Summary," "Risk Factors," "Management's  Discussion and Analysis of
Financial Condition and Results of  Operations," and "Business and  Properties".
Given  these  uncertainties, prospective  investors are  cautioned not  to place
undue reliance on  such forward-looking  statements. The  Company disclaims  any
obligation  to update any such factors or to publicly announce the result of any
revisions to any of the  forward-looking statements contained herein to  reflect
future events or developments.
 
                                  THE COMPANY
 
GENERAL
 
    The  Company  is  an  independent  energy company  that  is  engaged  in the
acquisition,  exploration,  exploitation   and  development  of   oil  and   gas
properties.  The Company's primary operations are in the Permian Basin, the Gulf
Coast and the Rocky Mountain regions. The Company recently acquired an  interest
in  an entity which has a concession for the development of mineral interests in
the Republic of Moldova, in Eastern Europe. The Company also has minor interests
in the domestic gas gathering and transmission business.
 
CORPORATE REORGANIZATION
 
    Costilla was  incorporated  in Delaware  in  June 1996  to  consolidate  and
continue the activities previously conducted by Costilla Energy, L.L.C., a Texas
limited  liability company  (the "LLC"), and  its wholly  owned subsidiaries, to
acquire the assets  of CSL  Management Corporation ("CSL")  (which owns  certain
office  equipment  used by  the Company),  and  to acquire  the stock  of Valley
Gathering Company ("Valley"). Costilla has been formed solely for the purpose of
conducting the Offerings, and has not commenced operations. Both CSL and  Valley
are owned by Messrs. Liedtke, Grella and Musselman. See "Certain Transactions."
 
    Contemporaneously  with the  closings of  the Offerings:  (1) the redeemable
membership interests of NationsBanc  Capital Corp. ("NBCC") in  the LLC will  be
redeemed  for  $15.5 million;  (2) the  LLC  will be  merged into  Costilla (the
"Merger") and an aggregate of 5,200,000 shares of Common Stock will be issued to
the four members of  the LLC; (3)  Costilla will acquire all  of the issued  and
outstanding stock of Valley and the assets of CSL for $0.7 million; and (4) $4.3
million in distributions will be made to the members of the LLC, $3.5 million of
which, in the case of Messrs. Liedtke, Grella and Musselman, will be provided to
such  persons  for certain  estimated income  tax effects  of the  Merger. These
transactions are  referred  to  throughout this  Prospectus  as  the  "Corporate
Reorganization." As a result of the Corporate Reorganization, Costilla will have
four  wholly  owned subsidiaries:  (i) Costilla  Petroleum Corporation,  a Texas
corporation ("CPC"), which operates properties owned by Costilla and owns  minor
interests  in  the  same  properties; (ii)  Statewide  Minerals,  Inc.,  a Texas
corporation ("Statewide"), which is engaged in the purchase of small royalty and
mineral interests; (iii) Valley, which owns several small gas gathering systems,
a small  gas processing  plant,  certain salt  water  disposal systems  and  gas
compressors;  and  (iv)  Costilla  Pipeline  Corporation,  a  Texas  corporation
("Pipeline") which owns a gas pipeline in Pennsylvania held for resale. CSL will
be dissolved.  Costilla  and CPC  are  the sole  members  of two  Texas  limited
liability   companies  through  which  the  Company's  Moldovan  operations  are
conducted. Costilla also owns a 40.5%  interest in a Delaware limited  liability
company  which owns  and operates  a gas  pipeline and  associated facilities in
Louisiana.
 
    The Company's  executive offices  are located  at 400  West Illinois,  Suite
1000, Midland, Texas, 79701 and its telephone number is (915) 683-3092.
 
                             COMMON STOCK OFFERING
 
    Concurrent with the Notes Offering, the Company is offering 4,800,000 shares
of  its Common Stock. The Notes Offering  and the Common Stock Offering are each
conditioned upon the consummation of the other.
 
                                       16
<PAGE>
                                USE OF PROCEEDS
 
    The net proceeds of the Offerings are estimated to be $151.5 million ($159.8
million if the underwriters'  over-allotment option with  respect to the  Common
Stock  Offering is  exercised). Approximately  $125.8 million  of such proceeds,
including all the net proceeds of the Notes Offering, will be used to repay  all
of  the  existing  senior  indebtedness of  the  Company  (the  "Existing Debt")
incurred in connection with the 1996 Acquisition, and to refinance its  previous
credit  facility. The  Existing Debt matures  in June  1999. Approximately $30.0
million of  the  Existing Debt  currently  bears  interest at  14.0%  per  annum
(increasing  to 14.5%  on September  13, 1996)  and the  balance currently bears
interest at a rate selected by the  Company equal to a base rate (generally  the
prime  rate established by NationsBank, N.A.) plus 0.75% or LIBOR plus 3.0%. See
"Description of  Other Indebtedness."  In  addition, $20.5  million of  the  net
proceeds  will be used  to pay certain  amounts incurred in  connection with the
Corporate Reorganization, including $15.5  million to redeem certain  membership
interests  of NBCC in the  LLC prior to the Merger,  $0.7 million to acquire the
stock of Valley and the assets of  CSL and $4.3 million in distributions to  the
members  of the  LLC, $3.5  million of  which, in  the case  of Messrs. Liedtke,
Grella and Musselman,  will be provided  to such persons  for certain  estimated
federal  income  tax  effects of  the  Merger. See  "Certain  Transactions." The
remaining estimated net proceeds of $5.1 million will be used by the Company for
general corporate purposes. Pending such  uses, the remaining net proceeds  will
be invested in short-term, investment grade, interest-bearing securities.
 
    The  following is  a description  of sources and  uses of  proceeds from the
Offerings, assuming the underwriters'  over-allotment option in connection  with
the Common Stock Offering is not exercised (in millions):
 
<TABLE>
<S>                                                                  <C>
Sources:
  Notes Offering...................................................  $   100.0
  Common Stock Offering............................................       60.0
                                                                     ---------
                                                                     $   160.0
                                                                     ---------
                                                                     ---------
Uses:
  Refinance Existing Debt..........................................  $   125.8
  Redeem membership interests......................................       15.5
  Distributions to individual members to pay estimated income tax
   liability of such members.......................................        3.5
  Pro rata distribution to remaining member........................        0.8
  Purchase of stock of Valley and assets of CSL....................        0.7
  Working capital..................................................        5.1
  Estimated fees, commissions, underwriting discounts and expenses
   related to the Offerings........................................        8.6
                                                                     ---------
                                                                     $   160.0
                                                                     ---------
                                                                     ---------
</TABLE>
 
                                       17
<PAGE>
                                 CAPITALIZATION
 
    The  following table sets forth the  unaudited capitalization of the Company
as of June  30, 1996, on  an historical basis  and on a  pro forma basis  giving
effect  to the Corporate Reorganization and the Offerings and the application of
the net proceeds therefrom, as if  such transactions had been consummated as  of
June  30,  1996. The  following table  should  be read  in conjunction  with the
Consolidated Financial Statements of the LLC, the unaudited Pro Forma  Condensed
Financial  Statements, the  related notes,  and the  other information contained
elsewhere  in  this   Prospectus,  including  the   information  set  forth   in
"Management's  Discussion  and Analysis  of Financial  Condition and  Results of
Operations." For further information regarding  the terms of the long-term  debt
reflected  in the following  table, see "Description  of Other Indebtedness" and
Note 7 and Note 12 of the Notes to Consolidated Financial Statements.
 
<TABLE>
<CAPTION>
                                                                                               JUNE 30, 1996
                                                                                          ------------------------
                                                                                          HISTORICAL    PRO FORMA
                                                                                          -----------  -----------
                                                                                               (IN THOUSANDS)
<S>                                                                                       <C>          <C>
Long-term debt:
  Existing debt.........................................................................  $   122,365  $       365
  Credit Facility.......................................................................           --           --
  10 1/4% Senior Notes due 2006.........................................................           --      100,000
                                                                                          -----------  -----------
Total long-term debt....................................................................      122,365      100,365
                                                                                          -----------  -----------
Redeemable members' capital.............................................................       13,171           --
                                                                                          -----------  -----------
Members' capital and stockholders' equity:
  Members' capital......................................................................      (11,326)          --
  Preferred stock, $.10 par value (3,000,000 shares authorized; no shares issued or
   outstanding).........................................................................           --           --
  Common Stock, $.10 par value (20,000,000 shares authorized; no shares outstanding
   actual, 10,000,000 shares outstanding pro forma).....................................           --        1,000
  Paid-in capital.......................................................................           --       34,232
                                                                                          -----------  -----------
Total members' capital and stockholders' equity.........................................      (11,326)      35,232
                                                                                          -----------  -----------
Total capitalization....................................................................  $   124,210  $   135,597
                                                                                          -----------  -----------
                                                                                          -----------  -----------
</TABLE>
 
                                       18
<PAGE>
                    PRO FORMA CONDENSED FINANCIAL STATEMENTS
 
    The  unaudited Pro Forma Condensed Financial  Statements of the Company have
been prepared to give effect to  the 1995 Acquisition and the 1996  Acquisition,
the  Corporate  Reorganization, and  the Offerings  and  the application  of the
estimated net proceeds  therefrom as  if such  transactions (to  the extent  not
already  reflected) had  taken place on  June 30,  1996 for purposes  of the Pro
Forma Condensed Balance  Sheet and  as if the  transactions had  taken place  on
January  1,  1995  for  purposes  of  the  Pro  Forma  Condensed  Statements  of
Operations. The Pro Forma Condensed Financial Statements of the Company are  not
necessarily  indicative of  the results for  the periods presented  had the 1995
Acquisition and  the 1996  Acquisition, the  Corporate Reorganization,  and  the
Offerings  and the  application of  the estimated  net proceeds  therefrom taken
place on January  1, 1995. In  addition, future results  may vary  significantly
from  the results  reflected in the  accompanying Pro  Forma Condensed Financial
Statements because of normal production declines, changes in product prices, and
the success  of  future  exploration and  development  activities,  among  other
factors.  This information should  be read in  conjunction with the Consolidated
Financial Statements  of  Costilla  Energy, L.L.C.  and  subsidiaries,  and  the
Statements  of  Revenues  and  Direct Operating  Expenses  with  respect  to the
properties acquired  in  the 1995  Acquisition  and the  1996  Acquisition,  all
included elsewhere herein.
 
                                       19
<PAGE>
                             COSTILLA ENERGY, INC.
 
                 PRO FORMA CONDENSED BALANCE SHEET -- UNAUDITED
 
                                 JUNE 30, 1996
 
                                 (IN THOUSANDS)
<TABLE>
<CAPTION>
                                                                                                       PRO FORMA
                                                                                         PRO FORMA     COSTILLA
                                                                           COSTILLA       OFFERING      ENERGY,
                                ASSETS                                      L.L.C.      ADJUSTMENTS      INC.
- -----------------------------------------------------------------------  -------------  ------------  -----------
<S>                                                                      <C>            <C>           <C>
Current assets:
  Cash and cash equivalents............................................    $   1,164    $     (700)(1)  $  10,655
                                                                                           151,450(3)
                                                                                          (141,259)(4)
  Accounts receivable..................................................        8,785                       8,785
  Prepaid and other current assets.....................................        2,629                       2,629
                                                                         -------------                -----------
      Total current assets.............................................       12,578                      22,069
Oil and gas properties, using the successful efforts method of
 accounting:
  Proved properties....................................................      126,809                     126,809
  Unproved properties..................................................        4,615                       4,615
  Accumulated depreciation, depletion and amortization.................      (13,933)                    (13,933)
                                                                         -------------                -----------
                                                                             117,491                     117,491
Other property and equipment, net......................................        1,640           700(1)      2,340
Deferred charges (Note 2)..............................................        2,654         3,850(3)      3,850
                                                                                            (2,654)(4)
Note receivable -- affiliate...........................................          684                         684
                                                                         -------------                -----------
                                                                           $ 135,047                   $ 146,434
                                                                         -------------                -----------
                                                                         -------------                -----------
 
<CAPTION>
          LIABILITIES, REDEEMABLE MEMBERS' CAPITAL AND EQUITY
- -----------------------------------------------------------------------
<S>                                                                      <C>            <C>           <C>
Current liabilities:
  Current maturities of long-term debt.................................    $      98                   $      98
  Trade accounts payable...............................................        4,587                       4,587
  Undistributed revenue................................................        1,524                       1,524
  Other current liabilities............................................        2,103                       2,103
                                                                         -------------                -----------
      Total current liabilities........................................        8,312                       8,312
Long-term debt, less current maturities................................      122,267       100,000(3)    100,267
                                                                                          (122,000)(4)
Deferred income........................................................        2,623                       2,623
                                                                         -------------                -----------
      Total liabilities................................................      133,202                     111,202
Redeemable members' capital............................................       13,171       (13,171)(4)         --
Members' capital and stockholders' equity:
  Members' capital.....................................................      (11,326)       11,326(2)         --
  Preferred stock, $.10 par value (3,000,000 shares authorized; no
   shares outstanding).................................................           --                          --
  Common Stock, $.10 par value (20,000,000 shares authorized; no shares
   outstanding historical, 10,000,000 shares outstanding pro forma)....           --         1,000(3)      1,000
  Paid-in capital......................................................           --        (2,654)(4)     34,232
                                                                                            (1,829)(4)
                                                                                            (4,259)(4)
                                                                                            54,300(3)
                                                                                           (11,326)(2)
                                                                         -------------                -----------
  Total members' capital and stockholders' equity......................      (11,326)                     35,232
                                                                         -------------                -----------
                                                                           $ 135,047                   $ 146,434
                                                                         -------------                -----------
                                                                         -------------                -----------
</TABLE>
 
 See accompanying notes to unaudited pro forma condensed financial statements.
 
                                       20
<PAGE>
                             COSTILLA ENERGY, INC.
 
            PRO FORMA CONDENSED STATEMENT OF OPERATIONS -- UNAUDITED
 
                          YEAR ENDED DECEMBER 31, 1995
 
                     (IN THOUSANDS, EXCEPT PER SHARE DATA)
<TABLE>
<CAPTION>
                                                                                                PRE OFFERING     PRO FORMA
                                           COSTILLA        1995         1996       PRO FORMA      COSTILLA       OFFERING
                                            L.L.C.      ACQUISITION  ACQUISITION  ADJUSTMENTS      L.L.C.       ADJUSTMENTS
                                         -------------  -----------  -----------  ------------  -------------  -------------
<S>                                      <C>            <C>          <C>          <C>           <C>            <C>
Revenues...............................    $  21,816     $  10,930    $  19,891                   $  52,637
                                         -------------  -----------  -----------                -------------
Expenses:
  Oil and gas production...............       10,355         5,473       11,409    $    (300)(1)      26,937
  General and administrative...........        3,571            --           --         (172)(1)       4,850
                                                                                       1,451(5)
  Compensation related to option
   settlement..........................          656                                                    656
  Exploration and abandonments.........        1,650           109        1,002                       2,761
  Depreciation, depletion and
   amortization........................        5,958            --           --          100(1)      14,176
                                                                                       8,118(6)
  Interest.............................        4,591            --           --       10,046(7)      14,637    $  (14,637)(8)
                                                                                                                   10,635(8)
  Other................................            2            --           --                           2
                                         -------------  -----------  -----------                -------------
                                              26,783         5,582       12,411                      64,019
                                         -------------  -----------  -----------                -------------
Net income (loss) before federal income
 taxes.................................       (4,967)        5,348        7,480                     (11,382)
Provision for federal income taxes.....            3            --           --                           3
                                         -------------  -----------  -----------                -------------
Net income (loss)......................    $  (4,970)    $   5,348    $   7,480                   $ (11,385)
                                         -------------  -----------  -----------                -------------
                                         -------------  -----------  -----------                -------------
Net loss per share.....................
 
<CAPTION>
                                          PRO FORMA
                                          COSTILLA
                                           ENERGY,
                                            INC.
                                         -----------
<S>                                      <C>
Revenues...............................   $  52,637
                                         -----------
Expenses:
  Oil and gas production...............      26,937
  General and administrative...........       4,850
 
  Compensation related to option
   settlement..........................         656
  Exploration and abandonments.........       2,761
  Depreciation, depletion and
   amortization........................      14,176
 
  Interest.............................      10,635
 
  Other................................           2
                                         -----------
                                             60,017
                                         -----------
Net income (loss) before federal income
 taxes.................................      (7,380)
Provision for federal income taxes.....           3
                                         -----------
Net income (loss)......................   $  (7,383)
                                         -----------
                                         -----------
Net loss per share.....................   $    (.75)
                                         -----------
                                         -----------
</TABLE>
 
 See accompanying notes to unaudited pro forma condensed financial statements.
 
                                       21
<PAGE>
                             COSTILLA ENERGY, INC.
 
            PRO FORMA CONDENSED STATEMENT OF OPERATIONS -- UNAUDITED
 
                         SIX MONTHS ENDED JUNE 30, 1996
 
                     (IN THOUSANDS, EXCEPT PER SHARE DATA)
 
<TABLE>
<CAPTION>
                                                                                                                     PRO FORMA
                                                                                       PRE OFFERING    PRO FORMA     COSTILLA
                                             COSTILLA        1996        PRO FORMA       COSTILLA       OFFERING      ENERGY,
                                              L.L.C.      ACQUISITION   ADJUSTMENTS       L.L.C.      ADJUSTMENTS      INC.
                                           -------------  -----------  --------------  -------------  ------------  -----------
<S>                                        <C>            <C>          <C>             <C>            <C>           <C>
Revenues.................................    $  19,525     $   9,223                     $  28,748                   $  28,748
Expenses:
  Oil and gas production.................        8,278         5,167    $    (150)(1)       13,295                      13,295
  General and administrative.............        2,809                        (86)(1)        3,010                       3,010
                                                                              287(5)
  Exploration and abandonments...........          308           247                           555                         555
  Depreciation, depletion and
   amortization..........................        4,620                         50(1)         6,981                       6,981
                                                                            2,311(6)
  Interest...............................        4,156                      2,832(7)         6,988     $  (6,988)(8)      5,317
                                                                                                           5,317(8)
                                           -------------  -----------                  -------------                -----------
                                                20,171         5,414                        30,829                      29,158
                                           -------------  -----------                  -------------                -----------
Net income (loss) before federal income
 taxes...................................         (646)        3,809                        (2,081)                       (410)
                                           -------------  -----------                  -------------                -----------
Net income (loss)........................    $    (646)    $   3,809                     $  (2,081)                  $    (410)
                                           -------------  -----------                  -------------                -----------
                                           -------------  -----------                  -------------                -----------
Net income (loss) per share..............                                                                            $   (0.04)
                                                                                                                    -----------
                                                                                                                    -----------
</TABLE>
 
 See accompanying notes to unaudited pro forma condensed financial statements.
 
                                       22
<PAGE>
                             COSTILLA ENERGY, INC.
          NOTES TO UNAUDITED PRO FORMA CONDENSED FINANCIAL STATEMENTS
 
NOTE 1. -- BASIS OF PRESENTATION
    The  Pro  Forma  Condensed Financial  Statements  of the  Company  have been
prepared to give effect  to the 1995 Acquisition  and the 1996 Acquisition,  the
Corporate  Reorganization and the Offerings and the application of estimated net
proceeds therefrom, as if such transactions had taken place on June 30, 1996 for
purposes of the  Pro Forma Condensed  Balance Sheet (with  the exception of  the
1995 Acquisition which was previously reflected in the balance sheet of Costilla
Energy,  L.L.C.), and as if each of  the transactions had taken place on January
1, 1995 for purposes  of the Pro Forma  Condensed Statements of Operations.  The
1995 Acquisition and 1996 Acquisition are accounted for by the purchase method.
 
        Costilla  L.L.C.  -- Represents  the  consolidated balance  sheet of
    Costilla Energy, L.L.C.  and subsidiaries as  of June 30,  1996 and  the
    related  consolidated  statements  of  operations  for  the  year  ended
    December 31, 1995 and the six months ended June 30, 1996.
 
        1995 Acquisition  -- Represents  the revenues  and direct  operating
    expenses  of the  properties acquired  in the  1995 Acquisition  for the
    period from  January  1,  1995  to  June 12,  1995  (date  of  the  1995
    Acquisition).
 
        1996  Acquisition --  Represents the  revenues and  direct operating
    expenses of  the properties  acquired in  the 1996  Acquisition for  the
    period  from  January  1,  1995  to June  14,  1996  (date  of  the 1996
    Acquisition).
 
NOTE 2. -- PRO FORMA ENTRIES
 
    (1) To record the acquisition of Valley Gathering Company and CSL Management
Corporation from certain members  of Costilla Energy, L.L.C.  and to record  the
related  additional depreciation and amortization, and  reduction in oil and gas
production and general and administrative expenses.
 
    (2) To  reflect  the  Corporate Reorganization  including  the  transfer  of
members' capital to stockholders' equity.
 
    (3)  To reflect the issuance of 4,800,000  shares of Common Stock at a price
of $12.50 per  share for  estimated proceeds  of $55,300,000,  net of  estimated
expenses   of  the  Common  Stock  Offering,   and  issuance  of  the  Notes  at
$100,000,000; and  to  reflect payment  of  related debt  issuance  expenses  of
$3,850,000.
 
    (4)  To  record the  repayment of  the  Existing Debt  and the  write-off of
related debt issuance costs,  the distribution of cash  to certain members,  and
the  repurchase of redeemable members capital for approximately $15,000,000 from
proceeds of the Offerings.
 
    The redemption amount is composed of the following:
 
<TABLE>
<S>                                                             <C>
Redeemable members' interest subject to preferred return......  $11,000,000
Redeemable members' interest not subject to preferred
 return.......................................................    1,500,000
Accrued 15% preferred return including associated 10%
 redemption premium...........................................    2,500,000
                                                                -----------
                                                                $15,000,000
                                                                -----------
                                                                -----------
</TABLE>
 
    (5) Estimated incremental general  and administrative expenses necessary  to
administer  the  properties  acquired  in the  1995  and  1996  acquisitions and
increased public reporting and administration costs include salary and  benefits
for  one executive level employee and  revised compensation arrangements for the
remaining executives, approximately 29 additional administrative personnel  (the
majority  of  which were  added prior  to December  31, 1995),  directors' fees,
insurance coverage and estimated costs to administer shareholder communications.
 
                                       23
<PAGE>
                             COSTILLA ENERGY, INC.
    NOTES TO UNAUDITED PRO FORMA CONDENSED FINANCIAL STATEMENTS (CONTINUED)
 
NOTE 2. -- PRO FORMA ENTRIES (CONTINUED)
    (6) To record  estimated incremental  depletion expense  for the  properties
acquired  in the  1995 Acquisition  from January 1,  1995 through  June 12, 1995
(date of  the 1995  Acquisition) and  for the  properties acquired  in the  1996
Acquisition  from  January 1,  1995  through June  14,  1996 (date  of  the 1996
Acquisition).
 
    (7) To  adjust interest  expense to  reflect additional  borrowings for  the
properties  acquired in the  1995 Acquisition from  January 1, 1995  to June 12,
1995 (date of the 1995 Acquisition) and for the properties acquired in the  1996
Acquisition   from  January  1,  1995  to  June  14,  1996  (date  of  the  1996
Acquisition). The adjustment also reflects adjusted interest expense due to  the
Existing  Debt. Also  included is  the amortization  of estimated  debt issuance
costs of $2,728,000 over a three-year period.
 
    Incremental interest expense includes the following components:
 
<TABLE>
<CAPTION>
                                                             YEAR ENDED      SIX MONTHS ENDED
                                                          DECEMBER 31, 1995    JUNE 30, 1996
                                                          -----------------  -----------------
<S>                                                       <C>                <C>
Additional interest on borrowings associated with the
 1995 Acquisition for the period of January 1 to June
 12, 1995 (Average rate 10.0%)..........................     $     2,350         $      --
Additional interest on borrowings for the 1996
 Acquisition through June 14, 1996 (including Tranche B
 interest ranging from 14% to 16.5%)....................           6,750             2,300
Adjustment of average interest rate on previously
 existing debt and amortization of loan fees............             946               532
                                                                --------           -------
                                                             $    10,046         $   2,832
                                                                --------           -------
                                                                --------           -------
</TABLE>
 
    (8) To reverse interest on the Existing Debt and to adjust interest  expense
to  reflect issuance of the  Notes at 10.25% plus  the amortization of estimated
debt issuance costs over 10 years ($385,000 annually).
 
NOTE 3. -- INCOME TAXES
    Upon consummation of  the Corporate Reorganization,  the Company intends  to
account  for income taxes  pursuant to the  provisions of SFAS  109. At June 30,
1996, the pro forma tax basis  of the Company's assets and liabilities  exceeded
the  pro forma book  basis by approximately $5,000,000.  The pro forma temporary
differences are primarily related  to the differences in  book and tax basis  of
oil  and gas properties due to the expensing of intangible development costs for
tax purposes and other income tax differences arising from the tax treatment  of
oil and gas producing activities.
 
NOTE 4. -- NET LOSS PER SHARE
    Net  loss per share  is calculated based  on the pro  forma weighted average
shares outstanding  during  the  respective  periods.  Weighted  average  shares
reflect  the pro  forma issuance of  936,000 shares  of Common Stock  to NBCC on
February 17, 1995 and the pro forma issuance of 4,264,000 shares of Common Stock
to the remaining holders prior to January 1, 1995. In addition, the issuance  of
4,800,000  shares in the Common Stock Offering is assumed to have taken place on
January 1, 1995 and assumes that the Underwriters' over-allotment option is  not
exercised.
 
NOTE 5. -- SUPPLEMENTAL OIL AND GAS RESERVE INFORMATION
    The  estimates of  proved oil  and gas  reserves, which  are located  in the
United States, were prepared by  the Company as of  December 31, 1993, 1994  and
1995, and Williamson as of April 1, 1996.
 
                                       24
<PAGE>
                             COSTILLA ENERGY, INC.
    NOTES TO UNAUDITED PRO FORMA CONDENSED FINANCIAL STATEMENTS (CONTINUED)
 
NOTE 5. -- SUPPLEMENTAL OIL AND GAS RESERVE INFORMATION (CONTINUED)
Reserves  were  estimated  in  accordance  with  guidelines  established  by the
Securities and Exchange Commission and FASB which require that reserve estimates
be prepared under existing economic  and operating conditions with no  provision
for  price and cost escalations, except by contractual arrangements. The Company
has presented the pro forma reserve  estimates utilizing an oil price of  $17.79
per  Bbl and a gas  price of $2.03 per  Mcf as of December  31, 1995, and an oil
price of $20.91 per Bbl and  a gas price of $2.02 per  Mcf as of April 1,  1996.
The  pro forma information assumes  that both the 1995  Acquisition and the 1996
Acquisition took place on January 1, 1995.
 
OIL AND GAS PRODUCING ACTIVITIES
 
    Oil and gas reserve quantity estimates are subject to numerous uncertainties
inherent in  the  estimation  of  quantities  of  proved  reserves  and  in  the
projection  of  future  rates  of  production  and  the  timing  of  development
expenditures. The accuracy  of such estimates  is a function  of the quality  of
available  data and of  engineering and geological  interpretation and judgment.
Results of subsequent drilling, testing  and production may cause either  upward
or  downward revision of previous estimates.  Further, the volumes considered to
be commercially  recoverable  fluctuate with  changes  in prices  and  operating
costs.  The Company emphasizes  that reserve estimates  are inherently imprecise
and that estimates of new discoveries are more imprecise that those of currently
producing oil and gas properties.  Accordingly, these estimates are expected  to
change as additional information becomes available in the future.
 
<TABLE>
<CAPTION>
                                                                              OIL AND            GAS
                                                                        CONDENSATE (MBBLS)     (MMCF)
                                                                        -------------------  -----------
<S>                                                                     <C>                  <C>
Balance, January 1, 1995..............................................           17,990         115,281
  Revisions of previous estimates.....................................             (570)            425
  Extensions and discoveries..........................................              605           8,922
  Production..........................................................           (2,085)        (11,984)
                                                                                -------      -----------
Balance, December 31, 1995............................................           15,940         112,644
  Revisions of previous estimates.....................................              437           2,615
  Extensions and discoveries..........................................              592             296
  Production..........................................................             (492)         (2,634)
                                                                                -------      -----------
Balance, April 1, 1996................................................           16,477         112,921
                                                                                -------      -----------
                                                                                -------      -----------
Proved Developed Reserves:
  December 31, 1995...................................................           13,235          87,345
  April 1, 1996.......................................................           13,552          84,369
</TABLE>
 
STANDARDIZED MEASURE OF DISCOUNTED FUTURE NET CASH FLOWS RELATING TO PROVED OIL
AND GAS RESERVES
 
    The  standardized measure of discounted future net cash flows is computed by
applying period-end prices of oil and  gas (with consideration of price  changes
only to the extent provided by contractual arrangements) to the estimated future
production  of proved oil  and gas reserves less  estimated future production of
proved oil and gas reserves less estimated future expenditures (based on period-
end costs) to be incurred in developing and producing the proved reserves,  less
estimated  future income tax expenses (based  on period-end statutory tax rates,
with consideration of  future tax rates  already legislated) to  be incurred  on
pretax  net cash flows less  tax basis of properties  and available credits, and
assuming continuation of existing economic conditions. The estimated future  net
cash  flows are  then discounted  using a rate  of 10%  per year  to reflect the
estimated timing of the future cash flows.
 
    Discounted future  cash  flow  estimates  like those  shown  below  are  not
intended  to represent estimates  of the fair  value of oil  and gas properties.
Estimates of fair value should also consider
 
                                       25
<PAGE>
                             COSTILLA ENERGY, INC.
    NOTES TO UNAUDITED PRO FORMA CONDENSED FINANCIAL STATEMENTS (CONTINUED)
 
NOTE 5. -- SUPPLEMENTAL OIL AND GAS RESERVE INFORMATION (CONTINUED)
probable reserves,  anticipated  future  oil and  gas  prices,  interest  rates,
changes  in development  and production costs  and risks  associated with future
production. Because  of these  and other  considerations, any  estimate of  fair
value is necessarily subjective and imprecise.
 
<TABLE>
<CAPTION>
                                                                           DECEMBER 31,    MARCH 31,
                                                                               1995           1996
                                                                           ------------  --------------
                                                                                  (IN THOUSANDS)
<S>                                                                        <C>           <C>
Future cash flows........................................................   $  516,515    $    572,426
Future costs:
  Production.............................................................     (239,388)       (253,348)
  Development............................................................      (20,907)        (22,076)
                                                                           ------------  --------------
Future net cash flows before income taxes (a)............................      256,220         297,002
Future income taxes......................................................      (48,735)        (63,418)
                                                                           ------------  --------------
Future net cash flows....................................................      207,485         233,584
10% annual discount for estimated timing of cash flows...................      (66,851)        (76,359)
                                                                           ------------  --------------
Standardized measure of discounted net cash flows........................   $  140,634    $    157,225
                                                                           ------------  --------------
                                                                           ------------  --------------
</TABLE>
 
- ------------------------
(a) Present  value of estimated future net cash flows, before income taxes would
    be $155,984  and  $179,527 as  of  December 31,  1995  and March  31,  1996,
    respectively.
 
Changes in Standardized Measure of Discounted Future Net Cash Flows From Proved
Reserves:
 
<TABLE>
<CAPTION>
                                                                           YEAR ENDED    THREE MONTHS
                                                                          DECEMBER 31,  ENDED MARCH 31,
                                                                              1995           1996
                                                                          ------------  ---------------
                                                                                 (IN THOUSANDS)
<S>                                                                       <C>           <C>
Increase (decrease):
  Extensions and discoveries and improved recovery, net of future
   production and development costs.....................................   $    9,598     $     6,002
  Accretion of discount.................................................       14,147           3,516
  Net change in sales prices, net of production costs...................        2,992          20,807
  Changes in estimated future development costs.........................       (1,651)           (238)
  Revisions of quantity estimates.......................................       (2,392)          4,694
  Net change in income taxes............................................        1,633          (9,563)
  Sales, net of production costs........................................      (27,055)         (7,264)
  Changes of production rates (timing) and other........................        1,893          (1,363)
                                                                          ------------  ---------------
    Net increase (decrease).............................................         (835)         16,591
Standardized measure of discounted future net cash flows:
  Beginning of period...................................................      141,469         140,634
                                                                          ------------  ---------------
  End of period.........................................................   $  140,634     $   157,225
                                                                          ------------  ---------------
                                                                          ------------  ---------------
</TABLE>
 
                                       26
<PAGE>
                         SELECTED FINANCIAL INFORMATION
 
    The  following table sets forth selected  financial data of Costilla Energy,
L.L.C. See  "Management's Discussion  and Analysis  of Financial  Condition  and
Results of Operations." The historical information should be read in conjunction
with  the  Consolidated  Financial  Statements and  the  notes  thereto included
elsewhere in  this  Prospectus.  Costilla Energy,  L.L.C.  acquired  significant
producing  oil  and gas  properties in  certain of  the periods  presented which
affect the comparability of the historical financial and operating  information.
The  historical results are  not necessarily indicative  of the Company's future
operations or financial results.
 
<TABLE>
<CAPTION>
                                                                                                    SIX MONTHS ENDED
                                                          YEAR ENDED DECEMBER 31,                       JUNE 30,
                                           -----------------------------------------------------  --------------------
                                             1991       1992       1993       1994       1995       1995       1996
                                           ---------  ---------  ---------  ---------  ---------  ---------  ---------
                                                                  (IN THOUSANDS, EXCEPT RATIOS)
<S>                                        <C>        <C>        <C>        <C>        <C>        <C>        <C>
STATEMENT OF OPERATIONS DATA:
  Operating revenues.....................  $   1,623  $   2,362  $   4,231  $   7,637  $  21,693  $   5,568  $  19,445
  Total revenues.........................      2,134      2,887      4,397      7,836     21,816      5,573     19,525
  Expenses:
    Oil and gas production...............        769      1,340      1,688      2,351     10,355      2,413      8,278
    General and administrative...........        354        388        952      1,184      3,571      1,008      2,809
    Compensation related to option
     settlement..........................         --         --         --         --        656        656         --
    Exploration and abandonments.........        106          4        218        793      1,650      1,007        308
    Depreciation, depletion and
     amortization........................        494        404        884      1,847      5,958      1,367      4,620
    Interest.............................        179        365        605      1,458      4,591      1,046      4,156
    Other................................         --         --         --         --          2         --         --
                                           ---------  ---------  ---------  ---------  ---------  ---------  ---------
  Income (loss) before income taxes and
   extraordinary item....................        232        386         50        203     (4,967)    (1,924)      (646)
  Net income (loss)......................        234        368         73        163     (4,970)    (1,924)    (2,286)
STATEMENT OF CASH FLOWS DATA:
  Net cash provided by (used in):
    Operating activities.................  $     276  $     140  $     322  $   1,527  $   6,366  $  (3,040) $    (122)
    Investing activities.................     (2,659)    (1,432)    (6,731)   (12,146)   (62,467)   (57,773)   (49,723)
    Financing activities.................      2,440      1,450      6,315     10,618     58,830     62,094     48,143
OTHER FINANCIAL DATA:
  Capital expenditures...................  $   3,092  $   3,720  $   6,862  $  11,868  $  62,220  $  57,773  $  49,723
  Distributions to members...............         --         --        456        961         55         55         --
  Adjusted EBITDA (1)....................      1,011      1,159      1,757      4,301      7,232      1,496      8,438
  Adjusted EBITDA/interest expense (1)...        5.6x       3.2x       2.9x       2.9x       1.6x       1.4x       2.0x
  Ratio of earnings to fixed charges
   (2)...................................        1.3        1.5        1.0        1.1         --         --         --
BALANCE SHEET DATA (AS OF PERIOD END):
  Working capital........................  $    (580) $     185  $   1,612  $   1,081  $   2,496         --  $   4,266
  Total assets...........................      4,602      6,675     13,290     24,904     87,367         --    135,047
  Total debt.............................      3,610      5,352     12,034     23,613     71,494         --    122,365
  Redeemable members' capital............         --         --         --         --     11,576         --     13,171
  Members' capital.......................        504        434         51       (747)    (7,445)        --    (11,326)
</TABLE>
 
- ------------------------------
(1) Adjusted EBITDA  and the ratio  of Adjusted EBITDA  to interest expense  are
    presented  because of  their wide  acceptance as  financial indicators  of a
    company's ability to service or incur debt. Adjusted EBITDA (as used herein)
    is calculated by adding interest, income taxes, depreciation, depletion  and
    amortization,  exploration  and  abandonment  costs  and  extraordinary loss
    resulting from extinguishment  of debt to  net income (loss).  The ratio  of
    Adjusted  EBITDA  to interest  expense  is calculated  by  dividing Adjusted
    EBITDA  by  interest.  Interest   includes  interest  expense  accrual   and
    amortization  of deferred financing costs. Adjusted  EBITDA and the ratio of
    Adjusted EBITDA to interest expense should not be considered as alternatives
    to earnings (loss), or  operating earnings (loss),  as defined by  generally
    accepted  accounting principles,  as indicators  of the  Company's financial
    performance or to cash flow as a measure of liquidity.
 
(2) For  purposes  of  calculating  the ratio  of  earnings  to  fixed  charges,
    "earnings"  are net income  (loss) before extraordinary  loss resulting from
    extinguishment of debt, plus income  taxes and fixed charges. Fixed  charges
    are   comprised  of  interest  on  indebtedness,  amortization  of  deferred
    financing costs, and that portion of operating lease expense which is deemed
    to be representative of  an interest factor.  Earnings were insufficient  to
    cover   fixed  charges  by  $4,967,000,  $1,924,000  and  $646,000  for  the
    historical periods ended December 31, 1995, June 30, 1995 and June 30, 1996,
    respectively.
 
                                       27
<PAGE>
          MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION
                           AND RESULTS OF OPERATIONS
 
GENERAL
 
    Costilla  is  an  independent  energy company  engaged  in  the exploration,
acquisition and development of oil and gas properties. The Company's predecessor
began operating  in 1988  and through  mid-1995 had  grown primarily  through  a
series  of small acquisitions of oil and  gas properties and the exploitation of
those properties. In June 1995, Costilla consummated the 1995 Acquisition for  a
purchase  price  of approximately  $46.6  million, and  in  June 1996,  the 1996
Acquisition was consummated for a purchase price of approximately $42.5 million.
 
    To date, the Company has achieved its high rate of growth primarily  through
acquisitions.  This has impacted  its reported financial results  in a number of
ways. Properties sold by others  frequently have not received focused  attention
prior  to sale. After  acquisition, certain of  these properties are  in need of
maintenance,  workovers,   recompletions  and   other  remedial   activity   not
constituting  capital expenditures, which substantially increase lease operating
expenses. The increased production and revenue resulting from these expenditures
is predominately realized  in periods subsequent  to the period  of expense.  In
addition,  the rapid growth of the Company has required it to develop operating,
accounting and administrative personnel compatible with its increased size.  The
Company  believes it has  now achieved a  sufficient size to  expand its reserve
base without a corresponding increase in its general and administrative expense.
The Company also believes it now has a sufficient inventory of prospects and the
professional staff necessary to  follow a more  balanced program of  exploration
and exploitation activities to complement its acquisition efforts.
 
    Costilla's  strategy is to increase its oil and gas reserves, production and
cash flow  from operations  through  a two-pronged  approach which  combines  an
active  exploration  program with  the  acquisition and  exploitation  of proved
reserves. In  addition,  Costilla  continues  to  evaluate  the  acquisition  of
undeveloped   acreage  for  its  exploration   efforts.  Costilla  has  in-house
exploration expertise  using 3-D  seismic technology  to identify  new  drilling
opportunities as well as for the exploitation of acquired properties.
 
    Costilla  has shown a significant  increase in its oil  and gas reserves and
production, especially due to the 1995 Acquisition and the 1996 Acquisition. The
following table sets forth  certain operating data of  Costilla for the  periods
presented:
 
<TABLE>
<CAPTION>
                                                                                         SIX MONTHS ENDED
                                                          YEAR ENDED DECEMBER 31,            JUNE 30,
                                                      -------------------------------  --------------------
                                                        1993       1994       1995       1995       1996
                                                      ---------  ---------  ---------  ---------  ---------
<S>                                                   <C>        <C>        <C>        <C>        <C>
OIL AND GAS PRODUCTION:
  Oil (MBbls).......................................        158        330        950        233        709
  Gas (Mmcf)........................................        865      1,600      4,806      1,233      3,504
  MBOE..............................................        302        597      1,751        438      1,293
AVERAGE SALES PRICES (1):
  Oil (per Bbl).....................................  $   16.93  $   15.25  $   15.53  $   16.12  $   18.93
  Gas (per Mcf).....................................       1.82       1.63       1.45       1.46       1.91
PRODUCTION COST (2):
  Per BOE (3).......................................  $    5.59  $    3.94  $    5.91  $    5.51  $    6.40
  Per dollar of sales...............................       0.40       0.31       0.48       0.43       0.43
DEPRECIATION, DEPLETION AND AMORTIZATION:
  Per BOE...........................................  $    2.93  $    3.09  $    3.40  $    3.12  $    3.57
  Per dollar of sales...............................       0.21       0.24       0.27       0.25       0.24
</TABLE>
 
- ------------------------------
(1)  Before deduction of production taxes and net of hedging results.
 
(2)  Excludes depreciation, depletion and amortization. Production cost includes
     lease   operating  expenses  and  production   and  ad  valorem  taxes,  if
     applicable.
 
(3)  Production costs per BOE in 1995 and for the six months ended June 30, 1996
     were unusually high as a result  of relatively high workover expenses  with
     respect  to  properties  acquired in  the  1995 Acquisition  which  did not
     produce  related   production   improvement   until   subsequent   periods.
     Additionally,  the Company's 1995 production  costs were adversely affected
     by expenses  incurred in  connection  with plugging  wells to  comply  with
     applicable regulatory requirements.
 
                                       28
<PAGE>
    Costilla  uses the successful  efforts method of accounting  for its oil and
gas activities. Costs to acquire mineral interests in oil and gas properties, to
drill and equip exploratory wells that  result in proved reserves, and to  drill
and  equip development wells  are capitalized. Costs  to drill exploratory wells
that do  not result  in  proved reserves,  geological, geophysical  and  seismic
costs,  and costs  of carrying and  retaining unproved  properties are expensed.
Capitalized costs  of  producing  oil  and  gas  properties,  after  considering
estimated  dismantlement and abandonment costs and estimated salvage values, are
depreciated and depleted using the  unit-of-production method. Unproved oil  and
gas  properties that are individually  significant are periodically reviewed for
impairment of value,  and a  loss is  recognized at  the time  of impairment  by
providing an impairment allowance. Other unproved properties are amortized based
on the Company's experience of successful drilling and average holding period.
 
    The  Company utilizes option contracts to  hedge the effect of price changes
on a portion of  its future oil  and gas production.  Premiums paid and  amounts
receivable  under the option contracts are amortized  and accrued to oil and gas
sales, respectively. If market prices of oil and gas exceed the strike price  of
put  options,  the  options  will expire  unexercised,  therefore,  reducing the
effective price  received for  oil and  gas sales  by the  cost of  the  related
option.  Conversely, if market  prices of oil  and gas decline  below the strike
price of put options, the options  will be exercised, therefore, increasing  the
effective  price received for oil and gas sale by the proceeds received from the
related option. The  net effect  of the Company's  commodity hedging  activities
reduced  oil  and  gas  revenues  by  $9,000,  $80,000,  $80,000  and  $854,000,
respectively, for the years ended December 31, 1994 and 1995, and the six months
ended June 30, 1995 and 1996 and  increased oil and gas revenues by $71,000  for
the  year  ended  December  31,  1993.  See  "Business  and  Properties  -- Risk
Management."
 
    The Company utilizes interest rate  swap agreements to reduce the  potential
impact  of  increases in  interest rates  on floating-rate,  long term  debt. If
market rates of interest experienced during  the applicable swap term are  below
the  rate  of interest  effectively fixed  by  the swap  agreement, the  rate of
interest incurred  by the  Company will  exceed the  rate that  would have  been
experienced under the Credit Agreement. The net effect of the Company's interest
rate  hedging activities increased interest expense by $8,000 for the year ended
December 31, 1995 and $359,000 for the six months ended June 30, 1996.
 
    The Company's  predecessors  were  classified as  partnerships  for  federal
income tax purposes. Therefore, no income taxes were paid or provided for by the
Company  prior to the Offerings.  Future tax amounts, if  any, will be dependent
upon several factors,  including but  not limited  to the  Company's results  of
operations.
 
RESULTS OF OPERATIONS
 
  SIX MONTHS ENDED JUNE 30, 1996 COMPARED TO SIX MONTHS ENDED JUNE 30, 1995
 
    The  Company's total oil and gas revenues  for the six months ended June 30,
1996 were  $19,445,000,  representing an  increase  of $13,877,000  (249%)  over
revenues  of $5,568,000  for the  comparable period  in 1995.  This increase was
primarily  due  to  the  1995  Acquisition  which  accounted  for  approximately
$11,046,000  of the revenue increase. Prior to  accounting for the impact of the
1995 Acquisition  and the  1996 Acquisition,  the Company's  total oil  and  gas
revenues  for the six months  ended June 30, 1996  increased by $1,912,000 (45%)
over the same period in 1995.
 
    Oil and gas production  was 1,293 MBOE  in the 1996  period compared to  438
MBOE  in the 1995 period.  Of the 855 MBOE  increase, approximately 800 MBOE was
due to the  properties acquired in  the 1995 Acquisition.  The remainder of  the
increase  was due  to a  combination of  successful drilling  activities and the
enhancement of existing production.
 
    Interest and other revenues were $40,000  for the six months ended June  30,
1996  compared  to $5,000  for the  comparable period  in 1995,  representing an
increase of $35,000, which was primarily
 
                                       29
<PAGE>
comprised of an increase in interest income of $33,000 in 1996 due to  increased
funds  earning interest. Also in the 1996  period, the Company realized gains of
$40,000 on various transactions for which there were no comparable  transactions
for the six months ended June 30, 1995.
 
    Oil  and gas production costs in the  1996 period were $8,278,000 ($6.40 per
BOE), compared to $2,413,000 in 1995  ($5.51 per BOE), representing an  increase
of  $5,865,000 (243%),  due principally  to the 1995  Acquisition. On  a per BOE
basis, production  costs increased  $0.89  due primarily  to costs  incurred  to
exploit  the properties acquired  in the 1995 Acquisition  which did not produce
related production improvement for the full period.
 
    General and administrative expense  for the six months  ended June 30,  1996
was   $2,809,000,  representing  an  increase  of  $1,801,000  (179%)  from  the
comparable period in  1995 of $1,008,000.  The increase is  primarily due to  an
increase  in personnel and related costs  necessary to accommodate the increased
activities of the Company due to the 1995 Acquisition and in anticipation of the
1996 Acquisition.
 
    Results of  operations  for the  six  months  ended June  30,  1995  include
non-cash  compensation  expense of  $656,000 deemed  to have  been accrued  to a
minority owner  of  the  Company who  was  deemed  to have  benefited  from  the
cancellation of an option to purchase an additional interest in the Company held
by the other minority owner.
 
    Exploration and abandonment expense decreased to $308,000 in the 1996 period
compared to $1,007,000 in 1995. The Company incurred $4,000 of seismic costs for
the six months ended June 30, 1996, compared to $514,000 which were incurred for
the  comparable  period  in 1995.  Dry  hole  costs decreased  from  $493,000 to
$304,000 for the comparable periods in 1995 and 1996, respectively.
 
    Depreciation, depletion and  amortization expense  for the  1996 period  was
$4,620,000  compared to $1,367,000 for the 1995 period, representing an increase
of $3,253,000 (238%). During 1996,  depreciation, depletion and amortization  on
oil and gas production was provided at an average rate of $3.57 per BOE compared
to  $3.12  per  BOE  for  1995.  The increase  was  due  primarily  to  the 1995
Acquisition.
 
    Interest expense was $4,156,000 in  the 1996 period, compared to  $1,046,000
for   the  comparable  period  in  1995.  The  $3,110,000  (297%)  increase  was
attributable primarily to  increased levels of  debt which the  Company used  to
finance the 1995 Acquisition. The average amounts of applicable interest-bearing
debt  for  the  comparable  periods  in  1996  and  1995  were  $77,646,000  and
$25,145,000, respectively.
 
    Results of operations  for the  six months ended  June 30,  1996 include  an
extraordinary  charge of $1,640,000  related to the  early extinguishment of the
Company's  previous  credit  agreement.  The  charge  consisted  of   previously
capitalized  debt issuance costs. The previous  credit agreement was replaced by
the Existing Debt.
 
  YEAR ENDED DECEMBER 31, 1995 COMPARED TO YEAR ENDED DECEMBER 31, 1994
 
    The Company's  total  oil  and  gas  revenues  for  1995  were  $21,693,000,
representing  an increase of  $14,056,000 (184%) over  revenues of $7,637,000 in
1994. This increase was  primarily due to the  1995 Acquisition which  accounted
for approximately $13,373,000 of the revenue increase.
 
    Oil  and gas production was 1,751 MBOE in  1995 and 597 MBOE in 1994. Of the
1,154 MBOE increase, 1,099 MBOE was due  to the properties acquired in the  1995
Acquisition.
 
    Interest  and other  revenues were $123,000  in 1995 compared  to $87,000 in
1994, representing  an increase  of $36,000  (41%), which  was comprised  of  an
increase  in interest income  of $59,000 in  1995 due to  an increased amount of
funds earning  interest, partially  offset  by a  decrease  of other  income  of
$23,000. In 1994, the Company realized a gain of $112,000 on the sale of various
properties for which there were no comparable gains in 1995.
 
    Oil  and  gas production  costs in  1995 were  $10,355,000 ($5.91  per BOE),
compared to $2,351,000  in 1994  ($3.94 per  BOE), representing  an increase  of
$8,004,000 (340%). The major portion of the
 
                                       30
<PAGE>
increase  was due to increased production  associated with the 1995 Acquisition.
In addition, certain acquired properties  required remedial workovers and  other
activity  immediately following acquisition resulting in unusual operating costs
of approximately  $600,000 during  1995. In  addition, $1,605,000  of  operating
costs were incurred in connection with properties acquired in late 1994.
 
    General  and administrative expense for 1995 was $3,571,000, representing an
increase of $2,387,000 (202%) from 1994  expense of $1,184,000. The increase  is
primarily  due  to  an increase  in  personnel  and related  costs  necessary to
accommodate the increased activities of the Company due to the 1995 Acquisition.
 
    Results of operations for the year ended December 31, 1995 include  non-cash
compensation expense of $656,000 deemed to have been accrued to a minority owner
of  the Company in connection with the  cancellation of an option to purchase an
additional interest in the Company held by the other minority owner.
 
    Exploration and abandonment expense increased to $1,650,000 in 1995 compared
to $793,000 in 1994. The increase  of $857,000 (108%) was comprised  principally
of $790,000 of seismic costs.
 
    Depreciation,  depletion and  amortization expense  for 1995  was $5,958,000
compared to $1,847,000 for 1994, representing an increase of $4,111,000  (233%).
During  1995, depreciation, depletion and amortization on oil and gas production
was provided at an average rate of $3.40  per BOE compared to $3.09 per BOE  for
1994. The increase was due primarily to the 1995 Acquisition.
 
    Interest  expense was $4,591,000 in 1995 compared to $1,458,000 in 1994. The
$3,133,000 (215%) increase was  attributable to increased  levels of debt  which
the  Company  used  to finance  the  1995  Acquisition. The  average  amounts of
applicable  interest-bearing  debt  in  1995  and  1994  were  $49,972,000   and
$17,632,000, respectively.
 
  YEAR ENDED DECEMBER 31, 1994 COMPARED TO YEAR ENDED DECEMBER 31, 1993
 
    The  Company's  total  oil  and  gas  revenues  for  1994  were  $7,637,000,
representing an  increase of  $3,406,000 (81%)  over revenues  of $4,231,000  in
1993.  The  primary  reason  for  the  increase  in  revenues  was  due  to  two
acquisitions of properties in  1994, one of which  occurred in January 1994  and
the other in October 1994.
 
    Oil  and gas  production was  597 MBOE  in 1994  and 302  MBOE in  1993. The
increase in production of  295 MBOE was principally  due to properties  acquired
during 1994.
 
    Interest  and other  revenues were  $87,000 in  1994 compared  to $56,000 in
1993. The increase of $31,000 was comprised of an increase in interest income of
$26,000 in 1994,  due to  increased funds  earning interest,  and an  additional
$5,000 in other income.
 
    Oil  and  gas production  costs  in 1994  were  $2,351,000 ($3.94  per BOE),
compared to $1,688,000  in 1993  ($5.59 per  BOE), representing  an increase  of
$663,000.  The increase  in production  costs is  primarily attributable  to two
acquisitions in 1994.
 
    In 1994, general and administrative expense was $1,184,000, representing  an
increase of $232,000 (24%) from 1993 expense of $952,000. The increase is due to
an  increase in  personnel and costs  related primarily to  acquisitions made in
1994.
 
    Exploration and abandonment expense increased  to $793,000 in 1994  compared
to  $218,000 in 1993. The increase of $575,000  (264%) was due to an increase in
non-productive wells drilled in 1994 compared to 1993.
 
    Depreciation, depletion  and amortization  expense for  1994 was  $1,847,000
compared  to $884,000  for 1993,  representing an  increase of  $963,000 (109%),
primarily due to increased production. During 1994, depreciation, depletion  and
amortization  expense on oil and gas production  was provided at an average rate
of $3.09 per BOE  compared to $2.93 per  BOE for 1993. The  increase was due  to
increased   drilling  and   development,  and  the   acquisition  of  additional
properties.
 
                                       31
<PAGE>
    Interest expense was $1,458,000  in 1994 compared to  $605,000 in 1993.  The
$853,000 increase was attributable to increased debt levels related primarily to
the  Company's acquisition  of additional  oil and  gas properties  in 1994. The
average amount  of  applicable  interest-bearing  debt  in  1994  and  1993  was
$17,632,000 and $8,258,000, respectively.
 
LIQUIDITY AND CAPITAL RESOURCES
 
  NET CASH USED IN OPERATING ACTIVITIES
 
    For  the  six  months  ended  June 30,  1996,  net  cash  used  in operating
activities decreased to $0.1 million from $3.0 million for the comparable period
in 1995. Cash  provided by operations,  before changes in  operating assets  and
liabilities,  increased to  $4.2 million  from $0.1  million for  the comparable
period in 1995 due primarily to the 1995 Acquisition and the increase in results
of operations therefrom.
 
  NET CASH USED IN INVESTING ACTIVITIES
 
    Net cash used in investing activities for the six months ended June 30, 1996
was  $49.7  million.  Approximately  $42.5   million  was  used  for  the   1996
Acquisition,  $5.2 million was used for other  oil and gas expenditures and $2.0
million was used for other property  and equipment. For the year ended  December
31, 1995, net cash used in investing activities was $62.5 million. Approximately
$46.6  million was used  for the 1995 Acquisition,  $14.9 million for additional
acquisitions of producing oil and gas properties and exploration and development
activities and $1.0 million primarily for other property and equipment.
 
NET CASH PROVIDED BY FINANCING ACTIVITIES
 
    The Company entered into  a $125.0 million senior  credit agreement in  June
1996,  against which  $122.0 million  was initially  funded. Approximately $74.5
million was for the extension and  refinancing of prior debt, $42.5 million  was
used  for  the 1996  Acquisition  and approximately  $5.0  million was  used for
general corporate purposes.
 
  CAPITAL SOURCES
 
    Funding for the Company's business activities has historically been provided
by bank financings, cash  flow from operations,  private equity sales,  property
divestitures   and  joint  ventures  with  industry  participants.  The  Company
completed  a  $10.0   million  private  equity   placement  in  February   1995.
Subsequently,  the 1995 Acquisition and  the 1996 Acquisition were substantially
funded  by  bank  financings.  The  Company  plans  to  finance  its  continuing
operations and execute its business strategy with cash flow from operations, net
proceeds from the Offerings and borrowings under the Credit Facility.
 
    While  the Company  regularly engages  in discussions  relating to potential
acquisitions, the Company has no present agreement, commitment or  understanding
with  respect to any such acquisition, other than the acquisition of undeveloped
acreage and royalty  and overriding royalty  interests in its  normal course  of
business.  Any future acquisition  may require additional  financing and will be
dependent upon financing arrangements available at the time.
 
    The Company believes that cash flow  from operations will be sufficient  for
anticipated  operating  and capital  expenditure requirements.  However, because
future cash flows and the availability of  financing are subject to a number  of
variables  beyond  the Company's  control, there  can be  no assurance  that the
Company's capital resources  will be  sufficient to  maintain currently  planned
levels  of capital expenditures. The Company's historical and pro forma earnings
for the year ended December 31, 1995 and the six months ended June 30, 1996 were
insufficient to  cover  fixed  charges. Although  the  Company's  earnings  were
insufficient to cover fixed charges for these periods, the Company does not have
covenants  in  the Indenture  or the  Credit Facility  requiring the  Company to
maintain a specific ratio of earnings to fixed charges. However, if the  Company
is  unable to generate sufficient cash flow from operations to service its debt,
it may be  required to refinance  all or a  portion of its  debt, including  the
Notes,  or to obtain  additional financing. There  can be no  assurance that any
such refinancing would  be possible or  that any additional  financing could  be
obtained. See "Risk Factors -- Significant Leverage and Debt Service."
 
                                       32
<PAGE>
    The  Company has received a commitment  from NationsBank of Texas, N.A. (the
"Bank") to provide the Credit Facility  to the Company following the  Offerings.
The  Credit  Facility will  provide  for a  revolving  line of  credit  with the
availability of funds and  letters of credit being  subject to a borrowing  base
determination  at  least semiannually.  The borrowing  base  will provide  for a
maximum availability of $50.0 million (which  amount is also expected to be  the
initial borrowing base), none of which is expected to be outstanding immediately
following  the  Offerings. Availability  under the  borrowing base  is initially
limited to $20.0 million for working capital and $30.0 million for  acquisitions
of  oil and  gas properties  meeting certain  criteria established  by the Bank.
Borrowings under the Credit Facility will bear interest at the Company's  option
at  a floating rate which is at or above the NationsBank, N.A. prime rate or the
LIBOR rate,  depending on  the percentage  of committed  funds which  have  been
borrowed.  Interest will be payable quarterly and principal will be amortized in
twelve equal  installments  commencing  two years  following  the  execution  of
definitive  loan  documents.  Under the  Credit  Facility, the  Company  will be
obligated to pay certain fees to the  Bank, including a commitment fee based  on
the unused portion of the commitment. The Credit Facility will contain customary
restrictive  covenants (including restrictions  on the payment  of dividends and
the incurrence  of additional  indebtedness)  and will  require the  Company  to
maintain a current ratio of not less than 1.0 to 1.0, a ratio of Adjusted EBITDA
to  interest expense  of not  less than 2.0  to 1.0  and a  minimum tangible net
worth. At June 30, 1996, on a pro forma basis, the Company's current ratio would
have been 2.7 to  1.0, the ratio  of Adjusted EBITDA  to interest expense  would
have  been 2.3 to 1.0 and the Company would have exceeded the tangible net worth
test by $1.4 million. The  Company believes it will  be in compliance with  such
covenants  on the date of closing of  the Offerings. Borrowings under the Credit
Facility will be secured by substantially all  of the assets of the Company  and
any  subsidiary of the  Company that guarantees  the Company's obligations under
the  Credit  Facility.  Initially,  none  of  the  Company's  subsidiaries  will
guarantee  the  Company's  obligations  under the  Credit  Facility.  The Bank's
commitment is  subject  to  certain  conditions,  including  completion  of  the
Offerings  and the Corporate Reorganization and  application of the net proceeds
therefrom to  repay  the  Company's  prior secured  indebtedness.  See  "Use  of
Proceeds."
 
    Although  certain of  the Company's  costs and  expenses may  be affected by
inflation, inflationary costs have not had a significant effect on the Company's
results of operations.
 
  CAPITAL EXPENDITURES
 
    The Company requires capital primarily for the exploration, development  and
acquisition of oil and gas properties, the repayment of indebtedness and general
working capital needs.
 
    The  following  table  sets  forth  costs incurred  by  the  Company  in its
development,  exploration  and   acquisition  activities   during  the   periods
indicated.
 
<TABLE>
<CAPTION>
                                                                                           SIX MONTHS
                                                            YEARS ENDED DECEMBER 31,          ENDED
                                                         -------------------------------    JUNE 30,
                                                           1993       1994       1995         1996
                                                         ---------  ---------  ---------  -------------
                                                                       (IN THOUSANDS)
<S>                                                      <C>        <C>        <C>        <C>
Development costs......................................  $      --  $      --  $     158   $       607
Exploration costs......................................      2,017      2,167      5,627         3,881
Acquisition costs:
  Unproved properties..................................        829      1,232      1,742         1,712
  Proved properties....................................      4,665      9,649     52,470        41,791
                                                         ---------  ---------  ---------  -------------
Total..................................................  $   7,511  $  13,048  $  59,997   $    47,991
                                                         ---------  ---------  ---------  -------------
                                                         ---------  ---------  ---------  -------------
</TABLE>
 
    The  Company anticipates that costs incurred  for 1996 will be approximately
$64.8 million, of which  approximately $42.5 million was  expended for the  1996
Acquisition,  and  approximately  $5.2  million  was  expended  for acquisition,
exploration and  development activities  during the  six months  ended June  30,
1996.
 
  DELIVERY COMMITMENT
 
    In  November 1995, the Company entered  into gas sales agreements whereby it
committed to  delivery of  a total  of 2,379,000  Mmbtu, from  December 1,  1995
through December 1, 1996, for a total fixed price of $3,429,610. Income from the
agreements is recognized in the period of delivery.
 
                                       33
<PAGE>
                            BUSINESS AND PROPERTIES
 
GENERAL
 
    Costilla  is  an  independent  energy company  engaged  in  the exploration,
acquisition and development  of oil  and gas properties.  The Company's  primary
operations are in the Permian Basin area of Texas and New Mexico, the Gulf Coast
and  the Rocky  Mountain regions. The  Company's strategy  focuses on increasing
reserves through targeted exploration programs, the exploitation of its existing
properties  and  selective  property  acquisitions.  In  addition,  the  Company
recently  acquired  an interest  in an  entity  which has  a concession  for the
development of mineral interests in the Republic of Moldova, in Eastern  Europe.
The  Company  also  has  minor  interests  in  the  domestic  gas  gathering and
transmission business.
 
    The Company's  predecessor began  operating  in 1988  with the  strategy  of
acquiring and exploiting undervalued oil and gas properties, and at December 31,
1992  had net proved reserves  of 4.7 MMBOE. Since  January 1, 1993, the Company
has successfully closed seven  transactions for an  aggregate purchase price  of
approximately $101 million. As of April 1, 1996, the Company had total estimated
net  proved reserves (as defined  below) of 16.5 Mmbbls of  oil and 112.9 Bcf of
gas, aggregating 35.3 MMBOE, with a PV-10 Value of approximately $179.5 million,
assuming the 1996 Acquisition (as defined below) had occurred at April 1,  1996.
The  Company also has  a substantial undeveloped  acreage position consisting of
180,704 gross (165,166 net) acres at  June 30, 1996. The Company has  identified
in  excess of  185 drilling  locations of  which 64  are included  in its proved
reserves.
 
    Costilla  has  in-house  exploration   expertise  which  uses  3-D   seismic
technology  as  a  primary  tool  to  identify  drilling  opportunities  and has
experienced high rates of  success in each  of its first  two major 3-D  seismic
drilling  programs. Since 1994, the Company has  drilled 37 wells based on these
3-D surveys,  31  of  which  have been  productive.  The  Company  has  recently
completed  two additional 3-D surveys and intends to commence drilling on one of
these acreage blocks in the second half of 1996. The Company currently plans  to
drill 63 wells through 1997 based on its 3-D surveys.
 
    Since  1993,  Costilla  has  generated significant  growth  in  reserves and
production. The Company increased its  estimated proved reserves from 6.0  MMBOE
at  December 31, 1993  to 35.3 MMBOE  at April 1,  1996 (pro forma  for the 1996
Acquisition), representing a compound annual  growth rate of 114%. This  reserve
growth has been achieved at an average all-in finding cost of $3.60 per BOE over
such period, a level which the Company believes is lower than industry averages.
Concurrently,  the Company increased  its average net  daily production from 827
BOE for the  year ended December  31, 1993 to  10,231 BOE for  the three  months
ended  March  31, 1996  (pro  forma for  the  1996 Acquisition),  representing a
compound annual growth rate of 190%.
 
BUSINESS STRATEGY
 
    The Company's strategy is to increase  its oil and gas reserves,  production
and  cash flow from operations through  a two-pronged approach which combines an
active exploration program  using 3-D seismic  and other technological  advances
with the acquisition and exploitation of producing properties. The Company seeks
to  reduce its operating and commodity risks by holding a geographically diverse
portfolio of properties,  the reserves attributable  to which are  approximately
balanced  between oil and gas. The Company  also seeks to manage the elements of
its business strategy  through the  operation of  a significant  portion of  its
properties,  the use of a  rate of return analysis  and the direct marketing and
hedging of its oil  and gas production. The  elements of the Company's  strategy
may be further described as follows:
 
- - EXPLORATION  EFFORTS.  The Company uses extensive geological and geophysical
  analysis to carefully focus its 3-D  seismic surveys. This focus allows  the
  Company to successfully direct the size and scope of its exploration program
  in  order  to  improve  the likelihood  of  success  while  managing overall
  exploration  costs.  The  Company's  exploration  efforts  are  concentrated
  currently on
 
                                       34
<PAGE>
  known  producing regions.  The Company plans  to drill  24 exploratory wells
  during the  last half  of 1996  and 36  exploratory wells  in 1997.  Capital
  budgeted  for exploration activities is $8.1 million for the last six months
  of 1996 and $10.8 million for 1997.
 
- - EXPLOITATION  ACTIVITIES.    The  Company  is  actively  pursuing   numerous
  exploitation  opportunities within its  existing properties, including areas
  where no  proved reserves  are currently  assigned. Exploitation  activities
  currently  in  progress  include  a  carbon  dioxide  flood,  recompletions,
  workovers and  infill  and  horizontal drilling  and  a  secondary  recovery
  project.  The Company's capital  budget for such  activities is $8.4 million
  for the last six months  of 1996 and $9.2  million for 1997, which  includes
  the  drilling of 12  development wells in  1996 and 13  development wells in
  1997.
 
- - PROPERTY ACQUISITIONS.   The Company seeks  to acquire producing  properties
  where  it has identified  opportunities to increase  production and reserves
  through both  exploitation  and  exploration  activities.  The  Company  has
  increased  the  value  of  its  acquisitions  by  aggressively  managing the
  operations of existing proved properties and by successfully identifying and
  developing previously  unproved reserves  on acquired  acreage. The  Company
  seeks  to  acquire  reserves  which will  fit  its  existing  portfolio, are
  generally not being actively marketed and  where a negotiated sale would  be
  the  method of  purchase. The  Company does  not rely  on major  oil company
  divestitures or property auctions.
 
- - PROPERTY DIVERSIFICATION.   The Company  holds a  portfolio of  oil and  gas
  properties  located  in the  Permian  Basin, the  Gulf  Coast and  the Rocky
  Mountain regions. The Company believes that by conducting its activities  in
  distinct  regions it is able to reduce commodity price and other operational
  risks. The Company's Moldovan interest is an extension of this strategy  and
  can be characterized by low initial costs, significant reserve potential and
  the  availability of  technical data  that may  be further  developed by the
  Company.
 
- - CONTROL OF OPERATIONS.  The Company prefers to operate and own the  majority
  working  interest in its properties. This allows the Company greater control
  over  future  development,  drilling,  completing  and  lifting  costs   and
  marketing  of  production.  At April  1,  1996, the  Company  operated wells
  constituting approximately 72% of its total  PV-10 Value (pro forma for  the
  1996 Acquisition).
 
SIGNIFICANT ACQUISITIONS
 
    1995  ACQUISITION.  In  a $46.6 million acquisition  completed in June 1995,
the Company acquired a group  of oil and gas  properties located in the  Permian
Basin,  Gulf Coast and Rocky  Mountain regions. At the  date of acquisition, the
net proved reserves included 7.1 Mmbbls of oil and 44.1 Bcf of gas,  aggregating
14.4  MMBOE. From  the date  of acquisition  until March  31, 1996,  the Company
produced 1.1  MMBOE from  the acquired  properties  and sold  a portion  of  the
acquired  properties for approximately  $3.6 million. At April  1, 1996, the net
proved reserves  of  the remaining  properties  were 13.4  MMBOE.  The  acquired
properties also included 103,010 gross (93,787 net) undeveloped acres.
 
    1996 ACQUISITION.  In June 1996, the Company acquired a group of oil and gas
properties  located primarily  in the Permian  Basin and Gulf  Coast regions for
approximately $42.5  million.  This  acquisition included  properties  with  net
proved  reserves at  April 1, 1996  of 5.0  Mmbbls of oil  and 33.5  Bcf of gas,
aggregating 10.6  MMBOE.  The acquired  properties  also included  42,855  gross
(16,646  net) undeveloped acres and a pipeline located in Pennsylvania which had
an allocated purchase price of $3.5 million.
 
                                       35
<PAGE>
PRINCIPAL PROPERTIES
 
    The following table sets forth certain information, as of April 1, 1996 (pro
forma for the  1996 Acquisition),  which relates to  the principal  oil and  gas
properties owned by the Company.
 
<TABLE>
<CAPTION>
                                                                                 PROVED RESERVES
                                                            ----------------------------------------------------------
                                                                                              TOTAL OIL    PERCENT OF
                                                              GROSS       OIL        GAS     EQUIVALENT    TOTAL OIL
REGION                                                        WELLS     (MBBLS)    (MMCF)      (MBOE)      EQUIVALENT
- ----------------------------------------------------------  ---------  ---------  ---------  -----------  ------------
<S>                                                         <C>        <C>        <C>        <C>          <C>
Permian Basin.............................................      1,890      9,200     55,200      18,400         52.1%
Gulf Coast................................................        968      2,054     38,440       8,461         24.0
Rocky Mountain............................................        236      4,526     12,886       6,674         18.9
Other.....................................................        428        697      6,395       1,762          5.0
                                                            ---------  ---------  ---------  -----------       -----
Total.....................................................      3,522     16,477    112,921      35,297        100.0%
                                                            ---------  ---------  ---------  -----------       -----
                                                            ---------  ---------  ---------  -----------       -----
</TABLE>
 
    PERMIAN  BASIN.  At  April 1, 1996,  52.1% of the  Company's proved reserves
were concentrated in  the Permian  Basin, an approximately  70-county region  in
West  Texas and Southeast  New Mexico. The Company's  production comes from well
known fields such  as the  Spraberry Trend,  Sawyer Canyon,  Goldsmith Unit  and
Susan  Peak. The  majority of the  Company's producing intervals  in the Permian
Basin range from 4,500 feet to 9,500 feet in depth.
 
    The Company  has several  exploratory projects  in the  Permian Basin  based
primarily on 3-D seismic surveys. The most significant include:
 
    EDWARDS/MCELROY  RANCH PROSPECT, ECTOR AND  CRANE COUNTIES, TEXAS.  Costilla
has identified 68 drilling locations on  the Company's 11,513 gross (5,066  net)
acres  in this prospect  based on 3-D  seismic data. Since  January 1, 1996, the
Company has drilled seven successful wells on this prospect, three of which have
resulted in  three separate  field discoveries.  In addition,  these wells  have
confirmed  the Strawn  and Wolfcamp  trends defined  by the  Company's extensive
approximate 50-square mile  3-D seismic project  undertaken jointly with  Texaco
Exploration and Production Inc. ("Texaco"). One additional well is being drilled
on  seismic delineated  features. The  Company plans to  drill 21  wells in this
trend  through  1997.  The  Company's  working  interest  in  this  prospect  is
approximately 44%.
 
    Costilla  and Texaco are also developing  a Queen Sand field identified from
the Edwards/McElroy  Ranch seismic  program. The  four producing  wells  drilled
through June 30, 1996 are producing an aggregate of approximately 80 Bbls of oil
per  day and  the Company  has participated  in the  drilling of  two additional
productive wells subsequent to June 30,  1996. Drilling of six additional  Queen
Sand  wells  is  anticipated  through  1997,  with  the  field  ultimately being
developed on a planned waterflood pattern  in order to maximize recovery of  the
oil in place.
 
    MCGYVER-GREEN  ACRES  PROSPECT,  HOWARD  COUNTY,  TEXAS.    The  Company has
identified 34 drilling locations in  this prospect based on information  derived
from approximately 30 square miles of 3-D seismic data that the Company acquired
on  the area in 1994. The  Talbot Fuller well was the  first well drilled by the
Company on this prospect and was completed in the Canyon Lime formation at 8,200
feet in August  1994. From completion  to June  30, 1996, the  well produced  61
MBbls of oil and 238 Mmcf of gas, and had average capacity of 71 Bbls of oil per
day  and 278 Mcf of gas per day  during June 1996. Subsequent to the first well,
16 additional  wells  have  been  drilled  on this  prospect  of  which  13  are
productive.  The Company is  drilling or intends to  drill five additional wells
during the balance of 1996 on its  9,801 gross (7,057 net) acres. The  Company's
working interest in this prospect averages approximately 72%.
 
    The  following two 3-D programs currently being undertaken by the Company in
the Permian Basin are expected to provide additional drilling locations:
 
    WILSON RANCH 3-D PROJECT, PECOS COUNTY, TEXAS.  The Wilson Ranch is  located
in  northeastern Pecos County,  approximately 10 miles west  of the Yates field.
The Company recently completed an
 
                                       36
<PAGE>
approximate 17-square mile seismic survey on the project. A second phase will be
initiated in the first quarter of  1997. The project presents several  potential
exploration  targets, including  the Queen,  San Andres,  Wolfcamp, Devonian and
Ellenberger formations, found at  depths ranging from 1,600  to 8,000 feet.  The
Company  has agreed to lease  3,750 gross acres on  this 50,000 acre ranch. Upon
acquiring the  lease,  the  Company  intends  to sell  up  to  one-half  of  its
approximate 75% working interest. The Company believes that there is significant
additional potential in this area.
 
    DAVAN  UNIT 3-D PROJECT, STONEWALL COUNTY, TEXAS.  The Company has completed
another 3-D seismic project with Texaco to further develop the  Company-operated
Davan Unit. The project involves a 3-D seismic evaluation of approximately 3,200
gross  acres adjacent  to a  Company-operated waterflood  which has  produced in
excess of three Mmbbls of oil. An exploratory well is scheduled on this prospect
for the first quarter of 1997.
 
    Two examples of the  Company's current exploitation  efforts in the  Permian
Basin include:
 
    EAST  GOLDSMITH FIELD  C02 PROJECT, ECTOR  COUNTY, TEXAS.   The Company owns
3,053 gross  (2,656 net)  acres in  this  field located  20 miles  northwest  of
Midland,  Texas. Since  its discovery,  the field has  produced in  excess of 20
Mmbbls of  oil from  seven formations.  The most  productive zones  in the  East
Goldsmith Field have been the San Andres and Holt formations, both of which have
been  subject  to  secondary recovery  by  waterflooding. The  Company  has been
analyzing a tertiary recovery project in those formations using CO2, and intends
to initiate the  project in the  fourth quarter of  1996. The Company's  working
interest in this project averages approximately 87%.
 
    SUSAN PEAK FIELD WORKOVER AND HORIZONTAL DRILLING PROGRAM, TOM GREEN COUNTY,
TEXAS.   The Company recently completed the  first horizontal well in this field
located south of San  Angelo, Texas, in  which it owns  a 100% working  interest
until  payout. Production  from this  well drilled  in the  Strawn formation was
approximately 68 Bbls of oil per  day and 225 Mcf of  gas per day in July  1996.
With  only two workovers and the new  horizontal well, the Company has increased
Susan Peak production from approximately 31 Bbls  of oil per day and 760 Mcf  of
gas  per day in  the last half of  1995 to an average  rate of approximately 126
Bbls of oil per  day and 1,330  Mcf of gas  per day in  July 1996. Two  possible
horizontal  drilling locations and additional workover candidates remain on this
7,461 gross  (3,730 net)  acre lease.  The Company's  working interest  in  this
project is 50%.
 
    GULF  COAST.  At April 1, 1996,  24.0% of the Company's proved reserves were
concentrated in the Gulf Coast region.  The Company's production in this  region
primarily  comes from  known formations  such as  Frio, Yegua,  Austin Chalk and
Wilcox.
 
    The Company  plans  to use  its  expertise in  aggressively  developing  3-D
opportunities on the extensive acreage position it holds in the region. Examples
of such exploration projects in progress include:
 
    SEALY  PROSPECT, AUSTIN COUNTY, TEXAS.  The Sealy Field, consisting of 3,534
gross and net acres, was acquired in the 1995 Acquisition. The Wilcox  formation
in  this  field  has  produced over  66  Bcf  of gas  and  there  are subsurface
indications of  the presence  of several  fault blocks  that lie  untested.  The
Company's  working interest in  this prospect is 100%.  The Company is currently
attempting to acquire additional acreage in this prospect prior to initiating  a
3-D survey in late 1996 or 1997.
 
    SOUTHWEST  SPEAKS, LAVACA COUNTY, TEXAS.   This project, consisting of 5,078
gross (2,539 net) acres, was also acquired  in the 1995 Acquisition and is  held
by  several  shallow Company-operated  wells.  Multiple producing  horizons from
shallow depths to below 14,000 feet have produced over 122 Bcf of gas from  this
highly  faulted field. A recent well was completed in the Rainbow Wilcox sand on
acreage adjoining  Costilla's  lease. A  well,  in  which Costilla  holds  a  5%
interest  as a result of a farmout, has also been completed on Costilla's lease.
The Company's plans are to conduct a 3-D survey in the Speaks area in late  1996
or 1997. The Company's working interest in this prospect is approximately 50%.
 
                                       37
<PAGE>
    BORCHERS  FIELD,  LAVACA COUNTY,  TEXAS.   This  field  was acquired  by the
Company in  the 1996  Acquisition. The  property  is on  trend with  the  Speaks
project  and is  also a highly  faulted field providing  opportunity for further
development. The Company's lease in the Borchers Field area has produced a total
of 21.2 Bcf of gas from two  Wilcox sands. Costilla has a 100% working  interest
in  this field  consisting of 1,322  gross and  net acres. The  Company plans to
conduct a 3-D survey in the Borchers Field in 1997.
 
    Examples of exploitation activities in this region include:
 
    JOSEY RANCH LEASE,  HARRIS COUNTY,  TEXAS.   Two examples  of the  Company's
production  enhancement  of  Gulf  Coast  properties  were  undertaken  on  this
prospect. When the lease  was acquired in the  1995 Acquisition, production  had
nearly  ceased. Through  a series of  workovers, the Company  has improved daily
capacity, as of June 30, 1996, to 63 Bbls  of oil per day and 73 Mcf of gas  per
day.  In addition, Costilla has  participated in a 10,900  foot test well on the
Josey Ranch lease to test the Wilcox formation. The well was completed in  April
1996  and has consistently produced approximately 1,000  Mcf of gas per day. The
Josey Ranch lease covers 1,661 gross (650 net) acres, and the Company's  working
interest in this prospect is approximately 39%.
 
    PERSONVILLE, LIMESTONE COUNTY, TEXAS.  The Company has recently completed an
11,000  foot Cotton Valley well, which is producing 2.8 Mmcf of gas per day. The
Company is  currently drilling  an additional  well on  this prospect.  Costilla
leases  411  gross (119  net) acres  in  this prospect,  and has  identified one
additional drilling location. The Company is  the operator of this prospect  and
its working interest is approximately 29%.
 
    AUSTIN  CHALK, BRAZOS, BURLESON, FAYETTE AND  LEE COUNTIES, TEXAS.  Costilla
acquired the majority of the working  interest in nine gross Austin Chalk  wells
in  the 1995  Acquisition and  an additional  80 gross  Austin Chalk  wells were
included in the 1996 Acquisition. The  Company intends to enhance production  on
certain  of these wells through stimulation and workover activities, and analyze
further development potential. Costilla has  30,414 gross (20,985 net) acres  in
the  Austin  Chalk  area,  and  its  working  interest  in  this  area  averages
approximately 69%.
 
    ROCKY MOUNTAIN.  At  April 1, 1996, 18.9%  of the Company's proved  reserves
were  concentrated in the  Rocky Mountain region,  which includes Montana, North
Dakota, Wyoming, Colorado and Utah.
 
    The Company  has a  number of  opportunities in  the Rocky  Mountain  region
involving 3-D seismic surveys, exploratory drilling and exploitation activities.
Examples of each of these opportunities are:
 
    RAYMOND  FIELD, SHERIDAN COUNTY, MONTANA.   Since its discovery in 1972, the
Raymond Field  has  produced  over  five  Mmbbls  of  oil  from  five  different
formations.  Daily production from the field has  increased from 179 Bbls of oil
per day since its acquisition in  June 1995 to 368 Bbls  of oil per day in  June
1996  primarily as  a result of  the Company's improved  operations. The Company
plans a 3-D program on  its 960 gross and net  acres in this field. The  Company
owns a 100% working interest in this prospect.
 
    OUTLOOK  FIELD, SHERIDAN COUNTY,  MONTANA.  The  Company undertook its first
Rocky Mountain 3-D  seismic survey in  the Outlook area  to further develop  the
field.  Three  drilling locations  were identified  from  the data.  The Company
anticipates commencing an Outlook test well in October 1996 that will be drilled
to 10,500  feet,  a  depth  sufficient to  test  several  different  formations.
Costilla  leases 5,169 gross (1,292 net) acres in the Outlook prospect, and owns
an approximate 25% working interest in this prospect.
 
    NATURAL BUTTES FIELD, UINTAH COUNTY, UTAH.  The Company owns a 100%  working
interest  in 4,640 gross  and net acres  in this prospect.  Development by prior
owners was  on 640-acre  spacing  while offset  acreage  has been  developed  on
80-acre  spacing. Low gas  prices in the  area have precluded  the assignment of
proved reserves to  any undeveloped acres.  As gas prices  improve, the  Company
plans to drill additional wells on the prospect.
 
                                       38
<PAGE>
    The  Company owns an interest in  significant acreage positions in the Rocky
Mountain region  which are  operated by  third parties  and are  the subject  of
active exploitation efforts. The most significant property is:
 
    CIRCLE  RIDGE FIELD,  FREMONT COUNTY, WYOMING.   The Circle  Ridge Field, in
which the  Company has  an  approximate 18%  working  interest, is  operated  by
Marathon Oil Company. This field is an approximate 1,100 acre waterflood located
in  the Wind River Basin  of Wyoming, approximately 30  miles north of Riverton,
Wyoming. There are 97  active producing wells and  10 active injection wells  in
the  field.  Production  originates  from the  Phosphoria,  Tensleep  and Amsden
formations that are  present at  depths ranging from  500 to  2,000 feet.  Since
January  1995,  45 projects  have been  completed in  the field.  These projects
include recompletions,  stimulation  treatments and  reactivations,  which  have
increased production from 1,469 Bbls of oil per day in January 1995 to a rate of
1,778 Bbls of oil per day for June 1996. The operator has several other projects
scheduled  for the remainder of 1996 and is evaluating various different methods
of enhanced oil recovery for the field.
 
MARKETING ARRANGEMENTS
 
    The Company utilizes an active marketing program for a portion of its  crude
oil  production in order to enhance the net price it receives. The Company sells
its crude oil production from operated  properties in North Dakota, Montana  and
Wyoming,  at the  lease level  to an oil  transportation company  for the posted
price, plus an agreed upon bonus,  with a corresponding agreement to  repurchase
this production at its delivery point (typically, Cushing, Oklahoma) for a price
equal  to the then  posted price for  West Texas Intermediate  crude oil less an
agreed upon  deduction for  transportation and  quality differentials,  if  any,
between  the repurchased  crude oil and  West Texas Intermediate  crude oil. The
Company then employs  a broker to  resell its crude  oil to end  users (such  as
refineries)  on a  month-to-month basis.  The lease  level sales  and repurchase
contracts are  typically of  six  months duration.  With  respect to  its  other
operated  oil production  (primarily located  in Texas),  the Company  employs a
similar price enhancement strategy, although  the repurchase feature is  absent.
Instead,  the lease level  purchaser resells the  crude oil to  end users at the
delivery point for the account of the Company. While these arrangements have the
effect of increasing the net price the Company receives for its crude oil,  such
arrangements  do  not have  the  effect of  limiting  the Company's  exposure to
movements in crude  oil prices. The  Company markets its  gas production at  the
lease  level  pursuant to  month-to-month contracts.  Phibro Energy  USA, Inc.'s
purchases of the Company's oil production  for the year ended December 31,  1995
accounted  for 17.7% of the Company's 1995 consolidated revenues. Because of the
number of crude oil purchasers, the  Company does not anticipate any  difficulty
in replacing Phibro Energy USA, Inc.
 
RISK MANAGEMENT
 
    The  Company typically  employs a  strategy of  purchasing put  options on a
portion of its anticipated oil and gas production. This strategy is designed  to
protect  the  Company from  significant downward  movements in  commodity prices
while preserving the benefit  of rising prices. The  Company does not  establish
hedges  in  excess  of  its anticipated  production.  Upon  consummation  of the
Offerings, substantially  all of  the Company's  debt will  be fixed  rate.  The
Company's  current  position with  regard  to its  1996  commodity hedges  is as
follows:
 
    OIL SALES.  The Company has purchased  put options to provide a floor  price
for  3,000 Bbls  of oil per  day of its  oil production for  August 1996 through
December 1996. These put options currently in place represent approximately  52%
of the Company's estimated oil production for August 1996 through December 1996.
The  floor price  the Company  has an  agreement to  receive is  $18.00 per Bbl,
irrespective of the prices actually paid by  purchasers of the oil at the  lease
level.
 
    GAS  SALES.   The Company  has purchased put  options which  provide a floor
price for 900,000 Mmbtu's per month of its gas production through October  1996.
The  put options currently in place represent approximately 84% of the Company's
estimated gas production for  July 1996 through October  1996. The floor  prices
with  respect to such put options varies from $1.65 to $1.75 per Mmbtu depending
on the area in which the gas is produced.
 
                                       39
<PAGE>
OIL AND GAS RESERVES
 
    The Company's estimated total  proved and proved  developed reserves of  oil
and  gas as of December 31, 1993, 1994 and 1995, and as of April 1, 1996 were as
follows:
 
<TABLE>
<CAPTION>
                                                       AS OF DECEMBER 31,
                               ------------------------------------------------------------------       PRO FORMA
                                                                                                         APRIL 1,
                                        1993                   1994                  1995                1996 (1)
                               ----------------------  --------------------  --------------------  --------------------
                                   OIL         GAS        OIL        GAS        OIL        GAS        OIL        GAS
                                 (MBBLS)     (MMCF)     (MBBLS)    (MMCF)     (MBBLS)    (MMCF)     (MBBLS)    (MMCF)
                               -----------  ---------  ---------  ---------  ---------  ---------  ---------  ---------
<S>                            <C>          <C>        <C>        <C>        <C>        <C>        <C>        <C>
Proved developed producing...       1,785      13,268      2,632     15,757      8,338     50,542     13,122     76,439
Proved developed non-
 producing...................           0           0          0        583        228      6,851        430      7,930
Proved undeveloped...........         580       8,351      1,377     11,172      2,222     20,759      2,925     28,552
                                    -----   ---------  ---------  ---------  ---------  ---------  ---------  ---------
  Total proved...............       2,365      21,619      4,009     27,512     10,788     78,152     16,477    112,921
                                    -----   ---------  ---------  ---------  ---------  ---------  ---------  ---------
                                    -----   ---------  ---------  ---------  ---------  ---------  ---------  ---------
</TABLE>
 
- ------------------------------
(1)  Assumes that the 1996 Acquisition had been consummated at April 1, 1996.
 
    The following table sets forth the future net cash flows from the  Company's
estimated proved reserves:
 
<TABLE>
<CAPTION>
                                                                            DECEMBER 31,              PRO FORMA
                                                                  ---------------------------------   APRIL 1,
                                                                    1993       1994        1995        1996(1)
                                                                  ---------  ---------  -----------  -----------
                                                                                  (IN THOUSANDS)
<S>                                                               <C>        <C>        <C>          <C>
Future net cash flows before income taxes.......................  $  47,213  $  68,596  $   188,337  $   297,002
Future net cash flows before income taxes, discounted at 10%....  $  26,377  $  36,779  $   113,296  $   179,527
</TABLE>
 
- ------------------------------
(1)  Assumes that the 1996 Acquisition had been consummated at April 1, 1996.
 
    The  reserve estimates reflected above for 1993, 1994 and 1995 were prepared
by the Company. The Company's 1995  estimates of gross reserves with respect  to
certain  of the Company's producing properties  were subject to a limited review
by Williamson of the Company's engineering analysis covering approximately 54.0%
of the Company's proved reserves at such date. The pro forma estimates for April
1, 1996,  including  the  properties  acquired in  the  1996  Acquisition,  were
prepared  by Williamson  and are part  of reports  on the Company's  oil and gas
properties prepared by  Williamson, a summary  of which is  set forth herein  as
Appendix A.
 
    The  reserve  data  set forth  herein  present estimates  only.  In general,
estimates of economically recoverable oil and gas reserves and of the future net
revenues therefrom are based upon an number of variable factors and assumptions,
such as historical production from  the subject properties, the assumed  effects
of regulation by governmental agencies and assumptions concerning future oil and
gas  prices and future operating costs, all  of which may vary considerably from
actual  results.  All  such  estimates  are  to  some  degree  speculative,  and
classifications   of  reserves  are  only  attempts  to  define  the  degree  of
speculation  involved.  For  these   reasons,  estimates  of  the   economically
recoverable  oil  and  gas  reserves attributable  to  any  particular  group of
properties, classifications  of such  reserves  based on  risk of  recovery  and
estimates  of the future net revenues  expected therefrom, prepared by different
engineers or by the same engineers  at different times, may vary  substantially.
The Company therefore emphasizes that the actual production, revenues, severance
and  excise taxes,  development and operating  expenditures with  respect to its
reserves will  likely vary  from such  estimates, and  such variances  could  be
material.
 
    Estimates with respect to proved reserves that may be developed and produced
in  the future are often based upon  volumetric calculations and upon analogy to
similar types of reserves rather than
 
                                       40
<PAGE>
actual production history. Estimates based  on these methods are generally  less
reliable than those based on actual production history. Subsequent evaluation of
the same reserves based upon production history will result in variations, which
may be substantial, in the estimated reserves.
 
    In  accordance with applicable  requirements of the  Securities and Exchange
Commission, the estimated discounted future  net revenues from estimated  proved
reserves  are based on  prices and costs as  of the date  of the estimate unless
such prices or costs  are contractually determined at  such date. Actual  future
prices  and costs may be materially higher  or lower. Actual future net revenues
also will be affected  by factors such as  actual production, supply and  demand
for oil and natural gas, curtailments or increases in consumption by natural gas
purchasers,  changes in governmental  regulations or taxation  and the impact of
inflation on costs.
 
EXPLORATION AND DEVELOPMENT ACTIVITIES
 
    The Company  drilled, or  participated  in the  drilling of,  the  following
number  of wells during the periods indicated. At June 30, 1996, the Company was
in the process of drilling two gross (0.49 net) wells and was in the process  of
completing  three gross (1.22 net) wells as producers which are not reflected in
the following table.
<TABLE>
<CAPTION>
                                                              1993                    1994                    1995
                                                     ----------------------  ----------------------  ----------------------
                                                        GROSS        NET        GROSS        NET        GROSS        NET
                                                     -----------  ---------  -----------  ---------  -----------  ---------
<S>                                                  <C>          <C>        <C>          <C>        <C>          <C>
Exploratory:
  Productive.......................................           3        0.83           9        2.27          10        4.58
  Dry..............................................           2        1.06          10        3.73           6        2.57
                                                            ---         ---         ---         ---         ---         ---
    Total..........................................           5        1.89          19        6.00          16        7.15
                                                            ---         ---         ---         ---         ---         ---
                                                            ---         ---         ---         ---         ---         ---
Development:
  Productive.......................................          --          --          --          --           1        0.44
  Dry..............................................          --          --          --          --          --          --
                                                            ---         ---         ---         ---         ---         ---
    Total..........................................          --          --          --          --           1        0.44
                                                            ---         ---         ---         ---         ---         ---
                                                            ---         ---         ---         ---         ---         ---
Total:
  Productive.......................................           3        0.83           9        2.27          11        5.02
  Dry..............................................           2        1.06          10        3.73           6        2.57
                                                            ---         ---         ---         ---         ---         ---
    Total..........................................           5        1.89          19        6.00          17        7.59
                                                            ---         ---         ---         ---         ---         ---
                                                            ---         ---         ---         ---         ---         ---
 
<CAPTION>
 
                                                     SIX MONTHS ENDED JUNE
                                                            30, 1996
                                                     ----------------------
                                                        GROSS        NET
                                                     -----------  ---------
<S>                                                  <C>          <C>
Exploratory:
  Productive.......................................           3        1.74
  Dry..............................................           1        0.72
                                                            ---         ---
    Total..........................................           4        2.46
                                                            ---         ---
                                                            ---         ---
Development:
  Productive.......................................           4        1.98
  Dry..............................................          --          --
                                                            ---         ---
    Total..........................................           4        1.98
                                                            ---         ---
                                                            ---         ---
Total:
  Productive.......................................           7        3.72
  Dry..............................................           1        0.72
                                                            ---         ---
    Total..........................................           8        4.44
                                                            ---         ---
                                                            ---         ---
</TABLE>
 
    The Company  does  not  own  any  drilling rigs  and  all  of  its  drilling
activities  are  conducted by  independent  contractors under  standard drilling
contracts.
 
PRODUCTIVE WELL SUMMARY
 
    The following table  sets forth  the Company's  gross and  net interests  in
productive oil and gas wells as of June 30, 1996. Productive wells are producing
wells and wells capable of production.
 
<TABLE>
<CAPTION>
                                                                                                      ACTUAL (1)
                                                                                                 --------------------
                                                                                                   GROSS       NET
                                                                                                 ---------  ---------
<S>                                                                                              <C>        <C>
Oil wells......................................................................................      2,248     678.54
Gas wells......................................................................................      1,278     231.11
                                                                                                 ---------  ---------
    Total......................................................................................      3,526     909.65
                                                                                                 ---------  ---------
                                                                                                 ---------  ---------
</TABLE>
 
- ------------------------------
(1)  Does  not  include  royalty  and  overriding  royalty  interests  owned  by
     Statewide or the Company. See "-- Other Activities -- Minerals  Acquisition
     Program."  In addition, one well with  multiple completions is counted as a
     single well.
 
                                       41
<PAGE>
ACREAGE
 
    The following table sets forth  certain information regarding the  Company's
developed  and undeveloped  leasehold acreage  as of  June 30,  1996. Acreage in
which the Company's interest is limited to royalty, overriding royalty,  mineral
and similar interests (such as all acreage owned by Statewide) is excluded.
 
<TABLE>
<CAPTION>
                                                  DEVELOPED            UNDEVELOPED              TOTAL
                                             --------------------  --------------------  --------------------
REGION                                         GROSS       NET       GROSS       NET       GROSS       NET
- -------------------------------------------  ---------  ---------  ---------  ---------  ---------  ---------
<S>                                          <C>        <C>        <C>        <C>        <C>        <C>
Permian Basin..............................    126,091     50,151     65,741     59,669    191,832    109,820
Gulf Coast.................................    197,650     65,547     46,040     39,713    243,690    105,260
Rocky Mountain.............................      8,534      6,126     24,757     24,650     33,291     30,776
Other......................................     43,651     26,108     44,166     41,134     87,817     67,242
                                             ---------  ---------  ---------  ---------  ---------  ---------
    Total..................................    375,926    147,932    180,704    165,166    556,630    313,098
                                             ---------  ---------  ---------  ---------  ---------  ---------
                                             ---------  ---------  ---------  ---------  ---------  ---------
</TABLE>
 
OTHER ACTIVITIES
 
    MOLDOVA  CONCESSION  AGREEMENT.    In July  1995,  the  Republic  of Moldova
(located in Eastern Europe between Romania and the Ukraine) granted a Concession
Agreement to Resource Development Company Limited, L.L.C. ("Redeco"), an  entity
not  affiliated with the Company. The Company has paid Redeco $90,000 and agreed
to bear the first $2.0 million of Concession expenses ($1.2 million of which had
been expended through June 30, 1996) in  return for a 50.0% interest in  Redeco.
After the initial $2.0 million expenditure, the Company and the other members of
Redeco are each responsible for bearing 50.0% of future expenses. The Concession
Agreement  covers  the entire  country with  respect  to oil  and gas  and other
minerals and continues for various time  periods depending on the nature of  the
activity  conducted.  In  connection  with  two  previously  producing  but  now
abandoned fields,  Redeco's exclusive  rights continue  for 20  years.  Redeco's
exclusive period to explore throughout the remainder of Moldova expires in 2005,
but  Redeco will  maintain exclusive development  rights with  respect to fields
discovered for a period of 20 years from the date of first production from  such
field.  The Company has no material  fixed financial commitments with respect to
the Concession.
 
    MINERALS  ACQUISITION  PROGRAM.    Statewide,  a  Company  subsidiary,   was
organized  for the purpose  of acquiring overriding  royalty interests and other
types of non  cost-bearing mineral  interests underlying producing  oil and  gas
fields  primarily in Texas. The strategy of such acquisitions is to make blanket
offers to holders  of small  interests. From  inception through  June 30,  1996,
Statewide   expended  approximately  $3.3  million  in  acquiring  interests  in
approximately 1,400 properties.  Through June 30,  1996, Statewide had  received
revenues  from such interests aggregating approximately $1.4 million, as well as
proceeds from sales of such interests of approximately $102,000.
 
    GAS GATHERING AND  TRANSMISSION.   In 1996,  the Company  purchased a  40.5%
membership  interest (which reduces to 32.4%  when the Company and certain other
members recoup their original  investment) in Republic  Gas Partners, L.L.C.,  a
Delaware  limited  liability company  ("Republic"), for  approximately $941,000.
Republic owns  all of  the stock  of Mid  Louisiana Gas  Company, Mid  Louisiana
Marketing  Company and Mid Louisiana Gas Transmission Company (collectively, the
"Midla Companies").  The assets  of the  Midla Companies  include 409  miles  of
mainly  22-inch pipeline extending from the Monroe  field area south of the city
of  Baton  Rouge,  serving  various  Louisiana  and  Mississippi  municipal  and
industrial  customers along its  route. Mid Louisiana  Gas Company's pipeline is
subject  to  the  jurisdiction  of  the  Federal  Energy  Regulatory  Commission
("FERC").
 
    Valley,  a Company  subsidiary, owns a  small gas  gathering system, several
small gas plants,  11 salt water  disposal wells  located in each  of its  three
principal  regions and compressors used in the compression of gas located in the
Gulf Coast region. For the year ended December 31, 1995, Valley had revenues  of
$553,000  and net income of $264,000, substantially all of which were related to
transactions with Costilla.
 
                                       42
<PAGE>
    In the 1996 Acquisition, Pipeline, a Company subsidiary, acquired a 120-mile
gas transportation pipeline in southwestern Pennsylvania for an allocated  value
of $3.5 million. The Company regards this asset as non-strategic to its business
activities and is presently marketing the pipeline for sale.
 
COMPETITION AND MARKETS
 
    Competition  in all areas of the  Company's operations is intense. Major and
independent oil and gas  companies and oil and  gas syndicates actively bid  for
desirable  oil  and gas  properties,  as well  as  for the  equipment  and labor
required to  operate and  develop such  properties. A  number of  the  Company's
competitors   have   financial  resources   and  acquisition,   exploration  and
development budgets that are  substantially greater than  those of the  Company,
which  may  adversely  affect  the  Company's  ability  to  compete  with  these
companies. Many of  the Company's competitors  have been engaged  in the  energy
business  for a much longer time than the Company. Such companies may be able to
pay more for productive oil and gas properties and exploratory prospects and  to
define,  evaluate,  bid for  and  purchase a  greater  number of  properties and
prospects than the Company's financial or human resources permit. The  Company's
ability  to acquire additional properties and to discover reserves in the future
will be dependent on its ability to evaluate and select suitable properties  and
to consummate transactions in a highly competitive environment.
 
    The  market for  oil, gas  and natural gas  liquids produced  by the Company
depends on factors beyond its control, including domestic and foreign  political
conditions,  the overall level of supply of  and demand for oil, gas and natural
gas liquids, the price of imports of oil and gas, weather conditions, the  price
and  availability  of  alternative  fuels, the  proximity  and  capacity  of gas
pipelines and other transportation  facilities and overall economic  conditions.
The  oil and  gas industry  as a  whole also  competes with  other industries in
supplying the  energy  and  fuel  requirements  of  industrial,  commercial  and
individual consumers.
 
REGULATION
 
    The Company's oil and gas exploration, production and related operations are
subject  to extensive  rules and regulations  promulgated by  federal, state and
local agencies. Failure to comply with such rules and regulations can result  in
substantial  penalties.  The  regulatory  burden on  the  oil  and  gas industry
increases the Company's cost  of doing business  and affects its  profitability.
Because  such rules and regulations are frequently amended or reinterpreted, the
Company is unable to predict  the future cost or  impact of complying with  such
laws.
 
    The  State  of Texas  and  many other  states  require permits  for drilling
operations, drilling bonds  and reports concerning  operations and impose  other
requirements  relating to  the exploration and  production of oil  and gas. Such
states also  have  statutes  or  regulations  addressing  conservation  matters,
including  provisions for the unitization or  pooling of oil and gas properties,
the establishment of maximum rates of production from oil and gas wells and  the
regulation  of spacing, plugging and abandonment of such wells. The statutes and
regulations of  certain states  limit  the rate  at which  oil  and gas  can  be
produced from the Company's properties.
 
    FERC  regulates  interstate  natural gas  transportation  rates  and service
conditions, which affect the marketing of  gas produced by the Company, as  well
as  the revenues received by the Company for sales of such production. Since the
mid-1980s, the FERC  has issued a  series of orders,  culminating in Order  Nos.
636,  636-A  and  636-B  ("Order  636"),  that  have  significantly  altered the
marketing  and  transportation  of  gas.   Order  636  mandates  a   fundamental
restructuring of interstate pipeline sales and transportation service, including
the unbundling by interstate pipelines of the sales, transportation, storage and
other  components  of the  city-gate  sales services  such  pipelines previously
performed. One  of the  FERC's purposes  in issuing  the orders  is to  increase
competition within all phases of the gas industry. Order 636 and subsequent FERC
orders  on rehearing have been appealed and are pending judicial review. Because
these orders may  be modified as  a result of  the appeals, it  is difficult  to
predict  the ultimate impact of the orders  on the Company and its gas marketing
efforts. Generally,
 
                                       43
<PAGE>
Order 636  has eliminated  or substantially  reduced the  interstate  pipelines'
traditional  role as wholesalers of natural gas, and has substantially increased
competition and volatility in natural gas markets. While significant  regulatory
uncertainty  remains, Order 636 may ultimately  enhance the Company's ability to
market and  transport its  gas, although  it  may also  subject the  Company  to
greater  competition and the more  restrictive pipeline imbalance tolerances and
greater associated penalties for violation of such tolerances.
 
    Sales of oil and natural  gas liquids by the  Company are not regulated  and
are made at market prices. The price the Company receives from the sale of these
products  is  affected  by the  cost  of  transporting the  products  to market.
Effective as of January 1,  1995, the FERC implemented regulations  establishing
an indexing system for transportation rates for oil pipelines, which, generally,
would  index  such  rates  to  inflation,  subject  to  certain  conditions  and
limitations. These regulations could increase  the cost of transporting oil  and
natural  gas liquids by pipeline, although  the most recent adjustment generally
decreased rates. These regulations are subject to pending petitions for judicial
review. The Company is not able to  predict with certainty what effect, if  any,
these  regulations  will  have  on  it,  but,  other  factors  being  equal, the
regulations may,  over time,  tend to  increase transportation  costs or  reduce
wellhead prices for oil and natural gas liquids.
 
ENVIRONMENTAL MATTERS
 
    Operations  of the Company  are subject to  numerous and constantly changing
federal, state  and  local  laws  and regulations  governing  the  discharge  of
materials   into  the   environment  or  otherwise   relating  to  environmental
protection. These laws and  regulations may require  the acquisition of  certain
permits,  restrict  or  prohibit  the  types,  quantities  and  concentration of
substances that can be released into the environment in connection with drilling
and production,  restrict  or prohibit  drilling  activities that  could  impact
wetlands,  endangered or threatened species or other protected natural resources
and impose substantial  liabilities for pollution  resulting from the  Company's
operations.  Such laws  and regulations may  substantially increase  the cost of
exploring for, developing or producing oil and gas and may prevent or delay  the
commencement or continuation of a given project. In the opinion of the Company's
management,  the Company  is in  substantial compliance  with current applicable
environmental laws and regulations,  and the cost of  compliance with such  laws
and  regulations has not been material and is not expected to be material during
the next fiscal year. Nevertheless,  changes in existing environmental laws  and
regulations or in interpretations thereof could have a significant impact on the
operating  costs of the Company, as well as the oil and gas industry in general.
For instance, legislation has been proposed  in Congress from time to time  that
would  reclassify certain oil  and gas production  wastes as "hazardous wastes,"
which reclassification would make exploration  and production wastes subject  to
much   more  stringent  handling,  disposal  and  clean-up  requirements.  State
initiatives to further regulate the disposal of oil and gas wastes and naturally
occurring radioactive materials  are also pending  in certain states,  including
Texas, and these various initiatives could have a similar impact on the Company.
 
    The  Comprehensive Environmental  Response, Compensation,  and Liability Act
("CERCLA"), also known as the "Superfund" law, imposes liability, without regard
to fault or the legality of the original conduct, on certain classes of  persons
that  are  considered  to  have  contributed  to  the  release  of  a "hazardous
substance" into the environment. These persons include the owner or operator  of
the  disposal site  or the  site where the  release occurred  and companies that
disposed or arranged for the disposal  of the hazardous substances found at  the
site.  Persons who are or were  responsible for releases of hazardous substances
found at  the site  and persons  who are  or were  responsible for  releases  of
hazardous  substances under CERCLA may be subject to joint and several liability
for the costs of  cleaning up the hazardous  substances that have been  released
into  the  environment and  for  damages to  natural  resources, and  it  is not
uncommon for neighboring landowners and other  third parties to file claims  for
personal injury and property damage allegedly caused by the hazardous substances
released  into  the environment.  The Company  is able  to control  directly the
operation of  only those  wells with  respect  to which  its acts  as  operator.
Notwithstanding the Company's lack of
 
                                       44
<PAGE>
control  over wells operated  by others, the  failure of the  operator to comply
with applicable  environmental regulations  may,  in certain  circumstances,  be
attributed  to the Company. The Company  has no material commitments for capital
expenditures to comply with existing environmental requirements.
 
EMPLOYEES
 
    At June  30, 1996,  the Company  had 109  full-time employees.  None of  the
Company's employees is subject to a collective bargaining agreement. The Company
considers its relations with its employees to be good.
 
LEGAL PROCEEDINGS
 
    The Company is a defendant or codefendant in minor lawsuits that have arisen
in  the ordinary  course of  business. While the  outcome of  the these lawsuits
cannot be predicted with certainty, management  does not expect any of these  to
have a material adverse effect on the Company's consolidated financial condition
or results of operations.
 
TITLE TO PROPERTIES
 
    The  Company  has  obtained  title  opinions  on  substantially  all  of its
producing properties  and  believes  that  it has  satisfactory  title  to  such
properties  in accordance with  standards generally accepted in  the oil and gas
industry. As is customary in  the oil and gas  industry, the Company performs  a
minimal  title investigation  before acquiring  undeveloped properties.  A title
opinion is obtained  prior to the  commencement of drilling  operations on  such
properties. The Company's properties are subject to customary royalty interests,
liens  incident  to  operating agreements,  liens  for current  taxes  and other
burdens which the Company believes do  not materially interfere with the use  of
or  affect the value of such properties.  Substantially all of the Company's oil
and gas  properties  are mortgaged  to  secure borrowings  under  the  Company's
Existing  Debt Facility and  will continue to be  mortgaged to secure borrowings
under  the  Credit  Facility.  See  "Management's  Discussion  and  Analysis  of
Financial  Conditions  and  Results  of  Operations  --  Liquidity  and  Capital
Resources," and "Description of Other Indebtedness."
 
OPERATIONAL HAZARDS AND INSURANCE
 
    The Company's operations are  subject to the hazards  and risks inherent  in
drilling  and production  and transportation  of oil  and gas,  including fires,
natural disasters, explosions, encountering formations with abnormal  pressures,
blowouts,  cratering, pipeline ruptures, and spills,  any of which can result in
loss of hydrocarbons, environmental pollution, personal injury or loss of  life,
severe  damage to and destruction  of properties of the  Company and others, and
suspension of operations. See "Risk Factors -- Drilling Risks" and "Risk Factors
- -- Operating Hazards and Uninsured Risks."
 
    The Company maintains insurance  of various types  to cover its  operations.
The limits provided under its liability policies total $21 million. In addition,
the Company maintains operator's extra expense coverage which provides for care,
custody  and control of all  material wells drilled by  the Company as operator.
The Company believes that its insurance is adequate and customary for  companies
of  a similar size  engaged in operations  similar to those  of the Company, but
losses could occur for uninsurable or uninsured risks or in amounts in excess of
existing insurance  coverage. The  Company's general  policy is  to only  engage
drilling  contractors who  provide substantial  insurance coverage  and name the
Company as an additional named insured. The occurrence of a significant  adverse
event,  the risks  of which  are not  fully covered  by insurance,  could have a
material adverse  effect on  the Company's  financial condition  and results  of
operations.  Moreover, no assurances can be given  that the Company will be able
to maintain adequate insurance in the future at rates it considers reasonable.
 
                                       45
<PAGE>
                                   MANAGEMENT
 
    The  executive officers and directors of the Company following completion of
the Corporate Reorganization are  listed below, together  with a description  of
their experience and certain other information (ages provided are as of June 30,
1996). Executive officers are appointed by the Board of Directors.
 
<TABLE>
<CAPTION>
NAME                             AGE       EMPLOYED SINCE                        POSITION WITH COMPANY
- ---------------------------      ---      -----------------  --------------------------------------------------------------
<S>                          <C>          <C>                <C>
Cadell S. Liedtke                    41            1988      Chairman of the Board, Chief Executive Officer and Director
Michael J. Grella                    47            1988      President, Chief Operating Officer and Director
Henry G. Musselman                   42            1992      Executive Vice President and Director
Jerry J. Langdon                     43             n/a      Director
W.D. Kennedy                         76             n/a      Director
Bobby W. Page                        53            1996      Senior Vice President, Treasurer and Chief Financial Officer
Clifford N. Hair, Jr.                49            1992      Vice President -- Land and Secretary
Roger J. Wetz                        47            1992      Vice President -- Exploration (Geology)
Roger A. Freidline                   46            1993      Vice President -- Exploration (Geophysics)
Brian K. Miller                      36            1992      Vice President -- Reservoir Engineering
Sal J. Pagano                        45            1995      Vice President -- Engineering and Operations
Keith Atwood                         42            1992      Vice President -- Field Operations
Celia A. Zinn                        48            1996      Controller
</TABLE>
 
    Cadell S. Liedtke entered the oil and gas business in Midland, Texas in 1977
as an independent landman generating oil and gas prospects in the Permian Basin.
He  founded the  Company's predecessor  with Michael J.  Grella in  1988 and has
served as managing partner and/or chief  executive officer since that time.  Mr.
Liedtke  has served  on the  Board of  Directors of  Texas Commerce Bank-Permian
Basin and has been appointed by Texas Governor George W. Bush to the Oil and Gas
Compact Commission.  Mr. Liedtke  is a  member of  the All-American  Wildcatters
Association, the Permian Basin Petroleum Association, the Permian Basin Landmans
Association  and the Independent Producer's  Association of America. Mr. Liedtke
graduated from the University of Texas at  Austin in 1977 with a B.A. degree  in
economics.
 
    Michael  J. Grella has served as Chief  Operating Officer of the Company and
its predecessor entities since their formation in 1988. He owned and operated an
independent oil and gas  company and has  invested in the  oil and gas  business
since  1982. Mr. Grella is a member  of the Permian Basin Petroleum Association,
the  Independent  Producer's  Association  of  America,  the  Texas  Independent
Producers   and  Royalty  Owners  Association  and  the  Permian  Basin  Landman
Association. Mr.  Grella  has  a  B.S.  degree  in  computer  science  from  the
University of California.
 
    Henry  G. Musselman  began his  oil and  gas career  in 1975  with Musselman
Petroleum and Land  Company where  he served as  Vice President  and a  Director
until forming Musselman, Owen & King in 1982. For the 10 years until merging his
company  into  Costilla's  predecessor  in  1992,  Mr.  Musselman  developed and
acquired oil and gas properties throughout the Permian Basin. Mr. Musselman is a
member and former director of the Independent Producer's Association of America.
Mr. Musselman graduated from the  University of Texas at  Austin in 1975 with  a
B.B.A. degree.
 
    Jerry  J. Langdon has previously held positions with WP Corporation, Houston
Pipeline Company, Texas  Oil &  Gas Corporation  and W.  Wilson Corporation.  In
1980,  Mr. Langdon formed  Texas IntraMark Gas Company,  Inc., an intrastate gas
gathering company engaging in the business of constructing and operating natural
gas gathering, treating and processing  facilities. In 1984, Mr. Langdon  formed
Langdon  &  Associates,  a  natural  gas  consulting  group  advising  petroleum
resource-oriented
 
                                       46
<PAGE>
companies, financial  institutions and  law  firms on  a variety  of  technical,
commercial  and regulatory issues.  Mr. Langdon served  as a member  of the FERC
from 1988 to June 1993. Since leaving the FERC, Mr. Langdon formed Republic  Gas
Corp.  to  acquire,  construct  and  operate  intrastate  natural  gas pipeline,
gathering, processing, treating  and marketing  facilities. Mr.  Langdon is  the
President  of  both Republic  and the  Midla  Companies. Mr.  Langdon is  a 1975
graduate of the University of Texas at Austin with a B.S. degree.
 
    W. D. Kennedy  has been  continually involved in  the oil  and gas  business
since  1948. From  1953 until  1980, Mr.  Kennedy was  an executive  officer and
director of C&K Petroleum, Inc., and its predecessor. C&K Petroleum, Inc. was  a
publicly held corporation from 1971 until 1980, when the company was sold for in
excess  of $200 million. Mr.  Kennedy remains an active  investor in the oil and
gas business. Mr. Kennedy is a graduate of the University of Texas, and a member
of the All-American  Wildcatters Association,  a past president  of the  Permian
Basin  Petroleum Association, a  former director of  the Texas Mid-Continent Oil
and Gas Association, and an advisory director of Norwest Bank Texas, Midland.
 
    Bobby W. Page began his oil and gas career with MGF Oil Corporation in 1967,
where he remained until  1988, ultimately serving  as Executive Vice  President,
Chief  Financial Officer and a  member of the Board  of Directors. Following two
years as  a self-employed  financial  consultant, Mr.  Page joined  Alta  Energy
Corporation  in 1990 as Executive Vice  President, Treasurer and Chief Financial
Officer. From  July 1993  until joining  the Company,  Mr. Page  served as  Vice
President, Chief Financial Officer and Secretary of Marcum Natural Gas Services,
Inc.  Mr. Page graduated from the University of Oklahoma with a B.B.A. degree in
accounting in 1965.
 
    Clifford N. Hair, Jr. has served in district and division landman roles,  as
well  as a corporate officer with Texas Gas Exploration Corporation, Samedan Oil
Corporation, Henry Petroleum Corporation and Donald C. Slawson Oil Producer. For
the two  year period  prior to  joining the  Company in  1992, Mr.  Hair was  an
independent  landman involved  in drilling projects  in Texas  and Oklahoma. Mr.
Hair is a Certified Petroleum Landman's and a member of the American Association
of Petroleum  Landmen and  the  Petroleum Basin  Landman Association.  Mr.  Hair
graduated  with honors  from the  University of  Houston in  1971 with  a B.B.A.
degree in accounting.
 
    Roger J. Wetz began his oil and gas career with IMCO Services, a division of
Halliburton, Inc. in 1974. He held  a variety of geological positions with  Gulf
Energy  & Minerals Company, TXO Production Corporation and Terra Resources, Inc.
from 1976 to 1989. From 1989 until joining the Company in 1992, Mr. Wetz was  an
independent  geologist  generating  prospects  in the  Permian  Basin.  Mr. Wetz
graduated from St. Mary's University in 1973 with a B.S. degree in geology.
 
    Roger A.  Freidline began  his industry  career with  Union Oil  Company  of
California.  From 1976 until  1985, Mr. Freidline  served in various geophysical
capacities with Forest  Oil Corporation,  Gifford, Mitchell  and Wisenbaker  and
Heritage Resources, Inc. Mr. Freidline was an independent geophysicist from 1985
until  joining  the  Company, except  for  a  period of  employment  as district
geologist for Hondo  Oil & Gas  Company prior to  its sale. Mr.  Freidline is  a
Certified  Petroleum  Geologist,  and a  member  of the  Society  of Exploration
Geophysicists,  the  Permian  Basin  Geophysical  Society  and  the  West  Texas
Geological Society. He has co-authored papers which have appeared in Geology and
The  Bulletin of the Seismological Society  of America. Mr. Freidline received a
B.S. degree with  highest honors  from the New  Mexico Institute  of Mining  and
Technology  in  1972 and  a Masters  of  Science degree  in geophysics  from the
University of Utah in 1974.
 
    Brian K. Miller entered the oil  and gas business as an operations  engineer
for  ARCO Oil and  Gas Company. From 1984  to 1987, he  was a reservoir engineer
with First City  National Bank of  Midland, Texas,  and from 1987  to 1989,  Mr.
Miller  was an independent consulting engineer.  Prior to joining the Company in
1992, Mr. Miller  served as  an oil  and gas  analyst under  appointment to  the
Federal  Deposit Insurance Corporation. Mr. Miller is a member of the Society of
Petroleum Engineers.
 
                                       47
<PAGE>
Mr. Miller received a B.S. degree  with highest honors in petroleum  engineering
from  the  University  of Texas  at  Austin in  1982  and a  Master  of Business
Administration degree with honors in finance in 1984.
 
    Sal J. Pagano  began his oil  and gas career  with Amoco Production  Company
where  he  was employed  until  1978. From  1978  through 1989,  Mr.  Pagano was
employed by several  independent oil and  gas companies in  Midland, Texas in  a
variety  of petroleum  engineering capacities. Prior  to joining  the Company in
1995, Mr. Pagano was employed by Midland  Resources Company from 1989 as a  vice
president.  Mr. Pagano is  a registered petroleum  engineer and a  member of the
Society of Petroleum Engineers. Mr. Pagano graduated in 1973 from the University
of Missouri at Rolla with a B.S. degree in petroleum engineering.
 
    Keith Atwood began  his oil and  gas career with  Otis Engineering Corp.  in
1974.  Mr. Atwood worked as an independent  consultant from 1979 to 1983 when he
joined Musselman,  Owen &  King Operating  Co. to  manage field  operations.  He
served  in that capacity until joining the  Company in 1992. Mr. Atwood attended
Southwest Texas State University and the University of Texas.
 
    Celia A. Zinn joined the Company in  1996. From 1992 to 1996, she  practiced
public  accounting in Midland. Ms.  Zinn has 18 years  experience in the oil and
gas industry, including  12 years as  Controller for Clayton  W. Williams,  Jr.,
Inc.  from 1981  to 1992. Ms.  Zinn is  a certified public  accountant. Ms. Zinn
graduated from  the  University  of  Texas-Arlington in  1978  with  a  B.A.  in
mathematics.
 
                                       48
<PAGE>
                    SECURITY OWNERSHIP OF CERTAIN BENEFICIAL
                             OWNERS AND MANAGEMENT
 
    The  following  table sets  forth the  names  and addresses  of each  of the
Company's stockholders  who  beneficially own  more  than five  percent  of  the
Company's  Common  Stock,  the  number  of  shares  beneficially  owned  by such
shareholders and the percentage of the Common  Stock so owned at June 30,  1996,
assuming  in each case the Corporate Reorganization had been consummated at June
30, 1996  and  that  the  Common  Stock  Offering  is  consummated  without  the
underwriters' over-allotment option being exercised.
 
<TABLE>
<CAPTION>
                                         AMOUNT AND NATURE OF
                                         BENEFICIAL OWNERSHIP    PERCENT OF
NAME AND ADDRESS OF BENEFICIAL OWNER             (1)                CLASS
- --------------------------------------  ----------------------  -------------
<S>                                     <C>                     <C>
Cadell S. Liedtke ....................          2,302,560             23.0%
400 W. Illinois
Midland, Texas 79701
Michael J. Grella ....................          1,350,267             13.5%
400 W. Illinois
Midland, Texas 79701
NationsBanc Capital Corp. ............            936,000              9.4%
100 North Tryon Street
Charlotte, North Carolina 28255
Henry G. Musselman ...................            611,173              6.1%
400 W. Illinois
Midland, Texas 79701
</TABLE>
 
- ------------------------------
(1)  All persons own the listed shares of record.
 
    The following table sets forth information as of June 30, 1996 (assuming the
Corporate  Reorganization had been consummated on such date) with respect to the
shares of Common Stock  beneficially owned by each  of the Company's  Directors,
the  Chief  Executive  Officer  and  the  three  other  most  highly compensated
executive officers for 1996 (whose  annualized compensation for such year  based
on  compensation levels following  the Offering is  expected to exceed $100,000)
and all Directors  and executive  officers as  a group  and the  percent of  the
outstanding  Common Stock owned by each, assuming that the Common Stock Offering
is consummated without the underwriters' over-allotment option being exercised.
 
<TABLE>
<CAPTION>
DIRECTORS AND NAMED                      AMOUNT AND NATURE OF    PERCENT OF
EXECUTIVE OFFICER                        BENEFICIAL OWNERSHIP     CLASS (1)
- --------------------------------------  ----------------------  -------------
<S>                                     <C>                     <C>
Cadell S. Liedtke.....................          2,302,560             23.0%
Michael J. Grella.....................          1,350,267             13.5%
Henry G. Musselman....................            611,173              6.1%
Bobby W. Page.........................             75,000(2)           0.7%
All Officers and Directors as a group
(13 persons)..........................          4,814,000(3)          45.6%
</TABLE>
 
- ------------------------------
(1)  For the sole purpose  of calculating these  percentages, the shares,  which
     the  named person has the  right to acquire within  60 days, by exercise of
     the options described  in these  footnotes, are  deemed outstanding  shares
     with  respect to that person's percentage ownership and with respect to the
     percentage ownership of all Officers and Directors as a group.
 
(2)  Includes 75,000 shares  issuable pursuant  to an option  granted under  the
     Company's   1996  Stock  Option  Plan  which  option  will  be  immediately
     exercisable upon closing of the Offerings  at a price equal to the  initial
     public offering price of the Common Stock.
 
(3)  Includes  550,000  shares issuable  pursuant to  options granted  under the
     Company's  1996  Stock  Option  Plan  which  options  will  be  immediately
     exercisable  upon closing of the Offerings at  a price equal to the initial
     public offering price of the Common Stock.
 
                                       49
<PAGE>
                  EXECUTIVE COMPENSATION AND OTHER INFORMATION
 
SUMMARY COMPENSATION TABLE
 
    The following table sets forth information for the Company's Chief Executive
Officer and the  three other  most highly compensated  executive officers  whose
annual  compensation for the fiscal year ending December 31, 1996 is expected to
exceed $100,000.  Information  is  presented  for  1995,  and  for  1996  on  an
annualized basis based on salaries to be effective following consummation of the
Offerings.  Information for  1994 and  prior years  is not  comparable since the
Company's predecessor was a general  partnership in which the partners  received
periodic partnership distributions in lieu of salary.
 
<TABLE>
<CAPTION>
                                                                                      LONG-TERM
                                                                                    COMPENSATION
                                                                                       AWARDS
                                                                                    -------------
                                                ANNUAL COMPENSATION                  SECURITIES
                                 -------------------------------------------------   UNDERLYING
                                                                    OTHER ANNUAL      OPTIONS/        ALL OTHER
NAME AND PRINCIPAL POSITION        YEAR     SALARY($)  BONUS($)   COMPENSATION($)    SARS(#)(2)    COMPENSATION($)
- -------------------------------  ---------  ---------  ---------  ----------------  -------------  ----------------
<S>                              <C>        <C>        <C>        <C>               <C>            <C>
Cadell S. Liedtke
  Chairman of the Board and           1995    185,700         --             --              --               --
   Chief Executive Officer            1996    300,000         --             --              --               --
Michael J. Grella
  President and Chief Operating       1995    261,750         --             --              --          656,000(3)
   Officer                            1996    300,000         --             --              --               --
Henry G. Musselman
                                      1995    139,800         --             --              --               --
  Executive Vice President            1996    215,000         --             --              --               --
Bobby W. Page
  Senior Vice President,
   Treasurer and Chief                1995(1)        --        --            --              --               --
   Financial Officer                  1996    150,000         --             --          75,000               --
</TABLE>
 
- ------------------------------
(1)  Mr. Page joined the Company in June 1996.
 
(2)  The  amount shown represents the number of shares subject to a stock option
     to be granted upon the closing  of the Offerings pursuant to the  Company's
     1996  Stock Option  Plan described  under "--  Benefit Plans  -- 1996 Stock
     Option Plan." The option will be  granted with an exercise price per  share
     equal  to the initial public offering price of the Common Stock and will be
     granted for a 10-year term.
 
(3)  The amount  shown  represents non-cash  compensation  deemed to  have  been
     accrued to Mr. Grella in connection with the cancellation of an option held
     by  a minority owner to purchase an additional interest in the Company. See
     Note 11 of the Notes to the Consolidated Financial Statements.
 
DIRECTORS' COMPENSATION
 
    Compensation for non-employee directors  (Messrs. Langdon and Kennedy)  will
consist  of an annual retainer fee of $10,000,  plus a $1,000 fee for each Board
meeting attended and a $1,000  fee for attending a  committee meeting held on  a
day  other than the same day of  a Board meeting. In addition, outside Directors
are participants in the Company's Outside Directors Stock Option Plan  described
under  "--  Benefit  Plans --  Outside  Directors Stock  Option  Plan." Employee
Directors do not receive  compensation for serving on  the Board or the  Board's
committees.
 
EMPLOYMENT AGREEMENTS
 
    Messrs.   Liedtke,  Grella  and  Musselman   have  entered  into  employment
agreements (the "Founders  Employment Agreements") with  the Company which  will
become  effective upon the closing of the Offerings and replace certain existing
agreements. The  Founders  Employment  Agreements  are  each  for  three  years,
commencing on the closing of the Offerings and each will automatically renew for
successive  one-year periods thereafter  unless the employee  is notified to the
contrary. The  Founders  Employment Agreements  provide  for salary  levels  for
Messrs.  Liedtke,  Grella  and  Musselman of  $300,000,  $300,000  and $215,000,
respectively.
 
    Each of Messrs. Liedtke, Grella and  Musselman would receive his salary  for
the  remaining  term  of the  applicable  Founders Employment  Agreement  if the
Company were to terminate such person's
 
                                       50
<PAGE>
employment other than for  cause. If such person  were to voluntarily leave  his
employment  with the Company prior to the second anniversary of the Agreement no
further payments would  be required. If  a voluntary termination  were to  occur
after  the second anniversary of the Agreement, such person would be entitled to
one year's  salary  from  the  date of  termination.  Each  Founders  Employment
Agreement  provides that the covered employee  will not compete with the Company
for a  one  year period  following  his  voluntary cessation  of  employment  or
termination  of employment for cause, in either case if such event occurs within
the initial three-year term of the Agreement. Competitive activities are defined
as engaging in the oil and gas business in any area in which the Company is then
active.
 
    Bobby W. Page has entered into an employment agreement (the "Page Employment
Agreement") with  the  Company effective  June  30, 1996.  The  Page  Employment
Agreement  is  for  a  period  of  three  years  from  June  30,  1996  and will
automatically renew for successive one-year  periods thereafter unless Mr.  Page
is  notified  to the  contrary  by the  Company.  The Page  Employment Agreement
provides a $25,000 bonus (which includes Mr. Page's cost of relocation), plus  a
base  salary of $150,000 until January 1,  1997; $175,000 until January 1, 1998;
and $185,000 thereafter. In addition, Mr. Page will receive options to  purchase
75,000  shares of  Common Stock, certain  insurance benefits  and other benefits
generally available  to the  Company's  employees. Mr.  Page would  receive  his
salary  for the remaining term  of the Page Employment  Agreement if the Company
were to terminate the Page Employment  Agreement other than for cause.  However,
if  Mr.  Page were  to voluntarily  leave  his employment  with the  Company, no
further payments would be required.
 
BENEFIT PLANS
 
    OUTSIDE DIRECTORS STOCK  OPTION PLAN.   The Outside  Directors Stock  Option
Plan  provides for the issuance of stock options to the outside directors of the
Company. A  total of  50,000 shares  of  Common Stock  has been  authorized  and
reserved  for issuance under the plan, subject to adjustments to reflect changes
in the Company's capitalization resulting from stock splits, stock dividends and
similar events. Only outside directors are eligible to participate in the  plan.
Outside  directors  are those  directors of  the Company  who are  not executive
officers or regular salaried employees of the  Company as of the date an  option
is  granted. Under the plan, an option for  1,000 shares of Common Stock will be
granted to each person who qualifies as an outside director each year that  such
person  is elected  as a  director of  the Company.  The exercise  price of each
option granted under the plan will be the fair market value (as reported on  the
Nasdaq  National Market) of the Common Stock  at the time the option is granted,
and may be  paid either in  cash, shares  of Common Stock  or a  broker-assisted
cashless  transaction.  Each option  will be  exercisable immediately,  and will
expire ten years from the date of grant. An option granted under the plan is not
transferrable other than by will or the laws of descent and distribution. In the
event a participant in the plan ceases to be an outside director, other than  by
reason  of death, such participant may  exercise an outstanding option under the
plan within six months after such termination, to the extent the participant was
entitled to exercise the option on the date of termination. In the event of  the
death  of  a participant  under the  plan, such  participant's option(s)  may be
exercised by the executors or administrators of the optionee's estate or by  the
legatees  of such participant  within one year  after his death,  so long as the
term  of  the  option  has  not  expired.  The  Company  does  not  receive  any
consideration  upon the  grant of  options under  the plan.  The options granted
under the plan are intended to be non-qualifying options for federal income  tax
purposes. Because options under the plan are not generally transferrable, do not
appear  to be subject to a substantial risk of forfeiture and the exercise price
will be the  fair market value  of the common  stock on the  date of grant,  the
options  should not be taxable  to an optionee until  the optionee exercises the
option, at which  time the  optionee would  recognize income  on the  difference
between  the exercise price and the fair market  value of the shares on the date
of exercise.  The  grant  of  options  under  the  plan  should  be  treated  as
compensation  paid by the  Company for purposes of  the Company's federal income
tax considerations.  The Board  of  Directors may  amend  the plan  without  the
approval  of  the stockholders  of the  Company  in any  respect other  than any
amendment which requires stockholder
 
                                       51
<PAGE>
approval by  law  and may  only  modify  an outstanding  option,  including  the
repricing  of such options, with  the consent of the  option holder. The Company
currently has five  directors, two of  whom are eligible  to participate in  the
plan.
 
    1996  STOCK OPTION PLAN.  The 1996  Stock Option Plan provides for the grant
of both incentive  stock options and  non-qualifying stock options,  as well  as
limited  stock appreciation rights and supplemental bonuses, to the employees of
the Company  and its  subsidiaries,  including officers  and directors  who  are
salaried  employees.  A  total  of  850,000  shares  of  Common  Stock  has been
authorized and reserved for  issuance under the plan,  subject to adjustment  to
reflect  changes in  the Company's  capitalization resulting  from stock splits,
stock dividends and  similar events. The  plan is administered  by the Board  of
Directors.  The Board of Directors has the sole authority to interpret the plan,
to determine the persons to whom options will be granted, to determine the basis
upon which the  options will be  granted, and to  determine the exercise  price,
duration  and other terms of the options  to be granted under the plan; provided
that (a) the exercise  price of each  option granted under the  plan may not  be
less  than the fair market value  of the Common Stock on  the date the option is
granted (and  for incentive  stock options,  110% of  fair market  value if  the
employee  is  the  beneficial owner  of  10%  or more  of  the  Company's voting
securities), (b)  the exercise  price  must be  paid  in cash,  by  surrendering
previously  owned shares of Common Stock upon the exercise of the option or by a
promissory note or broker-assisted  cashless exercise approved  by the Board  of
Directors,  (c) the  term of  the option may  not exceed  ten years,  and (d) no
option is transferrable other than by will, the laws of descent and distribution
or pursuant to  a qualified  domestic relations  order. Upon  termination of  an
optionee's  employment (other than  by death or  disability), an incentive stock
option may be exercised  prior to the  expiration date of  the option or  within
three  months after the date of such termination, whichever is earlier, but only
to the extent the optionee had the right to exercise the option upon the date of
such termination,  while the  rights of  the holder  of a  non-qualifying  stock
option  will  be  set  forth in  each  option  agreement. In  the  event  of the
disability of an optionee,  the option may  be exercised by  such person or  his
personal  representative at any time within one  year of the termination of such
person's employment,  but only  to the  extent  the optionee  had the  right  to
exercise  the option as of the date of  his disability. In the event of death of
the optionee, the  option may  be exercised  by his  personal representative  or
successor  in interest  at any  time until  the later  of the  expiration of the
option or one  year after the  optionee's death,  to the extent  the option  was
exercisable at the time of the optionee's death. Incentive stock options may not
be  granted under the plan  to any individual if the  effect of such grant would
permit that person to  have the first opportunity  to exercise such options,  in
any calendar year, for the purchase of shares having a fair market value (at the
time  of grant of the option) in excess of $100,000. Neither the Company nor any
of its subsidiaries will receive any  consideration for the granting of  options
under  the plan. Limited stock appreciation rights may be granted under the plan
with respect to  specified options, allowing  the option holder  to receive,  in
cash,  the difference  between the  exercise price and  the market  value in the
event of a change  of control of  the Company. The Board  of Directors may  also
grant  supplemental bonuses under the plan which  are cash bonuses not to exceed
the amount  of income  tax liability  incurred by  a plan  participant upon  the
exercise  of a non-qualifying stock option or a limited stock appreciation right
with respect to  which the bonus  was granted. Incentive  stock options  granted
under  the plan are  intended to have  the federal income  tax consequences of a
qualified stock option. As a result,  the exercise of an incentive stock  option
will  not be a taxable event; the taxable event occurs at the time the shares of
Common Stock acquired  upon exercise  of the option  are sold.  If the  optionee
holds  such shares  for the  later of  two years  from the  date the  option was
granted or one  year from the  date of  exercise of the  option, the  difference
between  the price paid for the shares at exercise and the price for which those
shares are sold will be  treated as capital gains  income. If the optionee  does
not hold the shares for the required holding period, the income would be treated
as  ordinary income rather  than capital gains  income. The non-qualifying stock
options granted under the plan should  be taxable when the option is  exercised,
at  which  time  the optionee  would  recognize ordinary  income  the difference
between the exercise price and the fair  market value of the shares on the  date
of exercise. The grant of options under the plan will be treated as compensation
by the Company for federal income tax purposes. The Board of Directors may amend
 
                                       52
<PAGE>
the  plan, without stockholder approval, in any respect other than any amendment
that requires stockholder approval by law, and may modify an outstanding option,
including the  repricing of  non-qualifying  options, with  the consent  of  the
option holder. There are currently approximately 100 persons who are eligible to
participate under the plan.
 
    BONUS  INCENTIVE PLAN.  The Company has  adopted the Bonus Incentive Plan to
become effective upon the  completion of the Offerings.  The plan provides  that
the  Board of Directors  each year may  award bonuses in  cash, Common Stock, or
some combination thereof, to those  officers, directors, employees and  advisors
of  the  Company or  a subsidiary  of the  Company, who  the Board  of Directors
determines have contributed to  the success of the  Company. A total of  100,000
shares  of Common Stock has been authorized  and reserved for issuance under the
plan, subject to adjustments to reflect changes in the Company's  capitalization
resulting  from stock splits, stock dividends  and similar events. All officers,
directors, employees and advisors of the Company or a subsidiary of the  Company
who have completed a minimum of 180 days of service and are employed or retained
by  the Company or such subsidiary on the  last day of the plan year, other than
such persons who own  ten percent or  more of the  outstanding shares of  Common
Stock  during that year  are eligible to  participate in the  plan. Bonus awards
will be  determined based  on a  number of  factors, including  performance  and
salary level of the participant and the financial performance of the Company and
its  subsidiaries. Bonuses will be awarded after review and upon approval of the
Board of Directors, subject to the terms and conditions of the plan.
 
                              CERTAIN TRANSACTIONS
 
    A&P supplies meter  reading services  which measures gas  production to  the
Company,  as well as to unaffiliated oil  and gas companies. A&P is also engaged
in the sale  of gas meter  and regulating  equipment, and in  certain other  oil
field  related  businesses. For  the fiscal  year ended  December 31,  1995, the
Company accounted for approximately  27% of A&P's gross  revenues. From time  to
time,  the Company has  advanced funds to  A&P for working  capital needs. These
advances have been consolidated into two promissory notes. One note was executed
December 31, 1994 in the original  principal amount of $370,000. The note  bears
interest at a floating rate equal to the "prime rate" plus 1.0%. No principal or
interest  payments are due until the maturity  of the note at December 31, 2004.
The note is secured by a second lien on A&P's accounts receivable, inventory and
equipment. The second note is in  the original principal amount of $247,000  and
is  dated May 22, 1996. The note bears  interest at 6.0% per annum, is unsecured
and is payable upon demand. During the fiscal year ended December 31, 1995,  A&P
received $612,139 from the Company for meter reading, meter repair, calibration,
flow  line installation and other related  services provided to the Company. The
Company believes that the services and charges therefor are comparable to  those
the Company could have obtained from unaffiliated third parties.
 
    During 1995 the Company paid $440,884 to Valley for gas compression and salt
water  disposal  charges.  During 1995,  Valley  paid the  Company  $109,399 for
operating costs  of its  salt water  disposal wells  and gas  compressors.  Also
during  1995,  the  Company paid  CSL  $592,920  for management  fees  and lease
payments on equipment.
 
    During a portion of 1995, the Company leased office space from 511 Tex L.C.,
in which Messrs. Liedtke, Grella and Musselman are the sole members. The  amount
of  rental payments  to 511  Tex L.C.  during 1995  was $67,896.  The Company no
longer leases office space from any affiliated party.
 
    The Company  has  agreed that,  upon  the request  of  NBCC, on  up  to  two
occasions,  the  Company will  register  under the  Securities  Act of  1933, as
amended (the "Securities Act"), and applicable state securities laws the sale of
the Common Stock owned by NBCC.  The Company's obligation is subject to  certain
limitations regarding the timing of registrations and certain other matters. The
Company  is also  obligated to  offer to  NBCC and  Messrs. Liedtke,  Grella and
Musselman (collectively, the  "Affiliated Holders") the  opportunity to  include
shares of the Common Stock owned by them in certain
 
                                       53
<PAGE>
registration  statements  filed by  the Company.  In  addition, the  Company has
agreed to indemnify  the Affiliated  Holders and their  respective officers  and
directors  against securities  law liabilities  arising in  connection with such
offerings, other than liabilities arising  as a result of information  furnished
to  the Company by the Affiliated Holders participating in the registration. The
Company is obligated to pay all  expenses incident to such registration,  except
underwriters'  discounts  and commissions  allocable to  the  sale of  shares by
Affiliated Holders  and  any professional  fees  and expenses  incurred  by  the
Affiliated  Holders incident to  such registration. The  Affiliated Holders have
agreed that they will not  sell any shares of Common  Stock for a period of  180
days   after  the  Offerings  without   the  consent  of  Prudential  Securities
Incorporated.
 
    In 1995, non-cash compensation of $656,000 was deemed to have accrued to Mr.
Grella in connection with the cancellation of an option held by Mr. Musselman to
acquire an interest in the Company's predecessors. To effect such  cancellation,
Mr.  Liedtke  agreed to  transfer a  portion  of his  ownership interest  in the
Company's predecessors directly to  Mr. Musselman. See Note  11 of the Notes  to
the Consolidated Financial Statements.
 
    Certain   of  the  transactions   comprising  the  Corporate  Reorganization
represent transactions  between  the  Company,  or  its  predecessors,  and  its
affiliates.  Messrs. Liedtke, Grella  and Musselman, the  shareholders of Valley
and CSL will sell the stock of Valley  and the assets of CSL to the Company  for
$0.7  million.  The  purchase price  is  based on  negotiations  between Messrs.
Liedtke, Grella and Musselman, on the one hand, and NBCC, considering the  value
to  the Company of the stock and assets being acquired. No third party conducted
an appraisal of either Valley or CSL.
 
    Messrs. Liedtke, Grella and Musselman will receive an aggregate distribution
from the LLC of approximately $3.5 million which is estimated to be the  federal
income  tax  liability (as  well as  the  federal income  tax liability  on such
distribution) which will be owed by  Messrs. Liedtke, Grella and Musselman as  a
result  of the  Corporate Reorganization.  However, the  precise amount  of such
liability will be dependent upon a number of factors which cannot be  determined
with  certainty until subsequent  to December 31,  1996. While the  amount to be
distributed has been  determined in good  faith by the  Company's tax  advisors,
there  can  be no  assurance that  the actual  tax liability  of any  of Messrs.
Liedtke, Grella or Musselman  will not be less  or greater than the  distributed
amounts. If the distributed amounts exceed the ultimate tax liabilities, none of
such  persons will reimburse the Company.  Correspondingly, if the tax liability
exceeds the amount of such distributions, the Company will not make any  further
distributions to cover such short-fall. NBCC is also receiving a distribution of
$0.8  million, which represents its  post-redemption ownership percentage of the
distribution made to Messrs. Liedtke, Grella and Musselman. However, NBCC has no
tax or other liability with respect  to such distribution. In addition,  Messrs.
Liedtke,  Grella and Musselman  and NBCC will receive  an aggregate of 5,200,000
shares of Common Stock in the Merger in exchange for their interests in the LLC.
 
                                       54
<PAGE>
                              DESCRIPTION OF NOTES
 
GENERAL
 
    The Notes will be issued pursuant to an Indenture (the "Indenture")  between
the Company and State Street Bank and Trust Company, as trustee (the "Trustee").
A  copy of the Indenture in substantially the  form in which it will be executed
has been  filed  as an  Exhibit  to the  Registration  Statement of  which  this
Prospectus  is  a part.  The  terms of  the Notes  include  those stated  in the
Indenture and  those  made part  of  the Indenture  by  reference to  the  Trust
Indenture  Act of 1939,  as amended (the  "Trust Indenture Act").  The Notes are
subject to all such terms,  and Holders of Notes  are referred to the  Indenture
and  the Trust Indenture Act  for a statement thereof.  The following summary of
certain provisions  of the  Indenture does  not purport  to be  complete and  is
qualified  in  its  entirety  by  reference  to  the  Indenture,  including  the
definitions therein of certain terms used below and those terms that are made  a
part  of the Indenture by reference to  the Trust Indenture Act. The definitions
of certain terms used  in the following  summary are set  forth below under  the
caption "Certain Definitions."
 
    As of the date of the Indenture, Costilla Redeco Energy, L.L.C. and Costilla
Redeco  Operating,  L.L.C.,  through  which the  Company  conducts  its Moldovan
operations,  will   be  Unrestricted   Subsidiaries.  However,   under   certain
circumstances,  the Company will be able to designate additional Subsidiaries as
Unrestricted Subsidiaries.  If  so designated,  such  Subsidiaries will  not  be
subject to many of the restrictive covenants set forth in the Indenture. As used
herein,  "Subsidiary" refers  to any  Subsidiary of the  Company that  is not an
Unrestricted Subsidiary.
 
PRINCIPAL, MATURITY AND INTEREST
 
    The Notes  will be  general  unsecured senior  obligations of  the  Company,
limited  in  aggregate principal  amount to  $100.0 million  and will  mature on
October 1, 2006. Interest  on the Notes  will accrue at the  rate of 10.25%  per
annum  and will  be payable  semiannually in  arrears on  April 1  and October 1
commencing on April 1, 1997, to  Holders of record on the immediately  preceding
March  15 and  September 15.  Interest on  the Notes  will accrue  from the most
recent date to which interest  has been paid or, if  no interest has been  paid,
from  October 8, 1996. Interest will be computed  on the basis of a 360-day year
comprised of twelve 30-day months. Principal,  premium, if any, and interest  on
the  Notes will be payable at the office or agency of the Company maintained for
such purpose within  the City and  State of New  York or, at  the option of  the
Company,  payment of interest may be made by  check mailed to the Holders of the
Notes at their  respective addresses  set forth in  the register  of Holders  of
Notes;  PROVIDED that all payments with respect to Global Notes and Certificated
Securities the Holders  of which have  given wire transfer  instructions to  the
Company  will be required to  be made by wire  transfer of immediately available
funds to  the  accounts  specified  by  the  Holders  thereof.  Until  otherwise
designated by the Company, the Company's office or agency in New York will be in
the  office of the Trustee maintained for such purpose. The Notes will be issued
in denominations of $1,000 and integral multiples thereof.
 
RANKING
 
    The Notes will be  general unsecured senior obligations  of the Company  and
will  rank  equally in  right of  payment  with all  existing and  future Senior
Indebtedness of the Company, and senior in right of payment to all existing  and
future  subordinated indebtedness  of the Company.  The Notes,  however, will be
effectively subordinated to secured Senior  Indebtedness of the Company and  its
subsidiaries  with respect to  the assets securing  such Indebtedness, including
indebtedness under the  Credit Facility, which  is expected to  be secured by  a
lien on substantially all of the assets of the Company and any subsidiary of the
Company that guarantees the Company's obligations under the Credit Facility. See
"Description  of Other Indebtedness  -- Credit Facility." On  a pro forma basis,
after giving  effect to  the  Corporate Reorganization,  the Offerings  and  the
application  of  proceeds  therefrom,  the  Company  would  have  had  no senior
unsecured indebtedness,  other  than the  Notes  and trade  payables,  and  $0.4
million  of  secured  Senior  Indebtedness.  On  such  a  pro  forma  basis,  no
Indebtedness was  junior  to the  Notes.  The  Notes will  also  be  effectively
subordinated    to    liabilities   of    the   Company's    subsidiaries   that
 
                                       55
<PAGE>
are not Subsidiary Guarantors.  On a pro forma  basis, the total liabilities  of
the Company's Subsidiaries were $6.5 million at June 30, 1996, all of which were
operating  liabilities. The Indenture  will limit, subject  to certain financial
tests,  the  amount  of  additional  Indebtedness  that  the  Company  and   its
Subsidiaries can incur. See "Certain Covenants -- Incurrence of Indebtedness and
Issuance  of Preferred Stock." The Indenture will  also limit the amount of such
Indebtedness that can be secured. See "Certain Covenants -- Liens."
 
SUBSIDIARY GUARANTEES
 
    The Indenture does not  require any Subsidiary to  guarantee the payment  of
the  Notes  unless  each such  Subsidiary  incurs Indebtedness  (other  than its
Indebtedness existing  on the  date of  the Indenture  and certain  intercompany
Indebtedness).  The Indenture requires  the Company to  cause such Subsidiary to
fully and  unconditionally, jointly  and  severally guarantee  (the  "Subsidiary
Guarantees")  the Company's  payment obligations  under the  Notes prior  to the
incurrence of  such  Indebtedness.  See  "Certain  Covenants  --  Incurrence  of
Indebtedness  and Issuance  of Preferred Stock."  On the date  of the Indenture,
there will be no Subsidiary Guarantors. So  long as a Person is an  Unrestricted
Subsidiary, such Person will not be required to become a Subsidiary Guarantor or
execute   a  Subsidiary  Guarantee.  See   "Certain  Covenants  --  Unrestricted
Subsidiaries." As a result, claims of creditors against the Subsidiaries and the
Unrestricted Subsidiaries, including their  trade creditors and tort  claimants,
will effectively have priority to the property and earnings of such subsidiaries
over  claims of creditors of the Company, including the Holders. The obligations
of each Subsidiary Guarantor under its Subsidiary Guarantee will be limited in a
manner intended  to  result in  such  Subsidiary Guarantee  not  constituting  a
fraudulent conveyance under applicable law.
 
    The Indenture will provide that no Subsidiary Guarantor may consolidate with
or merge with or into (whether or not such Subsidiary Guarantor is the surviving
Person)  another Person whether or not affiliated with such Subsidiary Guarantor
(other than the  consolidation or  merger of a  Wholly Owned  Subsidiary of  the
Company with another Wholly Owned Subsidiary of the Company or into the Company)
unless  (i) subject  to the  provisions of  the following  paragraph, the Person
formed by or  surviving any  such consolidation or  merger (if  other than  such
Subsidiary  Guarantor) becomes a Subsidiary Guarantor pursuant to a supplemental
indenture or other agreement  in form and  substance reasonably satisfactory  to
the  Trustee, and (ii) immediately after  giving effect to such transaction, (A)
no Default or Event of Default would  exist or be continuing and (B) other  than
in  the case of the consolidation or merger of two or more Subsidiary Guarantors
or of one or more Subsidiary Guarantors with the Company, the Company would  (A)
have  Consolidated  Net  Worth immediately  after  the transaction  equal  to or
greater than the Consolidated Net Worth of the Company immediately preceding the
transactions; and (B) at  the time of such  transaction and after giving  effect
thereto,  be  permitted  to  incur at  least  $1.00  of  additional Indebtedness
pursuant  to  the  Consolidated  Interest   Coverage  Ratio  and  the   Adjusted
Consolidated  Net Tangible Assets  to Consolidated Indebtedness  Ratio tests set
forth in the first paragraph of  the covenant described below under the  caption
"Certain  Covenants  -- Incurrence  of  Indebtedness and  Issuance  of Preferred
Stock."
 
    The Indenture  will  provide that  (i)  in the  event  of a  sale  or  other
disposition  of all of the assets of any Subsidiary Guarantor, by way of merger,
consolidation or otherwise, or a sale or other disposition of all of the capital
stock of  any  Subsidiary Guarantor  or  (ii) in  the  event that  a  Subsidiary
Guarantor is properly designated as an Unrestricted Subsidiary, in each case, in
accordance  with the provisions of the Indenture, then such Subsidiary Guarantor
(in the  event  of a  sale  or  other disposition,  by  way of  such  a  merger,
consolidation  or  otherwise, of  all of  the capital  stock of  such Subsidiary
Guarantor  or  the  proper  designation  of  such  Subsidiary  Guarantor  as  an
Unrestricted  Subsidiary in accordance with the  provisions of the Indenture) or
the corporation  acquiring  the  property (in  the  event  of a  sale  or  other
disposition  of  all  or substantially  all  of  the assets  of  such Subsidiary
Guarantor),  will  be  released  and  relieved  of  any  obligations  under  its
Subsidiary  Guarantee;  provided that  the Net  Proceeds of  such sale  or other
disposition are  applied in  accordance with  the applicable  provisions of  the
Indenture. See "Certain Covenants -- Merger, Consolidation or Sale of Assets."
 
                                       56
<PAGE>
OPTIONAL REDEMPTION
 
    The Notes will not be redeemable at the Company's option prior to October 1,
2001.  Thereafter, the Notes will be subject  to redemption at the option of the
Company, in whole  or in  part, upon not  less than  30 nor more  than 60  days'
notice,  at the redemption prices (expressed as percentages of principal amount)
set forth below plus  accrued and unpaid interest  to the applicable  redemption
date,  if redeemed during the twelve-month period  beginning on October 1 of the
years indicated below:
 
<TABLE>
<CAPTION>
YEAR                                                                               PERCENTAGE
- ---------------------------------------------------------------------------------  -----------
<S>                                                                                <C>
2001.............................................................................     105.125%
2002.............................................................................     103.417%
2003.............................................................................     101.708%
2004 and thereafter..............................................................     100.000%
</TABLE>
 
    Notwithstanding the foregoing, at any time on or before October 1, 1999, the
Company may (but  shall not  have the  obligation to) redeem  up to  30% of  the
original  aggregate  principal amount  of  the Notes  at  a redemption  price of
110.25% of  the  principal amount  thereof,  plus accrued  and  unpaid  interest
thereon to the redemption date, with the net proceeds of an Equity Offering made
by  the Company; PROVIDED that at least 70% of the aggregate principal amount of
Notes originally issued remain outstanding  immediately after the occurrence  of
such  redemption; and PROVIDED, FURTHER, that such redemption shall occur within
75 days of the date of the closing of such Equity Offering.
 
    If less than all of the Notes are  to be redeemed at any time, selection  of
Notes  for redemption will be made by the  Trustee on a pro rata basis; PROVIDED
that no Notes of $1,000 or less shall be redeemed in part. Notices of redemption
shall be mailed by first class mail at least 30 but not more than 60 days before
the redemption date to  each Holder of  Notes to be  redeemed at its  registered
address.  If any Note is  to be redeemed in part  only, the notice of redemption
that relates  to such  Note shall  state  the portion  of the  principal  amount
thereof  to be redeemed. A new Note  in principal amount equal to the unredeemed
portion thereof  will  be  issued  in  the  name  of  the  Holder  thereof  upon
cancellation  of the original  Note. On and after  the redemption date, interest
ceases to accrue on Notes or portions thereof called for redemption.
 
MANDATORY REDEMPTION
 
    The Company is  not required to  make mandatory redemption  or sinking  fund
payments with respect to the Notes.
 
REPURCHASE AT THE OPTION OF HOLDERS
 
  CHANGE OF CONTROL
 
    Upon  the occurrence of a Change of  Control, each Holder of Notes will have
the right to require the Company to repurchase all or any part (equal to  $1,000
or  an integral multiple thereof)  of such Holder's Notes  pursuant to the offer
described below (the "Change of Control Offer") at an offer price in cash  equal
to  101%  of the  aggregate  principal amount  thereof  plus accrued  and unpaid
interest thereon  (the  "Change of  Control  Purchase  Price") to  the  date  of
purchase  (the "Change of  Control Payment Date"). Within  30 days following any
Change of Control, the Company will mail a notice to each Holder describing  the
transaction  or transactions that constitute the  Change of Control and offering
to repurchase Notes  pursuant to the  procedures required by  the Indenture  and
described in such notice. The Change of Control Payment Date shall be a business
day not less than 30 days nor more than 60 days after such notice is mailed. The
Company  will comply with the requirements of  Rule 14e-1 under the Exchange Act
and any other securities laws and regulations thereunder to the extent such laws
and regulations are applicable in connection with the repurchase of the Notes as
a result of a Change of Control.
 
    On the  Change of  Control Payment  Date, the  Company will,  to the  extent
lawful,  (1) accept for payment all  Notes or portions thereof properly tendered
pursuant to the Change of  Control Offer, (2) deposit  with the Paying Agent  an
amount  equal to the Change of Control Purchase Price in respect of all Notes or
portions thereof so tendered  and (3) deliver  or cause to  be delivered to  the
Trustee the
 
                                       57
<PAGE>
Notes  so accepted together with an  Officers' Certificate stating the aggregate
principal amount of Notes  or portions thereof being  purchased by the  Company.
The  Paying Agent  will promptly mail  to each  Holder of Notes  so tendered the
Change of  Control  Payment  for  such Notes,  and  the  Trustee  will  promptly
authenticate  and mail (or cause to be transferred by book entry) to each Holder
a new Note equal  in principal amount  to any unpurchased  portion of the  Notes
surrendered,  if any; PROVIDED  that each such  new Note will  be in a principal
amount of $1,000 or an integral multiple thereof.
 
    Except as described above with respect to a Change of Control, the Indenture
does not contain provisions that permit the Holders of the Notes to require that
the Company repurchase or  redeem the Notes  in the event of  a takeover by  any
persons  other than the Approved Shareholders,  or a recapitalization or similar
restructuring.
 
    If a Change of  Control Offer is  made, there can be  no assurance that  the
Company  will  have available  funds  sufficient to  pay  the Change  of Control
Purchase Price for all of the Notes  that might be delivered by Holders  seeking
to  accept the Change of Control Offer. If  on a Change of Control Purchase Date
the Company  does not  have available  funds  sufficient to  pay the  Change  of
Control  Purchase Price or is prohibited from  purchasing the Notes, an Event of
Default would occur  under the Indenture.  The definition of  Change of  Control
includes  an event by which the Company  sells, conveys, transfers or leases all
or substantially  all  of its  properties  to any  Person.  The phrase  "all  or
substantially  all" is subject to applicable legal  precedent and as a result in
the future  there may  be uncertainty  as to  whether a  Change of  Control  has
occurred.  The  existence of  a Holder's  right to  require, subject  to certain
conditions, the Company  to repurchase the  Notes upon a  Change of Control  may
deter   a  third  party  from  acquiring  the  Company  in  a  transaction  that
constitutes, or results in, a Change of Control.
 
    The Credit  Facility provides  that certain  change of  control events  with
respect  to the Company would constitute  a default thereunder and prohibits the
Company from making a Change  of Control Offer or  Asset Sale Offer. Any  future
credit  agreements or other agreements relating  to Senior Indebtedness to which
the Company becomes a party may contain similar restrictions and provisions.  In
the  event a Change of  Control occurs at a time  when the Company is prohibited
from purchasing Notes, the Company could seek the consent of its lenders to  the
purchase  of Notes or  could attempt to  repay or refinance  the borrowings that
contain such prohibition. If the Company does not obtain such a consent or repay
such borrowings, the Company  will remain prohibited  from purchasing Notes.  In
such  case, the Company's failure to purchase tendered Notes would constitute an
Event of Default under the Indenture which would, in turn, constitute a  default
under  the Credit Facility. Even  if such consents to  an offer to purchase were
obtained or the lenders did not declare a default under the Credit Facility  and
other credit facilities, the Company's ability to pay cash to the holders of the
Notes  upon a repurchase may be limited by the Company's then existing financial
resources.
 
  ASSET SALES
 
    The Indenture will provide  that the Company will  not, and will not  permit
any  of its Subsidiaries to, engage in an  Asset Sale unless (i) the Company (or
such Subsidiary) receives consideration at the time of such Asset Sale at  least
equal to the fair market value, and in the case of a lease of assets under which
the Company or any of its Subsidiaries is the lessor, a lease providing for rent
and  other  conditions which  are  no less  favorable  to the  Company  (or such
Subsidiary) in any material respect  than the then prevailing market  conditions
(evidenced in each case by a resolution of the Board of Directors of such entity
set  forth in an Officers'  Certificate delivered to the  Trustee) of the assets
sold or otherwise disposed of, and (ii) at  least 85% (100% in the case of  such
lease  payments) of the  consideration therefor received by  the Company or such
Subsidiary is in the form of cash or Cash Equivalents or properties used in  the
Oil and Gas Business of the Company and its Subsidiaries.
 
    The  Company may apply Net Proceeds of an  Asset Sale, at its option, (a) to
permanently reduce Senior Indebtedness other than Senior Revolving Indebtedness,
(b) to permanently reduce Senior Revolving Indebtedness (and to  correspondingly
reduce  commitments with  respect thereto), or  (c) to invest  in properties and
assets that will  be used in  the Oil and  Gas Business of  the Company and  its
 
                                       58
<PAGE>
Subsidiaries.  Pending  the  final application  of  any such  Net  Proceeds, the
Company may temporarily reduce Senior Revolving Indebtedness or otherwise invest
such Net Proceeds in any manner that is not prohibited by the Indenture. Any Net
Proceeds from  Asset  Sales that  are  not applied  within  270 days  after  the
consummation  of  an  Asset Sale  as  provided  in the  first  sentence  of this
paragraph will be  deemed to  constitute "Excess Proceeds."  When the  aggregate
amount  of Excess Proceeds exceeds $5.0 million, the Company will be required to
make an offer to all  Holders of Notes (an "Asset  Sale Offer") to purchase  the
maximum  principal  amount of  Notes that  may  be purchased  out of  the Excess
Proceeds, at  a purchase  price  in cash  in  an amount  equal  to 100%  of  the
principal amount thereof plus accrued and unpaid interest thereon to the date of
purchase,  in accordance with the procedures set  forth in the Indenture. To the
extent that the aggregate unpaid amount  of Notes tendered pursuant to an  Asset
Sale  Offer is less than  the Excess Proceeds, the  Company may use such surplus
Excess Proceeds for general corporate  purposes. If the aggregate unpaid  amount
of  Notes surrendered by Holders thereof  exceeds the amount of Excess Proceeds,
the Trustee shall select  the Notes to  be purchased on a  pro rata basis.  Upon
completion  of such offer  to purchase, the  amount of Excess  Proceeds shall be
reset at zero.
 
    The Credit Facility prohibits the Company  from making an Asset Sale  Offer.
The  Indenture prohibits the Company from  directly or indirectly engaging in an
Asset Sale of any Principal Properties to any Subsidiary other than a Subsidiary
Guarantor.
 
CERTAIN COVENANTS
 
  OWNERSHIP OF CAPITAL STOCK
 
    The Indenture  will provide  that the  Company will  not permit  any  Person
(other  than the Company or  any Wholly Owned Subsidiary  of the Company) to own
any Capital Stock  of any Subsidiary  of the  Company, and will  not permit  any
Subsidiary  of the Company to issue Capital Stock (except to the Company or to a
Wholly Owned Subsidiary) in each  case except (a) directors' qualifying  shares,
(b)  Capital Stock issued prior to the  time such Person becomes a Subsidiary of
the Company, (c) if such Subsidiary merges with and into another Subsidiary, (d)
if another  Subsidiary  merges  with  and into  such  Subsidiary,  (e)  if  such
Subsidiary  ceases to be  a Subsidiary (as a  result of the sale  of 100% of the
shares of such Subsidiary, the Net Proceeds from which are applied in accordance
with "Repurchase at the Option of Holders -- Asset Sales") or (f) Capital  Stock
of a Subsidiary organized in a foreign jurisdiction required to be issued to, or
owned by, the government of such foreign jurisdiction or individual or corporate
citizens  of such foreign jurisdiction in  order for such Subsidiary to transact
business in such foreign jurisdiction.
 
  UNRESTRICTED SUBSIDIARIES
 
    The Board of Directors of the Company may designate any of its  Subsidiaries
as  an Unrestricted Subsidiary.  A Subsidiary may  only be so  designated if (i)
immediately after  giving effect  to such  designation no  Default or  Event  of
Default  exists, (ii)  the Company  would, at the  time of  such designation and
after giving pro forma effect thereto as if such designation had occurred at the
beginning of the applicable four-quarter period, have been permitted to incur at
least $1.00 of  additional Indebtedness  pursuant to  the Consolidated  Interest
Coverage Ratio and the Adjusted Consolidated Tangible Net Assets to Consolidated
Indebtedness  Ratio  tests set  forth  in the  first  paragraph of  the covenant
described under  the caption  "--  Incurrence of  Indebtedness and  Issuance  of
Preferred  Stock," and (iii) after  the date of the  Indenture and prior to such
designation, no  assets of  the Company  or  of any  Subsidiary of  the  Company
(including, without limitation, Capital Stock of any such Subsidiary) shall have
been  transferred, directly or indirectly, to any Unrestricted Subsidiary or any
of its Subsidiaries,  other than assets  transferred in the  ordinary course  of
business  and on terms that are no less favorable to the Company or the relevant
Subsidiary than those that would have been obtained in a comparable  transaction
by  the Company or  such Subsidiary with  an unrelated Person  and except to the
extent  permitted  under  the  caption   "--  Restricted  Payments."  Any   such
designation  by the Board of Directors of  the Company shall be evidenced to the
Trustee by filing with the Trustee a
 
                                       59
<PAGE>
certified copy of  the Board  Resolution of the  Company giving  effect to  such
designation  and an  Officers' Certificate of  the Company  certifying that such
designation complied with the foregoing conditions.
 
    Any  Subsidiary  of  the  Company  shall  continue  to  be  an  Unrestricted
Subsidiary   only  if  it  (a)  has  no  Indebtedness  other  than  Non-Recourse
Indebtedness; (b) is a Person with respect to which neither the Company nor  any
of  its Subsidiaries has any direct or  indirect obligation (x) to subscribe for
additional Equity  Interests  or  (y)  to maintain  or  preserve  such  Person's
financial  condition or to cause such Person  to achieve any specified levels of
operating  results;  and  (c)  has  not  guaranteed  or  otherwise  directly  or
indirectly provided credit support for any Indebtedness of the Company or any of
its Subsidiaries. If, at any time, any Unrestricted Subsidiary fails to meet the
foregoing  requirements, such Unrestricted Subsidiary  shall thereafter cease to
be an Unrestricted Subsidiary for  purposes of the Indenture, such  Unrestricted
Subsidiary  shall  execute  and  deliver  a  Subsidiary  Guarantee, supplemental
indenture or  other  agreement pursuant  to  which such  Person  guarantees  the
payment  of  the  Notes on  the  same  terms and  conditions  as  the Subsidiary
Guarantees and any Indebtedness of such Unrestricted Subsidiary shall be  deemed
to  be incurred by  a Subsidiary of  the Company as  of such date  (and, if such
Indebtedness is not permitted to be incurred as of such date under the  covenant
described  under  the caption  "-- Incurrence  of  Indebtedness and  Issuance of
Preferred Stock," the Company shall be in default of such covenant).
 
    The Board  of  Directors  of the  Company  may  at any  time  designate  any
Subsidiary,  if previously  designated as  an Unrestricted  Subsidiary, to  be a
Subsidiary; PROVIDED that such designation shall  be deemed to be an  incurrence
of  Indebtedness by a Subsidiary of  the Company of any outstanding Indebtedness
of such Subsidiary  and such  designation shall only  be permitted  if (i)  such
Indebtedness  is permitted  under the covenant  described under  the caption "--
Incurrence of Indebtedness and Issuance of Preferred Stock," (ii) no Default  or
Event of Default would be in existence following such designation and (iii) such
Subsidiary  shall execute and deliver a supplemental indenture pursuant to which
such Person guarantees the payment of the Notes on the same terms and conditions
as the Subsidiary Guarantees.
 
    As of the  date of the  Indenture, Costilla Redeco  Exploration, L.L.C.  and
Costilla  Redeco  Operating,  L.L.C.,  through which  the  Company  conducts its
Moldovan operations will be Unrestricted Subsidiaries.
 
  RESTRICTED PAYMENTS
 
    The Indenture will provide  that the Company will  not, and will not  permit
any  of its  Subsidiaries to,  directly or  indirectly: (i)  declare or  pay any
dividend or make  any distribution on  account of  the Company's or  any of  its
Subsidiaries' Equity Interests, other than dividends or distributions payable in
Equity  Interests (other than Disqualified Stock) of the Company or dividends or
distributions payable  to the  Company or  any Wholly  Owned Subsidiary  of  the
Company;  (ii) purchase,  redeem or  otherwise acquire  or retire  for value any
Equity Interests of the Company or any Subsidiary or Unrestricted Subsidiary  or
other  Affiliate of the Company (other than Equity Interests of the Company, any
Subsidiary or Unrestricted Subsidiary owned by  the Company or any Wholly  Owned
Subsidiary  of the Company);  (iii) make any principal  payment on, or purchase,
redeem, defease  or  otherwise acquire  or  retire for  value  any  Subordinated
Indebtedness  of the  Company or  any Subsidiary of  the Company,  in each case,
prior to a  scheduled mandatory sinking  fund payment date  or maturity date  or
(iv)  make any  Restricted Investment (all  such payments and  other actions set
forth in  clauses (i)  through  (iv) above  being  collectively referred  to  as
"Restricted  Payments"), unless, at the time of  and after giving effect to such
Restricted Payment:
 
        (a) no Default or Event of Default shall have occurred and be continuing
    or would occur as a consequence thereof;
 
        (b) the Company would, at the time of such Restricted Payment and  after
    giving  pro forma effect thereto as if such Restricted Payment had been made
    at the beginning of the applicable four-quarter period, have been  permitted
    to incur at least $1.00 of additional Indebtedness
 
                                       60
<PAGE>
    pursuant  to  the  Consolidated  Interest Coverage  Ratio  and  the Adjusted
    Consolidated Net Tangible  Assets to Consolidated  Indebtedness Ratio  tests
    set  forth in the first paragraph of  the covenant described below under the
    caption "-- Incurrence of Indebtedness and Issuance of Preferred Stock"; and
 
        (c) such Restricted Payment,  together with the  aggregate of all  other
    Restricted Payments made by the Company and its Subsidiaries on or after the
    date  of the Indenture  (excluding Restricted Payments  permitted by clauses
    (ii), (iii), (iv) and  (v) of the next  succeeding paragraph), is less  than
    the  sum of (i)  50% of the Consolidated  Net Income of  the Company and its
    Subsidiaries for  the  period (taken  as  one accounting  period)  from  the
    beginning  of the first day  of the fiscal month  during which the Indenture
    was executed and delivered to the  end of the Company's most recently  ended
    fiscal  quarter for which internal financial statements are available at the
    time of such  Restricted Payment (or,  if such Consolidated  Net Income  for
    such  period is a deficit, less 100% of such deficit), plus (ii) 100% of the
    aggregate net cash proceeds received by the Company as capital contributions
    to the Company or from the issue or sale after the date of the Indenture  of
    Equity  Interests of the Company  or of debt securities  of the Company that
    have been converted into such Equity Interests (other than Equity  Interests
    (or  convertible debt  securities) sold to  a Subsidiary  or an Unrestricted
    Subsidiary of  the  Company  and  other  than  Disqualified  Stock  or  debt
    securities  that have been converted into Disqualified Stock) other than the
    Common Stock sold in the Common Stock Offering.
 
    The foregoing  clauses (b)  and  (c), however,  will  not prohibit  (i)  the
payment of any dividend within 60 days after the date of declaration thereof, if
at said date of declaration such payment would have complied with the provisions
of  the Indenture; (ii) the  payment of any dividend  on Equity Interests of the
Company (other  than Disqualified  Stock)  payable solely  in shares  of  Equity
Interests  of the Company (other than Disqualified Stock); (iii) any dividend or
other distribution payable from  a Subsidiary of the  Company to the Company  or
any  Wholly Owned  Subsidiary; (iv) the  making of any  Restricted Investment in
exchange for, or  out of  the proceeds  of, the  substantially concurrent  sale,
issuance  or exchange (other than to a Subsidiary or any Unrestricted Subsidiary
of the Company)  of Equity  Interests of  the Company  (other than  Disqualified
Stock);  PROVIDED, that  any net  cash proceeds that  are utilized  for any such
Restricted Investment  shall  be  excluded  from clause  (c)  of  the  preceding
paragraph;  (v) the redemption,  repurchase, retirement or  other acquisition of
any Equity Interests of the Company in exchange for, or out of the proceeds  of,
the  substantially  concurrent  sale,  issuance or  exchange  (other  than  to a
Subsidiary or  any  Unrestricted Subsidiary  of  the Company)  of  other  Equity
Interests  of the Company (other than any Disqualified Stock); PROVIDED that any
net cash  proceeds  that  are  utilized for  any  such  redemption,  repurchase,
retirement  or  other  acquisition shall  be  excluded  from clause  (c)  of the
preceding paragraph;  and  (vi)  the defeasance,  redemption  or  repurchase  of
Subordinated  Indebtedness prior to  a scheduled mandatory  sinking fund payment
date or maturity date thereof with the  net cash proceeds from an incurrence  of
Permitted   Refinancing  Indebtedness  or  the  substantially  concurrent  sale,
issuance or exchange (other than to a Subsidiary or any Unrestricted  Subsidiary
of  the Company)  of Equity  Interests of  the Company  (other than Disqualified
Stock) or the purchase, redemption  or acquisition of Subordinated  Indebtedness
prior  to  a scheduled  mandatory  sinking fund  payment  date or  maturity date
thereof through the  issuance in  exchange thereof  of Equity  Interests of  the
Company  (other than Disqualified  Stock); PROVIDED, that  any net cash proceeds
that are utilized for any  such defeasance, redemption, repurchase, purchase  or
acquisition shall be excluded from clause (c) of the preceding paragraph.
 
    The  amount of all Restricted  Payments (other than cash)  shall be the fair
market value (evidenced by a resolution of  the Board of Directors set forth  in
an Officers' Certificate delivered to the Trustee) on the date of the Restricted
Payment  of  the asset(s)  proposed to  be  transferred by  the Company  or such
Subsidiary, as the case  may be, pursuant to  the Restricted Payment. Not  later
than the date of making any Restricted Payment, the Company shall deliver to the
Trustee  an Officers'  Certificate of the  Company stating  that such Restricted
Payment is permitted and setting forth the
 
                                       61
<PAGE>
basis  upon which the calculations required  by the covenant described under the
caption "-- Restricted Payments" were computed, which calculations may be  based
upon the Company's latest available financial statements.
 
  INCURRENCE OF INDEBTEDNESS AND ISSUANCE OF PREFERRED STOCK
 
    The  Indenture will provide that  the Company will not,  and will not permit
any of  its  Subsidiaries to,  directly  or indirectly,  create,  incur,  issue,
assume,   guarantee  or   otherwise  become   directly  or   indirectly  liable,
contingently  or  otherwise,  with   respect  to  (collectively,  "incur")   any
Indebtedness  (including Acquired  Indebtedness) and  that the  Company will not
issue any Disqualified  Stock and  will not permit  any of  its Subsidiaries  to
issue  any shares  of preferred stock;  PROVIDED, HOWEVER, that  the Company may
incur Indebtedness (including Acquired Indebtedness)  and the Company may  issue
shares  of Disqualified Stock  if: (i) the  Consolidated Interest Coverage Ratio
for the  Company's most  recently  ended four  full  fiscal quarters  for  which
internal  financial statements are  available immediately preceding  the date on
which such additional  Indebtedness is  incurred or such  Disqualified Stock  is
issued  would have been at least, during  the period until the first anniversary
of the date  of the Indenture,  2.25 to 1,  and, thereafter, 2.5  to 1, in  each
case,  determined on a pro forma basis (including a pro forma application of the
net proceeds therefrom), as if the additional Indebtedness had been incurred, or
the Disqualified Stock had been issued, as the case may be, at the beginning  of
such  four-quarter period;  (ii) the  Adjusted Consolidated  Net Tangible Assets
would have been at least 150% of Consolidated Indebtedness, determined on a  pro
forma  basis (including a  pro forma application of  the net proceeds therefrom)
and (iii) no Default or Event of  Default shall have occurred and be  continuing
or  would occur  as a  consequence thereof; PROVIDED,  that no  Guarantee may be
incurred pursuant  to  this paragraph,  unless  the guaranteed  Indebtedness  is
incurred by the Company pursuant to this paragraph.
 
    The foregoing provisions will not apply to:
 
        (i)  the  incurrence by  the Company  of  Indebtedness under  the Credit
    Facility (and the incurrence  by Subsidiaries of  Guarantees thereof) in  an
    aggregate  principal amount at any time  outstanding (with letters of credit
    being deemed  to have  a principal  amount equal  to the  maximum  potential
    liability  of the Company and its Subsidiaries thereunder) not to exceed the
    greater of (a) $50 million and (b) 15% of Adjusted Consolidated Net Tangible
    Assets, in each case, less the aggregate amount of all Net Proceeds of Asset
    Sales  applied  to  permanently  reduce   the  outstanding  amount  or   the
    commitments  with  respect to  such  Indebtedness pursuant  to  the covenant
    described above under the caption "-- Asset Sales";
 
        (ii) the incurrence by  the Company of  Indebtedness represented by  the
    Notes  and of its Subsidiaries of Indebtedness represented by the Subsidiary
    Guarantees;
 
       (iii) the  incurrence  by the  Company  or  any of  its  Subsidiaries  of
    Permitted  Refinancing Indebtedness in exchange for,  or the net proceeds of
    which are used to extend, refinance, renew, replace, defease or refund,  any
    Indebtedness described in the foregoing clause (ii);
 
       (iv)  the  incurrence  by  the  Company or  any  of  its  Subsidiaries of
    intercompany Indebtedness between or among the Company and any of its Wholly
    Owned Subsidiaries  or  between  or among  any  Wholly  Owned  Subsidiaries;
    PROVIDED  that, in the case of Indebtedness of the Company, such obligations
    shall be unsecured and subordinated  in case of an  event of default in  all
    respects  to the Company's obligations pursuant  to the Notes; and PROVIDED,
    however, that (i) any  subsequent issuance or  transfer of Equity  Interests
    that  results in any such  Indebtedness being held by  a Person other than a
    Wholly Owned Subsidiary  and (ii)  any sale or  other transfer  of any  such
    Indebtedness  to a Person that  is not either the  Company or a Wholly Owned
    Subsidiary shall be  deemed, in each  case, to constitute  an incurrence  of
    such Indebtedness by the Company or such Subsidiary, as the case may be;
 
        (v)  the  incurrence  by the  Company  of Hedging  Obligations  that are
    incurred for  the purpose  of  fixing or  hedging  interest rate  risk  with
    respect to any floating rate Indebtedness that is
 
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<PAGE>
    permitted  by  the Indenture  to be  incurred;  PROVIDED that,  the notional
    amount of such Hedging Obligations does  not exceed the principal amount  of
    the Indebtedness to which such Hedging Obligations relate;
 
       (vi) the incurrence by the Company of Hedging Obligations under commodity
    hedging  and currency  exchange agreements;  PROVIDED that,  such agreements
    were entered into  in the  ordinary course of  business for  the purpose  of
    limiting risks that arise in the ordinary course of business;
 
       (vii)  The incurrence by the Subsidiaries of Indebtedness in existence on
    the date of the Indenture; and
 
      (viii) the incurrence by the Company and its Subsidiaries of  Indebtedness
    (in  addition  to  Indebtedness  permitted  by  any  other  clause  of  this
    paragraph) in an aggregate principal amount  at any time outstanding not  to
    exceed $10.0 million:
 
provided  that no Subsidiary may incur  any Indebtedness other than Indebtedness
described in the foregoing  clauses (iv) or (vii)  unless such Subsidiary  shall
have executed and delivered a Subsidiary Guarantee and such Subsidiary Guarantee
remains  in  full force  and effect  (unless terminated  in accordance  with the
provisions of the Indenture).
 
    Further, the Company will  not, directly or indirectly,  in any event  incur
any  Indebtedness which by its terms (or by the terms of any agreement governing
such Indebtedness)  is subordinated  to any  other Indebtedness  of the  Company
unless  such Indebtedness is also by its terms (or by the terms of any agreement
governing such Indebtedness)  expressly subordinated  to the Notes  to the  same
extent  and in the same manner as  such Indebtedness is subordinated pursuant to
subordination provisions that  are most favorable  to the holders  of any  other
Indebtedness of the Company.
 
  LIENS
 
    The  Indenture will provide that  the Company will not,  and will not permit
any of its  Subsidiaries to, directly  or indirectly, create,  incur, assume  or
suffer to exist any Lien (other than Permitted Liens) on any of their respective
assets,  now owned or  hereafter acquired, securing  any Indebtedness unless the
Notes, in  the case  of such  Indebtedness of  the Company,  and the  Subsidiary
Guarantee  of such Subsidiary Guarantor,  in the case of  such Indebtedness of a
Subsidiary  Guarantor,  are  secured  equally   and  ratably  with  such   other
Indebtedness;  PROVIDED that,  if such  Indebtedness is  by its  terms expressly
subordinate to the Notes  or the Subsidiary Guarantees,  the Lien securing  such
subordinate  or junior Indebtedness shall be  subordinate and junior to the Lien
securing the Notes or the Subsidiary Guarantees with the same relative  priority
as such subordinated or junior Indebtedness shall have with respect to the Notes
or the Subsidiary Guarantees, as the case may be.
 
  DIVIDEND AND OTHER PAYMENT RESTRICTIONS AFFECTING SUBSIDIARIES
 
    The  Indenture will provide that  the Company will not,  and will not permit
any of its Subsidiaries to, directly or indirectly, create or otherwise cause or
suffer to  exist or  become  effective any  encumbrance  or restriction  on  the
ability   of  any  Subsidiary  to  (i)(a)   pay  dividends  or  make  any  other
distributions to the Company or any of its Subsidiaries on its Capital Stock  or
with  respect to  any other  interest or participation  in, or  measured by, its
profits, or  (b)  pay  any indebtedness  owed  to  the Company  or  any  of  its
Subsidiaries,  (ii)  make  loans  or  advances to  the  Company  or  any  of its
Subsidiaries or (iii) transfer any of its properties or assets to the Company or
any of its  Subsidiaries, (iv) transfer  any of  its property or  assets to  the
Company or any of its Subsidiaries, (v) grant liens or security interests on the
assets  in favor  of the Holders  of Notes, or  (vi) guarantee the  Notes or any
renewals or refinancings thereof, except  for such encumbrances or  restrictions
existing under or by reason of (A) the Credit Facility, the Indenture, the Notes
or any other agreement in existence on the date of the Indenture, (B) applicable
law,  (C) any instrument  governing Acquired Indebtedness of  Capital Stock of a
Person acquired by the Company  or any of its Subsidiaries  as in effect at  the
time  of such acquisition  (except to the extent  such Acquired Indebtedness was
incurred in  connection with  or in  contemplation of  such acquisition),  which
encumbrance or restriction is not applicable to any Person, or the properties or
assets  of  any Person,  other than  the Person,  or the  property or  assets of
 
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the Person, so acquired, PROVIDED that the Consolidated EBITDA of such Person is
not taken into account in determining whether such acquisition was permitted  by
the  terms of the Indenture, or (D) Permitted Refinancing Indebtedness, PROVIDED
that the  restrictions  contained in  the  agreements governing  such  Permitted
Refinancing  Indebtedness are  no more restrictive  than those  contained in the
agreements governing the Indebtedness being refinanced.
 
  LIMITATION ON LAYERING INDEBTEDNESS
 
    The Indenture will provide that the  Company will not incur, create,  issue,
assume,  guarantee  or  otherwise become  liable  for any  Indebtedness  that is
subordinate or junior in right of payment to any Senior Indebtedness and  senior
in any respect in right of payment to the Notes.
 
  MERGER, CONSOLIDATION OR SALE OF ASSETS
 
    The  Indenture will provide that  the Company will not,  and will not permit
any Subsidiary to,  in a single  transaction or series  of related  transactions
consolidate  or merge with or into (other  than the consolidation or merger of a
Wholly Owned Subsidiary of the Company  with another Wholly Owned Subsidiary  of
the  Company or into the Company) (whether or not the Company or such Subsidiary
is the  surviving  corporation),  or  directly  and/or  indirectly  through  its
Subsidiaries  sell, assign, transfer, lease, convey  or otherwise dispose of all
or substantially all of its properties  or assets (determined on a  consolidated
basis  for the  Company and its  Subsidiaries taken as  a whole) in  one or more
related transactions to, another corporation, Person or entity unless (i) either
(a) the Company, in  the case of  a transaction involving  the Company, or  such
Subsidiary,  in  the  case  of  a transaction  involving  a  Subsidiary,  is the
surviving corporation or (b) in the case of a transaction involving the Company,
the entity or the Person formed by or surviving any such consolidation or merger
(if other than the Company) or to which such sale, assignment, transfer,  lease,
conveyance  or other disposition shall have been made is a corporation organized
or existing  under the  laws of  the United  States, any  state thereof  or  the
District  of Columbia and assumes  all the obligations of  the Company under the
Notes and  the  Indenture  pursuant  to  a  supplemental  indenture  in  a  form
reasonably  satisfactory to the Trustee; (ii) immediately after such transaction
no Default or Event of Default exists;  and (iii) the Company or, if other  than
the  Company, the entity or Person formed by or surviving any such consolidation
or merger, or  to which such  sale, assignment, transfer,  lease, conveyance  or
other  disposition shall  have been  made (A)  will have  Consolidated Net Worth
immediately after the transaction equal to or greater than the Consolidated  Net
Worth  of the Company immediately preceding the transaction and (B) will, at the
time of such transaction and  after giving pro forma  effect thereto as if  such
transaction had occurred at the beginning of the applicable four-quarter period,
be  permitted to incur at least $1.00 of additional Indebtedness pursuant to the
Consolidated Interest Coverage Ratio and the Adjusted Consolidated Net  Tangible
Assets to Consolidated Indebtedness Ratio tests set forth in the first paragraph
of the covenant described above under the caption "-- Incurrence of Indebtedness
and Issuance of Preferred Stock."
 
  TRANSACTIONS WITH AFFILIATES
 
    The  Indenture will provide that  the Company will not,  and will not permit
any of  its Subsidiaries  to, after  the  date of  the Indenture,  sell,  lease,
transfer or otherwise dispose of any of its properties or assets to, or make any
payment  to, or purchase any property or assets from, or enter into or suffer to
exist any transaction or  series of transactions, or  make any agreement,  loan,
advance  or guarantee with,  or for the  benefit of, any  Affiliate (each of the
foregoing,  an   "Affiliate   Transaction"),   other   than   Exempt   Affiliate
Transactions, unless (i) such Affiliate Transaction is on terms that are no less
favorable to the Company or the relevant Subsidiary (as reasonably determined by
the  Company)  than  those  that  would  have  been  obtained  in  a  comparable
transaction by the Company or such Subsidiary with an unrelated Person and  (ii)
the  Company  delivers  to  the  Trustee  (a)  with  respect  to  any  Affiliate
Transaction entered into  after the  date of the  Indenture involving  aggregate
consideration  in excess of $1.0 million, a resolution of the Board of Directors
set forth in an Officers' Certificate certifying that such Affiliate Transaction
complies with clause  (i) above  and that  such Affiliate  Transaction has  been
approved  by a majority of  the disinterested members of  the Board of Directors
and (b) with respect to
 
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<PAGE>
any Affiliate Transaction  involving aggregate consideration  in excess of  $5.0
million, an opinion as to the fairness to the Company or such Subsidiary of such
Affiliate  Transaction from  a financial point  of view issued  by an investment
banking firm of national standing.
 
  SALE AND LEASEBACK
 
    The Company will not, and will not permit any of its Subsidiaries to,  enter
into  any  Sale  and  Leaseback  Transaction  unless  (a)  the  Company  or  its
Subsidiaries entering  into  such  Sale and  Leaseback  Transaction  could  have
incurred  the  Indebtedness  relating  to such  Sale  and  Leaseback Transaction
pursuant to the "-- Incurrence of Indebtedness and Issuance of Preferred  Stock"
and  "--  Liens" covenants,  (b) the  Net  Proceeds of  such Sale  and Leaseback
Transaction are at  least equal to  the fair  market value of  such property  as
determined  by the Board of  Directors of the Company  and (c) such Net Proceeds
are applied in the same  manner and to the same  extent as Net Proceeds from  an
Asset Sale pursuant to the "-- Asset Sales" covenant.
 
  REPORTS
 
    The  Indenture will provide that,  whether or not required  by the rules and
regulations of the Commission, so long as any Notes are outstanding, the Company
will furnish to the Holders of Notes, and file with the Trustee, within 15  days
after  it is, or would have been, required  to file such with the Commission (i)
all quarterly and annual financial information  that is or would be required  to
be  contained in  a filing  with the Commission  on Forms  10-Q and  10-K if the
Company is  or were  required  to file  such  Forms, including  a  "Management's
Discussion  and Analysis of Financial Condition  and Results of Operations" and,
with respect to the annual information  only, a report thereon by the  Company's
certified independent accountants and (ii) all current reports that are or would
be  required to be  filed with the Commission  on Form 8-K if  the Company is or
were required to file such reports. In addition, whether or not required by  the
rules  and regulations of  the Commission, the  Company will file  a copy of all
such information and reports with the Commission for public availability (unless
the Commission  will  not  accept  such a  filing)  and  make  such  information
available to securities analysts and prospective investors upon written request.
 
  EVENTS OF DEFAULT AND REMEDIES
 
    The  Indenture will provide that each  of the following constitutes an Event
of Default: (i) default for 30 days in  the payment when due of interest on  the
Notes  (whether  or  not  prohibited  by  the  subordination  provisions  of the
Indenture); (ii) default in payment when  due (upon redemption or otherwise)  of
the  principal of or premium, if any, on the Notes (whether or not prohibited by
the subordination provisions of the Indenture); (iii) failure by the Company  to
comply with the provisions described under the captions "Repurchase at Option of
Holders -- Change of Control," "Repurchase at Option of Holders -- Asset Sales,"
"--  Ownership of  Capital Stock," "--  Restricted Payments,"  "-- Incurrence of
Indebtedness and Issuance of  Preferred Stock" or  "-- Merger, Consolidation  or
Sale  of Assets"; (iv)  failure by the Company  or any of  its Subsidiary for 60
days after notice by  the Trustee or  Holders of at least  25% of the  aggregate
principal  amount  of the  Notes outstanding  to  comply with  any of  its other
agreements in  the Indenture  or  the Notes;  (v)  default under  any  mortgage,
indenture or instrument under which there may be issued or by which there may be
secured  or evidenced any Indebtedness for money  borrowed by the Company or any
of its Subsidiaries (or the payment of which is guaranteed by the Company or any
of its Subsidiaries) whether  such Indebtedness or Guarantee  now exists, or  is
created  after  the date  of the  Indenture, which  default (a)  is caused  by a
failure to  pay principal  of such  Indebtedness at  final maturity  thereof  (a
"Payment Default") or (b) results in the acceleration of such Indebtedness prior
to  its express  maturity and, in  each case,  the principal amount  of any such
Indebtedness, together with the principal amount of any other such  Indebtedness
under  which there has been a Payment Default  or the maturity of which has been
so accelerated, aggregates $10 million or  more; (vi) failure by the Company  or
any  of its Subsidiaries to pay final judgments (not fully covered by insurance)
aggregating in excess of $1 million, which judgments are not paid, discharged or
stayed for a period of 60 days; (vii) certain events of bankruptcy or insolvency
with respect  to the  Company or  any of  its Subsidiaries  or any  Unrestricted
Subsidiary; and (viii) any
 
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<PAGE>
Subsidiary  Guarantor attempts to revoke its Subsidiary Guarantee or contest its
validity or  any Subsidiary  Guarantee shall  not be  in full  force and  effect
(other than in accordance with the terms of the Indenture).
 
    If any Event of Default occurs and is continuing, the Trustee or the Holders
of  at least 25% in  principal amount of the  then outstanding Notes may declare
all the Notes to be due and payable immediately. Notwithstanding the  foregoing,
in  the case of an Event of Default arising from certain events of bankruptcy or
insolvency with respect  to the Company  or any Subsidiary  or any  Unrestricted
Subsidiary,  all outstanding Notes  will become due  and payable without further
action or notice.  Holders of the  Notes may  not enforce the  Indenture or  the
Notes  except  as provided  in the  Indenture.  Subject to  certain limitations,
Holders of a  majority in  principal amount of  the then  outstanding Notes  may
direct the Trustee in its exercise of any trust or power. The Indenture provides
that  if a Default occurs and is  continuing, generally the Trustee must, within
90 days after the occurrence of such default, give to the Holders notice of such
Default. The  Trustee may  withhold from  Holders  of the  Notes notice  of  any
continuing  Default or Event  of Default (except  a Default or  Event of Default
relating to the  payment of principal  or premium,  if any, or  interest) if  it
determines that withholding notice is in their interest.
 
    The  Holders of a majority  in aggregate principal amount  of the Notes then
outstanding by notice to the Trustee may on behalf of the Holders of all of  the
Notes  waive any existing Default or Event of Default and its consequences under
the Indenture except a continuing Default or Event of Default in the payment  of
interest  or premium  on, or  the principal  of, the  Notes or  in respect  of a
provision that cannot  be amended or  waived without the  consent of the  Holder
affected. See "Amendment, Supplement and Waiver."
 
    The  Company  is required  to deliver  to the  Trustee annually  a statement
regarding compliance  with  the Indenture,  and  the Company  is  required  upon
becoming  aware of any Default or Event of  Default, to deliver to the Trustee a
statement specifying such Default or Event of Default.
 
NO PERSONAL LIABILITY OF DIRECTORS, OFFICERS, EMPLOYEES AND STOCKHOLDERS
 
    No director, officer, employee, incorporator  or stockholder of the  Company
or  any Subsidiary, as such, shall have any liability for any obligations of the
Company under the  Notes or  the Indenture  or the  Subsidiary Guarantors  under
their  Subsidiary Guarantees  or for any  claim based  on, in respect  of, or by
reason of, such obligations or their creation. Each Holder of Notes by accepting
a Note waives and releases all such  liability. The waiver and release are  part
of the consideration for issuance of the Notes. Such waiver may not be effective
to waive liabilities under the federal securities laws and it is the view of the
Commission that such waiver is against public policy.
 
LEGAL DEFEASANCE AND COVENANT DEFEASANCE
 
    The  Company may, at  its option and at  any time, elect to  have all of its
obligations  discharged   with  respect   to  the   outstanding  Notes   ("Legal
Defeasance")  except  for (i)  the  rights of  Holders  of outstanding  Notes to
receive payments in respect of the  principal of, premium, if any, and  interest
on  such Notes when such payments are due from the trust referred to below, (ii)
the Company's obligations with respect to the Notes concerning issuing temporary
Notes, registration of Notes, mutilated, destroyed, lost or stolen Notes and the
maintenance of an office or agency  for payment and money for security  payments
held  in trust, (iii) the  rights, powers, trusts, duties  and immunities of the
Trustee, and  the Company's  obligations in  connection therewith  and (iv)  the
Legal  Defeasance provisions of the Indenture.  In addition, the Company may, at
its option  and at  any  time, elect  to have  the  obligations of  the  Company
released  with respect to certain covenants  that are described in the Indenture
("Covenant  Defeasance")  and  thereafter  any  omission  to  comply  with  such
obligations  shall not constitute a Default or  Event of Default with respect to
the Notes.  In  the  event  Covenant  Defeasance  occurs,  certain  events  (not
including  nonpayment, bankruptcy,  receivership, rehabilitation  and insolvency
events) described under "Events of Default"  will no longer constitute an  Event
of Default with respect to the Notes.
 
                                       66
<PAGE>
    In order to exercise either Legal Defeasance or Covenant Defeasance, (i) the
Company  must irrevocably deposit with the Trustee, in trust, for the benefit of
the Holders of Notes, cash  in U.S. dollars, noncallable Government  Securities,
or  a combination thereof, in such amounts as will be sufficient, in the opinion
of a nationally recognized  firm of independent public  accountants, to pay  the
principal  of, premium,  if any,  and interest on  the outstanding  Notes on the
stated maturity or on the  applicable redemption date, as  the case may be,  and
the  Company must specify whether the Notes are being defeased to maturity or to
a particular redemption date; (ii) in the case of Legal Defeasance, the  Company
shall  have delivered to the Trustee an  opinion of counsel in the United States
reasonably acceptable  to  the  Trustee  confirming that  (A)  the  Company  has
received  from, or there has  been published by, the  Internal Revenue Service a
ruling or (B) since the  date of the Indenture, there  has been a change in  the
applicable  federal income tax law, in either case to the effect that, and based
thereon such  opinion  of  counsel  shall  confirm  that,  the  Holders  of  the
outstanding Notes will not recognize income, gain or loss for federal income tax
purposes  as a result  of such Legal  Defeasance and will  be subject to federal
income tax on  the same amounts,  in the same  manner and at  the same times  as
would have been the case if such Legal Defeasance had not occurred; (iii) in the
case  of Covenant Defeasance, the Company shall have delivered to the Trustee an
opinion of counsel  in the United  States reasonably acceptable  to the  Trustee
confirming  that the Holders of the outstanding Notes will not recognize income,
gain or  loss for  federal income  tax purposes  as a  result of  such  Covenant
Defeasance and will be subject to federal income tax on the same amounts, in the
same  manner and at the same times as  would have been the case if such Covenant
Defeasance had not  occurred; (iv)  no Default or  Event of  Default shall  have
occurred  and be continuing on the date of such deposit (other than a Default or
Event of Default resulting  from the borrowing  of funds to  be applied to  such
deposit)  or insofar as  Events of Default from  bankruptcy or insolvency events
are concerned, at any time in the period ending on the 123rd day after the  date
of  deposit; (v) such Legal Defeasance or Covenant Defeasance will not result in
a breach or violation of, or  constitute a default under any material  agreement
or  instrument (other  than the Indenture)  to which  the Company or  any of its
Subsidiaries is a party or  by which the Company or  any of its Subsidiaries  is
bound;  (vi) the  Company must deliver  to the Trustee  an Officers' Certificate
stating that  the  deposit was  not  made by  the  Company with  the  intent  of
preferring  the Holders of  Notes over other  creditors of the  Company with the
intent of defeating, hindering, delaying or defrauding creditors of the  Company
or  others;  and (vii)  the Company  must  deliver to  the Trustee  an Officers'
Certificate and  an  opinion  of  counsel,  each  stating  that  all  conditions
precedent  relating to the Legal Defeasance or the Covenant Defeasance have been
complied with.
 
TRANSFER AND EXCHANGE
 
    A Holder may transfer  or exchange Notes in  accordance with the  Indenture.
The  Company, the Registrar  and the Trustee  may require a  Holder, among other
things, to furnish appropriate  endorsements and transfer  documents as well  as
certifications, legal opinions and other information and the Company may require
a  Holder  to  pay any  taxes  and fees  required  by  law or  permitted  by the
Indenture. The Company is not required to transfer or exchange any Note selected
for redemption. Also, the  Company is not required  to transfer or exchange  any
Note for a period of 15 days before a selection of Notes to be redeemed.
 
    The  registered Holder of a Note will be  treated as the owner of it for all
purposes.
 
AMENDMENT, SUPPLEMENT AND WAIVER
 
    Except as provided in the next two succeeding paragraphs, the Indenture, the
Subsidiary Guarantees  or the  Notes may  be amended  or supplemented  with  the
consent  of the Holders of at least a  majority in principal amount of the Notes
then outstanding (including consents obtained in connection with a tender  offer
or  exchange offer for Notes),  and any existing default  or compliance with any
provision of the Indenture, the Subsidiary Guarantees or the Notes may be waived
with the consent of the  Holders of a majority in  principal amount of the  then
outstanding Notes (including consents obtained in connection with a tender offer
or exchange offer for Notes).
 
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<PAGE>
    Without  the consent of each Holder affected, an amendment or waiver may not
(with respect  to any  Notes held  by a  nonconsenting Holder):  (i) reduce  the
principal amount of Notes whose Holders must consent to an amendment, supplement
or waiver, (ii) reduce the principal of or change the fixed maturity of any Note
or  alter the provisions with respect to the redemption of the Notes (other than
provisions  relating  to  the  covenants  described  above  under  the   caption
"Repurchase  at the Option of Holders"), (iii)  reduce the rate of or change the
time for payment  of interest  on any  Note, (iv) waive  a Default  or Event  of
Default  in the payment of  principal of or premium, if  any, or interest on the
Notes (except a rescission  of acceleration of  the Notes by  the Holders of  at
least  a majority in aggregate principal amount of the Notes and a waiver of the
payment default that resulted from such acceleration), (v) make any Note payable
in money  other than  that stated  in the  Notes, (vi)  make any  change in  the
provisions of the Indenture, or the Subsidiary Guarantees relating to waivers of
past Defaults or the rights of Holders of Notes to receive payments of principal
of  or  premium, if  any, or  interest on  the Notes,  (vii) waive  a redemption
payment with respect to any  Note (other than a payment  required by one of  the
covenants  described  above  under  the caption  "Repurchase  at  the  Option of
Holders") or  (viii) make  any  change in  the  foregoing amendment  and  waiver
provisions.
 
    Notwithstanding  the foregoing, without the consent  of any Holder of Notes,
the Company  and  the  Trustee  may  amend  or  supplement  the  Indenture,  the
Subsidiary   Guarantees  or  the   Notes  to  cure   any  ambiguity,  defect  or
inconsistency, to provide for uncertificated Notes in addition to or in place of
certificated Notes, to provide for  the assumption of the Company's  obligations
to Holders of Notes in the case of a merger or consolidation, to make any change
that  would provide any additional rights or benefits to the Holders of Notes or
that does not adversely affect the legal rights under the Indenture of any  such
Holder,  or to comply with requirements of  the Commission in order to effect or
maintain the qualification of the Indenture under the Trust Indenture Act.
 
CONCERNING THE TRUSTEE
 
    The State  Street Bank  and Trust  Company  will be  the Trustee  under  the
Indenture.  The  Trustee's current  address is  Corporate Trust  Department, Two
International Place, 4th Floor, Boston, Massachusetts 02110.
 
    The Holders of a majority in principal amount of the then outstanding  Notes
will  have the  right to  direct the  time, method  and place  of conducting any
proceeding for  exercising  any remedy  available  to the  Trustee,  subject  to
certain  exceptions. The  Indenture provides  that in  case an  Event of Default
shall occur (which shall  not be cured),  the Trustee will  be required, in  the
exercise of its power, to use the degree of care of a prudent man in the conduct
of  his own affairs.  Subject to such  provisions, the Trustee  will be under no
obligation to exercise any of  its rights or powers  under the Indenture at  the
request  of any Holder  of Notes, unless  such Holder shall  have offered to the
Trustee security and indemnity satisfactory to it against any loss, liability or
expense.
 
ADDITIONAL INFORMATION
 
    Anyone who  receives this  Prospectus may  obtain a  copy of  the  Indenture
without  charge by  writing to  Costilla Energy,  Inc., 400  West Illinois, 10th
Floor, Midland, Texas 79701, Attention: Chief Financial Officer.
 
BOOK-ENTRY, DELIVERY AND FORM
 
    The Notes to be  sold as set  forth herein will initially  be issued in  the
form  of one Global Note (the "Global  Note"). The Global Note will be deposited
on the date of the closing of the sale of the Notes offered hereby (the "Closing
Date") with  the Trustee  as custodian  for The  Depository Trust  Company  (the
"Depositary")  and  registered in  the name  of Cede  & Co.,  as nominee  of the
Depositary (such nominee being referred to herein as the "Global Note Holder").
 
    The Depositary is a limited-purpose trust  company that was created to  hold
securities for its participating organizations (collectively, the "Participants"
or  the  "Depositary's  Participants")  and  to  facilitate  the  clearance  and
settlement of  transactions  in  such securities  between  Participants  through
electronic  book-entry changes in accounts of its Participants. The Depositary's
Participants
 
                                       68
<PAGE>
include securities brokers and dealers  (including the Underwriters), banks  and
trust  companies, clearing corporations and  certain other organizations. Access
to the Depositary's system  is also available to  other entities such as  banks,
brokers,  dealers and trust companies (collectively, the "Indirect Participants"
or the "Depositary's Indirect  Participants") that clear  through or maintain  a
custodial  relationship  with  a  Participant,  either  directly  or indirectly.
Persons who are not Participants may  beneficially own securities held by or  on
behalf  of  the Depositary  only through  the  Depositary's Participants  or the
Depositary's Indirect Participants.
 
    The  Company  expects  that  pursuant  to  procedures  established  by   the
Depositary  (i) upon deposit of the Global  Note, the Depositary will credit the
accounts of Participants  designated by  the Underwriters with  portions of  the
principal  amount of the Global Note and  (ii) beneficial ownership of the Notes
evidenced by  the  Global Note  will  be shown  on,  and the  transfer  of  such
ownership  will be effected  only through, records  maintained by the Depositary
(with  respect  to  the  interests   of  the  Depositary's  Participants),   the
Depositary's   Participants   and   the   Depositary's   Indirect  Participants.
Prospective purchasers are  advised that the  laws of some  states require  that
certain  persons take  physical delivery in  definitive form  of securities that
they own. Consequently, the  ability to transfer Notes  evidenced by the  Global
Note will be limited to such extent.
 
    So  long as  the Global Note  Holder is  the registered owner  of the Global
Note, the Global Note Holder will be  considered the sole owner or Holder  under
the  Indenture of any Notes  evidenced by the Global  Note. Beneficial owners of
Notes evidenced by the Global Note will not be considered the owners or  Holders
thereof  under  the Indenture  for any  purpose, including  with respect  to the
giving of any directions, instructions  or approvals to the Trustee  thereunder.
Except as provided below, owners of beneficial interests in the Global Note will
not  be entitled to have Notes registered in their names and will not receive or
be entitled to receive  physical delivery of Notes  in definitive form.  Neither
the  Company nor the Trustee  will have any responsibility  or liability for any
aspect of  the records  of the  Depositary or  for maintaining,  supervising  or
reviewing any records of the Depositary relating to the Notes.
 
    Payments  in respect of the  principal of, premium, if  any, and interest on
any Notes registered in  the name of  the Global Note  Holder on the  applicable
record  date will be payable by the Company to or at the direction of the Global
Note Holder in its capacity as the registered Holder under the Indenture.  Under
the terms of the Indenture, the Company and the Trustee may treat the persons in
whose  names the Notes, including the Global  Note, are registered as the owners
thereof for the purpose  of receiving such  payments. Consequently, neither  the
Company nor the Trustee has or will have any responsibility or liability for the
payment  of such  amounts to beneficial  owners of Notes.  The Company believes,
however, that it is currently the policy of the Depositary to immediately credit
the accounts  of  the  relevant  Participants with  such  payments,  in  amounts
proportionate  to  their  respective  holdings of  beneficial  interests  in the
relevant security as  shown on the  records of the  Depositary. Payments by  the
Depositary's  Participants  and the  Depositary's  Indirect Participants  to the
beneficial owners  of  Notes  will  be governed  by  standing  instructions  and
customary   practice  and  will  be   the  responsibility  of  the  Depositary's
Participants or the Depositary's Indirect Participants.
 
    As long as  the Notes  are represented by  a Global  Note, the  Depositary's
nominee  will be the holder  of the Notes and therefore  will be the only entity
that can exercise  a right  to repayment  or repurchase  of the  Notes. See  "--
Repurchase   at   the   Option   of   Holders   --   Change   of   Control"  and
"-- Asset Sales." Notice  by the Depositary's  Participants or the  Depositary's
Indirect Participants or by owners of beneficial interests in a Global Note held
through such Participants or Indirect Participants of the exercise of the option
to elect repayment of beneficial interests in Notes represented by a Global Note
must  be transmitted to  the Depositary in  accordance with its  procedures on a
form required by the Depositary and provided to Participants. In order to ensure
that the Depositary's  nominee will timely  exercise a right  to repayment  with
respect  to a particular Note,  the beneficial owner of  such Note must instruct
the broker or other Participant or  Indirect Participant through which it  holds
an  interest in such Note  to notify the Depositary of  its desire to exercise a
right  to  repayment.   Different  firms  have   cut-off  times  for   accepting
instructions from their customers and,
 
                                       69
<PAGE>
accordingly,   each  beneficial  owner  should   consult  the  broker  or  other
Participant or Indirect Participant through which it holds an interest in a Note
in order to  ascertain the cut-off  time by  which such an  instruction must  be
given  in order for timely notice to be delivered to the Depositary. The Company
will not be liable for any delay in  delivery of notices of the exercise of  the
option to elect repayment.
 
    The  Company will issue Notes in definitive  form in exchange for the Global
Note if, and  only if, either  (1) the Depositary  is at any  time unwilling  or
unable  to continue as depositary and a successor depositary is not appointed by
the Company within  90 days,  or (2)  an Event of  Default has  occurred and  is
continuing and the Notes registrar has received a request from the Depositary to
issue  Notes in definitive form in lieu of  all or a portion of the Global Note.
In either instance, an owner of a beneficial interest in the Global Note will be
entitled to have  Notes equal in  principal amount to  such beneficial  interest
registered  in its name and will be  entitled to physical delivery of such Notes
in definitive  form.  Notes so  issued  in definitive  form  will be  issued  in
denominations  of $1,000  and integral multiples  thereof and will  be issued in
registered form only, without coupons.
 
  CERTIFICATED NOTES
 
    If the Company  notifies the Trustee  in writing that  the Depositary is  no
longer  willing or  able to  act as a  depositary and  the Company  is unable to
locate a qualified successor within 90  days then, upon surrender by the  Global
Note Holder of its Global Note, Notes in the form of registered definitive Notes
will  be issued to  each person that  the Global Note  Holder and the Depositary
identify as being the beneficial owner of the related Notes.
 
    Neither the Company  nor the Trustee  will be  liable for any  delay by  the
Global  Note Holder  or the Depositary  in identifying the  beneficial owners of
Notes and the  Company and the  Trustee may  conclusively rely on,  and will  be
protected  in  relying  on, instructions  from  the  Global Note  Holder  or the
Depositary for all purposes.
 
  SAME-DAY SETTLEMENT AND PAYMENT
 
    The Indenture will require that payments in respect of the Notes represented
by the Global Note  (including principal, premium, if  any, interest be made  by
wire  transfer of immediately  available funds to the  accounts specified by the
Global Note Holder. With  respect to Certificated Notes,  the Company will  make
all  payments  of  principal, premium,  if  any,  interest by  wire  transfer of
immediately available funds to the accounts specified by the Holders thereof or,
if no  such account  is specified,  by mailing  a check  to each  such  Holder's
registered  address.  Secondary trading  in  long-term notes  and  debentures of
corporate issuers is generally settled  in clearinghouse or next-day funds.  The
Company  expects that secondary  trading in the Certificated  Notes will also be
settled in immediately available funds.
 
GOVERNING LAW
 
    The Indenture, the Notes and the Subsidiary Guarantees will be governed  by,
and construed in accordance with, the laws of the State of New York.
 
CERTAIN DEFINITIONS
 
    Set  forth below are certain defined  terms used in the Indenture. Reference
is made to the Indenture for a full disclosure of all such terms, as well as any
other capitalized terms used herein for which no definition is provided.
 
    "ACQUIRED INDEBTEDNESS" means with respect to any specified Person, (i)  any
Indebtedness  of any  other Person  existing at  the time  such other  Person is
merged with or into or becomes a Subsidiary of such specified Person, including,
without  limitation,   Indebtedness  incurred   in   connection  with,   or   in
contemplation  of,  such  other  Person  merging  with  or  into  or  becoming a
Subsidiary of such  specified Person, and  (ii) Indebtedness secured  by a  Lien
encumbering any asset acquired by such specified Person.
 
    "ADJUSTED  CONSOLIDATED  NET  TANGIBLE  ASSETS" means,  as  of  the  date of
determination, without duplication,  (a) the  sum of (i)  discounted future  net
revenue  from proved oil  and gas reserves  of the Company  and its Subsidiaries
calculated  in   accordance  with   Commission  guidelines   before  any   state
 
                                       70
<PAGE>
or federal income taxes, as estimated in a reserve report prepared as of the end
of  the Company's most  recently completed fiscal year,  which reserve report is
prepared or audited by independent petroleum  engineers, as increased by, as  of
the  date of determination,  the discounted future net  revenue of (A) estimated
proved oil and gas reserves of the Company and its Subsidiaries attributable  to
any  acquisition consummated since the date of such year-end reserve report, and
(B)  estimated  oil  and  gas  reserves  of  the  Company  and  its   Subsidiary
attributable to extensions, discoveries and other additions and upward revisions
of  estimates of  proved oil and  gas reserves due  to exploration, development,
exploitation, production or  other activities conducted  or otherwise  occurring
since  the date  of such  year-end reserve  report which  would, in  the case of
determinations made pursuant to clauses (A) and (B), in accordance with standard
industry  practice,  result  in  such  additions  or  revisions,  in  each  case
calculated  in  accordance  with  Commission  guidelines  (utilizing  the prices
utilized in such year-end reserve report), and  decreased by, as of the date  of
determination, the discounted future net revenue of (C) estimated proved oil and
gas  reserves of the Company and its  Subsidiaries produced or disposed of since
the date of such year-end reserve report and (D) reductions in the estimated oil
and gas reserves  of the Company  and its  Subsidiaries since the  date of  such
year-end  reserve  report attributable  to  downward revisions  of  estimates of
proved oil  and  gas reserves  due  to exploration,  development,  exploitation,
production  or other activities conducted or  otherwise occurring since the date
of such year-end reserve report which would, in the case of determinations  made
pursuant  to clauses (C) and (D), in accordance with standard industry practice,
result in such revisions, in each case calculated in accordance with  Commission
guidelines  (utilizing  the prices  utilized in  such year-end  reserve report);
provided that,  in the  case of  each  of the  determinations made  pursuant  to
clauses  (A) through (D), such increases and  decreases shall be as estimated by
the Company's  engineers, except  that  if as  a  result of  such  acquisitions,
dispositions,  discoveries, extensions or revisions,  there is a Material Change
that is an increase, then such increases and decreases in the discounted  future
net  revenue shall be  confirmed in writing  by independent petroleum engineers,
(ii) the capitalized costs  that are attributable to  oil and gas properties  of
the  Company and its  Subsidiaries to which  no proved oil  and gas reserves are
attributed, based on the  Company's books and  records as of  a date no  earlier
than  the date of the Company's latest annual or quarterly financial statements,
(iii) the net working capital (which  shall be calculated as all current  assets
of the Company and its Subsidiaries minus all current liabilities of the Company
and  its Subsidiaries, except current liabilities  included in Indebtedness on a
date no  earlier than  the date  of  the Company's  latest annual  or  quarterly
financial  statements) and  (iv) the greater  of (I)  the net book  value of the
other tangible assets of the Company and  its Subsidiaries on a date no  earlier
than  the date of the Company's  latest annual or quarterly financial statements
and (II) the appraised value, as  estimated by independent appraisers, of  other
tangible assets of the Company and its Subsidiaries as of a date no earlier than
the date of the Company's latest audited financial statements, MINUS (b) the sum
of  (i)  minority interests  of  third parties  to  the extent  included  in the
calculation  of  the  immediately  preceding  clause  (a),  (ii)  the   positive
remainder,  if any, obtained  by subtracting (I)  gas balancing underpayments of
the Company  and its  Subsidiaries  reflected in  the Company's  latest  audited
financial  statements  and  not otherwise  included  in the  calculation  of the
immediately preceding clause (a) from (II) any gas balancing liabilities of  the
Company and its Subsidiaries reflected in the Company's latest audited financial
statements  and not  otherwise included  in the  calculation of  the immediately
preceding clause (a), and (iii) the discounted future net revenue, calculated in
accordance with Commission guidelines (utilizing the same prices utilized in the
Company's year-end reserve report), attributable to oil and gas reserves of  the
Company  and  its Subsidiaries  subject  to participation  interests, overriding
royalty  interests   or  other   interests  of   third  parties,   pursuant   to
participation, partnership, vendor financing or other agreements then in effect,
other than pursuant to Production Payments, or that otherwise are required to be
delivered to third parties, other than pursuant to Production Payments.
 
    "ADJUSTED  CONSOLIDATED  NET  TANGIBLE ASSETS  TO  CONSOLIDATED INDEBTEDNESS
RATIO" means,  at any  time, the  ratio of  Adjusted Consolidated  Net  Tangible
Assets at such time to Consolidated Indebtedness at such time.
 
                                       71
<PAGE>
    "AFFILIATE"  of any specified Person means  (i) any other Person directly or
indirectly controlling  or controlled  by  or under  direct or  indirect  common
control with such specified Person or (ii) any other Person who is a director or
executive  officer of (a) such  specified Person or (b)  any Person described in
the preceding clause (i). For purposes of this definition, "control" (including,
with correlative meanings, the terms  "controlling," "controlled by" and  "under
common  control  with"), as  used with  respect  to any  Person, shall  mean the
possession, directly  or  indirectly,  of  the power  to  direct  or  cause  the
direction  of the  management or  policies of  such Person,  whether through the
ownership of  voting  securities,  by  agreement  or  otherwise;  PROVIDED  that
beneficial ownership of 10% or more of any class, or any series of any class, of
equity  securities of  a Person, whether  or not  voting, shall be  deemed to be
control.
 
    "ASSET SALE" means with respect to  any Person, the sale, lease,  conveyance
or  other  disposition, that  does  not constitute  a  Restricted Payment  or an
Investment, by such Person of any of its assets (including, without  limitation,
by  way  of a  Sale  and Leaseback  and including  the  issuance, sale  or other
transfer of any Equity Interests in any Subsidiary or the sale or other transfer
of any Equity  Interests in any  Unrestricted Subsidiary of  such Person)  other
than  to the  Company (including  the receipt of  proceeds of  insurance paid on
account of the loss of or damage to any asset and awards of compensation for any
asset taken by condemnation, eminent domain or similar proceeding, and including
the receipt of proceeds  of business interruption insurance),  in each case,  in
one  or a  series of  related transactions;  PROVIDED that,  notwithstanding the
foregoing, the  term  "Asset Sale"  shall  not  include: (a)  the  sale,  lease,
conveyance,  disposition or  other transfer of  all or substantially  all of the
assets of the Company, as permitted  pursuant to the covenant entitled  "Merger,
Consolidation or Sale of Assets," (b) the sale or lease of hydrocarbons or other
mineral  interests in the ordinary  course of business and  customary in the Oil
and Gas Business, (c) any  Production Payment, (d) a  transfer of assets by  the
Company  to a Wholly Owned  Subsidiary of the Company  (other than any Principal
Properties to any Wholly  Owned Subsidiary not a  Subsidiary Guarantor) or by  a
Wholly Owned Subsidiary of the Company to the Company or to another Wholly Owned
Subsidiary of the Company, (e) an issuance of Equity Interests by a Wholly Owned
Subsidiary  of the Company to the Company  or to another Wholly Owned Subsidiary
of the Company, (e) sale  or other disposition of  cash or Cash Equivalents,  or
(f)  any lease, abandonment, disposition, relinquishment  or farm out of any oil
and gas property  that are  customary in  nature and scope  in the  Oil and  Gas
Business and are entered into in the ordinary course of the Oil and Gas Business
of the Company and its Subsidiaries.
 
    "BENEFICIARY",  when used with respect to  any individual, means the spouse,
lineal descendants, parents and siblings of any such individual, the estates and
the legal representatives of  any such individual and  any of the foregoing  and
the  trustee of any bona fide trust of  which any such individual and any of the
foregoing are the sole beneficiaries or grantors.
 
    "CAPITAL LEASE OBLIGATION" means, at  the time any determination thereof  is
to be made, the amount of the liability in respect of a capital lease that would
at such time be required to be capitalized on a balance sheet in accordance with
GAAP.
 
    "CAPITAL  STOCK" means (i) in the case of a corporation, capital stock, (ii)
in the case of an association or business entity, any and all shares, interests,
participations, rights  or other  equivalents  (however designated)  of  capital
stock,  (iii) in the case of partnership, partnership interests (whether general
or limited),  (iv)  in the  case  of  a limited  liability  company,  membership
interests,  and (v) any other interest or participation that confers on a Person
the right to receive a share of  the profits and losses of, or distributions  of
assets of, the issuing Person.
 
    "CASH  EQUIVALENT"  means  (a)  securities  issued  or  directly  and  fully
guaranteed or  insured  by  the  United  States of  America  or  any  agency  or
instrumentality  thereof (provided that the full  faith and credit of the United
States is pledged  in support thereof)  having maturities not  more than  twelve
months  from the  date of acquisition,  (b) U.S. dollar  denominated (or foreign
currency fully hedged) time deposits,  certificates of deposit, Eurodollar  time
deposits  or Eurodollar certificates  of deposit of  (i) any domestic commercial
bank of  recognized  standing having  capital  and  surplus in  excess  of  $500
 
                                       72
<PAGE>
million or (ii) any bank whose short-term commercial paper rating from S&P is at
least  A-1 or  the equivalent  thereof or from  Moody's is  at least  P-1 or the
equivalent thereof (any such bank being an "Approved Lender"), in each case with
maturities of not more than twelve months from the date of acquisition, and  (c)
commercial  paper  issued  by any  Approved  Lender  (or by  the  parent company
thereof) or any variable  rate notes issued by,  or guaranteed by, any  domestic
corporation  rated A-1 (or the  equivalent thereof) or better  by S&P or P-1 (or
the equivalent thereof) or better by  Moody's and maturing within twelve  months
of the date of acquisition.
 
    "CHANGE OF CONTROL" means such time as any of the following events occur:
 
        (i)  any "person" or "group"  (as such terms are  used in Sections 13(d)
    and 14(d) of  the Exchange Act),  other than Cadell  S. Liedtke, Michael  J.
    Grella and Henry G. Musselman and any of their respective Beneficiaries (the
    "Approved Shareholders), is or becomes the "beneficial owner" (as defined in
    Rule 13d-3 under the Exchange Act), directly or indirectly, of more than 50%
    of the total Voting Stock of the Company;
 
        (ii)  the Company  is merged with  or into or  consolidated with another
    Person and, immediately after giving effect to the merger or  consolidation,
    (A)  less than 50% of the total voting power of the outstanding Voting Stock
    of the surviving or  resulting Person is  then "beneficially owned"  (within
    the  meaning of Rule 13d-3  under the Exchange Act)  in the aggregate by the
    stockholders  of  the   Company  immediately   prior  to   such  merger   or
    consolidation,  and  (B)  any "person"  or  "group" (as  defined  in Section
    13(d)(3)  or  14(d)(2)  of  the  Exchange  Act)  other  than  the   Approved
    Stockholders  has  become  the  direct or  indirect  "beneficial  owner" (as
    defined in Rule 13d-3 under the Exchange Act) of more than 50% of the  total
    voting power of the Voting Stock of the surviving or resulting Person;
 
        (iii)  the Company,  either individually or  in conjunction  with one or
    more Subsidiaries, sells, assigns,  conveys, transfers, leases or  otherwise
    disposes  of, or the  Subsidiaries sell, assign,  convey, transfer, lease or
    otherwise dispose of,  all or  substantially all  of the  properties of  the
    Company and the Subsidiaries, taken as a whole (either in one transaction or
    a   series  of  related  transactions),   including  Capital  Stock  of  the
    Subsidiaries, to  any Person  (other  than the  Company  or a  Wholly  Owned
    Subsidiary);
 
        (iv)  during  any consecutive  two-year period,  individuals who  at the
    beginning of such period constituted the  Board of Directors of the  Company
    (together  with any new directors whose  election by such Board of Directors
    or whose nomination  for election  by the  stockholders of  the Company  was
    approved  by a vote of a majority of  the directors then still in office who
    were either directors at the beginning  of such period or whose election  or
    nomination  for election was previously so approved) cease for any reason to
    constitute a  majority of  the Board  of Directors  of the  Company then  in
    office; or
 
        (v) the liquidation or dissolution of the Company.
 
    "CONSOLIDATED  EBITDA" means, with respect to any Person for any period, the
sum of, without duplication, (i) the Consolidated Net Income of such Person  and
its  Subsidiaries  for such  period, plus  (ii)  to the  extent deducted  in the
computation of such Consolidated Net  Income, the Consolidated Interest  Expense
for  such period, plus (iii)  to the extent deducted  in the computation of such
Consolidated Net Income, amortization or write-off of deferred financing charges
for such period, plus (iv)  provision for taxes based  on income or profits  for
such  period (to the  extent such income  or profits were  included in computing
Consolidated Net Income for such period), plus (v) to the extent deducted in the
computation  of  such  Consolidated   Net  Income,  consolidated   depreciation,
depletion,  amortization  and  other  noncash charges  of  such  Person  and its
Subsidiaries required to be  reflected as expenses on  the books and records  of
such  Person,  plus (vi)  to  the extent  deducted  in the  computation  of such
Consolidated Net Income,  consolidated exploration and  abandonment expenses  of
such  Person and  its Subsidiaries for  such periods, minus  (vii) cash payments
with respect to any nonrecurring, noncash charges previously added back pursuant
to clause (v), and excluding (viii) the impact of
 
                                       73
<PAGE>
foreign currency translations. Notwithstanding the foregoing, the provision  for
taxes  based on the income or profits  of, and the depreciation and amortization
and other noncash charges of, and the exploration and abandonment expenses of, a
Subsidiary of a  Person shall  be added to  Consolidated Net  Income to  compute
Consolidated EBITDA only to the extent (and in the same proportion) that the Net
Income  of  such Subsidiary  was included  in  calculating the  Consolidated Net
Income of such Person and only if  a corresponding amount would be permitted  at
the  date of determination  to be dividended  to such Person  by such Subsidiary
without prior approval (unless such approval has been obtained), pursuant to the
terms of  its  charter  and all  agreements,  instruments,  judgments,  decrees,
orders,   statutes,  rules  and  governmental  regulations  applicable  to  that
Subsidiary or its stockholders.
 
    "CONSOLIDATED INDEBTEDNESS" means, with respect to any Person for any  time,
the  Indebtedness of such Person and its Subsidiaries at such time as determined
on a consolidated basis in accordance with GAAP.
 
    "CONSOLIDATED INTEREST COVERAGE RATIO" means with respect to any Person  for
any  period,  the  ratio of  (i)  Consolidated  EBITDA of  such  Person  and its
Subsidiaries for  such period  to  (ii) Consolidated  Interest Expense  of  such
Person  and its Subsidiaries for  such period. In the  event that the Company or
any of its  Subsidiaries incurs, assumes,  Guarantees or repays  or redeems  any
Indebtedness  (other  than revolving  credit  borrowings) or  issues  or redeems
preferred stock subsequent  to the  commencement of  the four-quarter  reference
period  for which the  Consolidated Interest Coverage  Ratio is being calculated
but on or prior to the date on which the event for which the calculation of  the
Consolidated  Interest Coverage Ratio is made (the "Calculation Date"), then the
Consolidated Interest Coverage Ratio shall be calculated giving pro forma effect
to  such  incurrence,   assumption,  Guarantee,  repayment   or  redemption   of
Indebtedness,  or such issuance or redemption of preferred stock, as if the same
had occurred at the beginning  of the applicable four-quarter reference  period.
For  purposes of making the computation referred to above, (i) acquisitions that
have been made  by the  Company or any  of its  Subsidiaries, including  through
mergers  or  consolidations and  including  any related  financing transactions,
during the four-quarter reference period or subsequent to such reference  period
and  on or prior to the Calculation Date shall be deemed to have occurred on the
first day of the four-quarter reference period, and (ii) the Consolidated EBITDA
attributable to discontinued operations, as determined in accordance with  GAAP,
and operations or businesses disposed of prior to the Calculation Date, shall be
excluded,   and  (iii)   the  Consolidated  Interest   Expense  attributable  to
discontinued operations, as determined in  accordance with GAAP, and  operations
or  businesses disposed of prior to the Calculation Date, shall be excluded, but
only to  the  extent that  the  obligations  giving rise  to  such  Consolidated
Interest  Expense will not be  obligations of the referent  Person or any of its
Subsidiaries following the Calculation Date.
 
    "CONSOLIDATED INTEREST EXPENSE" means,  with respect to  any Person for  any
period,  the sum, without duplication, of  (i) the consolidated interest expense
of  such  Person  and  its  Subsidiaries  for  such  period  including,  without
limitation,  amortization of original issue discount, noncash interest payments,
the interest  component  of  any  deferred  payment  obligations,  the  interest
component   of  all   payments  associated   with  Capital   Lease  Obligations,
commissions, discounts and other fees and charges incurred in respect of  letter
of  credit or bankers' acceptance financings, and net payments (if any) pursuant
to Hedging  Obligations, but  excluding amortization  or write-off  of  deferred
financing charges for such period, and (ii) the consolidated interest expense of
such  Person and its  Subsidiaries that was capitalized  during such period, and
(iii) any interest expense on Indebtedness of another Person that is  Guaranteed
by such Person or one of its Subsidiaries or secured by a Lien on assets of such
Person  or one  of its Subsidiaries  (whether or  not such Guarantee  or Lien is
called upon),  and (iv)  the product  of  (a) all  cash dividend  payments  (and
noncash  dividend payments in the case of a  Person that is a Subsidiary) on any
series of preferred  stock of  such Person  payable to  a party  other than  the
Company  or a Wholly  Owned Subsidiary, times  (b) a fraction,  the numerator of
which is one and the denominator of which is one minus the then current combined
federal, state  and local  statutory tax  rate of  such Person,  expressed as  a
decimal, on a consolidated basis and in accordance with GAAP.
 
                                       74
<PAGE>
    "CONSOLIDATED  NET INCOME" means, with respect to any Person for any period,
the aggregate of the  Net Income of  such Person and  its Subsidiaries for  such
period,  on a consolidated  basis, determined in  accordance with GAAP; provided
that (i) the Net Income (but not loss) of any Person that is not a Subsidiary or
that is accounted for by the equity method of accounting shall be included  only
to  the extent of the  amount of dividends or distributions  paid in cash to the
referent Person or a Wholly Owned Subsidiary thereof, (ii) the Net Income of any
Subsidiary shall be excluded  to the extent that  the declaration or payment  of
dividends  or similar distributions by that Subsidiary of that Net Income is not
at the date of determination  permitted without any prior governmental  approval
(unless  such  approval  has  been  obtained)  or,  directly  or  indirectly, by
operation of the terms  of its charter or  any agreement, instrument,  judgment,
decree,  order,  statute, rule  or  governmental regulation  applicable  to that
Subsidiary or its stockholders, (iii) the Net Income of any Person acquired in a
pooling of  interests transaction  for any  period  prior to  the date  of  such
acquisition  shall  be  excluded, (iv)  the  cumulative  effect of  a  change in
accounting principles  shall be  excluded and  (v)  the Net  Income of,  or  any
dividends  or  other distributions  from,  any Unrestricted  Subsidiary,  to the
extent otherwise included, shall be excluded  unless distributed in cash to  the
Company or one of its Subsidiaries.
 
    "CONSOLIDATED  NET WORTH" means, with respect to  any Person as of any date,
the consolidated  stockholders'  equity  of such  Person  and  its  consolidated
Subsidiaries  as of such date  less (w) the amount  of such stockholders' equity
attributable to Disqualified Stock, (x) all write-ups subsequent to the date  of
the  Indenture  in  the book  value  of any  asset  owned  by such  Person  or a
consolidated  Subsidiary  of  such   Person  (other  than  purchase   accounting
adjustments  made, in connection with any acquisition of any entity that becomes
a consolidated Subsidiary of such Person after the date of the Indenture to  the
book value of the assets of such entity), (y) all investments as of such date in
unconsolidated Subsidiaries and in Persons that are not Subsidiaries (except, in
each  case, Permitted  Investments), and (z)  all unamortized  debt discount and
expense and unamortized deferred charges as  of such date, all of the  foregoing
determined in accordance with GAAP.
 
    "CREDIT FACILITY" means a credit facility that may be entered into among the
Company  and  the lenders  parties thereto  (which shall  initially be  a credit
facility among the Company, NationsBank of Texas, N.A. or one of its affiliates,
as agent, and the other lenders  parties thereto), including any related  notes,
guarantees,   collateral  documents,  instruments  and  agreements  executed  in
connection therewith, and in each case as amended, modified, renewed,  extended,
refunded, replaced, restated or refinanced from time to time.
 
    "DEFAULT"  means any event that is or with the passage of time or the giving
of notice or both would be an Event of Default.
 
    "DISQUALIFIED STOCK" means (a) with respect to any Person, Capital Stock  of
such Person that, by its terms (or by the terms of any security into which it is
convertible or for which it is exchangeable), or upon the happening of any event
(unless  any redemption or repurchase of  such Capital Stock upon the occurrence
of such event  is required  by any such  terms, but  only to the  extent that  a
payment in respect thereof would be permitted under the covenant set forth under
the  caption  "Restricted  Payments"),  matures  or  is  mandatorily redeemable,
pursuant to a  sinking fund  obligation or otherwise,  or is  redeemable at  the
option of the Holder thereof, in whole or in part, on or prior to the date which
is one year after the date on which the Notes mature and (b) with respect to any
Subsidiary  of  such Person  (including with  respect to  any Subsidiary  of the
Company), any Capital  Stock other  than any  common stock  with no  preference,
privileges, or redemption or repayment provisions.
 
    "DOLLAR-DENOMINATED  PRODUCTION  PAYMENTS" mean  dollar  denominated payment
obligations of the  Company or any  of its  Subsidiaries that are  or, upon  the
occurrence of a contingent event, would be recorded as liabilities in accordance
with  GAAP, together with all undertakings and obligations of the Company or any
of its Subsidiaries in connection therewith, which obligations will be deemed to
constitute Indebtedness for borrowed money for purposes of the Indenture.
 
                                       75
<PAGE>
    "EQUITY INTERESTS" means Capital  Stock and all  warrants, options or  other
rights  to  acquire  Capital Stock  (but  excluding  any debt  security  that is
convertible into, or exchangeable for, Capital Stock), whether outstanding prior
to, on or after the date of the Indenture.
 
    "EQUITY OFFERING" means an offer and sale of Qualified Stock of the  Company
to a Person other than an Affiliate of the Company.
 
    "EXCHANGE ACT" means the Securities Exchange Act of 1934, as amended.
 
    "EXEMPT  AFFILIATE TRANSACTIONS" means (a) transactions between or among the
Company and/or  its  Wholly  Owned  Subsidiaries, (b)  advances  not  to  exceed
$1,000,000  at any time outstanding to officers of the Company or any Subsidiary
of the Company in the ordinary course of business to provide for the payment  of
reasonable  expenses  incurred  by  such persons  in  the  performance  of their
responsibilities to the  Company or such  Subsidiary or in  connection with  any
relocation,  (c) fees and compensation paid  to and indemnity provided on behalf
of directors, officers  or employees  of the Company  or any  Subsidiary of  the
Company in the ordinary course of business, (d) any employment agreement that is
in  effect on the date  of the Indenture in the  ordinary course of business and
any such agreement entered into by the Company or a Subsidiary after the date of
the Indenture  in  the  ordinary course  of  business  of the  Company  or  such
Subsidiary and (e) payments and transactions under the Indebtedness of A&P Meter
Service  and Supply, Inc. ("A&P") to the  Company outstanding on the date of the
Indenture and the performance of and payment for services provided by A&P to the
Company and its Subsidiaries in the ordinary course of business consistent  with
past practice. See "Certain Transactions."
 
    "GAAP"  means  generally accepted  accounting  principles set  forth  in the
opinions and pronouncements of the  Accounting Principles Board of the  American
Institute  of Certified Public Accountants  and statements and pronouncements of
the Financial Accounting  Standards Board or  in such other  statements by  such
other  entity as have been  approved by a significant  segment of the accounting
profession, which are in effect on the date of the Indenture.
 
    "GUARANTEE" means  a  guarantee (other  than  by endorsement  of  negotiable
instruments  for  collection  in the  ordinary  course of  business),  direct or
indirect, in any manner  (including, without limitation,  letters of credit  and
reimbursement  agreements  in  respect  thereof),  of all  or  any  part  of any
Indebtedness.
 
    "HEDGING OBLIGATIONS" means, with respect to any Person, the obligations  of
such  Person  under  (i)  interest  rate  swap  agreements,  interest  rate  cap
agreements and  interest rate  collar agreements  and (ii)  other agreements  or
arrangements  designed  to  protect  such  Person  against  fluctuations  in (a)
interest rates, (b) the value of foreign currencies and (c) Oil and Gas Purchase
and Sales Contracts.
 
    "INDEBTEDNESS" means, with respect to  any Person, without duplication,  (a)
all  liabilities of such Person for borrowed  money or for the deferred purchase
price of property or  services (excluding any trade  accounts payable and  other
accrued  current liabilities incurred  in the ordinary  course of business), and
all liabilities  of such  Person  incurred in  connection  with any  letters  of
credit,  bankers'  acceptances  or  other  similar  credit  transactions  or any
agreement to purchase, redeem, exchange, convert or otherwise acquire for  value
any  Capital Stock of such Person, or any warrants, rights or options to acquire
such Capital Stock outstanding on the  date of the Indenture or thereafter,  if,
and  to the  extent, any  of the foregoing  would appear  as a  liability upon a
balance sheet  of  such  Person  prepared  in  accordance  with  GAAP,  (b)  all
obligations  of  such  Person evidenced  by  bonds, notes,  debentures  or other
similar instruments, if, and to the extent, any of the foregoing would appear as
a liability upon  a balance  sheet of such  Person prepared  in accordance  with
GAAP,  (c)  all  Indebtedness  of  such  Person  created  or  arising  under any
conditional sale or  other title  retention agreement with  respect to  property
acquired by such Person (even if the rights and remedies of the seller or lender
under such agreement in the event of default are limited to repossession or sale
of  such property), but excluding trade accounts payable arising in the ordinary
course of business, (d)  all Capitalized Lease Obligations  of such Person,  (e)
all  Indebtedness referred to in the preceding  clauses of other Persons and all
dividends of other Persons, the payment of which is secured by (or for which the
holder of such
 
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<PAGE>
Indebtedness has an  existing right  to be secured  by) any  Lien upon  property
(including,  without  limitation, accounts  and contract  rights) owned  by such
Person, even though such Person has not assumed or become liable for the payment
of such  Indebtedness (the  amount of  such obligation  being deemed  to be  the
lesser of the value of such property or asset or the amount of the obligation so
secured)  (f) all Production Payments of such Person, (g) all guarantees by such
Person of  Indebtedness referred  to in  this definition,  (h) all  Disqualified
Stock  of such  Person valued  at the  greater of  its voluntary  or involuntary
maximum fixed repurchase price plus accrued dividends and (i) all obligations of
such Person under or in respect  to currency exchange contracts, oil or  natural
gas price hedging arrangements and Hedging Obligations. For purposes hereof, the
"maximum  fixed repurchase  price" of Disqualified  Stock which does  not have a
fixed repurchase price shall be calculated in accordance with the terms of  such
Disqualified  Stock as if Disqualified Stock were purchased on any date on which
Indebtedness shall be required to be  determined pursuant to the Indenture,  and
if  such price  is based  upon, or measured  by, the  fair market  value of such
Disqualified Stock, such fair market value shall be determined in good faith  by
the  board  of directors  of the  issuer of  such Disqualified  Stock; provided,
however, that if  such Disqualified Stock  is not at  the date of  determination
permitted  or required  to be repurchase,  the "maximum  fixed repurchase price"
shall be the book value of such Disqualified Stock.
 
    "INVESTMENTS" means, with  respect to  any Person, all  investments by  such
Person in other Persons (including Affiliates) in the form of direct or indirect
loans  (including guarantees of Indebtedness  or other obligations), advances or
capital contributions (excluding advances to officers and employees of the  type
specified  in clause  (b) of the  definition of  Exempt Affiliate Transactions),
purchases or  other  acquisitions  for  consideration  of  Indebtedness,  Equity
Interests  or  other  securities  and  all other  items  that  are  or  would be
classified as investments on  a balance sheet prepared  in accordance with  GAAP
and  the acquisition, by purchase  or otherwise, of all  or substantially all of
the business or assets of any other Person.
 
    "LIEN" means, with respect to any asset, any mortgage, lien, pledge, charge,
security interest or encumbrance of any  kind in respect of such asset,  whether
or  not filed, recorded  or otherwise perfected  under applicable law (including
any conditional sale or other title retention agreement, any lease in the nature
thereof, any option or other  agreement to sell or  give a security interest  in
and any filing of or agreement to give any financing statement under the Uniform
Commercial Code (or equivalent statutes) of any jurisdiction).
 
    "MATERIAL  CHANGE"  means an  increase or  decrease (excluding  changes that
result solely from changes in prices) of  more than 10% during a fiscal  quarter
in  the discounted future net cash flows from proved oil and gas reserves of the
Company and its Subsidiaries calculated in accordance with clause (a)(i) of  the
definition of Adjusted Consolidated Net Tangible Assets; PROVIDED, however, that
the  following will be excluded from the calculation of Material Change: (i) any
acquisition during the quarter of oil and gas reserves that have been  estimated
by  independent petroleum engineers and on which  a report or reports exists and
(ii) any disposition  of properties existing  at the beginning  of such  quarter
that have been disposed of pursuant to the provisions of the Indenture described
under the caption "Redemption of the Option of the Holders."
 
    "NET  INCOME" means, with  respect to any  Person, the net  income (loss) of
such Person, determined  in accordance  with GAAP  and before  any reduction  in
respect  of preferred stock dividends, excluding, however, (i) any gain (but not
loss, except as provided in (b) below), together with any related provision  for
taxes  on such gain  (but not loss),  realized in connection  with (a) any Asset
Sale (including, without limitation, dispositions pursuant to Sale and Leaseback
transactions) or (b) the disposition of any securities by such Person or any  of
its  Subsidiary or the extinguishment of any  Indebtedness of such Person or any
of its Subsidiary,  other than  any loss arising  out of  the extinguishment  of
Indebtedness  refinanced with  the proceeds  of the  Notes and  other securities
issued contemporaneously with the Notes  or Indebtedness that was refinanced  in
June   1996  by  such   refinanced  Indebtedness,  (ii)   any  extraordinary  or
nonrecurring   gain   (but    not   loss,    except   as    provided   in    (i)
 
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<PAGE>
above),  together with any related provision  for taxes on such extraordinary or
nonrecurring gain  (but  not loss),  and  (iii) any  gain  (but not  loss)  from
currency exchange transactions not in the ordinary course of business consistent
with past practice.
 
    "NET  PROCEEDS" means the aggregate cash proceeds received by the Company or
any of  its  Subsidiaries in  respect  of  any Asset  Sale  (including,  without
limitation,  any cash received upon the sale or other disposition of any noncash
consideration received in any Asset Sale),  net of the direct costs relating  to
such Asset Sale (including, without limitation, legal, accounting and investment
banking  fees, and sales commissions) and  any relocation expenses incurred as a
result thereof, taxes paid or payable as  a result thereof, and any reserve  for
adjustment  in respect of the sale price  of such asset or assets established in
accordance with GAAP.
 
    "NON-RECOURSE INDEBTEDNESS" means Indebtedness (i)  as to which neither  the
Company  nor any  of its  Subsidiaries (a) provides  credit support  of any kind
(including any  undertaking,  agreement  or  instrument  that  would  constitute
Indebtedness),  (b)  is  directly  or  indirectly  liable  (as  a  guarantor  or
otherwise), or (c) constitutes the lender;  and (ii) no default with respect  to
which  (including  any  rights  that  the  holders  thereof  may  have  to  take
enforcement action  against  an  Unrestricted  Subsidiary)  would  permit  (upon
notice,  lapse of  time or  both) any  holder of  any other  Indebtedness of the
Company or  any  of  its  Subsidiaries  to  declare  a  default  on  such  other
Indebtedness  or cause the payment thereof to be accelerated or payable prior to
its stated maturity.
 
    "OBLIGATIONS"   means    any   principal,    interest,   penalties,    fees,
indemnifications,  reimbursements, damages  and other  liabilities payable under
the documentation governing any Indebtedness.
 
    "OIL AND  GAS BUSINESS"  means  the business  of  the exploration  for,  and
development,   acquisition,  and  production   of  hydrocarbons,  together  with
activities  ancillary  thereto  (including   with  limitation,  the   gathering,
processing,  treatment,  marketing and  transportation  of such  production) and
other related energy and natural resources businesses.
 
    "OIL AND GAS PURCHASE AND SALE  CONTRACT" means with respect to any  Person,
any  oil  and  gas  agreements  and  other  agreements  or  arrangements  or any
combination thereof  entered into  by  such Person  in  the ordinary  course  of
business  and that is designed to provide protection against oil and natural gas
price fluctuations.
 
    "PERMITTED INVESTMENTS" means (a) any Investments by the Subsidiaries of the
Company in the Company; (b) any Investments in Cash Equivalents; (c) Investments
made as a result of the receipt of noncash consideration from an Asset Sale that
was made pursuant to and in  compliance with the covenant described above  under
the  caption  "Repurchase  at  the  Option  of  Holders  --  Asset  Sales";  (d)
Investments outstanding as  of the  date of  the Indenture;  (e) Investments  in
Wholly Owned Subsidiaries engaged in the Oil and Gas Business and Investments in
any  Person that, as a  result of such Investment  (or a series of substantially
contemporaneous Investments  made pursuant  to  a single  plan) (x)  such  other
Person  becomes a Wholly Owned Subsidiary engaged in the Oil and Gas Business or
(y) such other Person that is engaged in  the Oil and Gas Business is merged  or
consolidated  with or into, or transfers or  conveys all or substantially all of
its assets  to  the  Company or  a  Wholly  Owned Subsidiary  in  a  transaction
permitted  under  the  Indenture;  (f) entry  into  operating  agreements, joint
ventures, partnership agreements, working interests, royalty interests,  mineral
leases,  processing  agreements, farm-out  agreements,  contracts for  the sale,
transportation or  exchange  of oil  and  natural gas,  unitization  agreements,
pooling  arrangements, area  of mutual interest  agreements or  other similar or
customary agreements, transactions, properties,  interests or arrangements,  and
Investments  and expenditures  in connection  therewith or  pursuant thereto, in
each case  made or  entered into  in  the ordinary  course of  the Oil  and  Gas
Business,  excluding, however, Investments  in corporations; (g)  entry into any
hedging arrangements  in the  ordinary course  of business  for the  purpose  of
protecting  the Company's or any  Subsidiaries's production against fluctuations
in oil or natural gas prices; (h) shares of money mutual or similar funds having
assets in excess of $500,000,000, and (i) Investments in an aggregate amount not
to exceed $5,000,000 at any one time outstanding.
 
                                       78
<PAGE>
    "PERMITTED LIENS" means (a) Liens existing on the date of the Indenture; (b)
Liens under the Credit Facility  securing Indebtedness permitted to be  incurred
in  accordance with  clause (i)  of the second  paragraph under  the caption "--
Incurrence of Indebtedness and  Issuance of Preferred Stock";  (c) Liens now  or
hereafter  securing any Hedging Obligations so  long as the related Indebtedness
is permitted under clauses (v) or (vi) of the second paragraph under the caption
"-- Incurrence  of Indebtedness  and  Issuance of  Preferred Stock";  (d)  Liens
securing Permitted Refinancing Indebtedness; PROVIDED, that such Liens extend to
or  cover only the property or  assets currently securing the Indebtedness being
refinanced; (e) Liens for taxes,  assessments and governmental charges not  then
due  or the validity  of which is  being contested in  good faith by appropriate
proceedings,  promptly  instituted  and  diligently  conducted,  and  for  which
adequate  reserves have  been established  to the  extent required  by GAAP; (f)
statutory   landlords',   carriers',   mechanics',   workmen's,   materialman's,
operator's  or similar Liens arising in the ordinary course of business for sums
not delinquent  or being  contested in  good faith  by appropriate  proceedings,
promptly  instituted and diligently  conducted, and for  which adequate reserves
have been established to the extent  required by GAAP; (g) easements, rights  of
way, restrictions and other similar encumbrances or minor imperfections in title
that,  in the  case of  any of the  foregoing, were  not incurred  or created to
secure the payment of borrowed money or the deferred purchase price of  property
or  services, and in  the aggregate do  not materially and  adversely affect the
value of such properties or materially impair use for the purposes of which such
properties are held by the Company or any Subsidiaries; (h) Liens on, or related
to, properties to  secure all  or part of  the costs  (other than  Indebtedness)
incurred   in  the  ordinary  course   of  business  of  exploration,  drilling,
development or operation thereof; (i)  judgment and attachment liens not  giving
rise  to an Event of Default or liens created by or existing from any litigation
or legal  proceeding  that  are  currently being  contested  in  good  faith  by
appropriate  proceedings, promptly instituted and  diligently conducted, and for
which adequate reserves have been made to the extent required by GAAP; (j) Liens
on deposits made  in the  ordinary course  of business;  (k) Liens  in favor  of
collecting  or  payor banks  having a  right of  set-off, revocation,  refund or
chargeback with respect to money or instruments of the Company or any Subsidiary
on deposit with or in possession of such bank; (l) Liens on pipeline or pipeline
facilities which arise out of operation of law; (m) Liens on deposits to  secure
public  or statutory obligations  or in lieu  of surety or  appeal bonds entered
into in the  ordinary course  of business;  (n) liens  reserved in  oil and  gas
leases  for bonus or rental  payments and for compliance  with the terms of such
leases; (o)  Liens arising  under partnership  agreements, oil  and gas  leases,
farmout agreements, division orders, contracts for the sale, purchase, exchange,
transportation  or processing of oil, gas or other hydrocarbons, unitization and
pooling  declarations   and   agreements,  development   agreements,   operating
agreements,  area of  mutual interest agreements  and other  agreements that are
customary in the Oil and Gas Business  and that do not secure Indebtedness;  (p)
(i)  Liens upon any property  of any Person existing  at the time of acquisition
thereof by the Company or a Subsidiary, (ii) Liens upon any property of a Person
existing at the time such Person is  merged or consolidated with the Company  or
any  Subsidiary or  existing at  the time of  the sale  or transfer  of any such
property of such Person to  the Company or any  Subsidiary, or (iii) Liens  upon
any  property of a Person existing at the time such Person becomes a Subsidiary;
PROVIDED, that in each case such Lien  has not been created in contemplation  of
such  sale, merger, consolidation, transfer or acquisition, and PROVIDED that in
each such case no such Lien shall extend to or cover any property of the Company
or any  Subsidiary  other than  the  property being  acquired  and  improvements
thereon;  (q) purchase money Liens granted in connection with the acquisition of
assets, PROVIDED, that (i) such Liens attach only to the assets so acquired with
the purchase money  indebtedness secured  thereby, (ii) such  Liens secure  only
Indebtedness that is not in excess of 100% of the purchase price of such assets,
and (iii) such Liens attach no later than 180 days after the acquisition of such
assets;  and (r) Liens securing Indebtedness incurred as a result of extensions,
renewals or replacements of Indebtedness  secured by Liens permitted by  clauses
(p)  or (q),  PROVIDED, that  (i) the  principal amount  of the  Indebtedness so
issued and secured by  such Lien shall  not exceed the  principal amount of  the
Indebtedness  so extended, renewed,  replaced, exchanged or  refinanced and (ii)
the Indebtedness so
 
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issued and secured by such Lien shall  not be secured by any property or  assets
of  the Company or any  Subsidiary other than the  property or assets subject to
the Liens securing such Indebtedness being exchanged or refinanced.
 
    "PERMITTED REFINANCING INDEBTEDNESS" means  any Indebtedness of the  Company
or  any of its Subsidiaries issued in exchange for, or the net proceeds of which
are  used  to  extend,  refinance,  renew,  replace,  defease  or  refund  other
Indebtedness  of the Company or any of  its Subsidiaries; PROVIDED that: (i) the
principal amount of such Permitted Refinancing Indebtedness does not exceed  the
principal  or  accrued  amount  of  the  Indebtedness  so  extended, refinanced,
renewed,  replaced,  defeased  or  refunded;  (ii)  such  Permitted  Refinancing
Indebtedness  has a Weighted Average Life to  Maturity and a final maturity date
equal to  or  greater than  the  Weighted Average  Life  to Maturity  and  final
maturity  date, respectively,  of the  Indebtedness being  extended, refinanced,
renewed, replaced,  defeased  or  refunded;  (iii)  if  the  Indebtedness  being
extended, refinanced, renewed, replaced, defeased or refunded is subordinated in
right  of  payment to  the Notes  or any  Subsidiary Guarantees,  such Permitted
Refinancing Indebtedness has a final maturity date later than the final maturity
date of, and is subordinated in right of payment to the Notes and any Subsidiary
Guarantees on terms at least  as favorable to the Holders  of the Note as  those
contained  in  the  documentation  governing  the  Indebtedness  being extended,
refinanced, renewed, replaced, defeased or refunded; and (iv) such  Indebtedness
is incurred either by the Company or by the Subsidiary who is the obligor on the
Indebtedness   being  extended,  refinanced,   renewed,  replaced,  defeased  or
refunded.
 
    "PRINCIPAL PROPERTIES" means the oil  and gas properties and other  tangible
assets   and  properties  owned  by  Company   on  the  date  of  the  Indenture
(collectively, the "Original Principal Properties") and assets and properties of
the Company obtained in exchange for any of the Original Principal Properties.
 
    "PRODUCTION PAYMENTS"  means,  collectively,  Dollar-Denominated  Production
Payments and Volumetric Production Payments.
 
    "QUALIFIED  STOCK" means, for any Person, any  and all Capital Stock of such
Person, other than Disqualified Stock.
 
    "RESTRICTED  INVESTMENT"  means  an   Investment  other  than  a   Permitted
Investment.
 
    "SALE  AND LEASEBACK TRANSACTION" means, with  respect to the Company or any
of its Subsidiaries, any arrangement with  any Person providing for the  leasing
by  the Company or any of its  Subsidiaries as lessee of any principal property,
acquired or placed  into service more  than 180 days  prior to such  arrangement
(except leases of two years or less), whereby such property has been or is to be
sold  or transferred by the Company or any of its Subsidiaries to such Person or
its Affiliates.
 
    "SENIOR BANK  INDEBTEDNESS" means  the  Indebtedness outstanding  under  the
Credit Facility.
 
    "SENIOR  INDEBTEDNESS" means (i)  the Senior Bank  Indebtedness and (ii) any
other Indebtedness  permitted  to be  incurred  by the  Company  or any  of  its
Subsidiaries under the terms of the Indenture, unless the instrument under which
such  Indebtedness is  incurred expressly  provides that  it is  subordinated in
right of payment to any Indebtedness for money borrowed.
 
    "SENIOR  REVOLVING  INDEBTEDNESS"  means  revolving  credit  borrowings  and
letters  of credit  under the Credit  Facility and/or any  successor facility or
facilities.
 
    "SUBORDINATED INDEBTEDNESS" means any Indebtedness of the Company or any  of
its Subsidiaries (whether outstanding on the date of the Indenture or thereafter
incurred)  that is contractually  subordinated or junior in  right of payment of
principal, premium and interest to the Notes or the Subsidiary Guarantees.
 
    "SUBSIDIARY" means,  with  respect  to  any  Person,  (i)  any  corporation,
association  or other business entity of which more than 50% of the total voting
power of shares of Capital Stock  entitled (without regard to the occurrence  of
any  contingency) to  vote in  the election  of directors,  managers or trustees
thereof is at  the time  owned or controlled,  directly or  indirectly, by  such
Person or one or more of the other Subsidiaries of that Person (or a combination
thereof)  and (ii) any partnership (a) the  sole general partner or the managing
general  partner   of  which   is  such   Person  or   a  Subsidiary   of   such
 
                                       80
<PAGE>
Person  or (b) the only general  partners of which are such  Person or of one or
more Subsidiaries of such Person  (or any combination thereof).  Notwithstanding
the  foregoing,  an Unrestricted  Subsidiary shall  not be  a Subsidiary  of the
Company for any purposes of the Indenture.
 
    "UNRESTRICTED SUBSIDIARY" means any Subsidiary,  if designated by the  Board
of  Directors of the Company  as an Unrestricted Subsidiary  pursuant to a Board
Resolution and  permitted to  be so  designated  pursuant to  the terms  of  the
Indenture.
 
    "VOLUMETRIC   PRODUCTION  PAYMENTS"  means   volumetric  production  payment
obligations of the  Company or any  of its  Subsidiaries that are  or, upon  the
occurrence  of  a contingent  event, would  be recorded  as deferred  revenue in
accordance with  GAAP, together  with all  undertakings and  obligations of  the
Company or any of its Subsidiaries in connection therewith, which will be deemed
to constitute debt for borrowed money for purpose of the Indenture.
 
    "VOTING  STOCK" of a corporation means all  classes of Capital Stock of such
corporation then outstanding and  normally entitled to vote  in the election  of
directors.
 
    "WEIGHTED  AVERAGE LIFE TO MATURITY" means, when applied to any Indebtedness
at any date, the number of years obtained by dividing (i) the sum of the product
obtained by  multiplying (a)  the  amount of  each then  remaining  installment,
sinking fund, serial maturity or other required payments of principal, including
payments  at final  maturity, in  respect thereof,  by (b)  the number  of years
(calculated to the nearest one-twelfth) that  will elapse between such date  and
the  making of such  payment, by (ii)  the then outstanding  principal amount of
such Indebtedness.
 
    "WHOLLY OWNED SUBSIDIARY" of  any Person means a  Subsidiary of such  Person
(i)  all of the outstanding Capital Stock  or other ownership interests of which
(other than directors  qualifying shares)  shall at the  time be  owned by  such
Person  or  by one  or more  Wholly Owned  Subsidiaries of  such Person  or (ii)
organized in a foreign jurisdiction and  is required by the applicable laws  and
regulations of such foreign jurisdiction to be partially owned by the government
of such foreign jurisdiction or individual or corporate citizens of such foreign
jurisdiction  in order for such Subsidiary  to transact business in such foreign
jurisdiction, provided that such Person or one or more Wholly Owned Subsidiaries
of such Person, owns the remaining  Capital Stock or ownership interest in  such
Subsidiary  and, by contract or otherwise,  controls the management and business
of such  Subsidiary and  derives  the economic  benefits  of ownership  of  such
Subsidiary  to substantially the same extent as if such Subsidiary were a wholly
owned Subsidiary.  Unrestricted  Subsidiaries  shall  not  be  included  in  the
definition of Wholly Owned Subsidiary for any purposes of the Indenture.
 
                                       81
<PAGE>
                       DESCRIPTION OF OTHER INDEBTEDNESS
 
EXISTING DEBT FACILITY
 
    In  June 1996,  the Company entered  into a credit  agreement (the "Existing
Debt Facility")  provided by  NationsBridge, L.L.C.  and NationsBank,  N.A.  and
consisting  of a $95.0  million revolving credit  loan (the "Existing Revolver")
and a $30.0  million term  loan (the "Existing  Term Loan").  The Existing  Debt
Facility  provided funds to consummate the 1996 Acquisition and to refinance the
Company's prior senior bank facility.  Prudential Securities Group Inc.  ("PGI")
has purchased an interest in the Existing Debt Facility.
 
    The Existing Revolver and the Existing Term Loan each matures June 10, 1999.
No  periodic principal reductions are required with respect to the Existing Term
Loan; however, quarterly principal reductions in the amount of $3.0 million  are
required  with respect to the Existing  Revolver, commencing January 1, 1997. In
addition, the Existing  Debt Facility requires  that the net  proceeds from  the
Notes  Offering be applied to reduce  the amounts outstanding under the Existing
Revolver and the  Existing Term  Loan, and  100% of  the net  proceeds from  the
Common  Stock  Offering  are  required  to be  utilized  to  reduce  the amounts
outstanding under the Existing Term Loan and Existing Revolver.
 
    Interest accrues on  the Existing Term  Loan initially at  14.0% per  annum,
increasing  by 0.5% per annum  at the end of  each successive three month period
(commencing September 13, 1996) up to a maximum of 16.5% per annum. Interest may
be paid in cash  or "in kind"  by delivery of additional  notes having the  same
terms as the notes issued pursuant to the Existing Term Loan. Interest under the
Existing  Revolver accrues, at the option of  the Company, at a margin in excess
of either NationsBank, N.A. "LIBOR" rate, up to a maximum of 5.0% per annum,  or
NationsBank, N.A. fluctuating "prime rate" up to a maximum of 2.75% per annum.
 
    The  Existing Debt Facility is  secured by a pledge  of substantially all of
the Company's  assets,  guaranties by  the  Company's subsidiaries  and  limited
guaranties  by  Messrs. Liedtke,  Grella  and Musselman  proportionate  to their
membership interests in the LLC.
 
CREDIT FACILITY
 
    The Credit Facility will provide the Company with a revolving facility based
on the borrowing base of its oil and  gas assets which will initially be set  at
$50.0 million, none of which is expected to be outstanding at its inception. The
Credit  Facility is expected to  be secured by a  pledge of substantially all of
the assets of the Company and any subsidiary of the Company that guarantees  the
Company's  obligations  under  the  Credit  Facility.  Initially,  none  of  the
Company's subsidiaries will guarantee the  obligations of the Company under  the
Credit Facility. See "Mangement's Discussion and Analysis of Financial Condition
and  Results of  Operations --  Liquidity and  Capital Resources"  for a further
description of the Credit Facility.
 
                          DESCRIPTION OF CAPITAL STOCK
 
    The authorized capital stock of the Company consists of 20,000,000 shares of
Common Stock, par value $0.10 per share ("Common Stock") and 3,000,000 shares of
preferred stock,  par  value  $0.10  per share  ("Preferred  Stock").  Upon  the
completion  of the  Offerings and the  Corporate Reorganization,  the issued and
outstanding capital stock of  the Company will consist  of 10,000,000 shares  of
Common Stock (or 10,720,000 shares if the underwriters' over-allotment option is
exercised in full).
 
    The  following description of certain matters  relating to the capital stock
of the Company  is summary in  nature and is  qualified in its  entirety by  the
provisions  of the Company's Certificate of  Incorporation and Bylaws, copies of
which have been filed  as exhibits to the  Registration Statement of which  this
Prospectus is a part.
 
                                       82
<PAGE>
COMMON STOCK
 
    The  holders  of Common  Stock are  entitled to  one vote  per share  on all
matters submitted to a  vote of stockholders of  the Company. In addition,  such
holders  are  entitled to  receive ratably  such  dividends, if  any, as  may be
declared from  time to  time by  the Board  of Directors  out of  funds  legally
available  therefor,  subject  to  the payment  of  preferential  dividends with
respect to any Preferred Stock that from time to time may be outstanding. In the
event of the dissolution, liquidation or winding-up of the Company, the  holders
of  Common Stock  are entitled  to share ratably  in all  assets remaining after
payment of all liabilities of the Company and subject to the prior  distribution
rights  of the holders  of any Preferred  Stock that may  be outstanding at that
time. The  holders of  Common Stock  do  not have  cumulative voting  rights  or
preemptive  rights. All shares of Common Stock outstanding and to be outstanding
after the Common Stock Offering will be fully paid and nonassessable.
 
PREFERRED STOCK
 
    The Board  of Directors  has  the authority  to  issue 3,000,000  shares  of
Preferred  Stock, in  one or  more series, and  to fix  the rights, preferences,
qualifications, privileges,  limitations or  restrictions  of each  such  series
without  any further vote or action  by the stockholders, including the dividend
rights, dividend rate,  conversion rights,  voting rights,  terms of  redemption
(including  sinking fund  provisions), redemption  price or  prices, liquidation
preferences and the number of shares constituting any series or the designations
of such series.  No shares of  Preferred Stock  have ever been  issued, and  the
Company  has no present plans to issue any Preferred Stock. In certain instances
the Indenture limits the  ability of the Company  to issue Preferred Stock.  See
"Description  of Notes  -- Certain Covenants  -- Incurrence  of Indebtedness and
Issuance of Preferred Stock."
 
                                       83
<PAGE>
                                  UNDERWRITING
 
    Upon the terms and subject to  the conditions of the Underwriting  Agreement
(the  "Underwriting Agreement") among the  Company, NationsBanc Capital Markets,
Inc.  and   Prudential  Securities   Incorporated  (the   "Underwriters"),   the
Underwriters  severally have agreed to purchase from the Company and the Company
has agreed to sell to the  Underwriters severally the principal amount of  Notes
set forth opposite the names of such Underwriters below:
 
<TABLE>
<CAPTION>
                                                                                                     PRINCIPAL
UNDERWRITER                                                                                            AMOUNT
- ------------------------------------------------------------------------------------------------  ----------------
<S>                                                                                               <C>
NationsBanc Capital Markets, Inc................................................................  $     90,000,000
Prudential Securities Incorporated..............................................................        10,000,000
                                                                                                  ----------------
    Total.......................................................................................  $    100,000,000
                                                                                                  ----------------
                                                                                                  ----------------
</TABLE>
 
    In the Underwriting Agreement, the several Underwriters have agreed, subject
to  certain conditions, to purchase all of  the Notes, if any are purchased. The
Underwriting  Agreement  provides  that,  in  the  event  of  a  default  by  an
Underwriter,   in   certain   circumstances,   the   purchase   commitments   of
non-defaulting Underwriters may be increased  or the Underwriting Agreement  may
be terminated.
 
    The  Company has been advised by the Underwriters that they propose to offer
the Notes to the public  initially at the price set  forth on the cover page  of
this Prospectus, to certain securities dealers (who may include Underwriters) at
such  price less a concession not in excess  of 0.50% of the amount per Note and
that the Underwriters and such dealers may  reallow a discount not in excess  of
0.25% of the amount per Note to other dealers, including the Underwriters. After
the  closing of the  public offering, the public  offering price, the concession
and the discount to other dealers may be changed by the Underwriters.
 
    There is no currently  existing trading market for  the Notes, and  although
the  Underwriters have advised the Company that  they currently intend to make a
market in the Notes, they are not obligated to do so and any such market  making
may  be discontinued at any time, without  notice, in the sole discretion of the
Underwriters. Accordingly, there can  be no assurance as  to the development  or
liquidity of any market that may develop for the Notes.
 
    The  Company  has  agreed  to  indemnify  the  Underwriters  against certain
liabilities, including liabilities under the Securities Act of 1933, as  amended
(the  "Securities Act"),  or to contribute  to payments the  Underwriters may be
required to make in respect thereof.
 
    The Underwriters  have informed  the  Company that  they  do not  expect  to
confirm  sales of Notes offered hereby to  any accounts over which they exercise
discretionary authority.
 
    NationsBanc Capital Markets, Inc. is an affiliate of NationsBank, N.A., NBCC
and NationsBridge,  L.L.C.  NationsBridge,  L.L.C.  and  NationsBank,  N.A.  are
lenders  under the  Existing Credit  Facility. PGI,  an affiliate  of Prudential
Securities Incorporated, is also  a lender under  the Existing Credit  Facility.
See  "Description  of Other  Indebtedness." NationsBridge,  L.L.C., NationsBank,
N.A. and PGI will receive their respective proportionate shares of the repayment
by the  Company of  borrowings under  the Existing  Debt Facility  from the  net
proceeds  of the Offerings. Prudential Securities Incorporated is also acting as
an underwriter in the  Company's concurrent Common Stock  Offering for which  it
will  receive customary underwriting discounts and commissions. In addition, the
Underwriters and their respective affiliates  provide or have provided  banking,
advisory  and other financial services for the Company in the ordinary course of
business for which they have received customary compensation.
 
    NBCC is a stockholder  of the Company and  will receive approximately  $16.3
million  of the  proceeds of  the Offerings  in redemption  of a  portion of the
membership interests owned by it  and in a distribution  to it in the  Corporate
Reorganization. See "Use of Proceeds," "The Company -- Corporate Reorganization"
and  "Security  Ownership of  Certain Beneficial  Owners  and Management."  As a
result of such ownership, The  National Association of Securities Dealers,  Inc.
("NASD") may view
 
                                       84
<PAGE>
this  offering as  a participation by  NationsBanc Capital Markets,  Inc. in the
distribution in a  public offering of  the securities of  an affiliate and  this
Notes  Offering is  being made pursuant  to the  provisions of Rule  2720 of the
NASD's Conduct  Rules.  In  accordance with  Rule  2720,  Prudential  Securities
Incorporated  is  acting as  a qualified  independent  underwriter in  the Notes
Offering and is assuming the responsibilities  of acting as such in pricing  the
Notes Offering and conducting due diligence.
 
                                 LEGAL MATTERS
 
    Certain  legal matters related to the  Notes offered hereby are being passed
upon for  the  Company  by  Cotton, Bledsoe,  Tighe  &  Dawson,  a  Professional
Corporation,  Midland,  Texas.  Certain  matters will  be  passed  upon  for the
Underwriters by Baker & Botts, L.L.P., Houston, Texas.
 
                                    EXPERTS
 
    The  consolidated  financial  statements  of  Costilla  Energy,  L.L.C.  and
subsidiaries as of December 31, 1995 and for the year then ended, the statements
of  revenues and direct operating expenses of the 1996 Acquisition for the years
ended December  31, 1993,  1994 and  1995, and  the statements  of revenues  and
direct  operating expenses of the 1995  Acquisition for the years ended December
31, 1993 and 1994, and the period ended June 12, 1995, have been included herein
and in  the registration  statement in  reliance upon  the report  of KPMG  Peat
Marwick  LLP,  independent  certified  public  accountants,  appearing elsewhere
herein, and  upon  the authority  of  said firm  as  experts in  accounting  and
auditing.
 
    The  consolidated  financial  statements  of  Costilla  Energy,  L.L.C.  and
subsidiaries as of December 31, 1994, and for the years ended December 31,  1993
and  1994,  have  been included  herein  and  in the  registration  statement in
reliance upon  the report  of Elms,  Faris &  Co., P.C.,  independent  certified
public  accountants, appearing elsewhere herein, and  upon the authority of said
firm as experts in accounting and auditing.
 
    In September 1995, the Company changed its principal accountants from  Elms,
Faris  & Co., P.C. to KPMG  Peat Marwick LLP. The reports  of Elms, Faris & Co.,
P.C. on the Company's financial statements for the year ended December 31,  1994
did  not  contain an  adverse opinion  or a  disclaimer of  opinion, nor  was it
qualified or modified in  any way as to  uncertainty, audit scope or  accounting
principles.  Moreover, there were no disagreements  with Elms, Faris & Co., P.C.
on any  matter  of  accounting  principles  or  practices,  financial  statement
disclosure,  or auditing  scope or  procedure. The members  of the  LLC made the
decision to change the LLC's principal accountants.
 
    Certain  information  appearing  in  this  Prospectus  regarding   estimated
quantities  of oil and gas  reserves and the discounted  present value of future
pre-tax cash flows therefrom attributable to the Company's properties and to the
properties included in  the 1996  Acquisition is  based upon  estimates of  such
reserves  and present values prepared  by Williamson Petroleum Consultants, Inc.
All of  such  information has  been  so included  herein  in reliance  upon  the
authority  of such firm as  experts in such matters. Set  forth as Appendix A is
Williamson's Summary Reserve Report dated July 23, 1996 with respect to the  oil
and  gas interests of the Company and with respect to properties acquired in the
1996 Acquisition.
 
                             AVAILABLE INFORMATION
 
    Upon completion  of  the Offerings,  the  Company  will be  subject  to  the
informational  requirements of the  Securities Exchange Act  of 1934, as amended
(the "Exchange Act"),  and, in  accordance therewith, will  file reports,  proxy
statements  and other  information with  the Securities  and Exchange Commission
(the "Commission"). Such reports, proxy  statements and other information  filed
by  the Company  with the  Commission can be  inspected at  the Public Reference
Section of the Commission at Room 1024, Judiciary Plaza, 450 Fifth Street, N.W.,
Washington, D.C. 20549, and the Regional  Offices of the Commission at  Citicorp
Center,    500   West   Madison   Street,    Suite   1400,   Chicago,   Illinois
 
                                       85
<PAGE>
60661-2511, and 7 World Trade Center, New  York, New York 10048. Copies of  such
material  can  also  be  obtained  from  the  Public  Reference  Section  of the
Commission at Room 1024,  Judiciary Plaza, 450  Fifth Street, N.W.,  Washington,
D.C.  20549, at prescribed rates. The Commission maintains a World Wide Web site
on  the  Internet  at  HTTP:\\WWW.SEC.GOV  that  contains  reports,  proxy   and
information  statements and  other information  regarding registrants  that file
electronically with the Commission.
 
    The Company has filed with the  Commission a Registration Statement on  Form
S-1  under the  Securities Act  with respect to  the Notes  offered hereby. This
Prospectus, which constitutes  a part  of the Registration  Statement, does  not
contain  all of the information set forth in the Registration Statement, certain
items of  which are  contained  in exhibits  to  the Registration  Statement  as
permitted   by  the  rules  and  regulations  of  the  Commission.  For  further
information with respect to the Company and the Notes offered hereby,  reference
is made to the Registration Statement, including the exhibits thereto, which may
be inspected without charge at the public reference facilities maintained by the
Commission at 450 Fifth Street, N.W., Washington, D.C. 20549 and at the Regional
Offices  of  the  Commission, and  copies  of  which may  be  obtained  from the
Commission at prescribed  rates. Statements made  in this Prospectus  concerning
the  contents of any  document referred to herein  are not necessarily complete.
With respect to each such  document filed with the  Commission as an exhibit  to
the Registration Statement, reference is made to the exhibit for a more complete
description of the matter involved, and each such statement made herein shall be
deemed qualified by such reference.
 
                                       86
<PAGE>
                                    GLOSSARY
 
    The terms defined in this section are used throughout this Prospectus.
 
    ADJUSTED EBITDA.  Calculated by adding interest, income taxes, depreciation,
depletion  and amortization, exploration and abandonment costs and extraordinary
loss resulting from extinguishment of debt to net income (loss).
 
    ALL-IN FINDING COSTS.  The  amount of total capital expenditures,  including
acquisition  costs,  and  exploration  and abandonment  costs  for  oil  and gas
activities divided by  the amount of  proved reserves (expressed  in BOE)  added
during  the specified period (including the effect on proved reserves of reserve
revisions).
 
    BBL.  One stock tank barrel, or  42 U.S. gallons liquid volume, used  herein
in reference to crude oil or other liquid hydrocarbons.
 
    BCF.  One billion cubic feet.
 
    BOE.   Equivalent barrels of  oil. In reference to  natural gas, natural gas
equivalents are determined using the ratio of six Mcf of natural gas to one  Bbl
of crude oil, condensate or natural gas liquids.
 
    BTU.   One British thermal unit. The  quantity of heat required to raise the
temperature of one pound of water one degree Fahrenheit.
 
    DEVELOPED ACREAGE.  The number of acres which are allocated or assignable to
producing wells or wells capable of production.
 
    DEVELOPMENT WELL.  A well  drilled within the proved area  of an oil or  gas
reservoir to the depth of a stratigraphic horizon known to be productive.
 
    DRY  WELL.  A well found  to be incapable of producing  either oil or gas in
sufficient quantifies to justify completion of an oil or gas well.
 
    EXPLORATORY WELL.   A well  drilled to  find and produce  oil or  gas in  an
unproved  area,  to find  a  new reservoir  in a  field  previously found  to be
productive of oil or gas in another reservoir, or to extend a known reservoir.
 
    GROSS ACRES OR GROSS WELLS.  The total  acres or wells, as the case may  be,
in which a working interest is owned.
 
    MBBL.  One thousand barrels of crude oil or other liquid hydrocarbons.
 
    MBOE.  One thousand barrels of oil equivalent.
 
    MMBOE.  One million barrels of oil equivalent.
 
    MMBBLS.  One million barrels of crude oil or other liquid hydrocarbons.
 
    MMBTU.  One million Btu's.
 
    MCF.  One thousand cubic feet.
 
    MMCF.  One million cubic feet.
 
    NET  ACRES OR NET WELLS.  The  sum of the fractional working interests owned
in gross acres or gross wells.
 
    PRESENT VALUE OF ESTIMATED FUTURE NET REVENUES OR PV-10 VALUE.  The  present
value  of estimated future  net revenues is  an estimate of  future net revenues
from a property at  its acquisition date, at  a specified date, after  deducting
production  and ad valorem  taxes, future capital  costs and operating expenses,
but before deducting  federal income taxes.  The future net  revenues have  been
discounted  at an  annual rate  of 10% to  determine their  "present value." The
present value is shown to indicate the
 
                                       87
<PAGE>
effect of time on the  value of the revenue stream  and should not construed  as
being  the fair market value  of the properties. Estimates  have been made using
constant oil and natural gas prices and operating costs at the specified date.
 
    PRODUCTIVE WELL.  A  well that is  producing oil or gas  that is capable  of
production.
 
    PROVED  DEVELOPED RESERVES.   Reserves that can be  expected to be recovered
through existing wells with existing equipment and operating methods.
 
    PROVED RESERVES.   The estimated quantities  of crude oil,  natural gas  and
natural  gas  liquids which  geological  and engineering  data  demonstrate with
reasonable certainty to  be recoverable  in future years  from known  reservoirs
under existing economic and operating conditions.
 
    PROVED  UNDEVELOPED RESERVES.   Reserves that  are expected  to be recovered
from new wells on undrilled acreage,  or from existing wells where a  relatively
major expenditure is required for recompletion.
 
    ROYALTY  INTEREST.   An interest  in an oil  and gas  property entitling the
owner to a share of oil and gas production free of costs of production.
 
    3-D SEISMIC.    Advanced technology  method  of detecting  accumulations  of
hydrocarbons  identified by the collection and  measurement of the intensity and
timing of sound waves  transmitted into the  earth as they  reflect back to  the
surface.
 
    UNDEVELOPED  ACREAGE.  Lease acreage on which wells have not been drilled or
completed to a point that would  permit the production of commercial  quantities
of oil and gas regardless of whether such acreage contains proved reserves.
 
    WORKING INTEREST.  The operating interest which gives the owner the right to
drill,  produce and conduct operating activities on  the property and a share of
production.
 
                                       88
<PAGE>
                         INDEX TO FINANCIAL STATEMENTS
 
<TABLE>
<S>                                                                                    <C>
Financial Statements of Costilla Energy, L.L.C.:
  Independent Auditors' Reports......................................................        F-2
  Consolidated Balance Sheets as of December 31, 1994 and 1995, and June 30, 1996
   (unaudited).......................................................................        F-4
  Consolidated Statements of Operations for the years ended December 31, 1993, 1994,
   and 1995, and the six months ended June 30, 1995 and 1996 (unaudited).............        F-5
  Consolidated Statements of Members' Capital for the years ended December 31, 1993,
   1994, and 1995, and the six months ended June 30, 1995 (unaudited)................        F-6
  Consolidated Statements of Cash Flows for the years ended December 31, 1993, 1994,
   and 1995, and the six months ended June 30, 1995 and 1996 (unaudited).............        F-7
  Notes to Consolidated Financial Statements.........................................        F-8
 
Financial Statements of the 1995 Acquisition:
  Independent Auditors' Report.......................................................       F-23
  Statements of Revenues and Direct Operating Expenses for the years ended December
   31, 1993 and 1994 and the period ended June 12, 1995..............................       F-24
  Notes to the Statements of Revenues and Direct Operating Expenses..................       F-25
 
Financial Statements of the 1996 Acquisition:
  Independent Auditors' Report.......................................................       F-28
  Statements of Revenues and Direct Operating Expenses for the years ended December
   31, 1993, 1994 and 1995, and the periods ended June 14, 1995 and 1996
   (unaudited).......................................................................       F-29
  Notes to the Statements of Revenues and Direct Operating Expenses..................       F-30
</TABLE>
 
Note:  The  financial statements of Costilla  Energy, Inc. (incorporated in June
       1996) are not presented, as Costilla Energy, Inc. has no material assets,
       liabilities or equity,  and has  not generated any  material revenues  or
       incurred any material expenses.
 
                                      F-1
<PAGE>
                          INDEPENDENT AUDITORS' REPORT
 
The Members
Costilla Energy, L.L.C. (a Texas limited liability company):
 
    We  have  audited the  accompanying consolidated  balance sheet  of Costilla
Energy, L.L.C.  (a  Texas limited  liability  company) and  subsidiaries  as  of
December  31,  1995,  and  the  related  consolidated  statement  of operations,
members' capital, and  cash flows for  the year then  ended. These  consolidated
financial  statements are  the responsibility  of the  Company's management. Our
responsibility is to express an opinion  on these financial statements based  on
our audit.
 
    We  conducted  our  audit  in accordance  with  generally  accepted auditing
standards. Those standards require that we plan and perform the audit to  obtain
reasonable assurance about whether the financial statements are free of material
misstatement.  An audit includes examining, on a test basis, evidence supporting
the amounts and disclosures in the financial statements. An audit also  includes
assessing  the  accounting principles  used  and significant  estimates  made by
management, as well as evaluating the overall financial statement  presentation.
We believe that our audit provides a reasonable basis for our opinion.
 
    In  our  opinion, the  consolidated financial  statements referred  to above
present fairly, in  all material  respects, the financial  position of  Costilla
Energy,  L.L.C. and  subsidiaries as  of December 31,  1995, and  the results of
their operations and  their cash flows  for the year  then ended, in  conformity
with generally accepted accounting principles.
 
                                          KPMG PEAT MARWICK LLP
 
Midland, Texas
April 16, 1996 (except  with respect to matters  discussed in the last paragraph
               of Note 7 and Note 12, as to which the date is June 14, 1996.)
 
                                      F-2
<PAGE>
                          INDEPENDENT AUDITORS' REPORT
 
The Members
Costilla Energy, L.L.C.:
 
    We have  audited the  accompanying consolidated  balance sheet  of  Costilla
Energy,  L.L.C.  (a  Texas  limited  liability  company)  and  subsidiaries (the
combination of  CSL  Partners,  Costilla  Petroleum  Corporation  and  Statewide
Minerals, L.C.) as of December 31, 1994, and the related consolidated statements
of operations, members' capital, and cash flows for the years ended December 31,
1993 and 1994. These consolidated financial statements are the responsibility of
the  Company's management. Our responsibility is  to express an opinion on these
financial statements based on our audits.
 
    We conducted  our  audits in  accordance  with generally  accepted  auditing
standards. Those standards require that we plan and perform the audits to obtain
reasonable assurance about whether the financial statements are free of material
misstatement.  An audit includes examining, on a test basis, evidence supporting
the amounts and disclosures in the financial statements. An audit also  includes
assessing  the  accounting principles  used  and significant  estimates  made by
management, as well as evaluating the overall financial statement  presentation.
We believe that our audits provides a reasonable basis for our opinion.
 
    In  our  opinion, the  consolidated financial  statements referred  to above
present fairly, in all material respects, the consolidated financial position of
Costilla Energy,  L.L.C. and  subsidiaries  as of  December  31, 1994,  and  the
results  of their operations and  their cash flows for  the years ended December
31, 1993 and 1994, in conformity with generally accepted accounting principles.
 
                                          ELMS, FARIS & CO., P.C.
 
Midland, Texas
March 31, 1995
 
                                      F-3
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
                          CONSOLIDATED BALANCE SHEETS
                                 (IN THOUSANDS)
 
                                     ASSETS
<TABLE>
<CAPTION>
                                                                                              DECEMBER 31,
                                                                                            ----------------   JUNE 30,
                                                                                             1994     1995       1996
                                                                                            -------  -------  -----------
                                                                                                              (UNAUDITED)
<S>                                                                                         <C>      <C>      <C>
Current assets:
  Cash and cash equivalents...............................................................  $   137  $ 2,866    $ 1,164
  Accounts receivable:
    Trade, net............................................................................    1,042    3,154      2,521
    Affiliates............................................................................       --      507        927
    Oil and gas sales.....................................................................    1,715    3,915      5,337
  Prepaid and other current assets........................................................      223      439      2,629
                                                                                            -------  -------  -----------
        Total current assets..............................................................    3,117   10,881     12,578
                                                                                            -------  -------  -----------
Property, plant and equipment, at cost:
  Oil and gas properties, using the successful efforts method of accounting:
    Proved properties.....................................................................   22,794   79,897    126,809
    Unproved properties...................................................................    2,060    2,903      4,615
  Accumulated depletion, depreciation and amortization....................................   (3,562)  (9,413)   (13,933)
                                                                                            -------  -------  -----------
                                                                                             21,292   73,387    117,491
                                                                                            -------  -------  -----------
Other property and equipment, net.........................................................       76      679      1,640
Deferred charges (Note 2).................................................................       29    1,736      2,654
Note receivable -- affiliate..............................................................      390      684        684
                                                                                            -------  -------  -----------
                                                                                            $24,904  $87,367    $135,047
                                                                                            -------  -------  -----------
                                                                                            -------  -------  -----------
 
<CAPTION>
 
                              LIABILITIES, REDEEMABLE MEMBERS' CAPITAL AND MEMBERS' CAPITAL
<S>                                                                                         <C>      <C>      <C>
Current liabilities:
  Current maturities of long-term debt....................................................  $    22  $    --    $    98
  Trade accounts payable..................................................................    1,712    5,467      4,587
  Undistributed revenue...................................................................      110    1,227      1,524
  Other current liabilities...............................................................      192    1,691      2,103
                                                                                            -------  -------  -----------
        Total current liabilities.........................................................    2,036    8,385      8,312
                                                                                            -------  -------  -----------
Long-term debt, less current maturities (Note 7)..........................................   23,591   71,494    122,267
Deferred income (Note 2)..................................................................       24    3,319      2,623
Other noncurrent liabilities..............................................................       --       38         --
                                                                                            -------  -------  -----------
        Total liabilities.................................................................   25,651   83,236    133,202
                                                                                            -------  -------  -----------
Redeemable members' capital (Note 10).....................................................       --   11,576     13,171
                                                                                            -------  -------  -----------
Members' capital (Note 10)................................................................     (747)  (7,445)   (11,326)
Commitments and contingencies (Note 8)....................................................       --       --         --
                                                                                            -------  -------  -----------
                                                                                            $24,904  $87,367    $135,047
                                                                                            -------  -------  -----------
                                                                                            -------  -------  -----------
</TABLE>
 
          See accompanying notes to consolidated financial statements.
 
                                      F-4
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
                     CONSOLIDATED STATEMENTS OF OPERATIONS
                                 (IN THOUSANDS)
 
<TABLE>
<CAPTION>
                                                                                                SIX MONTHS ENDED
                                                                YEARS ENDED DECEMBER 31,            JUNE 30,
                                                             -------------------------------  --------------------
                                                               1993       1994       1995       1995       1996
                                                             ---------  ---------  ---------  ---------  ---------
                                                                                                  (UNAUDITED)
<S>                                                          <C>        <C>        <C>        <C>        <C>
Revenues:
  Oil and gas sales........................................  $   4,231  $   7,637  $  21,693  $   5,568  $  19,445
  Interest and other.......................................         56         87        123          5         40
  Gain on sale of assets...................................        110        112         --         --         40
                                                             ---------  ---------  ---------  ---------  ---------
                                                                 4,397      7,836     21,816      5,573     19,525
                                                             ---------  ---------  ---------  ---------  ---------
Expenses:
  Oil and gas production...................................      1,688      2,349     10,024      2,268      8,093
  Oil and gas production -- affiliates.....................         --          2        331        145        185
  General and administrative...............................        639        634      2,910        678      2,439
  General and administrative -- affiliates.................        313        550        661        330        370
  Compensation related to option settlement (Note 11)......         --         --        656        656         --
  Exploration and abandonments.............................        218        793      1,650      1,007        308
  Depreciation, depletion and amortization.................        884      1,847      5,958      1,367      4,620
  Interest.................................................        605      1,458      4,591      1,046      4,156
  Other....................................................         --         --          2         --         --
                                                             ---------  ---------  ---------  ---------  ---------
                                                                 4,347      7,633     26,783      7,497     20,171
                                                             ---------  ---------  ---------  ---------  ---------
    Income (loss) before federal income taxes and
     extraordinary item....................................         50        203     (4,967)    (1,924)      (646)
Provision for federal income taxes
  Current..................................................        (25)         8          3         --         --
  Deferred.................................................          2         32         --         --         --
                                                             ---------  ---------  ---------  ---------  ---------
    Income (loss) before extraordinary item................         73        163     (4,970)    (1,924)      (646)
    Extraordinary loss resulting from extinguishment of
     debt (Note 7).........................................         --         --         --         --     (1,640)
                                                             ---------  ---------  ---------  ---------  ---------
Net income (loss)..........................................         73        163     (4,970)    (1,924)    (2,286)
  Preferred return and accretion of redeemable members'
   capital.................................................         --         --     (2,842)      (975)    (1,595)
                                                             ---------  ---------  ---------  ---------  ---------
  Net income (loss) applicable to members' capital.........  $      73  $     163  $  (7,812) $  (2,899) $  (3,881)
                                                             ---------  ---------  ---------  ---------  ---------
                                                             ---------  ---------  ---------  ---------  ---------
</TABLE>
 
          See accompanying notes to consolidated financial statements.
 
                                      F-5
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
                  CONSOLIDATED STATEMENTS OF MEMBERS' CAPITAL
                                 (IN THOUSANDS)
 
<TABLE>
<CAPTION>
                                                                                                         MEMBERS'
                                                                                                         CAPITAL
                                                                                                        ----------
<S>                                                                                                     <C>
Balance at January 1, 1993............................................................................  $      433
  Net income..........................................................................................          73
  Contributions.......................................................................................           1
  Withdrawals.........................................................................................        (456)
                                                                                                        ----------
Balance at December 31, 1993..........................................................................          51
  Net income..........................................................................................         163
  Withdrawals.........................................................................................        (961)
                                                                                                        ----------
Balance at December 31, 1994..........................................................................        (747)
  Issuance of members' interest (Note 10).............................................................       1,266
  Issuance costs (Note 10)............................................................................        (753)
  Net loss............................................................................................      (4,970)
  Withdrawals.........................................................................................         (55)
  Imputed capital contribution on settlement of option (Note 11)......................................         656
  Preferred return and accretion of redeemable members' capital.......................................      (2,842)
                                                                                                        ----------
Balance at December 31, 1995..........................................................................      (7,445)
  Net loss (unaudited)................................................................................      (2,286)
  Preferred return and accretion of redeemable members' capital (unaudited)...........................      (1,595)
                                                                                                        ----------
Balance at June 30, 1996 (unaudited)..................................................................  $  (11,326)
                                                                                                        ----------
                                                                                                        ----------
</TABLE>
 
          See accompanying notes to consolidated financial statements.
 
                                      F-6
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
                     CONSOLIDATED STATEMENTS OF CASH FLOWS
                                 (IN THOUSANDS)
 
<TABLE>
<CAPTION>
                                                                                                                SIX MONTHS ENDED
                                                                                   YEARS ENDED DECEMBER 31,         JUNE 30,
                                                                                  ---------------------------  ------------------
                                                                                   1993      1994      1995      1995      1996
                                                                                  -------  --------  --------  --------  --------
                                                                                                                  (UNAUDITED)
<S>                                                                               <C>      <C>       <C>       <C>       <C>
Cash flows from operating activities:
  Net income (loss).............................................................  $    73  $    163  $ (4,970) $ (1,924) $ (2,286)
  Adjustments to reconcile net income (loss) to net cash provided by operating
   activities:
    Depreciation, depletion and amortization....................................      884     1,847     5,958     1,367     4,620
    Amortization of deferred charges............................................       --        --       137        --       169
    Other noncash...............................................................      (21)       35       (75)      (28)       79
    Compensation related to option settlement...................................       --        --       656       656        --
    Gain on sale of oil and gas properties......................................     (110)     (112)       --        --       (40)
    Extraordinary loss resulting from extinguishment of debt....................       --        --        --        --     1,640
    Change in operating assets and liabilities:
      Increase in accounts receivable...........................................     (837)   (1,535)   (4,818)   (3,568)   (1,209)
      Decrease (increase) in other assets.......................................       20       301      (216)     (107)   (2,190)
      Increase (decrease) in accounts payable...................................      262       723     4,863     2,188      (880)
      Increase (decrease) in other liabilities..................................       59       102     1,537    (1,624)      671
      Increase (decrease) in deferred income....................................       (8)        3     3,294        --      (696)
                                                                                  -------  --------  --------  --------  --------
        Total adjustments.......................................................      249     1,364    11,336    (1,116)    2,164
                                                                                  -------  --------  --------  --------  --------
        Net cash provided by (used in) operating activities.....................      322     1,527     6,366    (3,040)     (122)
                                                                                  -------  --------  --------  --------  --------
Cash flows from investing activities:
  Capital expenditures for oil and gas properties...............................   (6,634)  (11,819)  (61,500)  (57,261)  (47,727)
  Proceeds from sale of oil and gas properties..................................      131       112        --        --        --
  Additions to other property and equipment.....................................     (228)      (49)     (720)     (512)   (1,996)
  Advances on affiliate notes receivable........................................       --      (390)     (247)       --        --
                                                                                  -------  --------  --------  --------  --------
        Net cash used in investing activities...................................   (6,731)  (12,146)  (62,467)  (57,773)  (49,723)
                                                                                  -------  --------  --------  --------  --------
Cash flows from financing activities:
  Borrowings under long-term debt...............................................    6,770    11,579    62,704    62,680   125,390
  Payments of long-term debt....................................................       --        --   (11,232)   (7,902)  (74,519)
  Deferred loan and financing costs.............................................       --        --    (2,587)   (2,587)   (2,728)
  Proceeds from redeemable members' capital.....................................       --        --    10,000    10,000        --
  Contributions.................................................................        1        --        --        --        --
  Withdrawals...................................................................     (456)     (961)      (55)      (97)       --
                                                                                  -------  --------  --------  --------  --------
        Net cash provided by financing activities...............................    6,315    10,618    58,830    62,094    48,143
                                                                                  -------  --------  --------  --------  --------
Net increase (decrease) in cash and cash equivalents............................      (94)       (1)    2,729     1,281    (1,702)
Cash and cash equivalents, beginning of period..................................      232       138       137       137     2,866
                                                                                  -------  --------  --------  --------  --------
Cash and cash equivalents, end of period........................................  $   138  $    137  $  2,866  $  1,418  $  1,164
                                                                                  -------  --------  --------  --------  --------
                                                                                  -------  --------  --------  --------  --------
</TABLE>
 
          See accompanying notes to consolidated financial statements.
 
                                      F-7
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(1) ORGANIZATION AND NATURE OF OPERATIONS
    Costilla  Energy, L.L.C. (the "Company"), a Texas limited liability company,
was formed on  February 14,  1995, as  the successor  to CSL  Partners, a  Texas
general  partnership, which was organized on January 11, 1989. The Company is an
unincorporated association of  several individuals  and a  corporation and  will
cease  to exist thirty (30)  years from the date  of formation. Its members have
limited personal liability for the Company's obligations and debts. The  Company
is classified as a partnership for federal income tax purposes.
 
    The  Company  is an  oil  and gas  exploration  and production  concern with
properties located  principally  in  West  Texas, South  Texas,  and  the  Rocky
Mountain regions of the United States.
 
(2) SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
 
    PRINCIPLES OF CONSOLIDATION
 
    As  of December 31, 1995, the  consolidated financial statements include the
accounts  of  the  Company  and  its  wholly-owned  subsidiaries.  The   Company
proportionately  consolidates less-than-100%-owned oil  and gas partnerships and
joint ventures in  accordance with industry  practice. All significant  accounts
and  transactions between the Company and its subsidiaries have been eliminated.
At December 31, 1993 and 1994, the  financial statements of the Company and  its
affiliates   were  combined.  The  combining   companies  were  owned  by  three
individuals prior the formation of  the Company. Such individuals owned  exactly
the  same proportionate  interest in  each of  the combining  companies prior to
their combination into the  Company on February 14,  1995. Each individual  held
exactly  the same proportionate interest in the combining companies as was their
proportionate interest in the Company after its formation. Management  believes,
based  on the  exact same  proportionate interests  being held  in the combining
companies and the Company before and after  the date of its formation, that  the
combination lacks substance and is not the purchase of a minority interest.
 
    Subsequent  to the formation of the Company, NBCC acquired a 30% interest in
the Company as described in Note 10.
 
    Significant intercompany transactions were eliminated.
 
    USE OF ESTIMATES
 
    Preparation  of  the  accompanying  consolidated  financial  statements   in
conformity  with generally accepted accounting principles requires management to
make estimates and assumptions  that affect the reported  amounts of assets  and
liabilities  and disclosure of contingent assets  and liabilities at the date of
the financial  statements and  the  reported amounts  of revenues  and  expenses
during the reporting period. Actual results could differ from those estimates.
 
    CASH AND CASH EQUIVALENTS
 
    For  purposes of  the statements  of cash  flows, cash  and cash equivalents
include cash on hand and depository accounts held by banks.
 
    CONCENTRATIONS OF CREDIT RISK
 
    Financial instruments that potentially expose the Company to  concentrations
of   credit  risk  consist  primarily  of  unsecured  accounts  receivable  from
unaffiliated working interest owners and  crude oil and natural gas  purchasers.
During  the year ended December 31, 1995,  the Company had sales to one customer
which accounted for 17.7% of total revenues.
 
                                      F-8
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(2) SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (CONTINUED)
    TRADE RECEIVABLES
 
    Trade receivables  generally consist  of amounts  due from  outside  working
interest  owners for their  proportionate share of  drilling and operating costs
incurred by the Company, as operator of the related properties.
 
    HEDGING
 
    Premiums  paid  for  commodity  option  contracts  and  interest  rate  swap
agreements   are  amortized  to   oil  and  gas   sales  and  interest  expense,
respectively, over  the  terms  of  the  agreements.  Unamortized  premiums  are
included  in other assets in the  consolidated balance sheet. Amounts receivable
under the  commodity option  contracts  and interest  rate swap  agreements  are
accrued as an increase in oil and gas sales and a reduction of interest expense,
respectively, for the applicable periods.
 
    OIL AND GAS PROPERTIES
 
    The Company uses the successful efforts method of accounting for oil and gas
producing  activities.  Costs  to  acquire  mineral  interests  in  oil  and gas
properties, to drill and equip exploratory wells that find proved reserves,  and
to drill and equip development wells are capitalized. Costs to drill exploratory
wells  that do not  find proved reserves, geological  and geophysical costs, and
costs of carrying and retaining unproved properties are expensed.
 
    Unproved oil  and  gas  properties that  are  individually  significant  are
periodically  assessed for impairment of value, and  a loss is recognized at the
time  of  impairment  by  providing  an  impairment  allowance.  Other  unproved
properties  are  amortized  based  on  the  Company's  experience  of successful
drilling and average holding period. Capitalized costs of producing oil and  gas
properties,  after considering estimated dismantlement and abandonment costs and
estimated salvage values, are depreciated and depleted by the unit-of-production
method. Support equipment and other property and equipment are depreciated on  a
straight-line  basis over the estimated useful  lives of the assets, which range
from 5 to 7 years.
 
    Prior to the adoption of FAS 121 on January 1, 1995, the Company's aggregate
oil and gas properties were  carried at cost, not  in excess of total  estimated
undiscounted future net revenues, on a worldwide basis.
 
    On  sale or retirement of a complete unit of a proved property, the cost and
related accumulated  depreciation, depletion,  and amortization  are  eliminated
from  the property accounts,  and the resultant  gain or loss  is recognized. On
retirement or sale of a partial unit of proved property, the cost is charged  to
accumulated  depreciation, depletion, and amortization  with a resulting gain or
loss recognized in income.
 
    On sale of  an entire  interest in  an unproved  property for  cash or  cash
equivalent,  gain or loss  on the sale is  recognized, taking into consideration
the amount  of  any  recorded  impairment if  the  property  had  been  assessed
individually.  If a partial interest in an unproved property is sold, the amount
received is treated as a reduction of the cost of the interest retained.
 
    IMPAIRMENT OF LONG-LIVED ASSETS
 
    As of January 1,  1995, the Company adopted  the provisions of Statement  of
Financial  Accounting  Standards No.  121 --  ACCOUNTING  FOR THE  IMPAIRMENT OF
LONG-LIVED ASSETS  AND FOR  LONG-LIVED ASSETS  TO BE  DISPOSED OF  ("FAS  121").
Consequently,  the Company  reviews its long-lived  assets to be  held and used,
including oil  and gas  properties accounted  for under  the successful  efforts
method of accounting,
 
                                      F-9
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(2) SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (CONTINUED)
whenever  events  or circumstances  indicate that  the  carrying value  of those
assets may not be recoverable. An impairment loss is indicated if the sum of the
expected future cash flows is  less than the carrying  amount of the assets.  In
this  circumstance, the Company recognizes an  impairment loss for the amount by
which the carrying amount of the asset exceeds the fair value of the asset.
 
    DEFERRED CHARGES
 
    The Company capitalized certain costs incurred in connection with  obtaining
the  Credit Agreement and  the related revolver  and term notes  (see Note 7 for
definitions and descriptions of each). These costs are being amortized over  the
lives of the notes.
 
    DEFERRED INCOME
 
    In  November 1995, the Company entered  into gas sales agreements whereby it
committed to  delivery of  a total  of 2,379,000  Mmbtu, from  December 1,  1995
through December 1, 1996, for a total fixed price of $3,429,610. Income from the
agreements is recognized in the period of delivery.
 
    REVENUE RECOGNITION
 
    The  Company uses  the sales  method of  accounting for  crude oil revenues.
Under this method, revenues are recognized  based on actual volumes of oil  sold
to purchasers.
 
    The  Company  uses the  entitlements method  of  accounting for  natural gas
revenues. Under this method, revenues are recognized based on actual  production
of  natural gas. Natural gas revenues  would not have been significantly altered
in any period  had the  sales method of  recognizing natural  gas revenues  been
utilized.
 
    ENVIRONMENTAL
 
    The  Company is subject to extensive  Federal, state and local environmental
laws and regulations. These  laws, which are  constantly changing, regulate  the
discharge  of  materials into  the environment  and may  require the  Company to
remove or  mitigate the  environmental effects  of the  disposal or  release  of
petroleum  or chemical  substances at various  sites. Environmental expenditures
are  expensed  or  capitalized  depending  on  their  future  economic  benefit.
Expenditures  that relate to an existing condition caused by past operations and
that have no future economic benefits are expensed. Liabilities for expenditures
of a  noncapital  nature are  recorded  when environmental  assessment  and/  or
remediation is probable, and the costs can be reasonably estimated.
 
    RECLASSIFICATIONS
 
    Certain  reclassifications have  been made  to the  1993 and  1994 financial
statements to conform to the 1995 presentation.
 
    INTERIM FINANCIAL STATEMENTS
 
    The interim  financial information  as of  June 30,  1996, and  for the  six
months  ended June 30, 1995  and 1996, is unaudited.  However, in the opinion of
management,  these  interim  financial  statements  include  all  the  necessary
adjustments  to fairly present the results of  the interim periods, and all such
adjustments are of a normal  recurring nature. The interim financial  statements
should  be read  in conjunction  with the  audited financial  statements for the
years ended December 31, 1993, 1994 and 1995.
 
(3) ACQUISITION OF OIL AND GAS PROPERTIES
    On June 12, 1995, the Company  completed the acquisition of certain oil  and
gas  properties and  related assets from  Parker & Parsley  Development L.P. and
Parker & Parsley Producing L.P.  for $46,621,371. The transaction was  accounted
for  using  the  purchase method.  The  results  of operations  of  the acquired
properties are included in the  Consolidated Statements of Operations  beginning
 
                                      F-10
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(3) ACQUISITION OF OIL AND GAS PROPERTIES (CONTINUED)
June  12, 1995.  The Company funded  the acquisition under  the Credit Agreement
described in Note  7. Certain  of the  acquired properties,  which were  located
outside of the Company's areas of strategic focus, were sold in 1995. No gain or
loss was recorded on these sales.
 
    PRO FORMA RESULTS OF OPERATIONS (UNAUDITED)
 
    The  following table reflects the pro  forma results of operations as though
the acquisition, net of the related properties sold, had occurred on January  1,
1994.  The pro forma amounts are not  necessarily indicative of the results that
may be reported in the future.
 
<TABLE>
<CAPTION>
                                                                           YEARS ENDED DECEMBER
                                                                                   31,
                                                                             1994       1995
                                                                           ---------  ---------
                                                                              (IN THOUSANDS)
<S>                                                                        <C>        <C>
Revenues.................................................................     35,460     32,746
Net income (loss)........................................................      1,563     (4,655)
</TABLE>
 
(4) IMPAIRMENT OF LONG-LIVED ASSETS
    The Company  adopted  FAS 121  effective  as of  January  1, 1995.  FAS  121
requires  that long-lived assets held  and used by an  entity, including oil and
gas properties accounted for under the successful efforts method of  accounting,
be  reviewed for impairment whenever events or changes in circumstances indicate
that the carrying amount of an  asset may not be recoverable. Long-lived  assets
to  be disposed of  are to be accounted  for at the lower  of carrying amount or
fair value less cost to sell when management has committed to a plan to  dispose
of  the  assets.  All  companies,  including  successful  efforts  oil  and  gas
companies, are  required to  adopt  FAS 121  for  fiscal years  beginning  after
December 15, 1995.
 
    In  order  to  determine whether  an  impairment had  occurred,  the Company
estimated the  expected future  cash flows  of its  oil and  gas properties  and
compared  such  future cash  flows to  the carrying  amount of  the oil  and gas
properties to determine if  the carrying amount was  recoverable. Based on  this
process,  no writedown in the carrying amount of the Company's proved properties
was necessary at December 31, 1995.
 
(5) DERIVATIVE FINANCIAL INSTRUMENTS
    The Company utilizes derivative financial instruments to manage well-defined
interest rate and commodity price risks. The Company is exposed to credit losses
in the event of nonperformance by  the counterparties to its interest rate  swap
agreements and its commodity hedges. The Company anticipates, however, that such
counterparties  will  be  able  to fully  satisfy  their  obligations  under the
contracts. The Company does not obtain  collateral or other security to  support
financial instruments subject to credit risk but monitors the credit standing of
the counterparties.
 
    COMMODITY HEDGES.  The Company utilizes option contracts to hedge the effect
of  price changes on future oil and gas  production. If market prices of oil and
gas exceed the strike price of put options,
 
                                      F-11
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(5) DERIVATIVE FINANCIAL INSTRUMENTS (CONTINUED)
the options  will expire  unexercised, therefore  reducing the  effective  price
received  for oil and gas sales by the cost of the related option. The following
table sets forth  the future  volumes hedged  by year  and the  weighted-average
strike price of the option contracts at December 31, 1995:
 
<TABLE>
<CAPTION>
                                                           OIL          GAS
                                                         VOLUME       VOLUME         STRIKE PRICE
                                                         (BBLS)       (MMBTU)        PER BBL/MMBTU
                                                       -----------  -----------  ---------------------
<S>                                                    <C>          <C>          <C>
Oil:
  1996...............................................    1,830,000           --   $16.00 - $20.38(a)
  1997...............................................      912,500           --   $16.00 - $20.65(a)
Gas:
  1996...............................................           --    1,500,000        $1.65(b)
  1997...............................................           --    1,350,000        $1.65(b)
</TABLE>
 
- ------------------------
(a) Represents  the  weighted-average  price  of  collars  established  with the
    purchase of put option contracts and the sale of call option contracts.
 
(b) Represents the strike price on purchased put option contracts.
 
    INTEREST RATE  SWAP AGREEMENTS.   The  Company utilizes  interest rate  swap
agreements  to reduce  the potential  impact of  increases in  interest rates on
floating-rate, long-term debt.  If market rates  of interest experienced  during
the  applicable swap term are  below the rates of  interest effectively fixed by
the swap agreement, the rate of interest experienced by the Company will  exceed
the  rate  that  would have  been  experienced  under the  Credit  Agreement. At
December 31, 1995, the Company was a party to two interest rate swap agreements,
providing the  Company  with  a  fixed  interest  rate  for  the  terms  of  the
agreements.  The following table sets forth the terms, fixed rates, and notional
amounts of the agreements in place as of December 31, 1995:
 
<TABLE>
<CAPTION>
                                        NOTIONAL
                                        PRINCIPAL                 FIXED
                TERM                     AMOUNT               INTEREST RATE
- ------------------------------------  -------------  -------------------------------
<S>                                   <C>            <C>
Jan. 25, 1996 to Jan. 25, 1999         $24 million      ranging from 7.5% to 8.5%
May 24, 1995 to May 27, 1997           $60 million                5.99%
</TABLE>
 
                                      F-12
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(6) FAIR VALUE OF FINANCIAL INSTRUMENTS
    The following table presents the carrying amounts and estimated fair  values
of  the  Company's financial  instruments at  December 31,  1994 and  1995. FASB
Statement No.  107,  DISCLOSURES  ABOUT FAIR  VALUE  OF  FINANCIAL  INSTRUMENTS,
defines  the fair  value of a  financial instrument  as the amount  at which the
instrument could be exchanged in a current transaction between willing parties.
 
<TABLE>
<CAPTION>
                                                                             1994                  1995
                                                                     --------------------  --------------------
                                                                     CARRYING     FAIR     CARRYING     FAIR
                                                                      AMOUNT      VALUE     AMOUNT      VALUE
                                                                     ---------  ---------  ---------  ---------
<S>                                                                  <C>        <C>        <C>        <C>
                                                                                   (IN THOUSANDS)
Financial assets:
  Cash, cash equivalents and restricted cash.......................  $     137  $     137  $   2,866  $   2,866
  Receivables (trade)..............................................      1,042      1,042      3,154      3,154
  Receivables (oil and gas sales)..................................      1,715      1,715      3,915      3,915
  Commodity option contracts.......................................         --         --        165        555
  Interest rate swap and option agreements.........................        203         --        146     (2,970)
  Notes receivable -- affiliate....................................        390        390        684        684
Financial liabilities:
  Payables (trade).................................................      1,712      1,712      5,467      5,467
  Deferred income..................................................         --         --      3,319      2,950
  Long-term debt...................................................     23,613     23,613     71,494     71,494
</TABLE>
 
    The carrying amounts  shown in the  table are included  in the statement  of
financial position under the indicated captions.
 
    The  following methods and assumptions were  used to estimate the fair value
of each class of financial instruments:
 
    CASH,  TRADE  RECEIVABLES,  AND  TRADE  PAYABLES:    The  carrying   amounts
approximate fair value because of the short maturity of those instruments.
 
    OTHER  CURRENT ASSETS:  The amounts  reported relate to the commodity option
contracts and interest rate  swap agreements described in  Note 5. The  carrying
amount comprises the unamortized premiums paid for the contracts. The fair value
is  estimated using option  pricing models and  essentially values the potential
for the  contracts and  agreements  to become  in-the-money through  changes  in
commodity prices and interest rates during the remaining terms.
 
    NOTES RECEIVABLE-AFFILIATE:  The amounts reported relate to notes receivable
from  an affiliated company. The carrying amount approximates fair value because
the rate given  to the affiliate  company is not  materially different from  the
affiliate company's bank debt.
 
    DEFERRED INCOME:  The amounts reported relate to the gas purchase agreements
described  in Note 2. The carrying amount represents the payments received under
the agreements for  which subsequent  delivery is  required. The  fair value  is
estimated  based upon the  commodity price at  December 31, 1995,  for a similar
agreement.
 
    LONG-TERM DEBT:  The fair value of the Company's long-term debt is estimated
by discounting expected cash flows at the rates currently offered to the Company
for debt of the same remaining maturities, as advised by the Company's bankers.
 
                                      F-13
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(7) LONG-TERM DEBT
    Long-term debt consists of the following:
 
<TABLE>
<CAPTION>
                                                                             DECEMBER 31,
                                                                         --------------------
                                                                           1994       1995
                                                                         ---------  ---------
                                                                            (IN THOUSANDS)
<S>                                                                      <C>        <C>
Revolver note..........................................................  $      --  $  59,824
Term notes.............................................................         --     11,670
Note payable to bank...................................................     23,591         --
Note payable to member.................................................         22         --
                                                                         ---------  ---------
                                                                            23,613     71,494
    Less current maturities............................................         22         --
                                                                         ---------  ---------
                                                                         $  23,591  $  71,494
                                                                         ---------  ---------
                                                                         ---------  ---------
</TABLE>
 
    At December  31, 1995,  the  Company and  certain  of its  subsidiaries  are
parties  to a  Credit Agreement  with a  syndicate of  banks (the  "Banks"). The
Credit Agreement provides for an aggregate $185 million senior secured revolving
line of credit  ("Revolver Notes")  and an aggregate  of $15  million in  senior
secured  term notes ("Term Notes"). All notes are secured with the assets of the
Company and  are guaranteed  by the  Company's subsidiaries  and, to  a  limited
extent, its individual members.
 
    The  Revolver Notes  and Term  Notes are  subject to  an aggregate borrowing
base, as determined by the Banks or their agents in their sole discretion and is
redetermined at least bi-annually  as of January 15  and July 15, utilizing  oil
and  gas reserve information as  of the immediately preceding  period end. As of
January 15, 1996, the borrowing base was $71,670,000.
 
    All outstanding balances under  the Credit Agreement  may be designated,  at
the  Company's option, as  either "Base Rate Portions"  or "Fixed Rate Portions"
(both as  defined in  the Credit  Agreement), provided  that no  more than  five
Eurodollar  Tranches may be outstanding  at any time. The  Base Rate Portions of
the Revolver Notes bear interest at  the fluctuating Base Rate, plus a  Revolver
Base  Rate Spread  ranging from 0.25%  to 0.75%, depending  upon the outstanding
principal balances of the Term Notes. The  Base Rate Portions of the Term  Notes
bear  interest at the fluctuating Base Rate  plus 0.75%. The Fixed Rate Portions
of the Revolver Notes bear interest at the Eurodollar Rate for a fixed period of
time elected by  the Company,  plus a Revolver  Fixed Rate  Spread ranging  from
2.25%  to 3.00%,  depending on  the outstanding  principal balances  of the Term
Notes. The Fixed Rate Portions of the Term Notes bear interest at the Eurodollar
Rate for a fixed period of time elected by the Company, plus a Fixed Rate Spread
of 3.00%. As of December 31, 1995, the Company had elected a fixed rate of 8.82%
for the Revolver Notes and had elected  fixed rates ranging from 8.82% to  8.94%
for  $14,000,000  of  the  outstanding  Term Notes  at  December  31,  1995. The
remaining balances  of  the  Term  Notes  bear interest  at  the  Base  Rate  of
NationsBank Prime plus 1.50% at December 31, 1995.
 
    The  outstanding principal balance of the  Revolver Notes is due and payable
in sixty  (60) monthly  installments beginning  August 1,  1996, and  continuing
regularly  thereafter until July  1, 2001. The  outstanding principal balance of
the Term Notes is due and payable  in two (2) installments, each of which  shall
be  equal to one-half of  the unpaid principal balance of  each note, on July 1,
1996, and January 1, 1997.
 
    The Credit Agreement requires the Company to hedge not less than 60% of  the
Company's  total  sales  volume,  through December  31,  1997,  from  its proved
developed producing oil and gas  reserves, with a floor  price of not less  than
$16 per Bbl of oil or $1.50 per Mcf of gas.
 
                                      F-14
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(7) LONG-TERM DEBT (CONTINUED)
    Additionally,  the Credit  Agreement contains  various restrictive covenants
and compliance requirements,  which include: (a)  restrictions on dividends  and
the  incurrence of additional indebtedness; (b)  restrictions as to merger, sale
or transfer of  assets; (c) limiting  total lease payments  and total  aggregate
executive  compensation to  $750,000 and  $500,000, respectively,  in any fiscal
year; and (d) compliance with certain financial ratios.
 
    The  Company  was   in  violation  of   certain  covenants  and   compliance
requirements  as of  December 31,  1995. Subsequent  to December  31, 1995, such
violations were waived by the Banks.
 
    Maturities of  long-term debt  at  December 31,  1995,  are as  follows  (in
thousands):
 
<TABLE>
<S>                                                         <C>
1996......................................................  $  10,820
1997......................................................     17,800
1998......................................................     11,965
1999......................................................     11,965
2000......................................................     11,965
Thereafter................................................      6,979
</TABLE>
 
    The  Company paid  interest on  long-term debt  of $546,147,  $1,356,604 and
$4,453,684 in 1993, 1994 and 1995, respectively.
 
    As described in  Note 12,  on June 10,  1996, the  Company demonstrated  its
intent  and  ability  to  refinance  the  current  maturities  under  the Credit
Agreement by entering into a new loan agreement, proceeds of which were used  to
repay the existing notes. Concurrently, the deferred charges associated with the
Credit Agreement were expensed as an extraordinary loss.
 
(8) COMMITMENTS AND CONTINGENCIES
 
    LEASES
 
    The Company leases equipment and office facilities under operating leases on
which  rental expense for the years ended  December 31, 1993, 1994 and 1995, was
$110,023, $197,533 and $311,221, respectively. Future minimum lease  commitments
under  noncancellable operating leases at December  31, 1995, are as follows (in
thousands):
 
<TABLE>
<S>                                                          <C>
1996.......................................................  $     257
1997.......................................................        272
1998.......................................................        268
1999.......................................................        195
2000.......................................................        188
Thereafter.................................................      1,190
</TABLE>
 
    SEVERANCE AGREEMENTS
 
    On February 17, 1995,  the Company entered  into employment agreements  with
each  of the officers which  are effective from the  above date through February
17, 2000, or  until terminated by  the officer  or the Company.  In addition  to
providing  a  base salary  and nominal  yearly increases  for each  officer, the
employment agreements provide  for severance  payments upon  termination of  any
such officer's employment or a significant reduction in that officer's duties or
responsibilities.
 
    In  the event  of such a  termination, the  Company is obligated  to pay the
officer an  amount  equal  to the  present  value  (discounted at  10%)  of  the
officer's  salary  which would  have been  paid through  February 17,  2000. The
current annual  base salaries  for the  officers covered  under such  employment
agreements total approximately $500,000.
 
                                      F-15
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(8) COMMITMENTS AND CONTINGENCIES (CONTINUED)
    EXPLORATION AND DEVELOPMENT
 
    In  July 1995,  the Republic of  Moldova (located in  Eastern Europe between
Romania and the Ukraine) granted a Concession Agreement to Resource  Development
Company  Limited, L.L.C. ("Redeco"), an entity  not affiliated with the Company.
The Company has paid Redeco $90,000 and agreed to bear the first $2.0 million of
Concession expenses ($214,178 of  which had been  expended through December  31,
1995)  in return for a 50.0% interest  in Redeco. After the initial $2.0 million
expenditure, Redeco and the  Company are responsible for  bearing 50.0% each  of
future expenses. The Concession Agreement covers the entire country with respect
to  oil  and gas  and  other minerals  and  continues for  various  time periods
depending on  the nature  of  the activity  conducted.  In connection  with  two
previously  producing  but  now  abandoned  fields,  Redeco's  exclusive  rights
continue for  20 years.  Redeco's  exclusive period  to explore  throughout  the
remainder  of  Moldova  expires  in 2005,  but  Redeco  will  maintain exclusive
development rights with respect  to fields discovered for  a period of 20  years
from  the date of first production from  such field. The Company has no material
fixed financial commitments with respect to the Concession.
 
    LETTERS OF CREDIT
 
    As a result of certain bonding and trade creditor requirements, the  Company
has  caused  irrevocable letters  of  credit to  be  issued by  a  bank totaling
$106,000. As of December 31, 1995, no amounts had been drawn on these letters of
credit.
 
(9) 401(K) PLAN
    The Company has established a  qualified cash or deferred arrangement  under
IRS  code section 401(k)  covering substantially all  employees. Under the plan,
the employees have  an option  to make elective  contributions of  a portion  of
their  eligible compensation, not to exceed specified annual limitations, to the
plan and the  Company has  an option  to match  a percentage  of the  employee's
contribution.  The Company has made matching  contributions to the plan totaling
$16,950, $8,921 and $22,531 in 1993, 1994 and 1995, respectively.
 
(10) REDEEMABLE MEMBERS' CAPITAL AND MEMBERS' CAPITAL
    During  1995,  NationsBanc  Capital  Corporation  ("NBCC")  contributed  $10
million  in exchange for a  30% ownership interest in  the Company including the
preferential return described below. Of  this amount, $1,266,000 was  attributed
to  the non-redeemable portion of members' capital and $8,734,000 was attributed
to redeemable  members'  capital. Preferred  return  and accretion  of  members'
capital   included  in  the  consolidated   statements  of  operations  and  the
consolidated statements of  members' capital  includes accretion  of the  amount
attributable  to  redeemable members'  capital to  $10,000,000  over a  two year
period beginning February 17,  1995. As described  below, the redemption  amount
will  ultimately  be  equal  to  $10,000,000  plus  a  preferred  return  and an
additional redemption amount related to  NBCC's redeemable interest not  subject
to preferential return. The Company incurred $751,737 in legal fees and broker's
commissions  in connection  with this  transaction and  recorded these  costs as
direct charges to members' capital in 1995.
 
    Redeemable members' capital is subject to  a preferential return of 15%  per
annum  on $10,000,000  and is  redeemable at any  time at  the Company's option,
subject to  a redemption  premium as  described below,  or at  NBCC's option  on
February  17,  2003 or  at an  earlier  date upon  occurrence of  certain events
including a change in  control, certain changes in  management, a change in  the
Company's  status as a limited liability  company for tax purposes, or violation
of any of various other restrictive  provisions contained in the Regulations  of
Costilla Energy, L.L.C. (the "Regulations"). The
 
                                      F-16
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(10) REDEEMABLE MEMBERS' CAPITAL AND MEMBERS' CAPITAL (CONTINUED)
15%  preferred  return  is  treated  as a  reduction  of  members'  capital. The
redemption price to be paid by the Company shall be equal to $10,000,000 plus  a
premium, determined in the year the units are purchased, as follows:
 
<TABLE>
<CAPTION>
      YEAR AFTER            PREMIUM
   FEBRUARY 17, 1995      PERCENTAGE
- -----------------------  -------------
<S>                      <C>
               1                  10%
               2                  10%
               3                   8%
               4                   6%
               5                   4%
               6                   2%
               7                   0%
               8                   0%
</TABLE>
 
    In addition, a portion of NBCC's interest not subject to preferential return
is  classified as redeemable members' capital as  the Company may be required to
repurchase such interest upon the occurrence of certain events similar to  those
events  requiring redemption of the  redeemable members' capital described above
and, in any  event, on or  after February 17,  2000. Such interest  may, at  the
Company's  option, be  repurchased to the  extent the Company  has exercised its
right to redeem all or a portion of the redeemable members' interest subject  to
the  preferential return. The  redemption price the Company  would pay in either
instance is determined by the year in which the members' capital is repurchased,
as follows:
 
<TABLE>
<CAPTION>
                                                                       AGGREGATE
BEFORE FEBRUARY 17                                                  REDEMPTION PRICE
- ------------------------------------------------------------------  ----------------
<S>                                                                 <C>
1996..............................................................   $            1
1997..............................................................        1,500,000
1998..............................................................        3,000,000
1999..............................................................        4,500,000
2000..............................................................        5,500,000
</TABLE>
 
    The ultimate redemption price  of $5,500,000 is  being accrued ratably  over
the  period from February 17, 1995 through February 17, 2000 and is treated as a
reduction of members' capital.
 
    NBCC would  retain an  18% interest  in the  Company after  the  redemptions
described above occur. Such interest is not subject to redemption.
 
    At  December 31, 1995,  the Company was in  violation of various restrictive
provisions of the Regulations. Subsequent to December 31, 1995, NBCC waived such
violations.
 
(11) RELATED PARTY TRANSACTIONS
    Certain members  and officers  of  the Company  own  interests in  and  hold
positions  with  A&P  Meter Service  and  Supply, Inc.  ("A&P"),  CSL Management
Corporation ("CSL"),  511 Tex  L.C.  ("511 Tex")  and Valley  Gathering  Company
("Valley").
 
    Advances  from the Company to A&P have been consolidated into two promissory
notes. The first note, which was  originally executed December 31, 1994,  totals
$390,000,  including accrued interest of $20,000  at December 31, 1995. The note
bears interest  at a  floating rate  equal to  the "prime  rate" plus  1.0%.  No
principal  or  interest payments  are  due until  the  maturity of  the  note at
December 31,  2004. The  note is  secured by  a second  lien on  A&P's  accounts
receivable, inventory and equipment. The
 
                                      F-17
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(11) RELATED PARTY TRANSACTIONS (CONTINUED)
second note is in the amount of $294,000, including accrued interest of $47,000,
and  is  dated May  22, 1996.  The note  bears  interest at  6.0% per  annum, is
unsecured and is payable upon demand. During 1995, the Company paid $612,139  to
A&P for goods and services provided.
 
    During  1993,  1994  and  1995,  the  Company  paid  $312,623,  $549,620 and
$592,920, respectively,  to  CSL  for  management fees  and  lease  payments  on
equipment.
 
    During 1995, the Company paid $67,896 to 511 Tex for office rent.
 
    During 1994 and 1995, the Company paid $2,458 and $440,884, respectively, to
Valley  for gas compression and salt water disposal charges. During 1995, Valley
paid the Company $109,399 for operating  costs of its salt water disposal  wells
and gas compressors.
 
    In connection with this acquisition of a 5% interest in the Company in 1992,
a  minority owner also acquired an option to purchase an additional 20% interest
in the Company for $750,000. In 1995,  the majority owner of the Company  agreed
to  settle this option on behalf of the Company by transferring a portion of his
ownership directly  to  such minority  owner.  The option  was  cancelled.  This
transaction was deemed to be compensation to the remaining minority owner in the
amount of $656,000.
 
(12) SUBSEQUENT EVENTS
    On  March 8, 1996, the  Company executed a Purchase  and Sale Agreement with
Parker and Parsley Petroleum Company to  acquire certain oil and gas  properties
for  an estimated  adjusted purchase price  of approximately  $42.5 million. The
properties are located primarily  in south and west  Texas. The transaction  was
accounted  for  using the  purchase  method. The  results  of operations  of the
acquired properties are included in the Consolidated Statements of Operations as
of the closing date, June 14, 1996.
 
    PRO FORMA RESULTS OF OPERATIONS (UNAUDITED)
 
    The following table  reflects the pro  forma results of  operations for  the
six-months  ended June  30, 1995  and 1996,  as though  both acquisitions, which
closed on June 12, 1995 and June 14,  1996, had occurred as of January 1,  1995.
The  pro forma amounts are not necessarily indicative of the results that may be
reported in the future.
 
<TABLE>
<CAPTION>
                                                                           SIX MONTHS ENDED
                                                                               JUNE 30,
                                                                         --------------------
                                                                           1995       1996
                                                                         ---------  ---------
                                                                            (IN THOUSANDS)
<S>                                                                      <C>        <C>
Revenues...............................................................  $  25,706  $  28,748
Net loss...............................................................     (4,435)    (2,081)
</TABLE>
 
    In connection  with the  foregoing,  the Company  entered  into a  new  loan
agreement  with  NationsBridge L.L.C.,  an  affiliate of  the  Company's current
lender, to provide financing  of up to $125  million in advances (the  "Loans"),
subject to certain terms and conditions. Proceeds of the Loans were used to fund
the  Acquisition, to  refinance substantially  all of  the Company's outstanding
indebtedness, and for other general corporate purposes.
 
    Advances under the Loans were to be  made in two portions, Tranche A was  up
to  $95,000,000  and  Tranche  B  was  $30,000,000.  Tranche  A  initially bears
interest, at the Company's option, at  the applicable prime rate ("Prime")  plus
0.75%  or LIBOR plus 3.0%. Each margin  above Prime and LIBOR increases by 0.50%
at the end of each successive three-month  period, up to a maximum of 2.75%  and
5.0%  for Prime and  LIBOR, respectively. Tranche B  initially bears interest at
14.00% per annum,  increasing 0.50% at  the end of  each successive  three-month
period, up to a maximum of 16.5%.
 
                                      F-18
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
 
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(12) SUBSEQUENT EVENTS (CONTINUED)
    Tranche  A loans are subject to  a borrowing base determination. The initial
borrowing base is $95,000,000 which  is automatically reduced by $3,000,000  per
quarter  beginning  January  1, 1997.  The  borrowing  base is  also  subject to
periodic redetermination by NationsBridge L.L.C.  based on its determination  of
the  collateral value of the Company's oil and gas properties. Final maturity of
loans made under Tranches A and B is June 10, 1999.
 
    The Loans  are secured  by first  priority liens,  assignments and  security
interests  in all oil and gas properties, pipelines and gathering systems of the
Company and stock  of the  Company's subsidiaries. Additionally,  the Loans  are
subject  to various restrictive covenants and compliance requirements, including
but not  limited  to  (a)  restrictions  on  dividends  and  the  incurrence  of
additional  indebtedness, (b) minimum limitations on the Company's current ratio
and tangible net  worth, (c) limitations  on payments for  leases and  executive
compensation,  (d) maximum  limitations on general  and administrative expenses,
capital expenditures and the Company's ratio of debt to adjusted cash flow,  and
(e)  a requirement to pay to the lender all net oil and gas revenues (as defined
and as adjusted for capital expenditures) on a quarterly basis.
 
    The Company paid the lender's fees and expenses in connection with obtaining
the Loans.  The fees  were  approximately $2,625,000  and  will increase  by  an
additional  $625,000 if the Tranche B Loans  remain outstanding for more than 90
days. As the Company believes that  it is probable that the additional  $625,000
in  fees  will be  paid, the  total fees  of $3,250,000  are being  amortized as
additional interest expense  over a  period of  one year  from the  date of  the
Loans.  In addition, if the Tranche B amounts are not repaid within one year, an
additional amount of $4,800,000 will accrue. If such additional fee is incurred,
it will be amortized over the remaining period that the Loans are expected to be
outstanding.
 
(13) OIL AND GAS EXPENDITURES
    The following  table  reflects  costs  incurred  in  oil  and  gas  property
acquisition, exploration and development activities:
 
<TABLE>
<CAPTION>
                                                                     YEARS ENDED DECEMBER 31,       THREE MONTHS
                                                                  -------------------------------  ENDED MARCH 31,
                                                                    1993       1994       1995          1996
                                                                  ---------  ---------  ---------  ---------------
                                                                          (IN THOUSANDS)             (UNAUDITED)
<S>                                                               <C>        <C>        <C>        <C>
Property acquisition costs:
  Proved........................................................  $   4,665  $   9,649  $  52,470     $   2,246
  Unproved......................................................        829      1,232      1,742           677
Exploration.....................................................      2,017      2,167      5,627         1,822
Development.....................................................         --         --        158           232
                                                                  ---------  ---------  ---------       -------
                                                                  $   7,511  $  13,048  $  59,997     $   4,977
                                                                  ---------  ---------  ---------       -------
                                                                  ---------  ---------  ---------       -------
</TABLE>
 
(14) SUPPLEMENTAL OIL AND GAS RESERVE INFORMATION (UNAUDITED)
    The  estimates of proved oil and gas reserves, which are located principally
in the United States, were prepared by the Company as of December 31, 1993, 1994
and 1995, and Williamson  Petroleum Consultants as of  March 31, 1996.  Reserves
were  estimated in  accordance with guidelines  established by the  SEC and FASB
which require that  reserve estimates  be prepared under  existing economic  and
operating conditions with no provision for price and cost escalations, except by
contractual  arrangements.  The  Company  has  presented  the  reserve estimates
utilizing an oil price of $17.79 per Bbl and a gas price of $2.03 per Mcf as  of
December  31, 1995, and an oil price of $20.71  per Bbl and a gas price of $2.00
per Mcf as of March 31, 1996.
 
                                      F-19
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(14) SUPPLEMENTAL OIL AND GAS RESERVE INFORMATION (UNAUDITED) (CONTINUED)
 
    OIL AND GAS PRODUCING ACTIVITIES
 
    Oil and gas reserve quantity estimates are subject to numerous uncertainties
inherent  in  the  estimation  of  quantities  of  proved  reserves  and  in the
projection  of  future  rates  of  production  and  the  timing  of  development
expenditures.  The accuracy of  such estimates is  a function of  the quality of
available data and  of engineering and  geological interpretation and  judgment.
Results  of subsequent drilling, testing and  production may cause either upward
or downward revision of previous  estimates. Further, the volumes considered  to
be  commercially  recoverable fluctuate  with  changes in  prices  and operating
costs. The Company  emphasizes that reserve  estimates are inherently  imprecise
and that estimates of new discoveries are more imprecise than those of currently
producing  oil and gas properties. Accordingly,  these estimates are expected to
change as additional information becomes available in the future.
 
<TABLE>
<CAPTION>
                                                                                 OIL AND CONDENSATE       GAS
                                                                                       (MBBLS)          (MMCF)
                                                                                 -------------------  -----------
<S>                                                                              <C>                  <C>
Total Proved Reserves:
Balance, January 1, 1993.......................................................           1,985           16,418
  Revisions of previous estimates..............................................              57            1,160
  Extensions and discoveries...................................................             380              591
  Production...................................................................            (158)            (865)
  Purchases of minerals-in-place...............................................             101            4,315
                                                                                        -------       -----------
Balance, December 31, 1993.....................................................           2,365           21,619
  Revisions of previous estimates..............................................            (460)          (5,424)
  Extensions and discoveries...................................................             761            1,520
  Production...................................................................            (330)          (1,600)
  Purchases of minerals-in-place...............................................           1,673           11,397
                                                                                        -------       -----------
Balance, December 31, 1994.....................................................           4,009           27,512
  Revisions of previous estimates..............................................            (570)             425
  Extensions and discoveries...................................................             605            8,922
  Production...................................................................            (950)          (4,806)
  Purchases of minerals-in-place...............................................           7,694           46,099
                                                                                        -------       -----------
Balance, December 31, 1995.....................................................          10,788           78,152
  Revisions of previous estimates..............................................             437            2,615
  Extensions and discoveries...................................................             592              296
  Production...................................................................            (338)          (1,643)
  Purchases of minerals-in-place...............................................              --               --
                                                                                        -------       -----------
Balance, March 31, 1996........................................................          11,479           79,420
                                                                                        -------       -----------
                                                                                        -------       -----------
Proved Developed Reserves:
  January 1, 1993..............................................................           1,488           10,055
  December 31, 1993............................................................           1,785           13,268
  December 31, 1994............................................................           2,632           16,340
  December 31, 1995............................................................           8,566           57,393
  March 31, 1996...............................................................           9,037           55,408
</TABLE>
 
                                      F-20
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(14) SUPPLEMENTAL OIL AND GAS RESERVE INFORMATION (UNAUDITED) (CONTINUED)
 
    STANDARDIZED MEASURE OF DISCOUNTED FUTURE NET CASH FLOWS RELATING TO PROVED
OIL AND GAS RESERVES
 
    The standardized measure of discounted future net cash flows is computed  by
applying  year-end prices  of oil and  gas (with consideration  of price changes
only to the extent provided by contractual arrangements) to the estimated future
production of proved oil  and gas reserves,  less estimated future  expenditures
(based  on year-end costs) to be incurred in developing and producing the proved
reserves, less estimated future income tax expenses (based on year-end statutory
tax rates, with  consideration of  future tax  rates already  legislated) to  be
incurred  on  pretax  net cash  flows,  less  tax basis  of  the  properties and
available credits, and  assuming continuation of  existing economic  conditions.
The  estimated future net cash flows are then discounted using a rate of 10% per
year to reflect the estimated timing of the future cash flows.
 
    Discounted future  cash  flow  estimates  like those  shown  below  are  not
intended  to represent estimates  of the fair  value of oil  and gas properties.
Estimates of  fair value  should also  consider probable  reserves,  anticipated
future oil and gas prices, interest rates, changes in development and production
costs  and risks associated  with future production. Because  of these and other
considerations, any  estimate  of  fair  value  is  necessarily  subjective  and
imprecise.
 
<TABLE>
<CAPTION>
                                                                                                   THREE MONTHS
                                                                 YEARS ENDED DECEMBER 31,         ENDED MARCH 31,
                                                           -------------------------------------  ---------------
                                                              1993        1994          1995           1996
                                                           ----------  -----------  ------------  ---------------
                                                                               (IN THOUSANDS)
<S>                                                        <C>         <C>          <C>           <C>
Future cash flows........................................  $   83,510  $   122,098  $    350,653   $     396,919
Future costs:
  Production.............................................     (31,811)     (46,345)     (145,510)       (162,146)
  Development............................................      (4,486)      (7,157)      (16,806)        (17,975)
                                                           ----------  -----------  ------------  ---------------
Future net cash flows....................................      47,213       68,596       188,337         216,798
10% annual discount for estimated timing of cash flows...     (20,836)     (31,817)      (75,041)        (87,707)
                                                           ----------  -----------  ------------  ---------------
Standardized measure of discounted net cash flows........  $   26,377  $    36,779  $    113,296   $     129,091
                                                           ----------  -----------  ------------  ---------------
                                                           ----------  -----------  ------------  ---------------
</TABLE>
 
                                      F-21
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                      (A TEXAS LIMITED LIABILITY COMPANY)
 
             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)
        (THE INFORMATION AND AMOUNTS FOR INTERIM PERIODS ARE UNAUDITED.)
 
(14) SUPPLEMENTAL OIL AND GAS RESERVE INFORMATION (UNAUDITED) (CONTINUED)
 
    CHANGES IN STANDARDIZED MEASURE OF DISCOUNTED FUTURE NET CASH FLOWS FROM
PROVED RESERVES
 
<TABLE>
<CAPTION>
                                                                                                  THREE MONTHS
                                                                  YEARS ENDED DECEMBER 31,       ENDED MARCH 31,
                                                              ---------------------------------  ---------------
                                                                1993       1994        1995           1996
                                                              ---------  ---------  -----------  ---------------
                                                                                (IN THOUSANDS)
<S>                                                           <C>        <C>        <C>          <C>
Increase (decrease):
  Purchase of minerals-in-place.............................  $   3,732  $  15,231  $    77,343    $        --
  Extensions and discoveries and improved recovery, net of
   future production and development costs..................      2,707      4,072        9,799          6,002
  Accretion of discount.....................................      2,056      2,638        3,678          2,832
  Net change in sales prices, net of production costs.......       (209)       503       (3,422)         9,229
  Changes in estimated future development costs.............        (16)       940       (2,419)          (235)
  Revisions of quantity estimates...........................      1,203     (7,248)      (2,855)         4,839
  Sales, net of production costs............................     (2,543)    (5,286)     (11,338)        (5,174)
  Changes of production rates (timing) and other............     (1,114)      (448)       5,731         (1,698)
                                                              ---------  ---------  -----------  ---------------
    Net increase............................................      5,816     10,402       76,517         15,795
Standardized measure of discounted future net cash flows:
    Beginning of period.....................................     20,561     26,377       36,779        113,296
                                                              ---------  ---------  -----------  ---------------
    End of period...........................................  $  26,377  $  36,779  $   113,296    $   129,091
                                                              ---------  ---------  -----------  ---------------
                                                              ---------  ---------  -----------  ---------------
</TABLE>
 
    The  1995  future cash  flows shown  above  include amounts  attributable to
proved undeveloped  reserves requiring  approximately  $15.0 million  of  future
development costs. If these reserves are not developed, the standardized measure
of  discounted future net  cash flows for  1995 shown above  would be reduced by
approximately $22.4 million.
 
                                      F-22
<PAGE>
                          INDEPENDENT AUDITORS' REPORT
 
The Members
Costilla Energy, L.L.C.:
 
    We have audited the accompanying statements of revenues and direct operating
expenses of the 1995 Acquisition (see Note  1) for the years ended December  31,
1993  and 1994,  and the period  ended June  12, 1995. These  statements are the
responsibility of the Company's management. Our responsibility is to express  an
opinion on these statements based on our audits.
 
    We  conducted  our audits  in  accordance with  generally  accepted auditing
standards. Those standards require that we plan and perform the audit to  obtain
reasonable  assurance  about  whether  the  statements  of  revenues  and direct
operating  expenses  are  free  of  material  misstatement.  An  audit  includes
examining,  on a test basis, evidence  supporting the amounts and disclosures in
the financial  statements.  An  audit also  includes  assessing  the  accounting
principles  used  and  significant  estimates made  by  management,  as  well as
evaluating the  overall  statement  presentation. We  believe  that  our  audits
provide a reasonable basis for our opinion.
 
    The  accompanying statements of revenues  and direct operating expenses were
prepared for the  purpose of  complying with the  rules and  regulations of  the
Securities  and  Exchange Commission  (for inclusion  in  Forms S-1  of Costilla
Energy, Inc. as  described in  Note 1)  and are not  intended to  be a  complete
presentation of the 1995 Acquisition interests' revenue and expenses.
 
    In  our opinion,  the statements of  revenues and  direct operating expenses
referred to above  present fairly, in  all material respects,  the revenues  and
direct  operating expenses of the 1995  Acquisition for the years ended December
31, 1993  and 1994,  and the  period ended  June 12,  1995, in  conformity  with
generally accepted accounting principles.
 
                                          KPMG PEAT MARWICK LLP
 
Midland, Texas
July 4, 1996
 
                                      F-23
<PAGE>
                            COSTILLA ENERGY, L.L.C.
 
                                1995 ACQUISITION
 
              STATEMENTS OF REVENUES AND DIRECT OPERATING EXPENSES
                                 (IN THOUSANDS)
 
<TABLE>
<CAPTION>
                                                                                YEARS ENDED
                                                                                DECEMBER 31,
                                                                            --------------------   PERIOD ENDED
                                                                              1993       1994     JUNE 12, 1995
                                                                            ---------  ---------  --------------
<S>                                                                         <C>        <C>        <C>
Revenues:
  Oil and condensate......................................................  $  18,542  $  16,217    $    7,572
  Natural gas.............................................................     13,780     11,407         3,358
                                                                            ---------  ---------  --------------
                                                                               32,322     27,624        10,930
Direct operating expenses:
  Lease operating.........................................................     13,376     11,220         4,550
  Workovers and dry hole costs............................................        462        470           109
  Production taxes........................................................      2,070      2,023           923
                                                                            ---------  ---------  --------------
                                                                               15,908     13,713         5,582
                                                                            ---------  ---------  --------------
Revenues in excess of direct operating expenses...........................  $  16,414  $  13,911    $    5,348
                                                                            ---------  ---------  --------------
                                                                            ---------  ---------  --------------
</TABLE>
 
                See the accompanying notes to these statements.
 
                                      F-24
<PAGE>
                            COSTILLA ENERGY, L.L.C.
 
                                1995 ACQUISITION
 
       NOTES TO THE STATEMENTS OF REVENUES AND DIRECT OPERATING EXPENSES
 
(1) BASIS OF PRESENTATION
    On June 12, 1995, Costilla Energy, L.L.C. and Costilla Petroleum Corporation
(collectively,  the "Company") acquired  from Parker &  Parsley Development L.P.
and Parker & Parsley Producing  L.P. (collectively, "Parker & Parsley")  certain
oil   and  gas  properties   (the  "1995  Acquisition")   for  $46,621,371.  The
accompanying statements of revenues and  direct operating expenses for the  1995
Acquisition  do not include general and administrative expenses, interest income
or expense, a  provision for  depreciation, depletion and  amortization, or  any
provision  for income taxes since historical expenses of this nature incurred by
Parker & Parsley are not necessarily indicative  of the costs to be incurred  by
the Company.
 
    Historical  financial information reflecting  financial position, results of
operations, and cash flows  of the 1995 Acquisition,  are not presented  because
the  purchase price was assigned to the oil and gas property interests acquired.
Other assets acquired  and liabilities assumed  were not material.  Accordingly,
the  historical statements of revenues and direct operating expenses of the 1995
Acquisition are presented  in lieu  of the financial  statements required  under
Rule 3-05 of Securities and Exchange Commission Regulation S-X.
 
    Revenues  in the  accompanying statements  of revenues  and direct operating
expenses are recognized  on the sales  method. Under this  method, revenues  are
recognized  based on actual  volumes of oil  and gas sold  to purchasers. Direct
operating expenses are recognized on the accrual method.
 
(2) SUPPLEMENTARY FINANCIAL INFORMATION FOR OIL AND GAS PRODUCING ACTIVITIES
(UNAUDITED)
 
    ESTIMATED QUANTITIES OF PROVED OIL AND GAS RESERVES
 
    Reserve information presented  below for  the 1995 Acquisition  is based  on
Company prepared reserve estimates, using prices and costs in effect at December
31,  1993  and 1994,  and the  period ended  June 12,  1995. Changes  in reserve
estimates were derived  by adjusting  the period-end quantities  and values  for
actual production using historical prices and costs.
 
    Proved  reserves are estimated quantities of crude oil and natural gas which
geological and  engineering data  demonstrate with  reasonable certainty  to  be
recoverable  in future years  from known reservoirs  under existing economic and
operating conditions. Proved developed reserves are those which are expected  to
be  recovered  through  existing  wells with  existing  equipment  and operating
methods. Oil  and  gas  reserve  quantity  estimates  are  subject  to  numerous
uncertainties inherent in the estimation of quantities of proved reserves and in
the  projection  of future  rates of  production and  the timing  of development
expenditures. The accuracy  of such estimates  is a function  of the quality  of
available  data and of  engineering and geological  interpretation and judgment.
Results of subsequent drilling, testing  and production may cause either  upward
or  downward revision of previous estimates.  Further, the volumes considered to
be commercially  recoverable  fluctuate with  changes  in prices  and  operating
costs.  The Company emphasizes  that reserve estimates  are inherently imprecise
and that estimates of new discoveries are more imprecise than those of currently
producing oil  and  gas properties.  Accordingly,  these reserve  estimates  are
expected to change as additional information becomes available in the future.
 
                                      F-25
<PAGE>
                            COSTILLA ENERGY, L.L.C.
 
                                1995 ACQUISITION
 
 NOTES TO THE STATEMENTS OF REVENUES AND DIRECT OPERATING EXPENSES (CONTINUED)
 
(2) SUPPLEMENTARY FINANCIAL INFORMATION FOR OIL AND GAS PRODUCING ACTIVITIES
(UNAUDITED)
    (CONTINUED)
    Below  are  the  net  estimated quantities  of  proved  reserves  and proved
developed reserves for the 1995 Acquisition.
 
<TABLE>
<CAPTION>
                                                                     OIL (MBBLS)  GAS (MMCF)
                                                                     -----------  -----------
<S>                                                                  <C>          <C>
Proved reserves at December 31, 1992...............................       9,880       60,199
Production.........................................................      (1,204)      (6,914)
                                                                     -----------  -----------
Proved reserves at December 31, 1993...............................       8,676       53,285
Production.........................................................      (1,142)      (6,778)
                                                                     -----------  -----------
Proved reserves at December 31, 1994...............................       7,534       46,507
Production.........................................................        (479)      (2,405)
                                                                     -----------  -----------
Proved reserves at June 12, 1995...................................       7,055       44,102
                                                                     -----------  -----------
                                                                     -----------  -----------
Proved developed reserves at June 12, 1995.........................       6,707       38,151
                                                                     -----------  -----------
                                                                     -----------  -----------
</TABLE>
 
    STANDARDIZED MEASURE OF DISCOUNTED FUTURE NET CASH FLOWS OF PROVED OIL AND
GAS RESERVES
 
    The Company has estimated the standardized measure of discounted future  net
cash  flows  and changes  therein relating  to  proved oil  and gas  reserves in
accordance with the standards established by the Financial Accounting  Standards
Board  through its  Statement No.  69. The  estimates of  future cash  flows and
future production and development costs are based on period-end sales prices for
oil and  gas, estimated  future  production of  proved reserves,  and  estimated
future  production and  development costs of  proved reserves,  based on current
costs and economic  conditions. The  estimated future  net cash  flows are  then
discounted at a rate of 10%.
 
    Discounted  future net  cash flow estimates  like those shown  below are not
intended to  represent  estimates  of the  fair  market  value of  oil  and  gas
properties.  Estimates  of  fair  market  value  should  also  consider probable
reserves, anticipated  future oil  and gas  prices, interest  rates, changes  in
development  and production costs  and risks associated  with future production.
Because of these and other considerations, any estimate of fair market value  is
necessarily subjective and imprecise.
 
    The following are the Company's estimated standardized measure of discounted
future net cash flows from proved reserves attributable to the 1995 Acquisition:
 
<TABLE>
<CAPTION>
                                                                       DECEMBER 31,
                                                                 ------------------------
                                                                    1993         1994      JUNE 12, 1995
                                                                 -----------  -----------  -------------
                                                                             (IN THOUSANDS)
<S>                                                              <C>          <C>          <C>
Future:
  Cash inflows.................................................  $   222,698  $   188,828   $   191,758
  Production costs.............................................     (111,619)     (97,988)      (93,268)
  Development costs............................................       (4,797)      (4,797)       (4,797)
                                                                 -----------  -----------  -------------
    Net cash flows before income taxes.........................      106,282       86,043        93,693
10% annual discount for estimated timing of cash flows.........      (37,518)     (30,373)      (33,074)
                                                                 -----------  -----------  -------------
Standardized measure of discounted future net cash flows before
 income taxes..................................................  $    68,764  $    55,670   $    60,619
                                                                 -----------  -----------  -------------
                                                                 -----------  -----------  -------------
</TABLE>
 
                                      F-26
<PAGE>
                            COSTILLA ENERGY, L.L.C.
 
                                1995 ACQUISITION
 
 NOTES TO THE STATEMENTS OF REVENUES AND DIRECT OPERATING EXPENSES (CONTINUED)
 
(2) SUPPLEMENTARY FINANCIAL INFORMATION FOR OIL AND GAS PRODUCING ACTIVITIES
(UNAUDITED)
    (CONTINUED)
    The  following are  the sources  of changes  in the  standardized measure of
discounted net cash flows:
 
<TABLE>
<CAPTION>
                                                                YEARS ENDED DECEMBER
                                                                        31,
                                                               ----------------------  PERIOD ENDED
                                                                  1993        1994     JUNE 12, 1995
                                                               ----------  ----------  -------------
                                                                          (IN THOUSANDS)
<S>                                                            <C>         <C>         <C>
Standardized measure, beginning of period....................  $   96,022  $   68,764   $    55,670
Sales, net of production costs...............................     (16,414)    (13,911)       (5,348)
Net change in prices.........................................     (15,892)     (3,910)        8,032
Accretion of discount........................................       9,602       6,876         2,517
Other........................................................      (4,554)     (2,149)         (252)
                                                               ----------  ----------  -------------
Standardized measure, end of period..........................  $   68,764  $   55,670   $    60,619
                                                               ----------  ----------  -------------
                                                               ----------  ----------  -------------
</TABLE>
 
                                      F-27
<PAGE>
                          INDEPENDENT AUDITORS' REPORT
 
The Members
Costilla Energy, L.L.C.:
 
    We have audited the accompanying statements of revenues and direct operating
expenses  of the 1996 Acquisition (see Note  1) for the years ended December 31,
1993, 1994 and 1995.  These statements are the  responsibility of the  Company's
management.  Our responsibility  is to  express an  opinion on  these statements
based on our audits.
 
    We conducted  our  audits in  accordance  with generally  accepted  auditing
standards.  Those standards require that we plan and perform the audit to obtain
reasonable assurance  about  whether  the  statements  of  revenues  and  direct
operating  expenses  are  free  of  material  misstatement.  An  audit  includes
examining, on a test basis, evidence  supporting the amounts and disclosures  in
the  financial  statements.  An  audit also  includes  assessing  the accounting
principles used  and  significant  estimates  made by  management,  as  well  as
evaluating  the  overall  statement  presentation. We  believe  that  our audits
provide a reasonable basis for our opinion.
 
    The accompanying statements of revenues  and direct operating expenses  were
prepared  for the  purpose of  complying with the  rules and  regulations of the
Securities and  Exchange Commission  (for  inclusion in  Forms S-1  of  Costilla
Energy,  Inc. as  described in  Note 1) and  are not  intended to  be a complete
presentation of the 1996 Acquisition interests' revenues and expenses.
 
    In our opinion,  the statements  of revenues and  direct operating  expenses
referred  to above  present fairly, in  all material respects,  the revenues and
direct operating expenses of the 1996  Acquisition for the years ended  December
31,  1993,  1994  and 1995,  in  conformity with  generally  accepted accounting
principles.
 
                                          KPMG PEAT MARWICK LLP
 
Midland, Texas
July 4, 1996
 
                                      F-28
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                                1996 ACQUISITION
              STATEMENTS OF REVENUES AND DIRECT OPERATING EXPENSES
 
                                 (IN THOUSANDS)
 
<TABLE>
<CAPTION>
                                                                      YEARS ENDED                  PERIODS
                                                                     DECEMBER 31,               ENDED JUNE 14,
                                                            -------------------------------  --------------------
                                                              1993       1994       1995       1995       1996
                                                            ---------  ---------  ---------  ---------  ---------
<S>                                                         <C>        <C>        <C>        <C>        <C>
                                                                                                 (UNAUDITED)
Revenues:
  Oil and condensate......................................  $  11,467  $  10,170  $  10,564  $   5,140  $   5,205
  Natural gas.............................................     11,294     10,105      8,645      3,763      3,434
  Gas plant...............................................         57         57        126         47         42
  Transportation..........................................         39        379        556        253        542
                                                            ---------  ---------  ---------  ---------  ---------
                                                               22,857     20,711     19,891      9,203      9,223
 
Direct operating expenses:
  Lease operating.........................................     10,977      9,053      9,232      3,965      4,020
  Workovers and dry hole costs............................        675        869      1,002        256        450
  Production taxes........................................      1,166      1,089        992        458        453
  Gas plant...............................................        131        350        598        393        269
  Transportation..........................................         10        394        587        268        222
                                                            ---------  ---------  ---------  ---------  ---------
                                                               12,959     11,755     12,411      5,340      5,414
                                                            ---------  ---------  ---------  ---------  ---------
Revenues in excess of direct operating expenses...........  $   9,898  $   8,956  $   7,480  $   3,863  $   3,809
                                                            ---------  ---------  ---------  ---------  ---------
                                                            ---------  ---------  ---------  ---------  ---------
</TABLE>
 
                See the accompanying notes to these statements.
 
                                      F-29
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                                1996 ACQUISITION
       NOTES TO THE STATEMENTS OF REVENUES AND DIRECT OPERATING EXPENSES
 
(1) BASIS OF PRESENTATION
    On June 14, 1996, Costilla Energy, L.L.C. and Costilla Petroleum Corporation
(collectively, the "Company") acquired from  Parker & Parsley Development  L.P.,
Parker  &  Parsley  Producing  L.P.  and Parker  &  Parsley  Gas  Processing Co.
(collectively, "Parker &  Parsley") certain  oil and gas  properties (the  "1996
Acquisition")  for approximately  $42.5 million. The  accompanying statements of
revenues and direct operating expenses for  the 1996 Acquisition do not  include
general and administrative expenses, interest income or expense, a provision for
depreciation,  depletion  and amortization,  or any  provision for  income taxes
since historical expenses of  this nature incurred by  Parker & Parsley are  not
necessarily indicative of the costs to be incurred by the Company.
 
    Historical  financial information reflecting  financial position, results of
operations, and cash flows  of the 1996 Acquisition,  are not presented  because
the  purchase price was assigned to the oil and gas property interests acquired.
Other assets acquired  and liabilities assumed  were not material.  Accordingly,
the  historical statements of revenues and direct operating expenses of the 1996
Acquisition are presented  in lieu  of the financial  statements required  under
Rule 3-05 of Securities and Exchange Commission Regulation S-X.
 
    Revenues  in the  accompanying statements  of revenues  and direct operating
expenses are recognized  on the sales  method. Under this  method, revenues  are
recognized  based on actual  volumes of oil  and gas sold  to purchasers. Direct
operating expenses are recognized on the accrual method.
 
    INTERIM STATEMENTS OF REVENUES AND DIRECT OPERATING EXPENSES
 
    The interim financial information  for the periods ended  June 14, 1995  and
1996,  is  unaudited.  However,  in  the  opinion  of  management,  the  interim
statements of revenues and direct  operating expenses include all the  necessary
adjustments  to fairly present the  results of the interim  periods and all such
adjustments are of a normal recurring nature. The interim statements of revenues
and direct operating  expenses should be  read in conjunction  with the  audited
statements  of  revenues  and  direct operating  expenses  for  the  years ended
December 31, 1993, 1994 and 1995.
 
(2) SUPPLEMENTARY FINANCIAL INFORMATION FOR OIL AND GAS PRODUCING ACTIVITIES
(UNAUDITED)
 
    ESTIMATED QUANTITIES OF PROVED OIL AND GAS RESERVES
 
    Reserve information presented below  for the 1996  Acquisition, as of  March
31,  1996,  is  based  on reserve  estimates  prepared  by  Williamson Petroleum
Consultants, using prices and costs in  effect at that date. Changes in  reserve
estimates  were  derived  by adjusting  such  quantities and  values  for actual
production using historical prices and costs.
 
    Proved reserves are estimated quantities of crude oil and natural gas  which
geological  and  engineering data  demonstrate with  reasonable certainty  to be
recoverable in future years  from known reservoirs  under existing economic  and
operating  conditions. Proved developed reserves are those which are expected to
be recovered  through  existing  wells with  existing  equipment  and  operating
methods.  Oil  and  gas  reserve  quantity  estimates  are  subject  to numerous
uncertainties inherent in the estimation of quantities of proved reserves and in
the projection  of future  rates of  production and  the timing  of  development
expenditures.  The accuracy of  such estimates is  a function of  the quality of
available data and  of engineering and  geological interpretation and  judgment.
Results  of subsequent drilling, testing and  production may cause either upward
or downward revision of previous  estimates. Further, the volumes considered  to
be  commercially  recoverable fluctuate  with  changes in  prices  and operating
costs. The Company  emphasizes that reserve  estimates are inherently  imprecise
and that
 
                                      F-30
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                                1996 ACQUISITION
 NOTES TO THE STATEMENTS OF REVENUES AND DIRECT OPERATING EXPENSES (CONTINUED)
 
(2) SUPPLEMENTARY FINANCIAL INFORMATION FOR OIL AND GAS PRODUCING ACTIVITIES
(UNAUDITED) (CONTINUED)
estimates  of  new  discoveries  are  more  imprecise  than  those  of currently
producing oil  and  gas properties.  Accordingly,  these reserve  estimates  are
expected to change as additional information becomes available in the future.
 
    Below  are  the  net  estimated quantities  of  proved  reserves  and proved
developed reserves for the 1996 Acquisition.
 
<TABLE>
<CAPTION>
                                                                                          OIL (MBBLS)  GAS (MMCF)
                                                                                          -----------  -----------
<S>                                                                                       <C>          <C>
Proved reserves at December 31, 1992....................................................       7,211       49,963
Production..............................................................................        (718)      (5,481)
                                                                                               -----   -----------
Proved reserves at December 31, 1993....................................................       6,493       44,482
Production..............................................................................        (685)      (5,217)
                                                                                               -----   -----------
Proved reserves at December 31, 1994....................................................       5,808       39,265
Production..............................................................................        (656)      (4,773)
                                                                                               -----   -----------
Proved reserves at December 31, 1995....................................................       5,152       34,492
Production..............................................................................        (154)        (991)
                                                                                               -----   -----------
Proved reserves at March 31, 1996.......................................................       4,998       33,501
                                                                                               -----   -----------
                                                                                               -----   -----------
Proved developed reserves at March 31, 1996.............................................       4,515       28,961
                                                                                               -----   -----------
                                                                                               -----   -----------
</TABLE>
 
STANDARDIZED MEASURE OF DISCOUNTED FUTURE NET CASH FLOWS OF PROVED OIL AND GAS
RESERVES
 
    The Company has estimated the standardized measure of discounted future  net
cash  flows  and changes  therein relating  to  proved oil  and gas  reserves in
accordance with the standards established by the Financial Accounting  Standards
Board  through its  Statement No.  69. The  estimates of  future cash  flows and
future production and development costs are  based on year-end sales prices  for
oil  and  gas, estimated  future production  of  proved reserves,  and estimated
future production and  development costs  of proved reserves,  based on  current
costs  and economic  conditions. The  estimated future  net cash  flows are then
discounted at a rate of 10%.
 
    Discounted future net  cash flow estimates  like those shown  below are  not
intended  to  represent  estimates of  the  fair  market value  of  oil  and gas
properties. Estimates  of  fair  market  value  should  also  consider  probable
reserves,  anticipated future  oil and  gas prices,  interest rates,  changes in
development and production  costs and risks  associated with future  production.
Because  of these and other considerations, any estimate of fair market value is
necessarily subjective and imprecise.
 
                                      F-31
<PAGE>
                            COSTILLA ENERGY, L.L.C.
                                1996 ACQUISITION
 NOTES TO THE STATEMENTS OF REVENUES AND DIRECT OPERATING EXPENSES (CONTINUED)
 
(2) SUPPLEMENTARY FINANCIAL INFORMATION FOR OIL AND GAS PRODUCING ACTIVITIES
(UNAUDITED) (CONTINUED)
    The following are the Company's estimated standardized measure of discounted
future net cash flows from proved reserves attributable to the 1996 Acquisition:
 
<TABLE>
<CAPTION>
                                                                           DECEMBER 31,
                                                              ---------------------------------------   MARCH 31,
                                                                  1993          1994         1995         1996
                                                              ------------  ------------  -----------  -----------
                                                                          (IN THOUSANDS)
<S>                                                           <C>           <C>           <C>          <C>
Future:
  Cash inflows..............................................  $    181,010  $    156,222  $   165,862  $   175,507
  Production costs..........................................      (116,115)     (105,104)     (93,878)     (91,202)
  Development costs.........................................        (4,101)       (4,101)      (4,101)      (4,101)
                                                              ------------  ------------  -----------  -----------
    Net cash flows before income taxes......................        60,794        47,017       67,883       80,204
10% annual discount for estimated timing of cash flows......       (22,564)      (17,451)     (25,195)     (29,768)
                                                              ------------  ------------  -----------  -----------
Standardized measure of discounted future net cash flows
 before income taxes........................................  $     38,230  $     29,566  $    42,688  $    50,436
                                                              ------------  ------------  -----------  -----------
                                                              ------------  ------------  -----------  -----------
</TABLE>
 
    The following are  the sources  of changes  in the  standardized measure  of
discounted net cash flows:
 
<TABLE>
<CAPTION>
                                                                                                   THREE MONTH
                                                                     YEARS ENDED DECEMBER 31,      PERIOD ENDED
                                                                  -------------------------------   MARCH 31,
                                                                    1993       1994       1995         1996
                                                                  ---------  ---------  ---------  ------------
                                                                          (IN THOUSANDS)
<S>                                                               <C>        <C>        <C>        <C>
Standardized measure, beginning of period.......................  $  56,372  $  38,230  $  29,566   $   42,688
Sales, net of production costs..................................     (9,943)    (9,264)    (7,983)      (2,090)
Net change in prices............................................    (11,890)    (2,838)    18,141        9,277
Accretion of discount...........................................      5,637      3,823      2,957        1,067
Other...........................................................     (1,946)      (385)         7         (506)
                                                                  ---------  ---------  ---------  ------------
Standardized measure, end of period.............................  $  38,230  $  29,566  $  42,688   $   50,436
                                                                  ---------  ---------  ---------  ------------
                                                                  ---------  ---------  ---------  ------------
</TABLE>
 
                                      F-32
<PAGE>
                                                                      APPENDIX A
 
                                 July 23, 1996
 
Costilla Energy, Inc.
400 West Illinois, Suite 1000
Midland, Texas 79701
 
Attention  Mr. Michael J. Grella
 
Gentlemen:
 
Subject:    Summary  Letter  (for Inclusion  in a  Prospectus Included  in a
            Registration Statement for  Costilla Energy, Inc.  on Form  S-1)
            Combining   Specific   Data   from   Two   Williamson  Petroleum
            Consultants, Inc. Evaluations (1)  to the Interests of  Costilla
            Petroleum  Corporation  in  Various Properties  and  (2)  to the
            Interests of Parker  & Parsley  Petroleum USA,  Inc. in  Various
            Properties Included in Their First Quarter 1996 Sales Package
            Effective April 1, 1996
            Williamson Project 6.8393
 
    In  accordance  with your  request,  Williamson Petroleum  Consultants, Inc.
(Williamson) has prepared  a summary letter  for inclusion in  a prospectus  for
Costilla  Energy, Inc. (Costilla). The filing of this Prospectus gives effect to
the conversion of Costilla Energy, L.L.C. to Costilla Energy, Inc. This  summary
letter  includes specific  data from two  evaluations the subjects  of which are
described in  Item I.  All values  and  discussion of  proved reserves  and  net
revenues,  data  utilized, assumptions,  and qualifications  are taken  from and
include by reference data from these two evaluations.
 
    Interests in this summary letter represent the April 1, 1996 effective  date
consolidation of the ownership interests of Costilla and the ownership interests
of  Parker & Parsley in various properties  included in their first quarter 1996
sales package  which Costilla  acquired on  June  14, 1996  but which  was  made
effective  as  of  January  1,  1996. The  Costilla  interests  include  all the
interests of  Costilla  Energy, L.L.C.  and  all its  wholly-owned  subsidiaries
including Costilla Petroleum Corporation.
 
I.  THE TWO SUBJECT EVALUATIONS
 
    This  summary  letter  combines  certain proved  oil  and  gas  reserves and
revenues from the following two Williamson evaluations:
 
    (1) Evaluation  of  Oil  and  Gas Reserves  to  the  Interests  of  Costilla
       Petroleum  Corporation in  Various Properties,  Effective April  1, 1996,
       Utilizing Nonescalated Economics,  for Disclosure to  the Securities  and
       Exchange Commission, Williamson Project 6.8393, transmitted July 18, 1996
       (the Costilla report)
 
    (2)  Evaluation of Oil and Gas Reserves to the Interests of Parker & Parsley
       Petroleum USA, Inc. in Various Properties Included in Their First Quarter
       1996 Sales  Package,  Effective  April 1,  1996,  Utilizing  Nonescalated
       Economics,  for  Disclosure to  the  Securities and  Exchange Commission,
       Williamson Project  6.8393, transmitted  July 18,  1996 (the  Acquisition
       report)
 
II.  ESTIMATED SEC RESERVES AND FUTURE NET REVENUES
 
    Projections  of  the  reserves  that are  attributable  to  the consolidated
interests in this summary letter were based on economic parameters and operating
conditions considered applicable  as of April  1, 1996 and  are pursuant to  the
requirements of the Securities and Exchange Commission (SEC).
 
    In  accordance with  instructions from  Costilla, Williamson  utilized lease
operating expenses for the Costilla-operated  properties in the Costilla  report
that excluded COPAS overhead and internal
 
                                      A-1
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 2
indirect  overhead  which are  billed to  outside  working interest  owners. The
exclusion of these costs for the operated properties results in the  calculation
of  a lower  economic limit  and causes  the economic  lifetime to  be extended.
Williamson has  not  quantified the  incremental  reserves resulting  from  this
procedure. COPAS overhead was excluded from the lease operating expenses for the
Parker & Parsley-operated properties in the Acquisition report.
 
    The present values of the estimated future net revenues from proved reserves
were  calculated  using a  discount  rate of  10.00  percent per  year  and were
computed in accordance  with the  financial reporting requirements  of the  SEC.
Following  is a summary of the results of the two evaluations effective April 1,
1996:
 
<TABLE>
<CAPTION>
                                                 PROVED           PROVED
                                               DEVELOPED         DEVELOPED           PROVED           TOTAL
                                               PRODUCING       NONPRODUCING       UNDEVELOPED         PROVED
                                             --------------  -----------------  ----------------  --------------
<S>                                          <C>             <C>                <C>               <C>
Net Reserves to the Evaluated Interests:
  Oil/Condensate, BBL......................      13,122,088          429,450          2,924,589       16,476,127
  Gas, MCF.................................      76,439,217        7,929,591         28,551,497      112,920,305
Future Net Revenue, $:
  Undiscounted.............................     212,071,507       18,097,949         66,832,632      297,002,088
  Discounted Per Annum at 10.00 Percent....     135,185,097        9,530,285         34,811,523      179,526,905
</TABLE>
 
- ------------------------
Note: The values presented in this table are taken from evaluations described in
      Item I and include by reference all data, qualifications, and  assumptions
      from  these  evaluations. Realization  of  these values  is  contingent on
      achieving successful results from the various schedules and assumptions in
      these evaluations.  The available  engineering data  and the  completeness
      and/or  quality of data utilized in evaluating the properties are detailed
      in the specific evaluation. Review of any additionally available data  may
      necessitate  revision to these interpretations  and assumptions and impact
      these values.
 
III.  DEFINITIONS OF SEC RESERVES (1)
 
    The estimated  reserves presented  in  this summary  letter are  net  proved
reserves,  including proved developed  producing, proved developed nonproducing,
and proved  undeveloped  reserves, and  were  computed in  accordance  with  the
financial  reporting requirements of the SEC. In preparing these evaluations, no
attempt has been made to quantify the element of uncertainty associated with any
category. Reserves were assigned to each category as warranted. The  definitions
of  oil and gas reserves pursuant to the requirements of the Securities Exchange
Act are:
 
PROVED RESERVES (2)
 
    Proved reserves are the estimated quantities of crude oil, natural gas,  and
natural  gas  liquids which  geological  and engineering  data  demonstrate with
reasonable certainty to  be recoverable  in future years  from known  reservoirs
under the economic criteria employed and existing operating
 
- ------------------------
(1)  For  evaluations prepared  for disclosure  to  the Securities  and Exchange
    Commission, see SEC ACCOUNTING RULES. Commerce Clearing House, Inc.  October
    1981, Paragraph 290, Regulation 210.4-10, p. 329.
 
(2) Any variations to these definitions will be clearly stated in the report.
 
                                      A-2
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 3
conditions,  i.e., prices and costs as of  the date the estimate is made. Prices
and  costs  include  consideration  of  changes  provided  only  by  contractual
arrangements but not on escalations based upon an estimate of future conditions.
 
A.   Reservoirs are considered proved  if economic producibility is supported by
    either actual  production  or  conclusive  formation test.  The  area  of  a
    reservoir considered proved includes:
 
    1.    that portion  delineated  by drilling  and  defined by  gas-oil and/or
       oil-water contacts, if any; and
 
    2.  the  immediately adjoining portions  not yet drilled,  but which can  be
       reasonably  judged as economically  productive on the  basis of available
       geological and engineering data. In  the absence of information on  fluid
       contacts, the lowest known structural occurrence of hydrocarbons controls
       the lower proved limit of the reservoir.
 
B.   Reserves which can be produced economically through application of improved
    recovery techniques (such as fluid  injection) are included in the  "proved"
    classification  when successful testing by a pilot project, or the operation
    of  an  installed  program  in  the  reservoir,  provides  support  for  the
    engineering analysis on which the project or program was based.
 
C.  Estimates of proved reserves do not include the following:
 
    1.   oil that may  become available from known  reservoirs but is classified
       separately as "indicated additional reserves;"
 
    2.  crude oil, natural gas, and  natural gas liquids, the recovery of  which
       is  subject to  reasonable doubt  because of  uncertainty as  to geology,
       reservoir characteristics, or economic factors;
 
    3.  crude  oil, natural  gas, and  natural gas  liquids, that  may occur  in
       undrilled prospects; and
 
    4.   crude oil, natural gas, and  natural gas liquids, that may be recovered
       from oil shales, coal (3), gilsonite and other such sources.
 
PROVED DEVELOPED RESERVES (4)
 
    Proved developed reserves are reserves that can be expected to be  recovered
through existing wells with existing equipment and operating methods. Additional
oil  and gas expected to be obtained  through the application of fluid injection
or other improved recovery techniques  for supplementing the natural forces  and
mechanisms of primary recovery should be included as "proved developed reserves"
only  after testing by  a pilot project  or after the  operation of an installed
program has confirmed through production  response that increased recovery  will
be achieved.
 
PROVED UNDEVELOPED RESERVES
 
    Proved  undeveloped reserves are reserves that  are expected to be recovered
from new wells on undrilled acreage,  or from existing wells where a  relatively
major  expenditure is required  for recompletion. Reserves  on undrilled acreage
shall   be   limited   to    those   drilling   units   offsetting    productive
 
- ------------------------
(3) According to Staff Accounting Bulletin 85, excluding certain coalbed methane
    gas.
 
(4)  Williamson Petroleum Consultants, Inc.  separates proved developed reserves
    into proved developed producing and proved developed nonproducing  reserves.
    This  is  to identify  proved developed  producing reserves  as those  to be
    recovered from  actively  producing  wells;  proved  developed  nonproducing
    reserves  as those  to be  recovered from  wells or  intervals within wells,
    which  are  completed  but  shut   in  waiting  on  equipment  or   pipeline
    connections,  or wells where a relatively  minor expenditure is required for
    recompletion to another zone.
 
                                      A-3
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 4
units that are reasonably  certain of production  when drilled. Proved  reserves
for  other undrilled units can be claimed only where it can be demonstrated with
certainty that there is  continuity of production  from the existing  productive
formation.  Under  no  circumstances  should  estimates  for  proved undeveloped
reserves be  attributable to  any  acreage for  which  an application  of  fluid
injection  or  other improved  recovery technique  is contemplated,  unless such
techniques have been proved  effective by actual  tests in the  area and in  the
same reservoir.
 
IV.  DISCUSSION OF SEC RESERVES
 
A.  THE COSTILLA REPORT
 
    A  total of 1,014  properties in 294  fields were evaluated  in the Costilla
    report. Nineteen individual properties had  values greater than 1.0  percent
    of the total future net revenue discounted at 10.00 percent per annum (DFNR)
    and  in aggregate represent 34.5 percent of the DFNR in the Costilla report.
    The most valued property, the T.B. Pruett  Gas Unit No. 3, Soda Lake  field,
    Ward  County, Texas, had a  value equal to 4.5 percent  of the total DFNR in
    the Costilla report. The top  eight major-value fields are Talbot  (Canyon),
    Howard  County, Texas; Spraberry (Trend Area), Various Counties, Texas; Soda
    Lake (Fusselman), Ward  County, Texas;  South Buffalo  Ridge, Crane  County,
    Texas;  Wattenberg,  Weld County,  Colorado;  East Goldsmith,  Ector County,
    Texas; Raymond,  Sheridan County,  Montana; and  South West  Speaks,  Lavaca
    County,  Texas. These fields contain ten of  the 19 top value properties and
    represent, in aggregate,  41.0 percent  of the  total DFNR  in the  Costilla
    report. The remaining 286 fields represent 59.0 percent with no field having
    more  than 2.9 percent of  the DFNR in the  Costilla report. A more detailed
    property review is included in the Costilla report.
 
    Area oil prices were provided by Costilla  to be used at the effective  date
    with  the written assurance that the use  of these area prices is reasonable
    on an aggregate basis  and would not materially  affect the income from  any
    major-value  property. These  area prices  were calculated  by adjusting the
    West Texas Intermediate oil April 1, 1996 posted price of $20.75 per barrel.
    The oil  price adjustments  for  each area  are the  calculated  differences
    between  the actual price received during 1995 and the posted price for West
    Texas Intermediate oil during  that same period.  After the effective  date,
    prices  were held constant  for the life  of the properties.  No attempt has
    been made to account for oil  price fluctuations which have occurred in  the
    market subsequent to the effective date of this report.
 
    Gas prices were provided by Costilla to be used at the effective date. These
    prices  were based on the April 1996 spot price of $1.75 per million British
    thermal units  (MMBTU) at  the Waha,  Texas receipt  point. This  price  was
    adjusted  with  an  area  price  adjustment  which  was  calculated  as  the
    difference between the actual price received during 1995 and the stop price.
    The resultant price was further adjusted for the BTU content of the gas  for
    each  well. If the BTU  content was unknown, it was  assumed to be one MMBTU
    per MCF of gas. After the effective date, prices were held constant for  the
    life  of the properties unless Costilla indicated that changes were provided
    for by contract. All gas prices were applied to projected wellhead volumes.
 
    It should  be  emphasized  that with  the  current  economic  uncertainties,
    fluctuation in market conditions could significantly change the economics of
    the properties included in this report.
 
    Operating expenses were provided by Costilla and represented, when possible,
    the  latest available 12-month  average of all  recurring expenses excluding
    COPAS and internal indirect overhead costs which are billable to the working
    interest owners.  These expenses  included,  but were  not limited  to,  all
    direct  operating expenses, field  overhead costs, and  any ad valorem taxes
    not
 
                                      A-4
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 5
    deducted separately. Expenses  for workovers, well  stimulations, and  other
    maintenance were not included in the operating expenses unless such work was
    expected   on  a  recurring  basis.  Judgments  for  the  exclusion  of  the
    nonrecurring expenses were made by Costilla. In accordance with instructions
    from Costilla, Williamson has excluded COPAS overhead and internal  indirect
    overhead  which are billed  to the outside working  interest owners from the
    operating expenses for Costilla-operated properties. The exclusion of  these
    costs for operated properties results in the calculation of a lower economic
    limit  and causes the  economic lifetime to be  extended. Williamson has not
    calculated the reserves that have been added as a result of this  procedure.
    For  new and  developing properties  where data  were unavailable, operating
    expenses were estimated by Costilla. Operating costs were held constant  for
    the life of the properties.
 
    State  production  taxes  have  been  deducted  at  the  published  rates as
    appropriate. For  operated  properties,  average  county  ad  valorem  taxes
    provided  by Costilla were  deducted for those  properties located in states
    for which the  data were  available. Any  ad valorem  taxes for  nonoperated
    properties  or for properties in other states were assumed to be included in
    the operating expenses.
 
    All capital  costs for  drilling and  completion of  wells and  nonrecurring
    workover  or operating costs  have been deducted  as applicable. These costs
    were provided  by Costilla.  No adjustments  were made  to account  for  the
    potential effect of inflation on these costs.
 
    Neither salvage values nor abandonment costs were provided by Costilla to be
    included in this evaluation.
 
B.  THE ACQUISITION REPORT
 
    A  total of 1,091 properties in 135 fields were evaluated in the Acquisition
    report. Eighteen individual properties had  values greater than 1.0  percent
    of the total DFNR and in aggregate represent 35.5 percent of the DFNR in the
    Acquisition  report.  The most  valued  property, the  H.W.  Glasscock Unit,
    Howard-Glasscock field, Glasscock  County, Texas, has  a projected value  of
    5.7  percent of  the total  DFNR in  the Acquisition  report. The  top eight
    major-value fields  are  World,  Crockett  County,  Texas;  Dimmitt,  Loving
    County,   Texas;  Panna  Maria,  Karnes  County,  Texas;  Giddings,  Various
    Counties, Texas; Caldwell, Burleson  County, Texas; Coletto Creek,  Victoria
    County,  Texas;  Sawyer, Sutton  County,  Texas; and  Jameson,  Coke County,
    Texas. These fields contain 11 of the 18 top value properties and represent,
    in aggregate, 51.9 percent of the total DFNR in the Acquisition report.  The
    remaining  fields represent 48.1 percent with  no field having more than 2.9
    percent of the  DFNR in  the Acquisition  report. A  more detailed  property
    review is included in the Acquisition report.
 
    Area oil prices were provided by Costilla and Parker & Parsley to be used at
    the  effective date with  the written assurance  that the use  of these area
    prices is reasonable on an aggregate  basis and would not materially  affect
    the  income from any major-value property. These area prices were calculated
    by adjusting the West Texas Intermediate  oil April 1, 1996 posted price  of
    $20.75  per  barrel. The  oil price  adjustments as  calculated by  Parker &
    Parsley for  each area  are the  calculated differences  between the  actual
    price  received during 1995 and the posted price for West Texas Intermediate
    oil during that  same period.  After the  effective date,  prices were  held
    constant for the life of the properties. No attempt has been made to account
    for  oil price fluctuations which have  occurred in the market subsequent to
    the effective date of this report.
 
    Gas prices were provided by Costilla and Parker & Parsley to be used at  the
    effective  date. These  prices were  based on the  April 1996  spot price of
    $1.75 per million British thermal units
 
                                      A-5
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 6
    (MMBTU) at the Waha,  Texas receipt point. This  price was adjusted with  an
    area  price adjustment  which was calculated  as the  difference between the
    actual price received during  1995 and the stop  price. The resultant  price
    was  further adjusted for the  BTU content of the gas  for each well. If the
    BTU content was  unknown, it was  assumed to be  one MMBTU per  MCF of  gas.
    After  the effective  date, prices  were held constant  for the  life of the
    properties unless  Costilla  indicated that  changes  were provided  for  by
    contract. All gas prices were applied to projected wellhead volumes.
 
    It  should  be  emphasized  that with  the  current  economic uncertainties,
    fluctuation in market conditions could significantly change the economics of
    the properties included in this report.
 
    Operating expenses  were  provided by  Costilla  and Parker  &  Parsley  and
    represented,  when possible,  the latest  available 12-month  average of all
    recurring expenses  excluding COPAS  and  internal indirect  overhead  costs
    which  are billable to the working interest owners. These expenses included,
    but were  not limited  to,  all direct  operating expenses,  field  overhead
    costs,  and  any  ad valorem  taxes  not deducted  separately.  Expenses for
    workovers, well stimulations, and other maintenance were not included in the
    operating expenses  unless such  work  was expected  on a  recurring  basis.
    Judgments  for  the  exclusion of  the  nonrecurring expenses  were  made by
    Costilla or Parker & Parsley. In accordance with instructions from Costilla,
    Williamson has  excluded  COPAS overhead  which  is billed  to  the  outside
    working   interest  owners  from   the  operating  expenses   for  Parker  &
    Parsley-operated properties.  The  exclusion  of these  costs  for  operated
    properties  results in the calculation of  a lower economic limit and causes
    the economic  lifetime to  be extended.  Williamson has  not calculated  the
    reserves  that have been  added as a  result of this  procedure. For new and
    developing properties where data  were unavailable, operating expenses  were
    estimated  by  Costilla  or  Parker &  Parsley.  Operating  costs  were held
    constant for the life of the properties.
 
    State production  taxes  have  been  deducted  at  the  published  rates  as
    appropriate.  For  operated  properties,  average  county  ad  valorem taxes
    provided by Costilla were  deducted for those  properties located in  states
    for  which the  data were  available. Any  ad valorem  taxes for nonoperated
    properties or for properties in other states were assumed to be included  in
    the operating expenses.
 
    All  capital costs  for drilling  and completion  of wells  and nonrecurring
    workover or operating costs  have been deducted  as applicable. These  costs
    were  provided by Costilla or Parker &  Parsley. No adjustments were made to
    account for the potential effect of inflation on these costs.
 
    Neither salvage values nor abandonment costs were provided by Costilla to be
    included in this evaluation.
 
V.  GENERAL EVALUATION CONSIDERATIONS PERTAINING TO THE COSTILLA AND ACQUISITION
    REPORTS
 
    The individual projections prepared to  produce this summary letter  include
data that describe the production forecasts and associated evaluation parameters
such  as interests, taxes, product prices, operating costs, investments, salvage
values, abandonment costs, and net profit interests, as applicable.
 
    Net income to the evaluated interests  is the future net revenue payable  to
others,  taxes,  operating  expenses, investments,  salvage  values, abandonment
costs, and net profit interests, as applicable. The future net revenue is before
federal income tax and  excludes consideration of  any encumbrances against  the
properties if such exist.
 
    No  opinion is expressed  by Williamson as  to the fair  market value of the
evaluated properties.
 
                                      A-6
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 7
 
    The future  net revenues  presented in  this summary  letter were  based  on
projections  of  oil  and gas  production.  It  was assumed  there  would  be no
significant delay between the date of oil and gas production and the receipt  of
the associated revenue for this production.
 
    This  summary  letter  includes  only those  costs  and  revenues  which are
considered by  Costilla to  be directly  attributable to  individual leases  and
areas.  There  could  exist  other  revenues,  overhead  costs,  or  other costs
associated with Costilla  which are not  included in this  summary letter.  Such
additional costs and revenues are outside the scope of this summary letter. This
summary  letter is not a financial statement for Costilla and should not be used
as the sole basis for any transaction concerning Costilla, Parker & Parsley,  or
the evaluated properties.
 
    The  reserves projections in this summary letter are based on the use of the
available data and accepted industry engineering methods. Future changes in  any
operational   or  economic  parameters  or  production  characteristics  of  the
evaluated properties  could  increase  or decrease  their  reserves.  Unforeseen
changes  in market demand or allowables set by various regulatory agencies could
also cause actual production  rates to vary from  those projected. The dates  of
first production for nonproducing properties were based on estimates by Costilla
or  Williamson and  the actual dates  may vary from  those estimated. Williamson
reserves the right to alter any  of the reserves projections and the  associated
economics  included in  this summary  letter in  any future  evaluation based on
additional data that may be acquired.
 
    All data utilized in the preparation of this summary letter with respect  to
interests, reversionary status, oil and gas prices, gas categories, gas contract
terms,  operating expenses, investments, salvage  values, abandonment costs, net
profit  interests,  well  information  and  current  operating  conditions,   as
applicable, were provided by Costilla, Parker & Parsley, and the operators. Data
obtained  after the effective date of the  report but prior to the completion of
the report were used only if such  data were applied consistently. If such  data
were  used, the reserves category assignments reflect the status of the wells as
of the effective date. In the Costilla report, daily production data after April
1, 1996 were utilized  for new wells  in the South  Buffalo Ridge, Concho  Bluff
(Queen),  East  Goldsmith (Queen),  King  Mountain (Penn),  and  Talbot (Canyon)
fields to  assist in  determining  initial producing  and decline  rates.  Daily
production  since the effective date was also used  for the Pyote Gas Unit 5 No.
1A, Block 16  (Devonian) field, Ward  County, Texas to  establish the  producing
rate  after the well was affected by gas  plant problems and for the State 16-05
well in the  Raymond field, Sheridan  County, Montana to  establish the  initial
rate of production subsequent to the installation of a downhole pump. Production
data  generally through December  1995 or January 1996  provided by Costilla for
the properties in  the Costilla  report and  through November  or December  1995
provided  by Parker & Parsley for the  properties in the Acquisition report were
utilized. All data  have been  reviewed for reasonableness  and, unless  obvious
errors  were detected,  have been accepted  as correct. It  should be emphasized
that revisions to  the projections of  reserves and economics  included in  this
summary  letter may be required if the provided data are revised for any reason.
No inspection of the properties was made  as this was not considered within  the
scope  of  these  projects.  No  investigation  was  made  of  any environmental
liabilities that  might apply  to the  evaluated properties,  and no  costs  are
included for any possible related expenses.
 
    Unless  specifically  identified  and  documented by  Costilla  or  Parker &
Parsley as having curtailment problems, gas production trends have been  assumed
to  be a function of well productivity  and not of market conditions. The effect
of "take or pay" clauses in gas contracts was not considered.
 
                                      A-7
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 8
 
    Oil reserves  are expressed  in  United States  (U.S.)  barrels of  42  U.S.
gallons.  Gas  volumes are  expressed in  thousands  of cubic  feet (MCF)  at 60
degrees Fahrenheit and at the legal pressure base that prevails in the state  in
which the reserves are located. No adjustment of the individual gas volumes to a
common pressure base has been made.
 
    Costilla  represented  to  Williamson  that it  has,  or  can  generate, the
financial and operational capabilities to accomplish those projects evaluated by
Williamson which require capital expenditures.
 
    The estimates of reserves contained  in this summary letter were  determined
by  accepted industry methods and in accordance  with the definitions of oil and
gas reserves set forth  above. Methods utilized in  this summary letter  include
extrapolation  of historical production trends, material balance determinations,
analogy to similar properties, and volumetric calculations.
 
    Where sufficient production history and other data were available,  reserves
for   producing  properties  were  determined  by  extrapolation  of  historical
production trends or through the use of material balance determinations. Analogy
to similar  properties or  volumetric calculations  were used  for  nonproducing
properties  and those  producing properties  which lacked  sufficient production
history and other  data to  yield a  definitive estimate  of reserves.  Reserves
projections  based on analogy are subject to change due to subsequent changes in
the analogous properties or subsequent production from the evaluated properties.
Volumetric calculations are often  based upon limited  log and/or core  analysis
data and incomplete reservoir fluid and formation rock data. Since these limited
data  must frequently be extrapolated over  an assumed drainage area, subsequent
production performance trends  or material  balance calculations  may cause  the
need for significant revisions to the estimates of reserves.
 
    It  should  be  emphasized  that with  the  current  economic uncertainties,
fluctuation in market  conditions could  significantly change  the economics  in
this summary letter.
 
VII.  DECLARATION OF INDEPENDENT STATUS AND CONSENT
 
    We  understand  that our  estimates  are to  be  included in  a Registration
Statement on Form S-1 (the Registration Statement) to be filed with the SEC  and
in  the  Prospectus as  included in  such Registration  Statement which  will be
registered under the Securities Act of 1933, as amended.
 
    Williamson is an independent consulting firm and does not own any  interests
in the oil and gas properties covered by this summary letter. Roy C. Williamson,
Jr.,  Chief Executive Officer, owns a 2.5  percent working interest in six wells
in the Outlook  field, Sheridan  County, Montana, which  have a  total value  of
$138,912  to  the interests  of Costilla.  No employee,  officer or  director of
Williamson is an employee, officer or director of Costilla or Parker &  Parsley.
Neither  the  employment  of  nor the  compensation  received  by  Williamson is
contingent upon the  values assigned to  the oil and  gas properties covered  by
this summary letter.
 
    We  consent  to the  inclusion of  this summary  letter in  the Registration
Statement, the inclusion in  the Registration Statement  of data extracted  from
this  summary  letter and  to  all references  to  our firm  in  the Prospectus,
including any references to our firm as Experts.
 
                                    Yours very truly,
 
                                    WILLIAMSON PETROLEUM CONSULTANTS, INC.
 
                                      A-8
<PAGE>
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    NO  DEALER, SALESMAN  OR ANY  OTHER PERSON HAS  BEEN AUTHORIZED  TO GIVE ANY
INFORMATION OR TO MAKE  ANY REPRESENTATIONS OTHER THAN  THOSE CONTAINED IN  THIS
PROSPECTUS  IN CONNECTION WITH THE OFFERING  COVERED BY THIS PROSPECTUS, AND, IF
GIVEN OR MADE, SUCH  INFORMATION OR REPRESENTATIONS MUST  NOT BE RELIED UPON  AS
HAVING  BEEN AUTHORIZED BY THE COMPANY OR THE UNDERWRITERS. THIS PROSPECTUS DOES
NOT CONSTITUTE AN  OFFER TO  SELL OR  THE SOLICITATION OF  AN OFFER  TO BUY  THE
SECURITIES  IN ANY JURISDICTION WHERE, OR ANY  PERSON TO WHOM, IT IS UNLAWFUL TO
MAKE SUCH OFFER OR SOLICITATION. NEITHER THE DELIVERY OF THIS PROSPECTUS NOR ANY
SALE MADE HEREUNDER SHALL, UNDER  ANY CIRCUMSTANCES, CREATE AN IMPLICATION  THAT
THERE  HAS NOT BEEN A CHANGE IN THE FACTS SET FORTH IN THIS PROSPECTUS OR IN THE
AFFAIRS OF THE COMPANY SINCE THE DATE HEREOF.
 
    UNTIL OCTOBER 28, 1996, ALL DEALERS EFFECTING TRANSACTIONS IN THE REGISTERED
SECURITIES, WHETHER OR NOT PARTICIPATING  IN THIS DISTRIBUTION, MAY BE  REQUIRED
TO  DELIVER A PROSPECTUS.  THIS IS IN  ADDITION TO THE  OBLIGATION OF DEALERS TO
DELIVER
A PROSPECTUS WHEN ACTING AS UNDERWRITERS AND
WITH RESPECT TO THEIR UNSOLD ALLOTMENTS OR SUBSCRIPTIONS.
                            ------------------------
 
                               TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                                    PAGE
                                                    -----
<S>                                              <C>
Prospectus Summary.............................           3
Risk Factors...................................          10
The Company....................................          16
Common Stock Offering..........................          16
Use of Proceeds................................          17
Capitalization.................................          18
Pro Forma Condensed Financial Statements.......          19
Selected Financial Information.................          27
Management's Discussion and Analysis of
 Financial Condition and Results of
 Operations....................................          28
Business and Properties........................          34
Management.....................................          46
Security Ownership of Certain Beneficial Owners
 and Management................................          49
Executive Compensation and Other Information...          50
Certain Transactions...........................          53
Description of Notes...........................          55
Description of Other Indebtedness..............          82
Description of Capital Stock...................          82
Underwriting...................................          84
Legal Matters..................................          85
Experts........................................          85
Available Information..........................          85
Glossary.......................................          87
Index to Financial Statements..................         F-1
Summary Reserve Report.........................         A-1
</TABLE>
 
                                     [LOGO]
 
                             COSTILLA ENERGY, INC.
 
                          $100,000,000 10 1/4% SENIOR
                                 NOTES DUE 2006
 
                             ---------------------
 
                                   PROSPECTUS
 
                             ---------------------
 
                       NATIONSBANC CAPITAL MARKETS, INC.
 
                       PRUDENTIAL SECURITIES INCORPORATED
 
                                OCTOBER 2, 1996
 
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