COSTILLA ENERGY INC
S-1/A, 1996-10-02
CRUDE PETROLEUM & NATURAL GAS
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<PAGE>
   
    AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON OCTOBER 2, 1996
    
 
                                                      REGISTRATION NO. 333-08913
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
 
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                           --------------------------
 
   
                                AMENDMENT NO. 3
                                       TO
                                    FORM S-1
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
    
                            ------------------------
 
                             COSTILLA ENERGY, INC.
             (Exact name of Registrant as specified in its charter)
 
<TABLE>
<S>                              <C>                             <C>
            DELAWARE                           1311                     75-2658940
 (State or other jurisdiction of   (Primary Standard Industrial      (I.R.S. Employer
 incorporation or organization)    Classification Code Number)     Identification No.)
</TABLE>
 
                           --------------------------
 
                         400 WEST ILLINOIS, SUITE 1000
                              MIDLAND, TEXAS 79701
                                 (915) 683-3092
              (Address, including zip code, and telephone number,
       including area code, of Registrant's principal executive offices)
 
                          MICHAEL J. GRELLA, PRESIDENT
                             COSTILLA ENERGY, INC.
                         400 WEST ILLINOIS, SUITE 1000
                              MIDLAND, TEXAS 79701
                                 (915) 683-3092
           (Name, address, including zip code, and telephone number,
                   including area code, of agent for service)
                           --------------------------
 
                                   COPIES TO:
 
<TABLE>
<S>                                                 <C>
               Richard T. McMillan                                   R. Joel Swanson
         Cotton, Bledsoe, Tighe & Dawson,                         Baker & Botts, L.L.P.
            a Professional Corporation                                910 Louisiana
                500 West Illinois                                  Houston, Texas 77002
                    Suite 300
               Midland, Texas 79701
</TABLE>
 
                           --------------------------
 
    APPROXIMATE  DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: As soon as
practicable after this Registration Statement becomes effective.
 
    If any of the securities being registered on this Form are to be offered  on
a  delayed or continuous basis pursuant to  Rule 415 under the Securities Act of
1933, check the following box: / /
 
    If this Form  is filed  to register  additional securities  for an  offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and  list  the  Securities  Act registration  statement  number  of  the earlier
effective registration statement for the same offering. / /
 
    If this Form  is a post-effective  amendment filed pursuant  to Rule  462(c)
under  the Securities Act, check  the following box and  list the Securities Act
registration statement number  of the earlier  effective registration  statement
for the same offering. / /
 
    If  delivery of the Prospectus is expected  to be made pursuant to Rule 434,
please check the following box. / /
                           --------------------------
 
    THE REGISTRANT HEREBY  AMENDS THIS  REGISTRATION STATEMENT ON  SUCH DATE  OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE  A  FURTHER  AMENDMENT  WHICH SPECIFICALLY  STATES  THAT  THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE  IN ACCORDANCE WITH SECTION 8(A)  OF
THE  SECURITIES ACT  OF 1933  OR UNTIL  THE REGISTRATION  STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID SECTION  8(A),
MAY DETERMINE.
 
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
<PAGE>
INFORMATION   CONTAINED  HEREIN  IS  SUBJECT   TO  COMPLETION  OR  AMENDMENT.  A
REGISTRATION STATEMENT  RELATING TO  THESE SECURITIES  HAS BEEN  FILED WITH  THE
SECURITIES  AND EXCHANGE  COMMISSION. THESE SECURITIES  MAY NOT BE  SOLD NOR ANY
OFFERS TO BUY BE ACCEPTED PRIOR  TO THE TIME THE REGISTRATION STATEMENT  BECOMES
EFFECTIVE.  THIS  PROSPECTUS  SHALL  NOT  CONSTITUTE AN  OFFER  TO  SELL  OR THE
SOLICITATION OF ANY OFFER TO BUY NOR SHALL THERE BE ANY SALE OF THESE SECURITIES
IN ANY STATE IN WHICH SUCH OFFER,  SOLICITATION OR SALE WOULD BE UNLAWFUL  PRIOR
TO REGISTRATION OR QUALIFICATION UNDER THE SECURITIES LAWS OF ANY SUCH STATE.
<PAGE>
   
                  SUBJECT TO COMPLETION--DATED OCTOBER 2, 1996
    
 
PROSPECTUS
- --------------------------------------------------------------------------------
 
                                4,000,000 Shares
 
[LOGO]                       COSTILLA ENERGY, INC.
 
                                  Common Stock
- -------------------------------------------------------------------------
 
All  of the shares  of the Common  Stock, $0.10 par  value (the "Common Stock"),
offered hereby (the "Common Stock Offering") are being sold by Costilla  Energy,
Inc., a Delaware corporation ("Costilla" or the "Company").
 
Concurrent  with the Common Stock Offering, the Company is offering $100,000,000
of     % Senior Notes due 2006 (the "Notes") for sale to the public (the  "Notes
Offering,"  and  together  with  the  Common  Stock  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.
 
Prior  to this Common  Stock Offering, there  has been no  public market for the
Common Stock of the Company. It is currently anticipated that the initial public
offering price will be between $14.00  and $16.00 per share. See  "Underwriting"
for  a discussion  of the  factors to be  considered in  determining the initial
public offering price. The Company has applied for inclusion of the Common Stock
in The  Nasdaq Stock  Market's National  Market (the  "Nasdaq National  Market")
under the trading symbol "COSE."
 
SEE  "RISK FACTORS" ON PAGES  8 TO 12 FOR A  DISCUSSION OF MATERIAL FACTORS THAT
SHOULD BE  CONSIDERED IN  CONNECTION  WITH AN  INVESTMENT  IN THE  COMMON  STOCK
OFFERED HEREBY.
- --------------------------------------------------------------------------------
 
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  THIS  PROSPECTUS.  ANY
       REPRESENTATION   TO   THE   CONTRARY   IS   A   CRIMINAL  OFFENSE.
 
<TABLE>
<CAPTION>
                                                                  Underwriting
                                                   Price to       Discounts and     Proceeds to
                                                    Public       Commissions (1)    Company (2)
<S>                                             <C>              <C>              <C>
Per Share.....................................         $                $                $
Total (3).....................................         $                $                $
</TABLE>
 
(1) The Company has agreed to indemnify the several Underwriters against certain
    liabilities, including liabilities  under the  Securities Act  of 1933.  See
    "Underwriting."
 
(2) Before deducting expenses payable by the Company estimated to be $500,000.
 
(3)  The Company  has granted the  several Underwriters  a 30-day over-allotment
    option to purchase up  to 600,000 additional shares  of Common Stock on  the
    same  terms and conditions as set forth above. If all such additional shares
    are purchased  by  the Underwriters,  the  total  Price to  Public  will  be
    $      , the total Underwriting Discounts and Commissions will be $      and
    the total Proceeds to Company will be $      . See "Underwriting."
- --------------------------------------------------------------------------------
 
The  shares of Common Stock  are offered by the  several Underwriters subject to
delivery by the Company and acceptance by the Underwriters, to prior sale and to
withdrawal, cancellation or modification of  the offer without notice.  Delivery
of  the shares  to the  Underwriters is  expected to  be made  at the  office of
Prudential Securities Incorporated, One New York  Plaza, New York, New York,  on
or about             , 1996.
 
PRUDENTIAL SECURITIES INCORPORATED  RAUSCHER PIERCE REFSNES, INC.
 
   
October   , 1996
    
<PAGE>
                            PRIMARY OPERATING AREAS
 
               [A GEOGRAPHIC MAP INDICATING WHERE THE COMPANY HAS
                      OIL AND GAS PROPERTIES AND OFFICES]
 
                            ------------------------
 
    IN  CONNECTION  WITH  THIS  COMMON  STOCK  OFFERING,  THE  UNDERWRITERS  MAY
OVER-ALLOT OR EFFECT TRANSACTIONS WHICH  STABILIZE OR MAINTAIN THE MARKET  PRICE
OF  THE COMMON STOCK OFFERED HEREBY AT  A LEVEL ABOVE THAT WHICH MIGHT OTHERWISE
PREVAIL  IN  THE  OPEN  MARKET.   SUCH  STABILIZATION,  IF  COMMENCED,  MAY   BE
DISCONTINUED AT ANY TIME.
 
                                       2
<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 WILL NOT BE EXERCISED. CERTAIN OIL AND GAS TERMS USED IN THIS  PROSPECTUS
ARE  DEFINED  IN  THE "GLOSSARY"  INCLUDED  HEREIN. "ADJUSTED  EBITDA,"  AS USED
HEREIN, MEANS  NET INCOME  (LOSS), PLUS  INTEREST, INCOME  TAXES,  DEPRECIATION,
DEPLETION  AND AMORTIZATION, EXPLORATION AND ABANDONMENT COSTS AND EXTRAORDINARY
LOSS RESULTING 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,
production  and  Adjusted EBITDA.  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%. Adjusted EBITDA increased at
a 234% compound annual growth rate from  $1.8 million for 1993 to $20.2  million
for 1995 (pro forma for the 1995 Acquisition and the 1996 Acquisition).
    
 
                                       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).
 
                            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.
 
                                       4
<PAGE>
    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.
 
                           THE COMMON STOCK OFFERING
 
<TABLE>
<S>                                              <C>
Common Stock Offered by the Company............  4,000,000 Shares(1)
 
Common Stock to be Outstanding after the
 Offering......................................  10,000,000 Shares(1)
 
Concurrent Notes Offering......................  Concurrent  with the Common Stock Offering,
                                                 the Company  is  offering  $100,000,000  of
                                                     %  Senior  Notes due  2006.  See "Notes
                                                 Offering."  The  Notes  Offering  and   the
                                                 Common  Stock Offering are each conditioned
                                                 upon the consummation of the other.
 
Use of Proceeds................................  To  repay  existing  indebtedness,  to  pay
                                                 certain  costs incurred  in connection with
                                                 the  Corporate   Reorganization,  and   for
                                                 general  corporate  purposes.  See  "Use of
                                                 Proceeds."
 
Nasdaq National Market Symbol..................  COSE
</TABLE>
 
- ------------------------
(1) Excludes 1,000,000 shares of  Common Stock reserved  for issuance under  the
    Company's  employee  benefit plans.  See  "Executive Compensation  and Other
    Information."
 
                                  RISK FACTORS
 
    Investors should consider the material  risk factors involved in  connection
with  an investment in the Common Stock,  including the fact that investors will
experience immediate and  substantial dilution  in pro forma  net tangible  book
value  per share of $11.86 (assuming an  initial public offering price of $15.00
per share,  the midpoint  of the  price range  set forth  on the  cover of  this
Prospectus)  and  the  impact  to  investors  from  various  events  which could
adversely affect the Company's business. See "Risk Factors."
 
                                       5
<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>
                                                                                                  SIX MONTHS ENDED
                                                       YEAR ENDED DECEMBER 31,                        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      11,635       1,046      4,156      5,817
    Other..................................         --         --          2           2          --         --         --
                                             ---------  ---------  ---------  -----------  ---------  ---------  ---------
  Net income (loss) before income taxes and
   extraordinary item......................         50        203     (4,967)     (8,380)     (1,924)      (646)      (910)
  Net income (loss)........................         73        163     (4,970)     (8,383)     (1,924)    (2,286)      (910)
  Pro forma earnings (loss) per common
   share...................................         --         --         --       (0.85)         --         --      (0.09)
  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        1.7x        1.4x       2.0x       2.1x
  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
</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  $8,380,000  and $910,000  for the  pro  forma
     periods ended December 31, 1995 and June 30, 1996, respectively.
    
 
                                       6
<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 (6 Mcf/Bbl).........................................      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.
 
                                       7
<PAGE>
   
                                  RISK FACTORS
    
 
    This   Prospectus   contains  certain   forward-looking   statements.  These
statements include, among  others, statements  regarding oil  and gas  reserves,
future  drilling and operations, future production of oil and gas and future net
cash flows. Actual results could differ  materially from those projected in  the
forward-looking  statements as a  result of various  factors, including, without
limitation, those set forth  below and elsewhere  in this Prospectus.  Investors
should   carefully  consider  the  following   factors,  in  addition  to  other
information contained in this  Prospectus, in connection  with an investment  in
the shares of Common Stock offered hereby.
 
    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  Certain  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
Certain 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 (the
"Indenture") 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 $8.4 million and $0.9 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.
    
 
    RESTRICTIONS IMPOSED BY LENDERS.  The instruments governing the indebtedness
of the Company and its  subsidiaries impose significant operating and  financial
restrictions  on the Company. The terms of the Indenture and the Credit Facility
affect, and  in  many respects  significantly  limit or  prohibit,  among  other
things,  the  ability  of  the Company  to  incur  additional  indebtedness, pay
dividends, repay  indebtedness prior  to  its stated  maturity, sell  assets  or
engage  in  mergers or  acquisitions. These  restrictions  could also  limit the
ability  of  the  Company  to  effect  future  financing,  make  needed  capital
expenditures,  withstand  a future  downturn in  the  Company's business  or the
economy in  general,  or otherwise  conduct  necessary corporate  activities.  A
failure by the Company to comply with these restrictions could lead to a default
under the terms
 
                                       8
<PAGE>
of  such indebtedness. In the event of default, the holders of such indebtedness
could elect to declare all of the funds borrowed pursuant thereto to be due  and
payable  together with accrued and unpaid interest.  In such event, there can be
no assurance that  the Company would  be able  to make such  payments or  borrow
sufficient  funds from  alternative sources  to make  any such  payment. Even if
additional financing could be obtained, there can be no assurance that it  would
be  on terms that are  favorable or acceptable to  the Company. In addition, the
Company's indebtedness under its  Credit Facility is expected  to be secured  by
substantially  all of the assets of the Company and its subsidiaries. The pledge
of such collateral  to existing lenders  could impair the  Company's ability  to
obtain additional financing. See "Description of Certain Indebtedness."
 
    POTENTIAL  INABILITY  TO FUND  A  CHANGE OF  CONTROL  OFFER.   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. If a Change of
Control occurs, there is no assurance that the Company will have available funds
sufficient to  pay  for  the Notes  tendered  for  repurchase. If  an  offer  to
repurchase  is required to be made and the Company does not have available funds
sufficient to pay for Notes tendered  for repurchase, an event of default  would
occur under the Indenture. The occurrence of an event of default could result in
the  acceleration of  maturity of  the Notes  and of  all amounts  due under the
Credit Facility. In addition,  the Change of Control  covenant in the  Indenture
could  have the effect of  discouraging a takeover of  the Company by making any
such 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
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  shares
of  Common Stock offered hereby 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
 
                                       9
<PAGE>
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.
 
    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.
 
    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."
 
    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."
 
    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."
 
                                       10
<PAGE>
    CONTROL OF THE COMPANY.   If the Offerings  are completed, Messrs.  Liedtke,
Grella  and Musselman will own directly  and indirectly, in the aggregate, 49.2%
of the outstanding Common  Stock (or 46.4%  if the Underwriters'  over-allotment
option  in the Common Stock Offering is exercised in full). Accordingly, Messrs.
Liedtke, Grella and  Musselman will  be able to  exercise significant  influence
over  the  election of  the  directors of  the Company  and  the control  of the
Company's management,  operations and  affairs. The  voting power  held by  such
principal  stockholders and their ability to exercise significant influence over
the election of directors may have  the effect of discouraging certain types  of
transactions  involving an actual or potential change of control of the Company,
including transactions  in which  the holders  of Common  Stock might  otherwise
receive  a  premium  for  their  shares over  then  current  market  prices. See
"Security  Ownership   of  Certain   Beneficial  Owners   and  Management"   and
"Description of Capital Stock."
 
    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.
 
    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 FOR THE  COMMON STOCK AND  POSSIBLE VOLATILITY OF
STOCK PRICE.   Prior to  the Common  Stock Offering,  there has  been no  public
market for the Common Stock of the Company and there can be no assurance that an
active  trading  market will  develop  or be  sustained  after the  Common Stock
Offering, or that the market  price of the Common  Stock will not decline  below
the  initial public  offering price.  The initial  public offering  price of the
Common Stock will be determined through negotiations between the Company and the
Underwriters, and may  bear no relationship  to the market  price of the  Common
Stock  after the  Common Stock 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 Common Stock to fluctuate substantially. See "Underwriting."
 
                                       11
<PAGE>
    DILUTION.   Purchasers  of the Common  Stock offered  hereby will experience
immediate and substantial dilution in the pro forma net tangible book value  per
share  of Common Stock from  the initial public offering  price set forth on the
cover of this  Prospectus. Such  dilution to new  investors will  be $11.86  per
share (assuming an initial public offering price of $15.00 per share), while the
pro  forma net tangible  book value of the  shares of Common  Stock owned by the
existing stockholders will increase by $5.77 per share. See "Dilution."
 
    NO INTENTION  TO  PAY DIVIDENDS.    The  Company intends  to  retain  future
earnings  for  use in  its  business and  does  not anticipate  paying  any cash
dividends in the foreseeable  future. In addition, the  payment of dividends  by
the Company is limited and restricted by the Credit Facility and under the terms
of  the Indenture. See  "-- Restrictions Imposed  by Lenders," "Dividend Policy"
and "Description of Certain Indebtedness."
 
    ANTI-TAKEOVER EFFECT OF CERTAIN PROVISIONS  OF THE COMPANY'S CERTIFICATE  OF
INCORPORATION  AND BYLAWS. The Company's Certificate of Incorporation and bylaws
provide that (i) the Company's Board of Directors will be divided into  classes,
with  the directors of each class serving staggered terms of three years each or
until their respective successors are elected and qualified, and (ii) the  Board
of  Directors may issue serial preferred  stock with such rights and preferences
as the Board may determine,  without stockholder approval. These provisions  may
have  the effect, either  alone or in  combination, of making  more difficult or
discouraging an acquisition  or potential  acquisition of the  Company which  is
deemed  undesirable by the Board of Directors. See "Description of Capital Stock
- -- Anti-Takeover Provisions."
 
   
    SHARES ELIGIBLE FOR  FUTURE SALE.   Sales of substantial  amounts of  Common
Stock  on  the public  market after  the Common  Stock Offering  could adversely
affect the market price of the Common Stock. Upon completion of the Common Stock
Offering, the Company  will have a  total of 10,000,000  shares of Common  Stock
outstanding.  Of  these shares,  the 4,000,000  shares  of Common  Stock offered
hereby (4,600,000 shares if the Underwriters' over-allotment option is exercised
in full) will be freely tradeable without restriction or registration under  the
Securities Act of 1933 (the "Securities Act") by persons other than "affiliates"
of  the Company,  as defined under  the Securities Act.  The remaining 6,000,000
shares of Common Stock outstanding will be "restricted securities" as that  term
is  defined by  Rule 144  as promulgated under  the Securities  Act. The Company
currently has  eight  options outstanding  for  550,000 shares.  See  "Executive
Compensation and Other Information."
    
 
    Under  Rule 144  (and subject  to the  conditions thereof,  including volume
limitations), approximately 4,919,992  of the 6,000,000  restricted shares  will
become  eligible for sale 90 days after the Common Stock Offering. All 6,000,000
of the restricted shares are subject to lockup restrictions as described  below.
The  holders of these  shares, which include certain  of the Company's executive
officers and directors and NationsBanc Capital Corp. ("NBCC"), have agreed  that
they  will not, directly or indirectly, offer,  sell, offer to sell, contract to
sell, pledge, grant  any option  to purchase or  otherwise sell  or dispose  (or
announce  any offer, sale, offer of sale, contract to sell, pledge, grant of any
options to purchase or  sale or disposition)  of any shares  of Common Stock  or
other  capital  stock of  the Company,  or any  securities convertible  into, or
exercisable or exchangeable  for, any shares  of Common Stock  or other  capital
stock  of the Company without the prior written consent of Prudential Securities
Incorporated, on behalf of the Underwriters, for  a period of 180 days from  the
date of this Prospectus. After such 180-day period, this restriction will expire
and  shares permitted to be  sold under Rule 144 would  be eligible for sale. In
addition, NBCC has demand registration rights to require the Company to file  up
to  two registration statements to effect  the registration under the Securities
Act of the shares of  Common Stock held by such  holder, and the other  majority
stockholders  have  "piggyback"  registration rights,  which  would  permit such
holders to resell such shares without complying with Rule 144. Registration  and
sale  of such shares  could have an adverse  effect on the  trading price of the
Common Stock.  See "Description  of Capital  Stock --  Registration Rights"  and
"Shares Eligible for Future Sale."
 
    Prior  to the Common Stock Offering, there has been no public market for the
Common Stock and no predictions can be made of the effect, if any, that the sale
or availability for sale of additional shares  of Common Stock will have on  the
market  price of the Common Stock. Nevertheless, sales of substantial amounts of
such shares in the public market, or the perception that such sales could occur,
could materially and adversely affect the  market price of the Common Stock  and
could  impair the Company's future ability  to raise capital through an offering
of its equity securities.
 
                                       12
<PAGE>
                                  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 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
6,000,000  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.
 
                                 NOTES OFFERING
 
    Concurrent  with  the  Common  Stock  Offering,  the  Company  is   offering
$100,000,000  of     % Senior Notes  due 2006. The Notes Offering and the Common
Stock Offering are each conditioned upon the consummation of the other.
 
                                       13
<PAGE>
                                USE OF PROCEEDS
 
    The net proceeds to the Company from the Common Stock Offering are  expected
to  be $55.3 million, based upon an  initial public offering price of $15.00 per
share after  deducting  underwriting  discounts and  commissions  and  estimated
offering  expenses  to the  Company ($63.7  million  if the  Underwriters' over-
allotment option is exercised). The net  proceeds to the Company from the  Notes
Offering are estimated to be $96.2 million. 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 Certain  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>
 
                                DIVIDEND POLICY
 
    The Company intends to  retain future earnings for  use in its business  and
does  not anticipate declaring  or paying any cash  dividends in the foreseeable
future. The terms of the Credit Facility are expected to prohibit the payment of
dividends by the Company.  In addition, the  Indenture also contains  provisions
restricting the payment of dividends (or other restricted payments) generally to
(i)  50% of consolidated net income of the Company (less 100% of losses) for the
period commencing the first  day of the  month during which  the closing of  the
Offerings  occurs to the most  recently ended fiscal quarter,  plus (ii) 100% of
certain  equity  sales  after  the  date  of  the  Indenture.  Subject  to   the
restrictions  imposed  by the  Company's  lenders, future  dividend  policy will
depend on a number of factors, including future earnings, capital  requirements,
the  financial  condition and  future prospects  of the  Company and  such other
factors as the Board of Directors may deem relevant.
 
                                       14
<PAGE>
                                    DILUTION
 
   
    Purchasers of Common Stock offered  hereby will experience an immediate  and
substantial dilution in the net tangible book value of the Common Stock from the
initial public offering price. At June 30, 1996, the net tangible book value per
share  of Common Stock of the Company, on  a pro forma basis after giving effect
to the issuance of 6,000,000 shares in the Merger and the redemption of  certain
membership interests in the LLC was ($2.64). Such amount does not give effect to
the  Offerings or to  the distributions to  members of the  LLC in the Corporate
Reorganization. Net tangible book value per  share represents the amount of  the
Company's  tangible book value  (total book value of  tangible assets less total
liabilities) divided by the total number of shares of Common Stock  outstanding.
After  giving effect to the receipt of  $55.3 million of estimated proceeds from
the Common Stock Offering, the issuance of  the Notes in the Notes Offering  and
the  completion of the Corporate Reorganization,  the net tangible book value of
the Common Stock outstanding at June 30,  1996 would have been $31.4 million  or
$3.14  per share, representing an immediate  increase in net tangible book value
of approximately  $5.78  per share  to  current stockholders  and  an  immediate
dilution  of $11.86 per share (the difference between the assumed initial public
offering price and the net tangible book value per share after the Offerings) to
persons purchasing Common Stock  at the assumed  initial public offering  price.
The following table illustrates such per share dilution:
    
 
   
<TABLE>
<S>                                                           <C>        <C>
Assumed initial public offering price.......................             $   15.00
  Net tangible book value before Offerings..................  $   (2.64)
  Increase in net tangible book value attributable to sale
   of Common Stock in the Common Stock Offering.............       5.78
                                                              ---------
Net tangible book value after giving effect to the
 Offerings..................................................             $    3.14
                                                                         ---------
Dilution in net tangible book value to new investors........             $   11.86
                                                                         ---------
                                                                         ---------
</TABLE>
    
 
   
    Messrs.  Liedtke,  Grella and  Musselman  will receive  4,919,992  shares of
Common Stock in the Corporate Reorganization. These shares of Common Stock  will
be issued to such persons in exchange for their membership interests in the LLC.
The  effective  cash  cost  of  these  shares  to  Messrs.  Liedtke,  Grella and
Musselman, on a  per share  basis, is not  significant. In  February 1995,  upon
formation of the LLC, NBCC contributed $10,000,000 in cash in exchange for a 30%
membership  interest in the LLC, forty percent of which was redeemable and sixty
percent was  non-redeemable. In  the Corporate  Reorganization, such  redeemable
interest will be redeemed and such non-redeemable interest will be exchanged for
1,080,008  shares of Common Stock issued to NBCC. As discussed in Note 10 of the
Notes to the Consolidated Financial Statements, the value of such non-redeemable
interest, and thus the effective cash cost  of the shares of Common Stock to  be
issued to NBCC in the Corporate Reorganization, is deemed to be $1,266,000.
    
 
                                       15
<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, assuming an initial public offering price for the Common Stock in
the  Common Stock Offering  of $15.00 per  share. 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 Certain 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.........................................................................          --          --
     % 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>
    
 
                                       16
<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.
 
                                       17
<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.
 
                                       18
<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
                                                              1995         1996        PRO FORMA      COSTILLA       OFFERING
                                          COSTILLA 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)
                                                                                                                       11,635(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..............................      11,635
 
  Other.................................           2
                                          -----------
                                              61,017
                                          -----------
Net income (loss) before federal income
 taxes..................................      (8,380)
Provision for federal income taxes......           3
                                          -----------
Net income (loss).......................   $  (8,383)
                                          -----------
                                          -----------
Net loss per share......................   $    (.85)
                                          -----------
                                          -----------
</TABLE>
    
 
 See accompanying notes to unaudited pro forma condensed financial statements.
 
                                       19
<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
                                                                1996         PRO FORMA      PRE OFFERING      OFFERING
                                           COSTILLA L.L.C.   ACQUISITION    ADJUSTMENTS    COSTILLA L.L.C.   ADJUSTMENTS
                                           ---------------  -------------  --------------  ---------------  -------------
<S>                                        <C>              <C>            <C>             <C>              <C>
Revenues.................................     $  19,525       $   9,223                       $  28,748
Expenses:
  Oil and gas production.................         8,278           5,167     $    (150)(1)        13,295
  General and administrative.............         2,809                           (86)(1)         3,010
                                                                                  287(5)
  Exploration and abandonments...........           308             247                             555
  Depreciation, depletion and
   amortization..........................         4,620                            50(1)          6,981
                                                                                2,311(6)
  Interest...............................         4,156                         2,832(7)          6,988      $  (6,988)(8)
                                                                                                                 5,817(8)
                                                -------          ------                         -------
                                                 20,171           5,414                          30,829
                                                -------          ------                         -------
Net income (loss) before federal income
 taxes...................................          (646)          3,809                          (2,081)
                                                -------          ------                         -------
Net income (loss)........................     $    (646)      $   3,809                       $  (2,081)
                                                -------          ------                         -------
                                                -------          ------                         -------
Net income (loss) per share..............
 
<CAPTION>
                                            PRO FORMA
                                            COSTILLA
                                             ENERGY,
                                              INC.
                                           -----------
<S>                                        <C>
Revenues.................................   $  28,748
Expenses:
  Oil and gas production.................      13,295
  General and administrative.............       3,010
 
  Exploration and abandonments...........         555
  Depreciation, depletion and
   amortization..........................       6,981
 
  Interest...............................       5,817
 
                                           -----------
                                               29,658
                                           -----------
Net income (loss) before federal income
 taxes...................................        (910)
                                           -----------
Net income (loss)........................   $    (910)
                                           -----------
                                           -----------
Net income (loss) per share..............   $   (0.09)
                                           -----------
                                           -----------
</TABLE>
 
 See accompanying notes to unaudited pro forma condensed financial statements.
 
                                       20
<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  4,000,000  shares of  Common Stock  at an
estimated price of $15.00 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.
 
                                       21
<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. A 1/8% change in interest rate on
the variable portion  of such borrowings  would result in  a $115,000 change  in
annual interest expense.
 
    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 11.25%  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 1,080,008 shares of  Common Stock to NBCC  on
February 17, 1995 and the pro forma issuance of 4,919,992 shares of Common Stock
to  the remaining holders prior to January 1, 1995. In addition, the issuance of
4,000,000 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. 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
 
                                       22
<PAGE>
                             COSTILLA ENERGY, INC.
    NOTES TO UNAUDITED PRO FORMA CONDENSED FINANCIAL STATEMENTS (CONTINUED)
 
NOTE 5. -- SUPPLEMENTAL OIL AND GAS RESERVE INFORMATION (CONTINUED)
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   probable    reserves,
 
                                       23
<PAGE>
                             COSTILLA ENERGY, INC.
    NOTES TO UNAUDITED PRO FORMA CONDENSED FINANCIAL STATEMENTS (CONTINUED)
 
NOTE 5. -- SUPPLEMENTAL OIL AND GAS RESERVE INFORMATION (CONTINUED)
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>
 
                                       24
<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.
    
 
                                       25
<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,
production and Adjusted EBITDA, 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.
 
                                       26
<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 sales 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 for the
years ended December 31, 1994 and 1995,  and the six months ended June 30,  1995
and  1996, respectively, 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
$12,300,000  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 $658,000 (12%) 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 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.
    
 
                                       27
<PAGE>
    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 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 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  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.
 
                                       28
<PAGE>
    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  (223%).
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  the  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.
 
    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.
 
                                       29
<PAGE>
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
acquisition 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 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  it 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."
 
    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  semi-annually. 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
 
                                       30
<PAGE>
   
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 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 2.1 to 1 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
obligations of the  Company under the  Credit Facility. Initially,  none of  the
Company's  subsidiaries will guarantee the obligations  of the Company 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
                                                                 YEAR 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.
 
                                       31
<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 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 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,
production and  Adjusted  EBITDA. 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%. Adjusted EBITDA increased at
a  234% compound annual growth rate from  $1.8 million for 1993 to $20.2 million
for 1995 (pro forma for the 1995 Acquisition and the 1996 Acquisition).
    
 
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
 
                                       32
<PAGE>
      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).
 
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.
 
                                       33
<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  approximate 17-square mile seismic survey  on
the  project. A second phase will be initiated in the first quarter of 1997. The
 
                                       34
<PAGE>
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 to a major oil company.  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%.
 
    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
 
                                       35
<PAGE>
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.
 
    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
 
                                       36
<PAGE>
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 revenue. 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.
 
                                       37
<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 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.
 
                                       38
<PAGE>
    In accordance with  applicable requirements of  the Securities and  Exchange
Commission (the "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.
 
                                       39
<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
                                                  ---------  ---------  ---------  ---------  ---------  ---------
                                                    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 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.
 
    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.
 
    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.
 
                                       40
<PAGE>
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.
 
    The 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,  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
 
                                       41
<PAGE>
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 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.
 
                                       42
<PAGE>
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 Certain 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.
 
                                       43
<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
Landman's 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's
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 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
 
                                       44
<PAGE>
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  and a member of the American  Association
of  Petroleum Landmen  and the Petroleum  Basin Landman's  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. 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,
 
                                       45
<PAGE>
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.
 
                                       46
<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 ..................          2,656,796              26.6%
400 W. Illinois
Midland, Texas 79701
Michael J. Grella ..................          1,558,161              15.6%
400 W. Illinois
Midland, Texas 79705
NationsBanc Capital Corp. ..........          1,080,008              10.8%
100 North Tryon Street
Charlotte, North Carolina 28255
Henry G. Musselman .................            705,035               7.0%
400 W. Illinois
Midland, Texas 79701
</TABLE>
 
- ------------------------
(1) All persons own the listed shares of record.
 
    The table  appearing  below 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...................          2,656,796              26.6%
Michael J. Grella...................          1,558,161              15.6%
Henry G. Musselman..................            705,035               7.0%
Bobby W. Page.......................             75,000(2)            0.7%
All Officers and Directors as a
group (13 persons)..................          5,469,992(3)           51.8%
</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.
    
 
                                       47
<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 Chief       1995    185,700         --             --              --             --
   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 Financial        1995(1)        --        --            --              --             --
   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 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 employment
 
                                       48
<PAGE>
other than for  cause. However,  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
subsequent to 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 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.
 
                                       49
<PAGE>
    1996  STOCK OPTION PLAN.   The 1996 Employee 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  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 will be treated as compensation
by the Company for federal income tax purposes. The Board of Directors may amend
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
 
                                       50
<PAGE>
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  and  advisors  of the  Company  or a
subsidiary of the Company and  all employees of the  Company or a subsidiary  of
the Company who have completed a minimum of 180 days of service and are employed
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 Meter Sales and Services, Inc.  ("A&P"), a corporation in which  Messrs.
Liedtke,  Grella  and  Musselman  own 60.0%  of  the  outstanding  common stock,
supplies meter reading services which measure 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 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.
 
                                       51
<PAGE>
   
    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 6,000,000
shares of Common Stock in the Merger in exchange for their interests in the LLC.
    
 
                      DESCRIPTION OF CERTAIN 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% at per  annum 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, a  guaranty 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
 
                                       52
<PAGE>
   
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  such  obligations.  Messrs.  Liedtke,  Grella and
Musselman will not  guarantee indebtedness  due under the  Credit Facility.  See
"Management's  Discussion  and Analysis  of Financial  Condition and  Results of
Operations -- Liquidity and Capital Resources" for a further description of  the
Credit Facility.
    
 
THE NOTES
 
    The  Company is  concurrently offering  $100 million  in aggregate principal
amount of the Notes by means of a separate prospectus. The closing of the Common
Stock Offering and the Notes Offering are conditioned upon each other.
 
    The Notes will be general unsecured obligations of the Company, will be pari
passu in right of payment to all existing and future senior indebtedness of  the
Company  and  will be  senior  in right  of  payment to  all  other subordinated
indebtedness of the Company. The  Notes will bear interest payable  semiannually
at  a rate to be determined at the pricing of the Notes Offering and will mature
in 2006.
 
    The Notes will not be redeemable  until               2001. Thereafter,  the
Notes will be redeemable at any time at the option of the Company in whole or in
part  at the  redemption prices  set forth  in the  Indenture. There  will be no
mandatory sinking  fund  for the  Notes.  Upon the  occurrence  of a  change  of
control, the Company will be required to make an offer to purchase all the Notes
at 101% of the principal amount thereof, plus accrued and unpaid interest to the
date of purchase.
 
    The Indenture will contain certain covenants that, among other things, limit
the  ability of the Company and certain  of its subsidiaries to pay dividends or
make certain other restricted payments, issue preferred stock, incur  additional
indebtedness,  enter into transactions  with affiliates, incur  liens, engage in
certain sale and  leaseback arrangements,  make certain  asset dispositions  and
merge  or consolidate with, or transfer substantially  all of its assets to, any
other person. The Indenture will also  contain certain covenants that limit  the
ability  of certain of  the Company's subsidiaries to  restrict their ability to
make payments to  the Company. The  Indenture will contain  customary events  of
default.
 
                          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 10,600,000 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.
 
COMMON STOCK
 
    Except as  otherwise  required by  law,  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.
 
                                       53
<PAGE>
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.  The Indenture
governing the Notes limits the issuance of Preferred Stock in some instances.
 
    While  providing   desirable  flexibility   in  connection   with   possible
acquisitions  and other  corporate purposes,  and eliminating  delays associated
with a stockholder vote on specific  issuances, the issuance of Preferred  Stock
could  adversely affect  the voting  power of  holders of  Common Stock  and the
likelihood that such holders  will receive dividend  payments and payments  upon
liquidation  and could  have the effect  of delaying, deferring  or preventing a
change in control of the Company.
 
REGISTRATION RIGHTS
 
    The Company  has  agreed that,  upon  the request  of  NBCC, on  up  to  two
occasions,  the Company  will register under  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  the Affiliated Holders the opportunity to include shares of the Common Stock
owned by  them in  certain  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 by the Company by the Affiliated Holders participating  in
the  registration. The Company is obligated to pay all expenses incident to such
registration, except  underwriting 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.
 
ANTI-TAKEOVER PROVISIONS
 
    The Certificate of Incorporation and Bylaws of the Company and the  Delaware
General  Corporation Law (the  "DGCL") include a number  of provisions which may
have the effect of encouraging persons considering unsolicited tender offers  or
other  unilateral takeover  proposals to negotiate  with the  Board of Directors
rather than pursue non-negotiated takeover attempts. These provisions include  a
classified   board  of  directors,  authorized   blank  check  preferred  stock,
restrictions on business  combinations and  the availability  of authorized  but
unissued Common Stock.
 
    CLASSIFIED  BOARD OF DIRECTORS.   The Company's Certificate of Incorporation
and Bylaws contain provisions for a board of directors divided into classes with
only one class standing  for election each year.  Directors can only be  removed
for  cause. A staggered board makes it more difficult for stockholders to change
the majority of  the directors  and instead  promotes a  continuity of  existing
management.
 
    BLANK  CHECK PREFERRED STOCK.   The Certificate  of Incorporation authorizes
blank check Preferred Stock.  See "-- Preferred Stock."  The Board of  Directors
can set the voting rights, redemption rights, conversion rights and other rights
relating  to such Preferred Stock and could issue such stock in either a private
or public transaction. In  some circumstances, the  blank check Preferred  Stock
could  be issued  and have the  effect of  preventing a merger,  tender offer or
other takeover attempt which the Board of Directors opposes.
 
DELAWARE TAKEOVER STATUTE
 
    The Company is subject to Section 203  of the DGCL. In general, Section  203
prevents  an "interested stockholder" from  engaging in a "business combination"
with a  Delaware corporation  for three  years following  the date  such  person
became  an  interested stockholder,  unless (i)  prior to  the date  such person
became an  interested stockholder,  the board  of directors  of the  corporation
approved   the  transaction  in  which  the  interested  stockholder  became  an
interested  stockholder  or  approved   the  business  combination;  (ii)   upon
consummation  of the transaction  that resulted in  the interested stockholder's
becoming an
 
                                       54
<PAGE>
interested stockholder,  the interested  stockholder owns  at least  85% of  the
voting  stock  of  the  corporation  outstanding  at  the  time  the transaction
commenced, excluding  stock held  by  directors who  are  also officers  of  the
corporation  and stock  held by  certain employee  stock plans;  or (iii)  on or
subsequent to  the  date of  the  transaction in  which  such person  became  an
interested  stockholder, the  business combination is  approved by  the board of
directors of the corporation and authorized at a meeting of the stockholders  by
the  affirmative vote  of the  holders of  two-thirds of  the outstanding voting
stock of the corporation not owned by the interested stockholder.
 
    Section 203 defines a  "business combination" to include  (i) any merger  or
consolidation  involving the corporation and an interested stockholder; (ii) any
sale, transfer, pledge or other disposition involving an interested  stockholder
of  10%  or more  of the  assets of  the corporation;  (iii) subject  to certain
exceptions, any transaction  which results in  the issuance or  transfer by  the
corporation  of any stock of the  corporation to an interested stockholder; (iv)
any transaction involving the corporation which has the effect of increasing the
proportionate share  of the  stock of  any class  or series  of the  corporation
beneficially  owned  by the  interested  stockholder or  (v)  the receipt  by an
interested stockholder  of any  loans, guarantees,  pledges or  other  financial
benefits  provided by or  through the corporation. For  purposes of Section 203,
the term "corporation" also includes the Company's majority-owned  subsidiaries.
In  addition, Section  203 defines an  "interested stockholder" as  an entity or
person beneficially owning 15%  or more of the  outstanding voting stock of  the
corporation  and  any  entity  or  person  affiliated  with  or  controlling  or
controlled by such entity or person.
 
STOCKHOLDER ACTION
 
    With respect to  any act  or action  required of or  by the  holders of  the
Common  Stock, the affirmative vote of a  majority of the shares of Common Stock
present in person  or represented by  proxy at  a meeting and  entitled to  vote
thereon  is sufficient to  authorize, affirm, ratify  or consent to  such act or
actions,  except  as  otherwise  provided  by  law  or  in  the  Certificate  of
Incorporation.  The DGCL requires the  approval of the holders  of a majority of
the outstanding  stock  entitled to  vote  for certain  extraordinary  corporate
transactions, such as a merger, sale of substantially all assets, dissolution or
amendment of the Certificate of Incorporation.
 
    Pursuant  to Delaware law, stockholders may take actions without the holding
of a  meeting  by  written consent  or  consents  signed by  the  holders  of  a
sufficient  number  of  shares  to  approve  the  transaction  had  all  of  the
outstanding shares of the capital stock of the Company entitled to vote  thereon
been  present  at  a meeting.  The  Affiliated  Holders will  own  49.2%  of the
outstanding Common Stock after the Offering. See "Security Ownership of  Certain
Beneficial  Owners and Management." Pursuant to the rules and regulations of the
Commission, if stockholder action is taken by written consent, the Company  will
be  required to send each  stockholder entitled to vote  on the matter acted on,
but  whose  consent  was  not  received,  an  information  statement  containing
information  substantially similar to that which  would have been contained in a
proxy statement.
 
TRANSFER AGENT AND REGISTRAR
 
    The transfer agent  and registrar  for the Common  Stock will  be The  First
National Bank of Boston, 100 Federal Street, Boston, Massachusetts 02110.
 
                                       55
<PAGE>
                        SHARES ELIGIBLE FOR FUTURE SALE
 
    Upon  completion of the Common Stock Offering, the Company will have a total
of 10,000,000 shares of Common Stock outstanding. Of these shares, the 4,000,000
shares of Common  Stock offered  hereby (4,600,000 shares  if the  Underwriters'
over-allotment  option is  exercised in  full) will  be freely  tradable without
restriction or  registration under  the  Securities Act  by persons  other  than
"affiliates"  of the Company, as defined under the Securities Act. The remaining
6,000,000 shares of Common Stock outstanding will be "restricted securities"  as
that term is defined by Rule 144 as promulgated under the Securities Act.
 
    In  general, under  Rule 144  as currently in  effect, a  person (or persons
whose shares are  aggregated) who has  beneficially owned restricted  securities
for  at least two years, including persons who may be deemed "affiliates" of the
Company, would be  entitled to sell  within any three-month  period a number  of
shares  that does not exceed the greater of  one percent of the number of shares
of Common Stock then outstanding  (approximately 100,000 shares upon  completion
of the Common Stock Offering) or the average weekly trading volume of the Common
Stock  during the four  calendar weeks preceding  the filing of  a Form 144 with
respect to such sale. Sales under Rule 144 are also subject to certain manner of
sale provisions  and notice  requirements, and  to the  availability of  current
public information about the Company. In addition, a person who is not deemed to
have been an affiliate of the Company at any time during the 90 days preceding a
sale, and who has beneficially owned the shares proposed to be sold for at least
three  years, would be  entitled to sell  such shares under  Rule 144(k) without
regard to the requirements described above.
 
   
    Under Rule 144 (and subject to the conditions thereof, including the  volume
limitations   described  above),   approximately  4,919,992   of  the  6,000,000
restricted shares will become eligible for  sale 90 days after the Common  Stock
Offering.  All  6,000,000  of  the  restricted  shares  are  subject  to  lockup
restrictions. Pursuant to  these restrictions  the holders  of these  restricted
shares, including certain of its executive officers and directors and NBCC, have
agreed   that  they  will  not,  subject  to  certain  exceptions,  directly  or
indirectly, offer, sell,  offer to  sell, contract  to sell,  pledge, grant  any
option  to purchase or otherwise  sell or dispose (or  announce any offer, sale,
offer of sale, contract  to sell, pledge,  grant of any  options to purchase  or
sale or disposition) of any shares of Common Stock or other capital stock of the
Company, or any securities convertible into, or exercisable or exchangeable for,
any  shares of Common  Stock or other  capital stock of  the Company without the
prior written consent of  Prudential Securities Incorporated,  on behalf of  the
Underwriters,  for a period of 180 days  from the date of this Prospectus. After
such 180-day period,  this restriction will  expire and shares  permitted to  be
sold  under Rule 144  would be eligible  for sale. In  addition, NBCC has demand
registration rights  to require  the  Company to  file  up to  two  registration
statements  under the Securities Act of the  shares of Common Stock held by such
holder and NBCC and the  other major stockholders have "piggyback"  registration
rights,  which would permit such holders to resell such shares without complying
with Rule 144. Registration and sale of such shares could have an adverse effect
on the trading price of the Common Stock. The Company currently has eight  stock
options  outstanding for 550,000  shares. See "Executive  Compensation and Other
Information."
    
 
    Prior to the Common Stock Offering, there has been no public market for  the
Common Stock and no predictions can be made of the effect, if any, that the sale
or  availability for sale of shares of  additional Common Stock will have on the
market price of the Common Stock. Nevertheless, sales of substantial amounts  of
such shares in the public market, or the perception that such sales could occur,
could  materially and adversely affect the market  price of the Common Stock and
could impair the Company's future ability  to raise capital through an  offering
of its equity securities.
 
                                       56
<PAGE>
                                  UNDERWRITING
 
    The  Underwriters  named  below (the  "Underwriters"),  for  whom Prudential
Securities  Incorporated  and  Rauscher  Pierce  Refsnes,  Inc.  are  acting  as
Representatives,  have  severally agreed,  subject to  the terms  and conditions
contained in the Underwriting Agreement, to purchase from the Company the number
of shares of Common Stock set forth below opposite their respective names:
 
<TABLE>
<CAPTION>
                                                                                           NUMBER
            UNDERWRITER                                                                  OF SHARES
- ---------------------------------------------------------------------------------------  ----------
<S>                                                                                      <C>
Prudential Securities Incorporated.....................................................
Rauscher Pierce Refsnes, Inc...........................................................
 
                                                                                         ----------
  Total................................................................................   4,000,000
                                                                                         ----------
                                                                                         ----------
</TABLE>
 
    The Company is  obligated to  sell, and  the Underwriters  are obligated  to
purchase, all of the shares of Common Stock offered hereby if any are purchased.
 
    The Underwriters, through the Representatives, have advised the Company that
they  propose  to offer  the  shares of  Common  Stock initially  at  the public
offering price  set  forth  on the  cover  page  of this  Prospectus;  that  the
Underwriters may allow to selected dealers a concession of $      per share; and
that  such dealers may reallow a concession of $      per share to certain other
dealers.  After  the  initial  public  offering,  the  offering  price  and  the
concessions may be changed by the Representatives.
 
    The  Company has granted  to the Underwriters an  option, exercisable for 30
days from the  date of  this Prospectus, to  purchase up  to 600,000  additional
shares  of Common Stock  at the initial public  offering price less underwriting
discounts and commissions, as  set forth on the  cover page of this  Prospectus.
The  Underwriters may  exercise such option  solely for the  purpose of covering
over-allotments incurred  in the  sale of  the shares  of Common  Stock  offered
hereby.  To the  extent such option  to purchase is  exercised, each Underwriter
will become obligated, subject to certain conditions, to purchase  approximately
the  same percentage of such additional shares as the number of shares set forth
opposite each Underwriters' name in the preceding table bears to 4,000,000.
 
    The Company, its executive officers and directors, and all of the  Company's
stockholders  have agreed  that they  will not,  directly or  indirectly, offer,
sell, offer to sell, contract to sell, pledge, grant any option to purchase,  or
otherwise  sell  or dispose  of (or  announce  any offer,  sale, offer  of sale,
contract of sale,  pledge, grant  of any  option to  purchase or  other sale  or
disposition)  of any  shares of  Common Stock of  the Company  or any securities
convertible into, or exercisable or exchangeable for, any shares of Common Stock
or other  capital stock  of the  Company without  the prior  written consent  of
Prudential  Securities Incorporated, on behalf of the Underwriters, for a period
of 180 days after the date of this Prospectus.
 
    The  Company  has  agreed  to  indemnify  the  several  Underwriters  or  to
contribute  to losses arising out  of certain liabilities, including liabilities
under the Securities Act.
 
    The Representatives have informed the  Company that the Underwriters do  not
intend  to confirm sales to any  accounts over which they exercise discretionary
authority.
 
                                       57
<PAGE>
    Prior to the Common Stock Offering, there has been no public market for  the
Common Stock of the Company. Consequently, the initial public offering price for
the  Common Stock will be determined through negotiation between the Company and
the Representatives of the Underwriters. Among  the factors to be considered  in
making  such determination will be the prevailing market conditions, the results
of operations of the Company in recent periods relevant to its prospects and the
prospects for its  industry in general,  the management of  the Company and  the
market  prices of securities for companies in  businesses similar to that of the
Company.
 
    PGI, an affiliate of Prudential  Securities Incorporated, is a lender  under
the  Existing  Debt Facility.  See  "Description of  Certain  Indebtedness." The
lenders under the Existing Debt Facility  will receive customary fees and  other
compensation.  PGI will receive its proportionate  share of the repayment by the
Company of borrowings under the Existing Debt Facility from the net proceeds  of
the  Notes Offering.  Prudential Securities  Incorporated is  also acting  as an
underwriter in the Company's concurrent Notes Offering for which it will receive
customary underwriting discounts and commissions.
 
                                 LEGAL MATTERS
 
    Certain legal matters related to the  Common Stock 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
 
                                       58
<PAGE>
file reports, proxy statements and  other information with 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 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  Common Stock 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 Common  Stock 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.
 
                                       59
<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 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.
 
                                       60
<PAGE>
    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.
 
                                       61
<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-22
  Statements of Revenues and Direct Operating Expenses for the years ended December
   31, 1993 and 1994 and the period ended June 12, 1995..............................       F-23
  Notes to the Statements of Revenues and Direct Operating Expenses..................       F-24
 
Financial Statements of the 1996 Acquisition:
  Independent Auditors' Report.......................................................       F-27
  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-28
  Notes to the Statements of Revenues and Direct Operating Expenses..................       F-29
</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  to  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,  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.
 
                                      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)
    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
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.
 
                                      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)
    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, 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>
 
                                      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)
- ------------------------
(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>
 
(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.
 
                                      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 (CONTINUED)
    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.
 
(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
 
                                      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 (CONTINUED)
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.
 
    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.
 
                                      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.)
 
(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.
 
    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
 
                                      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.)
 
(9) 401(K) PLAN (CONTINUED)
   
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 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
</TABLE>
 
                                      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)
<TABLE>
<S>                                                                 <C>
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  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 the 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.
 
                                      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.)
 
(12) SUBSEQUENT EVENTS (CONTINUED)
    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%.
 
    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.
    
 
                                      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.)
 
(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.
 
    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
 
                                      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)
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>
 
    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.
 
                                      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)
    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>
 
    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-21
<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-22
<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-23
<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-24
<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-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)
    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-26
<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-27
<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-28
<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 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-29
<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)
    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.
 
    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>
 
                                      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)
    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-31
<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 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
 
                                      A-1
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 2
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 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
 
- ------------------------
(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
 
    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 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.
 
- ------------------------
(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
 
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 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.
 
                                      A-4
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 5
 
    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  (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
 
                                      A-5
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 6
    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.
 
    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
 
                                      A-6
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 7
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.
 
    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
 
                                      A-7
<PAGE>
Costilla Energy, Inc.
Mr. Michael J. Grella
July 23, 1996
Page 8
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>
- -------------------------------------------------
                               -------------------------------------------------
- -------------------------------------------------
                               -------------------------------------------------
 
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 OFFER MADE 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 ANY OF  THE UNDERWRITERS. THIS PROSPECTUS DOES NOT
CONSTITUTE AN OFFER TO SELL OR THE SOLICITATION OF ANY OFFER TO BUY ANY SECURITY
OTHER THAN THE SHARES OF  COMMON STOCK OFFERED BY  THIS PROSPECTUS, NOR DOES  IT
CONSTITUTE  AN OFFER TO SELL OR A SOLICITATION OF ANY OFFER TO BUY THE SHARES OF
COMMON STOCK BY ANYONE IN ANY  JURISDICTION IN WHICH SUCH OFFER OR  SOLICITATION
IS  NOT AUTHORIZED, OR IN WHICH THE  PERSON MAKING SUCH OFFER OR SOLICITATION IS
NOT QUALIFIED TO DO  SO, OR TO 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  ANY  IMPLICATION  THAT THE
INFORMATION CONTAINED HEREIN IS  CORRECT AS OF ANY  TIME SUBSEQUENT TO THE  DATE
HEREOF.
 
UNTIL               , 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.
                              --------------------
 
                                ----------------
                              P R O S P E C T U S
 
                              --------------------
                               TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                                               PAGE
                                                               ----
<S>                                                     <C>
Prospectus Summary....................................           3
Risk Factors..........................................           8
The Company...........................................          13
Notes Offering........................................          13
Use of Proceeds.......................................          14
Dividend Policy.......................................          14
Dilution..............................................          15
Capitalization........................................          16
Pro Forma Condensed Financial Statements..............          17
Selected Financial Information........................          25
Management's Discussion and Analysis of Financial
 Condition and Results of Operations..................          26
Business and Properties...............................          32
Management............................................          44
Security Ownership of Certain Beneficial Owners and
 Management...........................................          47
Executive Compensation and Other Information..........          48
Certain Transactions..................................          51
Description of Certain Indebtedness...................          52
Description of Capital Stock..........................          53
Shares Eligible for Future Sale.......................          56
Underwriting..........................................          57
Legal Matters.........................................          58
Experts...............................................          58
Available Information.................................          58
Glossary..............................................          60
Index to Financial Statements.........................         F-1
Summary Reserve Report................................         A-1
</TABLE>
 
                                4,000,000 Shares
 
                                     [logo]
 
                             COSTILLA ENERGY, INC.
 
                                  Common Stock
 
                       PRUDENTIAL SECURITIES INCORPORATED
 
                         RAUSCHER PIERCE REFSNES, INC.
 
                               September   , 1996
 
- -------------------------------------------------
                               -------------------------------------------------
- -------------------------------------------------
                               -------------------------------------------------
<PAGE>
                                    PART II
 
                     INFORMATION NOT REQUIRED IN PROSPECTUS
 
ITEM 13.  OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.
 
<TABLE>
<S>                                                                 <C>
SEC registration fee..............................................  $  20,690
NASD filing fee...................................................      7,860
Nasdaq listing fee................................................     42,500
Blue Sky fees and expenses........................................     15,000
Accounting fees and expenses......................................     65,625
Engineering fees and expenses.....................................     68,250
Transfer Agent fees and expenses..................................      5,000
Legal fees and expenses...........................................     93,750
Printing and mailing expenses.....................................    175,000
Miscellaneous.....................................................      6,325
                                                                    ---------
      TOTAL.......................................................    500,000
                                                                    ---------
                                                                    ---------
</TABLE>
 
ITEM 14.  INDEMNIFICATION OF DIRECTORS AND OFFICERS.
 
    Section  145 of the General Corporation Law of the State of Delaware permits
a corporation to indemnify certain persons, including officers and directors and
former officers  and  directors,  and  to purchase  insurance  with  respect  to
liability  arising out  of their capacity  or status as  officers and directors.
Such law provides  further that the  indemnification permitted thereunder  shall
not  be deemed exclusive of any other rights to which officers and directors may
be entitled under the corporation's bylaws, any agreement or otherwise.  Article
IX  of  the  Company's Certificate  of  Incorporation, included  in  Exhibit 3.1
hereto, and Article VI of the Company's Bylaws, included in Exhibit 3.2  hereto,
provide, in general, that the Company shall indemnify its directors and officers
under the circumstances defined in Section 145 of the General Corporation Law of
the  State of Delaware and gives authority  to the Company to purchase insurance
with respect to  such indemnification.  The Company may  in the  future seek  to
obtain  insurance providing for indemnification of officers and directors of the
Company and certain other persons  against liabilities and expenses incurred  by
any of them in certain stated proceedings and under certain stated conditions.
 
    In  addition, Section 102(b)(7) of the  General Corporation Law of the State
of Delaware  permits a  corporation  to limit  the  liability of  its  directors
subject  to certain exceptions. In accordance with Section 102(b)(7), Article VI
of the Company's Certificate of  Incorporation, included in Exhibit 3.1  hereto,
provides, in general, that no director of the Company shall be personally liable
for  (i) any  breach of  the director's duty  of loyalty  to the  Company or its
stockholders, (ii)  acts  or  omissions  not in  good  faith  or  which  involve
intentional misconduct or a knowing violation of law, (iii) unlawful payments of
dividends  or unlawful stock  repurchases or redemptions  as provided in Section
174 of  the  General Corporation  Law  of the  State  of Delaware  or  (iv)  any
transaction from which the director derived an improper personal benefit.
 
    The  Underwriting Agreement provides for indemnification by the Underwriters
of the Registrant,  its directors  and officers, and  by the  Registrant of  the
Underwriters,  for certain liabilities, including  liabilities arising under the
Securities Act of 1933 (the "Securities Act").
 
ITEM 15.  RECENT SALES OF UNREGISTERED SECURITIES.
 
    Prior to the consummation of the Offerings, the Company issued an  aggregate
of  3 shares  of Common Stock  to Messrs.  Liedtke, Grella and  Musselman in its
initial capitalization,  which  shares were  cancelled  in connection  with  the
Corporate  Reorganization, and an aggregate of  6,000,000 shares of Common Stock
to the four  members of  the LLC  in the merger  of the  LLC with  and into  the
Company.  Such shares were  not registered under the  Securities Act in reliance
upon the exemption from registration provided by Section 4(2) thereof.
 
                                      II-1
<PAGE>
ITEM 16.  EXHIBITS AND FINANCIAL STATEMENT SCHEDULES.
 
    (a) Exhibits
 
   
<TABLE>
<CAPTION>
 EXHIBIT
  NUMBER                                    DESCRIPTION OF EXHIBIT
- ----------  --------------------------------------------------------------------------------------
<C>         <S>
     *1.1   Form of Underwriting Agreement
   ***3.1   Certificate of Incorporation of the Company
   ***3.2   Bylaws of the Company
   ***4.1   Form of Notes or Global Certificate (included as Exhibit A to the form of Indenture
            filed as Exhibit Number 4.2 to the Registration Statement on Form S-1, File No.
            333-08909, filed with respect to the Notes)
   ***4.2   Form of Indenture
     *4.3   Form of Stock Certificate
     *5.1   Opinion of Cotton, Bledsoe, Tighe & Dawson, a Professional Corporation
  ***10.1   Commitment Letter dated September 20, 1996 between NationsBank of Texas, N.A. and the
            Company
  ***10.2   Lease Agreement dated January 12, 1996 between Independence Plaza, Ltd. and Costilla
            Energy, L.L.C.
  ***10.3   Concession Agreement dated July 6, 1995 between the Government of the Republic of
            Moldova and the Resource Development Company, Limited
  ***10.4   Purchase and Joint Exploration Agreement dated February 21, 1996 between the Company
            and Resources Development Limited, L.L.C. (DE)
  ***10.5   Form of Consolidation Agreement to be effective contemporaneously with closing of the
            Offerings to consummate the Corporate Reorganization
  ***10.6   Form of 1996 Stock Option Plan
  ***10.7   Form of Outside Directors Stock Option Plan
  ***10.8   Employment Agreement between the Company and Bobby W. Page effective June 30, 1996
  ***10.9   Employment Agreement between the Company and Cadell S. Liedtke to be effective
            contemporaneously with the closing of the Offerings
  ***10.10  Employment Agreement between the Company and Michael J. Grella to be effective
            contemporaneously with the closing of the Offerings
  ***10.11  Employment Agreement between the Company and Henry G. Musselman to be effective
            contemporaneously with the closing of the Offerings
  ***10.12  Exchange Agreement dated January 5, 1995 between Costilla Petroleum Corporation and
            Koch Oil Company
  ***10.13  Agreement dated January 2, 1996 between Costilla Petroleum Corporation and Frontier
            Oil and Refining Company
  ***10.14  Purchase and Sale Agreement dated April 3, 1995 by and between Parker & Parsley
            Development L.P. and Parker & Parsley Producing L.P. and Parker & Parsley Gas
            Processing Co. as Seller and Costilla Petroleum Corporation and Costilla Energy,
            L.L.C. as Purchaser
  ***10.15  Purchase and Sale Agreement dated March 8, 1996 by and between Parker & Parsley
            Development L.P. and Parker & Parsley Producing L.P. and Parker & Parsley Gas
            Processing Co. as Seller and Costilla Petroleum Corporation and Costilla Energy,
            L.L.C. as Purchaser
  ***10.16  Supplemental Agreement to Purchase and Joint Exploration Agreement dated August 7,
            1996 among the Company, Costilla Redeco Energy, L.L.C. and Resource Development
            Company Limited, L.L.C. (DE)
</TABLE>
    
 
                                      II-2
<PAGE>
<TABLE>
<CAPTION>
 EXHIBIT
  NUMBER                                    DESCRIPTION OF EXHIBIT
- ----------  --------------------------------------------------------------------------------------
<C>         <S>
  ***10.17  Form of Bonus Incentive Plan
  ***12.1   Computation of Ratio of Adjusted EBITDA to Interest Expense
  ***12.2   Computation of Ratio of Earnings to Fixed Charges
    *16.1   Letter Regarding Change of Accountants
  ***21.1   Subsidiaries of the Registrant
   **23.1   Consent of KPMG Peat Marwick LLP
    *23.2   Consent of Williamson Petroleum Consultants, Inc.
   **23.3   Consent of Elms, Faris & Co., P.C.
    *23.4   Consent of Cotton, Bledsoe, Tighe & Dawson, a Professional Corporation (such consent
            is included in the opinion filed as Exhibit 5.1 to this Registration Statement)
    *24.1   Power of Attorney
    *24.2   Certified copy of resolution of Board of Directors of Costilla Energy, Inc.
            authorizing signature pursuant to Power of Attorney
    *27.1   Financial Data Schedule
</TABLE>
 
- ------------------------
*   Previously filed
**  Filed herewith
*** Incorporated by reference to Registration Statement on Form S-1, File No.
    333-08909, filed with respect to the Notes
 
    (b) Financial Statement Schedules.
 
ITEM 17.  UNDERTAKINGS
    Insofar as indemnification for liabilities arising under the Securities  Act
may  be  permitted  to  directors,  officers  and  controlling  persons  of  the
Registrant pursuant  to  the  provisions  described  under  Item  14  above,  or
otherwise, the Registrant has been advised that in the opinion of the Securities
and  Exchange  Commission,  such  indemnification is  against  public  policy as
expressed in the Securities Act and  is, therefore, unenforceable. In the  event
that  a  claim  for indemnification  against  such liabilities  (other  than the
payment by the Registrant of expenses incurred or paid by a director, officer of
controlling person of the  Registrant in the successful  defense of any  action,
suit or proceeding) is asserted by such directors, officer or controlling person
in  connection with the securities being registered, the Registrant will, unless
in the  opinion  of its  counsel  the matter  has  been settled  by  controlling
precedent,  submit to a  court of appropriate  jurisdiction the question whether
such indemnification  by  it  is  against public  policy  as  expressed  in  the
Securities Act and will be governed by the final adjudication of such issue.
 
    The   undersigned  Registrant  hereby  undertakes   that,  for  purposes  of
determining any liability under the Securities Act, (i) the information  omitted
from  the Prospectus  filed as part  of this Registration  Statement in reliance
upon Rule 430A under the  Securities Act and contained  in a form of  Prospectus
filed  by the Registrant pursuant  to Rule 424(b)(1) or  (4) or 497(h) under the
Securities Act shall be deemed to be part of this Registrant Statement as of the
time it  was declared  effective  and (ii)  each post-effective  amendment  that
contains a form of prospectus shall be deemed to be a new Registration Statement
relating  to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.
 
    The undersigned registrant hereby undertakes to provide to the Underwriters,
at the closing  specified in  the Underwriting Agreement,  certificates in  such
denominations  and registered in  such names as required  by the Underwriters to
permit prompt delivery to each purchaser.
 
                                      II-3
<PAGE>
                                   SIGNATURES
 
   
    Pursuant  to the requirements of the  Securities Act of 1933, the Registrant
has duly caused this Registration  Statement to be signed  on its behalf by  the
undersigned,  thereunto duly authorized in the  City of Midland, State of Texas,
on October 2, 1996.
    
 
                                          COSTILLA ENERGY, INC.
                                          (Registrant)
 
                                          By:                  *
 
                                             -----------------------------------
                                                      Michael J. Grella
                                                PRESIDENT AND CHIEF OPERATING
                                                           OFFICER
 
    Pursuant  to  the  requirements  of   the  Securities  Act  of  1933,   this
Registration Statement has been signed by the following persons on behalf of the
Registrant and in the capacities and on the dates indicated.
 
   
             SIGNATURE                         TITLE                  DATE
- -----------------------------------  -------------------------  ----------------
 
                     *               Chairman of the Board,
- -----------------------------------   Chief Executive Officer   October 2, 1996
         Cadell S. Liedtke            and Director
 
                     *               President, Chief
- -----------------------------------   Operating Officer and     October 2, 1996
         Michael J. Grella            Director
 
                     *
- -----------------------------------  Executive Vice President   October 2, 1996
        Henry G. Musselman            and Director
 
         /s/ BOBBY W. PAGE           Senior Vice Present,
- -----------------------------------   Treasurer and Chief       October 2, 1996
           Bobby W. Page              Financial Officer
 
                     *
- -----------------------------------  Director                   October 2, 1996
         Jerry J. Langdon
 
                     *
- -----------------------------------  Director                   October 2, 1996
           W.D. Kennedy
 
*By:           /s/ BOBBY W. PAGE
- -----------------------------------
           Bobby W. Page
         ATTORNEY-IN-FACT
 
    
 
                                      II-4
<PAGE>
                               INDEX TO EXHIBITS
 
   
<TABLE>
<CAPTION>
 EXHIBIT                                                                                               SEQUENTIALLY
  NUMBER                                     DESCRIPTION OF EXHIBIT                                    NUMBERED PAGE
- ----------  ----------------------------------------------------------------------------------------  ---------------
<C>         <S>                                                                                       <C>
     *1.1   Form of Underwriting Agreement
   ***3.1   Certificate of Incorporation of the Company
   ***3.2   Bylaws of the Company
   ***4.1   Form  of Notes  or Global Certificate  (included as Exhibit  A to the  form of Indenture
             filed as  Exhibit Number  4.2  to the  Registration Statement  on  Form S-1,  File  No.
             333-08909, filed with respect to the Notes)
   ***4.2   Form of Indenture
     *4.3   Form of Stock Certificate
     *5.1   Opinion of Cotton, Bledsoe, Tighe & Dawson, a Professional Corporation
  ***10.1   Commitment  Letter dated September 20,  1996 between NationsBank of  Texas, N.A. and the
             Company
  ***10.2   Lease Agreement dated  January 12, 1996  between Independence Plaza,  Ltd. and  Costilla
             Energy, L.L.C.
  ***10.3   Concession  Agreement  dated July  6, 1995  between  the Government  of the  Republic of
             Moldova and the Resource Development Company, Limited
  ***10.4   Purchase and Joint Exploration Agreement dated February 21, 1996 between the Company and
             Resources Development Limited, L.L.C. (DE)
  ***10.5   Form of Consolidation Agreement  to be effective contemporaneously  with closing of  the
             Offerings to consummate the Corporate Reorganization
  ***10.6   Form of 1996 Stock Option Plan
  ***10.7   Form of Outside Directors Stock Option Plan
  ***10.8   Employment Agreement between the Company and Bobby W. Page effective June 30, 1996
  ***10.9   Employment  Agreement  between  the  Company  and  Cadell  S.  Liedtke  to  be effective
             contemporaneously with the closing of the Offerings
  ***10.10  Employment Agreement  between  the  Company  and  Michael  J.  Grella  to  be  effective
             contemporaneously with the closing of the Offerings
  ***10.11  Employment  Agreement  between  the  Company  and Henry  G.  Musselman  to  be effective
             contemporaneously with the closing of the Offerings
  ***10.12  Exchange Agreement dated January 5, 1995 between Costilla Petroleum Corporation and Koch
             Oil Company
  ***10.13  Agreement dated January 2, 1996 between Costilla Petroleum Corporation and Frontier  Oil
             and Refining Company
  ***10.14  Purchase  and  Sale  Agreement dated  April  3, 1995  by  and between  Parker  & Parsley
             Development L.P.  and  Parker  &  Parsley  Producing L.P.  and  Parker  &  Parsley  Gas
             Processing Co. as Seller and Costilla Petroleum Corporation and Costilla Energy, L.L.C.
             as Purchaser
  ***10.15  Purchase  and  Sale  Agreement dated  March  8, 1996  by  and between  Parker  & Parsley
             Development L.P.  and  Parker  &  Parsley  Producing L.P.  and  Parker  &  Parsley  Gas
             Processing Co. as Seller and Costilla Petroleum Corporation and Costilla Energy, L.L.C.
             as Purchaser
  ***10.16  Supplemental  Agreement to Purchase and Joint Exploration Agreement dated August 7, 1996
             among the  Company, Costilla  Redeco Energy,  L.L.C. and  Resource Development  Company
             Limited, L.L.C. (DE)
  ***10.17  Form of Bonus Incentive Plan
  ***12.1   Computation of Ratio of Adjusted EBITDA to Interest Expense
  ***12.2   Computation of Ratio of Earnings to Fixed Charges
</TABLE>
    
<PAGE>
<TABLE>
<CAPTION>
 EXHIBIT                                                                                               SEQUENTIALLY
  NUMBER                                     DESCRIPTION OF EXHIBIT                                    NUMBERED PAGE
- ----------  ----------------------------------------------------------------------------------------  ---------------
<C>         <S>                                                                                       <C>
    *16.1   Letter Regarding Change of Accountants
  ***21.1   Subsidiaries of the Registrant
   **23.1   Consent of KPMG Peat Marwick LLP
    *23.2   Consent of Williamson Petroleum Consultants, Inc.
   **23.3   Consent of Elms, Faris & Co., P.C.
    *23.4   Consent  of Cotton, Bledsoe, Tighe & Dawson, a Professional Corporation (such consent is
             included in the opinion filed as Exhibit 5.1 to this Registration Statement)
    *24.1   Power of Attorney
    *24.2   Certified copy of resolution of Board of Directors of Costilla Energy, Inc.  authorizing
             signature pursuant to Power of Attorney
    *27.1   Financial Data Schedule
</TABLE>
 
- ------------------------
*   Previously filed
**  Filed herewith
*** Incorporated  by reference to  Registration Statement on  Form S-1, File No.
    333-08909, filed with respect to the Notes.

<PAGE>
                                                                    EXHIBIT 23.1
 
                        CONSENT OF INDEPENDENT AUDITORS
 
The Members
Costilla Energy, L.L.C.
 
    We consent to the use of our reports included herein and to the reference to
our firm under the heading "Experts" in the Prospectus.
 
                                          KPMG PEAT MARWICK LLP
 
Midland, Texas
   
October 2, 1996
    

<PAGE>
                                                                    EXHIBIT 23.3
 
                        CONSENT OF INDEPENDENT AUDITORS
 
The Members
Costilla Energy, L.L.C.
 
    We consent to the use of our reports included herein and to the reference to
our firm under the heading "Experts" in the Prospectus.
 
                                          ELMS, FARIS & CO., P.C.
 
Midland, Texas
   
October 2, 1996
    


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