PIONEER NATURAL RESOURCES CO
S-3/A, 1997-12-17
CRUDE PETROLEUM & NATURAL GAS
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<PAGE>   1
 
   
   AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON DECEMBER 17, 1997
    
 
   
                                                      REGISTRATION NO. 333-39381
    
================================================================================
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
 
   
                                AMENDMENT NO. 1
    
   
                                       to
    
                                    FORM S-3
            REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933
                             ---------------------
 
                       PIONEER NATURAL RESOURCES COMPANY
             (Exact name of Registrant as specified in its charter)
 
<TABLE>
<S>                                                 <C>
                     DELAWARE                                           75-2702753
          (State or other jurisdiction of                            (I.R.S. Employer
          incorporation or organization)                            Identification No.)
                                                                    SCOTT D. SHEFFIELD
                                                           PRESIDENT AND CHIEF EXECUTIVE OFFICER
                                                             PIONEER NATURAL RESOURCES COMPANY
             1400 WILLIAMS SQUARE WEST                           1400 WILLIAMS SQUARE WEST
             5205 NORTH O'CONNOR BLVD.                           5205 NORTH O'CONNOR BLVD.
                IRVING, TEXAS 75039                                 IRVING, TEXAS 75039
                  (972) 444-9001                                      (972) 444-9001
(Address, including zip code, and telephone number,  (Name, address, including zip code, and telephone
  including area code, of Registrant's principal    number, including area code, of agent for service)
                 executive offices)
</TABLE>
 
                             ---------------------
 
                                With copies to:
 
   
<TABLE>
<C>                                                 <C>
                  MARK L. WITHROW                                    ROBERT L. KIMBALL
   EXECUTIVE VICE PRESIDENT AND GENERAL COUNSEL                   VINSON & ELKINS L.L.P.
         PIONEER NATURAL RESOURCES COMPANY                           2001 ROSS AVENUE
             1400 WILLIAMS SQUARE WEST                                  SUITE 3700
             5205 NORTH O'CONNOR BLVD.                              DALLAS, TEXAS 75201
                IRVING, TEXAS 75039                                   (214) 220-7700
                  (972) 444-9001
</TABLE>
    
 
                             ---------------------
 
    APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: From time
to time after this Registration Statement becomes effective.
 
    If the only securities being registered on this Form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box. [ ]
 
    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, other than securities offered only in connection with dividend or interest
reinvestment plans, check the following box. [X]
 
    If the Registrant elects to deliver its latest annual report to security
holders, or a complete and legible facsimile thereof, pursuant to Item 11(a)(i)
of this Form, 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. [ ]
    
                             ---------------------
 
                        CALCULATION OF REGISTRATION FEE
 
   
<TABLE>
<CAPTION>
===========================================================================================================================
                                                                    PROPOSED              PROPOSED
         TITLE OF EACH CLASS OF              AMOUNT TO BE       MAXIMUM OFFERING     MAXIMUM AGGREGATE        AMOUNT OF
       SECURITIES TO BE REGISTERED            REGISTERED        PRICE PER SHARE        OFFERING PRICE     REGISTRATION FEE
- ---------------------------------------------------------------------------------------------------------------------------
<S>                                       <C>                 <C>                  <C>                    <C>
Common Stock.............................         (1)                 (2)                   (2)                  (3)
===========================================================================================================================
</TABLE>
    
 
   
(1) The total amount of shares of Pioneer Common Stock being registered is
    18,004,289 shares as described in the initial filing of this Registration
    Statement on November 3, 1997.
    
 
   
(2) Pursuant to Rule 457(f), the fee was computed on the basis of the market
    value of the 18,004,289 shares of Pioneer Common Stock assumed to be issued
    by the Registrant in connection with the exchange of the Exchangeable
    Shares, and in accordance with Rule 457(c) on the basis of the average
    ($40.41) of the high and low price per share of the shares of Pioneer Common
    Stock reported on the New York Stock Exchange on October 31, 1997.
    
 
   
(3) A fee of $2,204.70 was paid in connection with the initial filing of this
    Registration Statement on November 3, 1997. Pursuant to Rule 457(b), the
    initial fee of $220,470.70 was offset by the fee of $218,266 paid with the
    filing under the Securities Exchange Act of 1934 of preliminary copies of
    the Registrant's proxy materials relating to the Transaction.
    
 
     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>   2
 
     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
     MAY OFFERS TO BUY BE ACCEPTED TO THE TIME THE REGISTRATION BECOMES
     EFFECTIVE. THIS PROSPECTUS SHALL NOT CONSTITUTE AN OFFER TO SELL OR THE
     SOLICITATION OF AN 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.
 
   
                 SUBJECT TO COMPLETION DATED DECEMBER 17, 1997
    
 
   
PROSPECTUS
    
 
   
                               18,004,289 SHARES
    
 
                       PIONEER NATURAL RESOURCES COMPANY
 
                                  COMMON STOCK
                          (PAR VALUE $0.01 PER SHARE)
 
   
     Each of the 18,004,289 shares of common stock, par value $0.01 per share
(the "Pioneer Common Stock"), of Pioneer Natural Resources Company, a Delaware
corporation ("Pioneer"), offered hereby is issuable upon the exchange or
redemption of an exchangeable share (an "Exchangeable Share") of Pioneer Natural
Resources (Canada) Ltd., an indirectly owned subsidiary of Pioneer ("Pioneer
Canada"). Pursuant to the Plan of Arrangement (as defined below), at the
Effective Time (as defined herein) Pioneer Canada, which is currently a British
Columbia corporation, will be continued as an Alberta corporation. The
Exchangeable Shares will be issued by Pioneer Canada upon the transfer of common
shares ("Chauvco Common Shares") of Chauvco Resources Ltd., an Alberta
corporation ("Chauvco"), pursuant to the terms of a Combination Agreement (the
"Combination Agreement") dated as of September 3, 1997, between Pioneer and
Chauvco and the terms of a plan of arrangement proposed under Section 186 of the
Business Corporations Act (Alberta) (the "Plan of Arrangement") attached as an
exhibit to the Combination Agreement (the transactions contemplated by the
Combination Agreement and the Plan of Arrangement being referred to herein
collectively as the "Transaction"). Shares of Pioneer Common Stock are being
offered on a continuous basis pursuant to Rule 415 under the Securities Act of
1933 (the "Securities Act") for such period as the Registration Statement to
which this Prospectus relates remains effective.
    
 
   
     Pioneer and Pioneer Canada are offering shares of Pioneer Common Stock to
holders of Exchangeable Shares pursuant to the terms of the Exchangeable Shares,
which obligate Pioneer Canada to redeem such shares or permit Pioneer, at its
election, to acquire such shares when, as and if the Exchangeable Shares are
presented by the holders thereof. Hereinafter the term "exchange" means either
the redemption by Pioneer Canada or the acquisition by Pioneer of Exchangeable
Shares, unless otherwise specified. Upon such exchange, holders of the
Exchangeable Shares will be entitled to receive for each Exchangeable Share one
share of Pioneer Common Stock, plus an additional amount equivalent to the full
amount of all declared and unpaid dividends on such Exchangeable Share (see
"Plan of Distribution"). All expenses of registration incurred in connection
with this offering are being paid by Pioneer. The Pioneer Common Stock currently
trades on the New York Stock Exchange (the "NYSE") under the symbol "PXD," and
has been conditionally approved for listing on The Toronto Stock Exchange (the
"TSE"). On December 15, 1997, the closing price of the Pioneer Common Stock on
the NYSE was $26 3/8 per share.
    
 
                             ---------------------
 
      SEE "RISK FACTORS" BEGINNING ON PAGE 3 HEREOF FOR A DISCUSSION OF CERTAIN
FACTORS THAT SHOULD BE CONSIDERED IN CONNECTION WITH ANY INVESTMENT IN THE
PIONEER 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.
 
   
               The date of this Prospectus is December   , 1997.
    
<PAGE>   3
 
                             AVAILABLE INFORMATION
 
     Pioneer is subject to the informational requirements of the Securities
Exchange Act of 1934 (the "Exchange Act"). It files reports, proxy statements,
and other information with the Securities and Exchange Commission (the "SEC").
Those reports, proxy statements, and other information can be inspected and
copied at the public reference facilities maintained by the SEC at Room 1024,
450 Fifth Street, N.W., Washington, D.C. 20549, and at the regional offices of
the SEC at 7 World Trade Center, Suite 1300, New York, New York 10048, and 500
West Madison Street, Suite 1400, Chicago, Illinois 60611. Copies of these
materials can be obtained at prescribed rates from the Public Reference Section
of the SEC at 450 Fifth Street, N.W., Washington, D.C. 20549. These reports,
proxy statements and other information may also be obtained without charge from
the web site that the SEC maintains at http://www.sec.gov. These reports, proxy
statements, and other information also may be inspected at the offices of the
NYSE, 20 Broad Street, New York, New York 10005.
 
     Pioneer has filed with the SEC a Registration Statement on Form S-3 (the
"Registration Statement") under the Securities Act, with respect to the shares
of Pioneer Common Stock offered hereby. This Prospectus does not contain all the
information set forth in the Registration Statement, certain parts of which are
omitted in accordance with the rules and regulations of the SEC. For further
information with respect to Pioneer and the shares of Pioneer Common Stock
offered hereby, reference is made to the Registration Statement and to the
exhibits thereto. Statements contained herein concerning the provisions of
certain documents are not necessarily complete, and in each instance, reference
is made to the copy of the document filed as an exhibit to the Registration
Statement or otherwise filed with the SEC. Each such statement is qualified in
its entirety by that reference.
 
                INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE
 
   
     The following documents have been filed by Pioneer, Parker & Parsley
Petroleum Company ("Parker & Parsley") and MESA Inc. ("Mesa") with the SEC, are
incorporated by reference into this Prospectus, and are deemed to be a part of
this Prospectus:
    
 
   
      1. Mesa's Annual Report on Forms 10-K and 10-K/A for the year ended
         December 31, 1996;
    
 
   
      2. Mesa's Quarterly Report on Form 10-Q for the period ended March 31,
         1997;
    
 
   
      3. Mesa's Current Reports on Form 8-K, dated February 7, 1997, and April
         6, 1997, and Mesa's Current Report on Form 8-K/A, dated February 7,
         1997;
    
 
   
      4. Mesa's Quarterly Report on Form 10-Q for the period ended June 30,
         1997;
    
 
   
      5. Parker & Parsley's Annual Report on Forms 10-K and 10-K/A for the year
         ended December 31, 1996;
    
 
   
      6. Parker & Parsley's Current Report on Form 8-K, dated February 3, 1997;
    
 
   
      7. Parker & Parsley's Quarterly Report on Form 10-Q for the period ended
         March 31, 1997;
    
 
   
      8. Parker & Parsley's Current Reports on Form 8-K, dated April 3, 1997,
         April 6, 1997, July 28, 1997, and July 29, 1997;
    
 
   
      9. Parker & Parsley's Quarterly Report on Form 10-Q for the period ended
         June 30, 1997;
    
 
   
     10. Pioneer's Registration Statement on Form S-4 (No. 333-26951) initially
         filed on June 26, 1997, as amended;
    
 
   
     11. Pioneer's Quarterly Report on Form 10-Q for the period ended June 30,
         1997;
    
 
   
     12. Pioneer's Current Report on Form 8-K, dated August 7, 1997;
    
 
   
     13. The description of Pioneer's Common Stock contained in Pioneer's
         Registration Statement on Forms 8-A and 8-A/A (File No. 001-13245),
         declared effective by the SEC on August 8, 1997;
    
 
                                        2
<PAGE>   4
 
   
     14. Pioneer's Current Report on Form 8-K, dated September 3, 1997;
    
 
   
     15. Pioneer's Quarterly Report on Form 10-Q for the period ended September
         30, 1997;
    
 
   
     16. The Definitive Joint Management Information Circular and Proxy
         Statement of Pioneer and Chauvco (File No. 001-13245) filed with the
         SEC on November 17, 1997, including any amendment or report for the
         purpose of updating any such material; and
    
 
   
     17. Pioneer's Current Report on Form 8-K, dated December 5, 1997.
    
 
   
     All documents filed by Pioneer pursuant to Section 13(a), 14 or 15(d) of
the Exchange Act subsequent to the date of this Prospectus and prior to the
termination of the offering made hereby shall be deemed to be incorporated by
reference into this Prospectus and to be a part hereof from the date of the
filing of such documents. Any statement contained herein or in a document
incorporated or deemed to be incorporated by reference herein shall be deemed to
be modified or superseded for purposes of this Prospectus to the extent that a
statement contained herein or in any subsequently filed document that also is or
is deemed to be incorporated by reference herein modifies or supersedes such
statement. Any statement so modified or superseded shall not be deemed to
constitute a part of this Prospectus, except as so modified or superseded.
    
 
     Pioneer will provide without charge to each person to whom a copy of this
Prospectus has been delivered, on the written or oral request of any person, a
copy of any or all of the documents referred to above that have been or may be
incorporated by reference into this Prospectus, other than exhibits to the
documents (unless the exhibits are specifically incorporated by reference into
the documents). Written or telephone request for the copies should be directed
to Corporate Secretary, Pioneer Natural Resources Company, 1400 Williams Square
West, 5205 North O'Connor Boulevard, Irving, Texas 75039 (Telephone: (972)
444-9001).
 
                                  RISK FACTORS
 
     Investors should consider carefully the following factors, in addition to
the other information contained in this Prospectus, before exchanging their
Exchangeable Shares for the shares of Pioneer Common Stock offered hereby.
 
TAXABILITY OF THE EXCHANGE
 
     Based on the tax laws as of the date of this Prospectus, the exchange of
Exchangeable Shares for shares of Pioneer Common Stock is generally a taxable
event in Canada and the United States. A holder's tax consequences can vary
depending on a number of factors, including the residency of the holder, the
method of the exchange and the length of time that the Exchangeable Shares were
held prior to the exchange (see "Income Tax Considerations").
 
DIFFERENCES IN CANADA AND U.S. TRADING MARKETS
 
   
     The Pioneer Common Stock is listed on the NYSE and has been conditionally
approved for listing on the TSE. As a condition precedent to the consummation of
the arrangement under the Plan of Arrangement (the "Arrangement"), the
Exchangeable Shares must be listed for trading on the TSE. The TSE has
conditionally approved the listing of the Exchangeable Shares. Listing is
subject to fulfilling the requirements of the TSE on or before February 4, 1998,
including the distribution of the Exchangeable Shares to a minimum number of
public shareholders. There is no current intention to list the Exchangeable
Shares or Pioneer Common Stock on any other stock exchange in Canada or the
United States. As a result of the foregoing, the price at which the Exchangeable
Shares will trade will be based upon the market for such shares on the TSE, and
the price at which the shares of Pioneer Common Stock will trade will be based
upon the market for such shares on the NYSE and TSE. Although Pioneer believes
that the market price of the Exchangeable Shares on the TSE and the market price
of the Pioneer Common Stock on the NYSE and TSE will reflect essentially
equivalent
    
 
                                        3
<PAGE>   5
 
values, there can be no assurance that the market price of the Pioneer Common
Stock will be identical, or even similar, to the market price of the
Exchangeable Shares.
 
FOREIGN PROPERTY
 
     So long as the Exchangeable Shares are listed on a prescribed stock
exchange in Canada (which currently includes the TSE) and Pioneer Canada
maintains a substantial presence in Canada, the Exchangeable Shares will not be
foreign property under the Income Tax Act (Canada) (the "Canadian Tax Act") for
trusts governed by registered pension plans, registered retirement savings
plans, registered retirement income funds and deferred profit sharing plans or
for certain other tax-exempt persons. Pioneer Common Stock will, however, be
foreign property for such plans or persons.
 
                                    PIONEER
 
     Pioneer is one of the largest public independent oil and gas companies in
the United States, engaged principally in the acquisition, development, and
production of, and exploration for, oil and gas reserves and related activities.
 
   
     Pioneer's executive offices and operating headquarters are located at 1400
Williams Square West, 5205 North O'Connor Blvd., Irving, Texas 75039, and its
telephone number at those offices is (972) 444-9001.
    
 
                                USE OF PROCEEDS
 
   
     Because shares of Pioneer Common Stock will be issued on the exchange of
the Exchangeable Shares, Pioneer will receive no cash proceeds on that issuance.
    
 
                          DESCRIPTION OF CAPITAL STOCK
 
     The authorized capital stock of Pioneer consists of 500,000,000 shares of
common stock, par value $.01 per share, and 100,000,000 shares of preferred
stock, par value $.01 per share, of which one share has been designated as
Special Preferred Voting Stock.
 
PIONEER CAPITAL STOCK
 
  Pioneer Common Stock
 
     All shares of Pioneer Common Stock issued upon the exchange of the
Exchangeable Shares will be fully paid and nonassessable. The holders of Pioneer
Common Stock are entitled to one vote for each share held on all matters
submitted to a vote of common stockholders. The Pioneer Common Stock does not
have cumulative voting rights. Shares of Pioneer Common Stock have no preemptive
rights, conversion rights, redemption rights or sinking fund provisions. Pioneer
Common Stock is not subject to redemption by Pioneer.
 
     Subject to the rights of the holders of any class of capital stock of
Pioneer having any preference or priority over the Pioneer Common Stock, the
holders of Pioneer Common Stock are entitled to dividends in such amounts as may
be declared by the Board of Directors of Pioneer (the "Pioneer Board") from time
to time out of funds legally available for such payments and, in the event of
liquidation, to share ratably in any assets of Pioneer remaining after payment
in full of all creditors and provision for any liquidation preferences on any
outstanding preferred stock ranking prior to the Pioneer Common Stock.
 
  Pioneer Preferred Stock
 
     The Pioneer Board, without further stockholder action, is authorized to
issue up to 100,000,000 shares of preferred stock in one or more series and to
fix and determine as to any series all the relative rights and preferences of
shares in the series, including voting rights, dividend rights, liquidation
preferences, terms of redemption, and conversion rights.
 
                                        4
<PAGE>   6
 
  Pioneer Special Preferred Voting Stock
 
   
     The Pioneer Board has designated one (1) of the 100,000,000 authorized
shares of preferred stock as Special Preferred Voting Stock (the "Voting
Share"). The Montreal Trust Company of Canada, or any successor thereto (the
"Trustee"), shall hold the Voting Share as trustee for and on behalf of, and for
the use and benefit of, the holders of Exchangeable Shares and in accordance
with the Voting and Exchange Trust Agreement. The Certificate of Designations
for the Voting Share includes the following principal terms:
    
 
     Dividends. No dividend shall be paid to the Trustee as the holder of the
Voting Share.
 
     Voting Rights. The Trustee, as the holder of record of the Voting Share,
shall be entitled to all of the voting rights attached to the Voting Share,
including the right to consent to or vote in person or by proxy the Voting
Share, on any matter, question or proposition whatsoever that may properly come
before the Pioneer stockholders at a meeting thereof or with respect to any
written consent sought by Pioneer from its stockholders. For each Exchangeable
Share owned of record on the relevant record date, the holder thereof shall be
entitled to instruct the Trustee to cast and exercise, in the manner instructed,
a number of votes (including for purposes of a quorum) equal to the number of
votes to which a holder of one share of Pioneer Common Stock is entitled with
respect to any matter, proposition or question on which the holders of Pioneer
Common Stock are entitled to vote. Except as otherwise described herein or
required by law, the holder of the Voting Share will vote together with the
Pioneer Common Stock as a single class and not as a separate class or series
apart therefrom, including any vote to approve or adopt: (i) any plan of merger,
consolidation or share exchange for which Delaware law requires a stockholder
vote; (ii) any disposition of assets for which Delaware law requires a
stockholder vote; and (iii) any dissolution of Pioneer for which Delaware law
requires a stockholder vote.
 
   
     The holders of Exchangeable Shares will have the right to submit
stockholder proposals to the Trustee and the Trustee will agree pursuant to the
Voting and Exchange Trust Agreement to submit any such proposals to Pioneer.
Such stockholder proposals may be considered at any meeting of Pioneer at which
the holders of Pioneer Common Stock are entitled to submit stockholder
proposals. Pioneer will agree pursuant to the Voting and Exchange Trust
Agreement to accept all stockholder proposals submitted by the Trustee provided
that not more than one proposal is submitted by the Trustee on behalf of any one
holder of Exchangeable Shares.
    
 
     So long as any Exchangeable Shares are outstanding, the number of shares
comprising the Special Preferred Voting Stock will not be increased or
decreased, and no other term of the Special Preferred Voting Stock may be
amended, except upon the approval of the holder of the Voting Share.
 
     Conversion. The Voting Share is not convertible into any other class or
series of the capital stock of Pioneer or into cash, property or other rights.
 
     Redemption. The Voting Share may not be redeemed, except at such time as no
Exchangeable Shares shall be outstanding, in which case, the Voting Share shall
be automatically redeemed. The redemption price due and payable upon such
automatic redemption will be equal to a $1.00 liquidation preference. The Voting
Share will be deemed retired and will be canceled upon any purchase or other
acquisition thereof by Pioneer. After such cancellation, the Voting Share may
not be reissued or otherwise disposed of by Pioneer.
 
     Liquidation. The Voting Share will rank prior to each share of Pioneer
Common Stock with respect to the distribution of assets upon a liquidation,
dissolution or winding-up of Pioneer. In the event of any such liquidation,
dissolution or winding-up, the holder of the Voting Share will be entitled to
receive, before any distribution to the holders of Pioneer Common Stock, but
only after the liquidation preference of any other shares of preferred stock of
Pioneer has been paid in full, a liquidation preference equal to $1.00.
 
     Certain Covenants of Pioneer. For so long as the Voting Share is
outstanding, Pioneer will: (i) fully comply with all terms of the Exchangeable
Shares and with all contractual obligations of Pioneer associated therewith, and
(ii) not amend, alter or repeal the terms and conditions of the Special
Preferred Voting Stock, except with the approval of the holder of the Voting
Share.
 
                                        5
<PAGE>   7
 
     For a more detailed description of Pioneer's Special Preferred Voting Stock
and the Voting Share, see the terms and conditions thereof set forth on Exhibit
3.3 to the Registration Statement of which this Prospectus is a part. See also
"-- Voting and Exchange Trust Agreement."
 
  Certain Provisions of the Certificate of Incorporation and Bylaws
 
     The Pioneer Board is divided into three classes. The directors of each
class are elected for three-year terms, with the terms of the three classes
staggered so that directors from a single class are elected at each annual
meeting of stockholders. Stockholders may remove a director only for cause. In
general, the Pioneer Board, not the stockholders, has the right to appoint
persons to fill vacancies on the Pioneer Board.
 
     The amended and restated certificate of incorporation of Pioneer (the
"Pioneer Restated Certificate") contains a "fair price" provision that requires
the affirmative vote of the holders of least 80% of Pioneer's voting stock and
the affirmative vote of at least 66 2/3% of Pioneer's voting stock not owned,
directly or indirectly, by a Pioneer Related Person (as defined below) to
approve any merger, consolidation, sale or lease of all or substantially all of
Pioneer's assets, or certain other transactions involving a Pioneer Related
Person. For purposes of this fair price provision, a "Pioneer Related Person" is
any person beneficially owning 10% or more of the voting power of the
outstanding capital stock of Pioneer who is a party to the transaction at issue.
The voting requirement is not applicable to certain transactions, including
those that are approved by Pioneer's Continuing Directors (as defined in the
Pioneer Restated Certificate) or that meet certain "fair price" criteria
contained in the Pioneer Restated Certificate.
 
   
     The Pioneer Restated Certificate further provides that stockholders may act
only at annual or special meetings of stockholders and not by written consent,
that special meetings of stockholders may be called only by the Pioneer Board,
and that only business proposed by the Pioneer Board may be considered at
special meetings of stockholders.
    
 
     The Pioneer Restated Certificate also provides that the only business
(including election of directors) that may be considered at an annual meeting of
stockholders, in addition to business proposed (or persons nominated to be
directors) by the directors of Pioneer, is business proposed (or persons
nominated to be directors) by stockholders who comply with the notice and
disclosure requirements set forth in the Pioneer Restated Certificate. In
general, the Pioneer Restated Certificate requires that a stockholder give
Pioneer notice of proposed business or nominations no later than 60 days before
the annual meeting of stockholders (meaning the date on which the meeting is
first scheduled and not postponements or adjournments thereof) or (if later) 10
days after the first public notice of the annual meeting is sent to common
stockholders. In general, the notice must also contain information about the
stockholder proposing the business or nomination, his interest in the business,
and (with respect to nominations for director) information about the nominee of
the nature ordinarily required to be disclosed in public proxy solicitations.
The stockholder also must submit a notarized letter from each of his nominees
stating the nominee's acceptance of the nomination and indicating the nominee's
intention to serve as director if elected.
 
     The Pioneer Restated Certificate also restricts the ability of stockholders
to interfere with the powers of the Board of Directors in certain specified
ways, including the constitution and composition of committees and the election
and removal of officers.
 
   
     The Pioneer Restated Certificate provides that approval by the holders of
at least 66 2/3% of the outstanding Pioneer voting stock is required to amend
the provisions of the Pioneer Restated Certificate discussed above and certain
other provisions, except that (a) approval by the holders of at least 80% of the
outstanding Pioneer voting stock together with approval by the holders of at
least 66 2/3% of the outstanding voting stock not owned, directly or indirectly,
by the Related Person, is required to amend the fair price provisions, and (b)
approval of the holders of at least 80% of the outstanding voting stock is
required to amend the provisions prohibiting stockholders from acting by written
consent.
    
 
                                        6
<PAGE>   8
 
  Delaware Anti-Takeover Statute
 
     Pioneer is a Delaware corporation and is subject to Section 203 of the
Delaware General Corporation Law. In general, Section 203 prevents an
"interested stockholder" (defined generally as a person owning 15% or more of
Pioneer's outstanding voting stock) from engaging in a "business combination"
(as defined in Section 203) with Pioneer for three years following the date that
person becomes an interested stockholder unless (a) before that person became an
interested stockholder, the Pioneer Board approved the transaction in which the
interested stockholder became an interested stockholder or approved the business
combination, (b) upon completion of the transaction that resulted in the
interested stockholder's becoming an interested stockholder, the interested
stockholder owns at least 85% of Pioneer voting stock outstanding at the time
the transaction commenced (excluding stock held by directors who are also
officers of Pioneer and by employee stock plans that do not provide employees
with the right to determine confidentially whether shares held subject to the
plan will be tendered in a tender or exchange offer), or (c) following the
transaction in which that person became an interested stockholder, the business
combination is approved by the Pioneer Board and authorized at a meeting of
stockholders by the affirmative vote of the holders of at least two-thirds of
the outstanding Pioneer voting stock not owned by the interested stockholder.
 
     Under Section 203, these restrictions also do not apply to certain business
combinations proposed by an interested stockholder following the announcement or
notification of one or certain extraordinary transactions involving Pioneer and
a person who was not an interested stockholder during the previous three years
or who became an interested stockholder with the approval of a majority of
Pioneer's directors, if that extraordinary transaction is approved or not
opposed by a majority of the directors before any person became an interested
stockholder in the previous three years or who were recommended for election or
elected to succeed such directors by a majority of such directors then in
office.
 
CHAUVCO SHARE CAPITAL
 
     The share capital of Chauvco after the Effective Time (as defined below)
will consist of one class of common shares, unlimited in number, and all of such
shares which are issued and outstanding shall be held by Pioneer Canada. The
"Effective Time" means 12:01 a.m. (Calgary time) on the "Effective Date", which
means the date shown on the certificate of amendment issued by the Registrar
under the Business Corporations Act (Alberta) giving effect to the Arrangement.
 
PIONEER CANADA SHARE CAPITAL
 
   
     The share capital of Pioneer Canada after the Effective Time will have the
rights and preferences summarized below. Such summary is qualified in its
entirety by reference to the Plan of Arrangement and the rights, privileges,
restrictions and conditions attaching to the Exchangeable Shares (the
"Exchangeable Share Provisions") which are Exhibits 2.3 and 3.4, respectively,
to the Registration Statement of which this Prospectus is a part.
    
 
     Pioneer Canada Common Voting Shares. The holders of common voting shares of
Pioneer Canada ("Pioneer Canada Common Voting Shares") will be entitled to
receive notice of and to attend all meetings of the shareholders of Pioneer
Canada and will be entitled to one vote for each share held of record on all
matters submitted to a vote of holders of Pioneer Canada Common Voting Shares.
The holders of Pioneer Canada Common Voting Shares will be entitled to receive
such dividends as may be declared by the Pioneer Canada board of directors out
of funds legally available therefor. Holders of Pioneer Canada Common Voting
Shares will be entitled upon any liquidation, dissolution or winding-up of
Pioneer Canada, subject to the prior rights of the holders of the Exchangeable
Shares and to any other shares ranking senior to the Pioneer Canada Common
Voting Shares, to receive the remaining property and assets of Pioneer Canada.
 
  Exchangeable Shares of Pioneer Canada
 
     Ranking. The Exchangeable Shares will rank prior to the Pioneer Canada
Common Voting Shares and any other shares ranking junior to the Exchangeable
Shares with respect to the payment of dividends and the distribution of assets
in the event of the liquidation, dissolution or winding-up of Pioneer Canada.
 
                                        7
<PAGE>   9
 
     Dividends. Holders of Exchangeable Shares will be entitled to receive
dividends equivalent to dividends paid from time to time by Pioneer on shares of
Pioneer Common Stock. The declaration date, record date and payment date for
dividends on the Exchangeable Shares will be the same as that for the
corresponding dividends on the Pioneer Common Stock.
 
     Certain Restrictions. Without the approval of the holders of the
Exchangeable Shares, Pioneer Canada will not:
 
          (a) pay any dividend on the Pioneer Canada Common Voting Shares, or
     any other shares ranking junior to the Exchangeable Shares, other than
     stock dividends payable in such other shares ranking junior to the
     Exchangeable Shares;
 
          (b) redeem, purchase or make any capital distribution in respect of
     Pioneer Canada Common Voting Shares or any other shares ranking junior to
     the Exchangeable Shares;
 
          (c) redeem or purchase any other shares of Pioneer Canada ranking
     equally with the Exchangeable Shares with respect to the payment of
     dividends or on any liquidation distribution;
 
          (d) issue any Exchangeable Shares or any other shares of Pioneer
     Canada ranking equally with, or superior to, the Exchangeable Shares other
     than by stock dividends to the holders of the Exchangeable Shares or as
     contemplated in the Support Agreement to be entered into between Pioneer
     and Pioneer Canada (the "Support Agreement"); or
 
          (e) amend the articles or bylaws of Pioneer Canada.
 
     The restrictions in (a), (b) and (c) above will not apply at any time when
the dividends on the outstanding Exchangeable Shares corresponding to dividends
declared on the Pioneer Common Stock have been declared and paid in full.
 
     Liquidation. In the event of the liquidation, dissolution or winding-up of
Pioneer Canada a holder of Exchangeable Shares will be entitled to receive for
each Exchangeable Share one share of Pioneer Common Stock, together with a cash
amount equivalent to the full amount of all unpaid dividends on the Exchangeable
Shares. See "-- Voting and Exchange Trust Agreement."
 
     Retraction of Exchangeable Shares by Holders. A holder of Exchangeable
Shares will be entitled at any time to require Pioneer Canada to retract (i.e.,
require Pioneer Canada to redeem) any or all of the Exchangeable Shares held by
such holder for one share of Pioneer Common Stock for each Exchangeable Share
plus an additional amount equivalent to the full amount of all unpaid dividends
thereon, which shall be delivered to the retracting holder on the retraction
date specified by the holder (which shall not be less than three nor more than
ten business days after the date on which Pioneer Canada receives the retraction
request from the holder).
 
     If, as a result of liquidity or solvency provisions of applicable law,
Pioneer Canada is not permitted to redeem all Exchangeable Shares tendered by a
retracting holder, Pioneer Canada will redeem only those Exchangeable Shares
tendered by the holder (rounded down to a whole number of shares) as would not
be contrary to such provisions of applicable law. The holder of any Exchangeable
Shares not redeemed by Pioneer Canada will be deemed to have required Pioneer to
purchase such unretracted shares in exchange for Pioneer Common Stock, plus an
additional amount equivalent to the full amount of all unpaid dividends thereon,
on the retraction date pursuant to the optional Exchange Rights (as defined
herein). See "-- Voting and Exchange Trust Agreement."
 
     Redemption of Exchangeable Shares. Subject to Pioneer's Redemption Call
Right (as defined below), on the Automatic Redemption Date (as defined below)
Pioneer Canada will redeem all but not less than all of the then outstanding
Exchangeable Shares for one share of Pioneer Common Stock for each Exchangeable
Share plus an additional amount equivalent to the full amount of all unpaid
dividends thereon. Pioneer Canada shall, at least 120 days prior to the
Automatic Redemption Date, provide the registered holders of the Exchangeable
Shares with written notice of the proposed redemption of the Exchangeable
Shares.
 
                                        8
<PAGE>   10
 
     "Automatic Redemption Date" means the fifth anniversary of the date of the
first issuance of Exchangeable Shares unless (a) such date shall be extended at
any time or from time to time to a specified later date by the Board of
Directors of Pioneer Canada, but not later than December 31, 2005 or (b) such
date shall be accelerated at any time to a specified earlier date (but no
earlier than the third anniversary of the first issuance of Exchangeable Shares)
by the Board of Directors of Pioneer Canada if at such time there are issued and
outstanding less than 5% of the number of Exchangeable Shares initially issued
and outstanding pursuant to the Plan of Arrangement (other than Exchangeable
Shares held by Pioneer and its subsidiaries) and as such number of shares may be
adjusted as deemed appropriate by the Board of Directors of Pioneer Canada to
give effect to any subdivision or consolidation of or stock dividend on the
Exchangeable Shares, any issuance or distribution of rights to acquire
Exchangeable Shares or securities exchangeable for or convertible into
Exchangeable Shares, any issue or distribution of other securities or rights or
evidences of indebtedness or assets, or any other capital reorganization or
other transaction affecting the Exchangeable Shares, in which case the Automatic
Redemption Date shall be such later or earlier date; provided, however, that the
accidental failure or omission to give any such notice of extension or
acceleration, as the case may be, to less than 10% of such holders of
Exchangeable Shares shall not affect the validity of such extension or
acceleration.
 
     Voting Rights. Except as required by applicable law, the holders of the
Exchangeable Shares shall not be entitled as such to receive notice of or attend
any meeting of the shareholders of Pioneer Canada or to vote at any such
meeting.
 
     Amendment and Approval. The rights, privileges, restrictions and conditions
attaching to the Exchangeable Shares may be changed only with the approval of
the holders thereof. Any such approval or any other approval or consent to be
given by the holders of the Exchangeable Shares will be sufficiently given if
given in accordance with applicable law and subject to a minimum requirement
that such approval or consent be evidenced by a resolution passed by not less
than two-thirds of the votes cast thereon (other than shares beneficially owned
by Pioneer or entities controlled by Pioneer) at a meeting of the holders of
Exchangeable Shares duly called and held at which holders of at least 50% of the
then outstanding Exchangeable Shares are present or represented by proxy. In the
event that no such quorum is present at such meeting within one-half hour after
the time appointed therefor, then the meeting will be adjourned to such place
and time not less than 10 days later as may be determined at the original
meeting and the holders of Exchangeable Shares present or represented by proxy
at the adjourned meeting may transact the business for which the meeting was
originally called. At the adjourned meeting, a resolution passed by the
affirmative vote of not less than two-thirds of the votes cast thereon will
constitute the approval or consent of the holders of the Exchangeable Shares.
 
     Actions of Pioneer Canada under Support Agreement. Under the Exchangeable
Share Provisions, Pioneer Canada will agree to take all such actions and do all
such things as are necessary or advisable to perform and comply with its
obligations under, and to ensure the performance and compliance by Pioneer with
its obligations under, the Support Agreement.
 
SUPPORT AGREEMENT
 
   
     The following is a summary description of the material provisions of the
Support Agreement and is qualified in its entirety by reference to the full text
of the Support Agreement, which is Exhibit 2.4 to the Registration Statement of
which this Prospectus is a part.
    
 
   
     Under the Support Agreement, Pioneer will agree that: (i) it will not
declare or pay dividends on the Pioneer Common Stock unless Pioneer Canada is
able to and simultaneously pays an equivalent dividend on the Exchangeable
Shares; (ii) it will cause Pioneer Canada to declare and pay an equivalent
dividend on the Exchangeable Shares simultaneously with Pioneer's declaration
and payment of dividends on the Pioneer Common Stock; (iii) it will take all
actions and do all things necessary to ensure that Pioneer Canada is able to
provide to the holders of the Exchangeable Shares the equivalent number of
shares of Pioneer Common Stock in the event of a liquidation, dissolution, or
winding-up of Pioneer Canada, a Retraction Request (as defined herein) by a
holder of Exchangeable Shares, or a redemption of Exchangeable Shares of Pioneer
Canada; and (iv) it will not vote or otherwise take any action or omit to take
any action causing the liquidation, dissolution or winding-up of Pioneer Canada.
    
 
                                        9
<PAGE>   11
 
     The Support Agreement also provides that, without the prior approval of
Pioneer Canada and the holders of the Exchangeable Shares, Pioneer will not
distribute additional shares of Pioneer Common Stock or rights to subscribe
therefor or other property or assets to all or substantially all holders of
shares of Pioneer Common Stock, nor change the Pioneer Common Stock nor effect
any tender offer, share exchange offer, issuer bid, take-over bid or similar
transaction affecting the Pioneer Common Stock, unless the same or an equivalent
distribution on or change to the Exchangeable Shares (or in the rights of the
holders thereof) is made simultaneously.
 
   
     Pioneer will agree that so long as there remain outstanding any
Exchangeable Shares not owned by Pioneer or any entity controlled by Pioneer,
Pioneer will remain the beneficial owner, directly or indirectly, of all
outstanding shares of Pioneer Canada other than the Exchangeable Shares. In
addition, the Support Agreement obligates Pioneer to nominate Messrs. James
Baroffio and Guy J. Turcotte to the board of directors of Pioneer in accordance
with the same terms as the Combination Agreement.
    
 
     With the exception of administrative changes for the purpose of adding
covenants for the protection of the holders of the Exchangeable Shares, making
certain necessary amendments or curing ambiguities or clerical errors (in each
case provided that the board of directors of each of Pioneer and Pioneer Canada
is of the opinion that such amendments are not prejudicial to the interests of
the holders of the Exchangeable Shares), the Support Agreement may not be
amended without the approval of the holders of the Exchangeable Shares.
 
   
     Under the Support Agreement, Pioneer will agree not to exercise any voting
rights attached to the Exchangeable Shares owned by it or any entity controlled
by it on any matter considered at meetings of holders of Exchangeable Shares
(including any approval sought from such holders in respect of matters arising
under the Support Agreement).
    
 
VOTING AND EXCHANGE TRUST AGREEMENT
 
   
     The following is a summary description of the material provisions of the
Voting and Exchange Trust Agreement and is qualified in its entirety by
reference to the full text of the Voting and Exchange Trust Agreement which is
Exhibit 2.5 to the Registration Statement of which this Prospectus is a part.
Under the terms of the Voting and Exchange Trust Agreement, Pioneer will issue
and grant to the Trustee the (i) rights of the holders of Exchangeable Shares to
direct the voting of the Voting Share in accordance with the Voting and Exchange
Trust Agreement (the "Voting Rights") and (ii) the Automatic Exchange Rights (as
defined below) and the optional exchange right granted to the Trustee for the
use and benefit of the holders of the Exchangeable Shares pursuant to the Voting
and Exchange Trust Agreement to require Pioneer to purchase Exchangeable Shares
from the holders hereof in exchange for shares of Pioneer Common Stock upon the
occurrence of a Pioneer Canada Insolvency Event (as defined herein). "Automatic
Exchange Rights" means the rights granted to the Trustee for the benefit of the
holders of the Exchangeable Shares pursuant to the Voting and Exchange Trust
Agreement to automatically exchange the Exchangeable Shares for shares of
Pioneer Common Stock upon a Pioneer Liquidation Event (as defined herein).
    
 
     Voting Rights. Under the Voting and Exchange Trust Agreement, Pioneer will
issue the Voting Share to the Trustee for the benefit of the holders (other than
Pioneer and its subsidiaries) of the Exchangeable Shares. The Voting Share will
carry a number of votes, exercisable at any meeting at which Pioneer
Stockholders are entitled to vote, equal to the number of outstanding
Exchangeable Shares (other than shares held by Pioneer and its subsidiaries).
With respect to any written consent sought from the Pioneer Stockholders, each
vote attached to the Voting Share will be exercisable in the same manner as set
forth above.
 
     Each holder of an Exchangeable Share on the record date for any meeting at
which Pioneer Stockholders are entitled to vote will be entitled to instruct the
Trustee to exercise one of the votes attached to the Voting Share for such
Exchangeable Share. The Trustee will exercise each vote attached to the Voting
Share only as directed by the relevant holder and, in the absence of
instructions from a holder as to voting, will not exercise such votes. A holder
may, upon instructing the Trustee, obtain a proxy from the Trustee entitling the
holder to vote directly at the relevant meeting the votes attached to the Voting
Share to which the holder is entitled.
 
                                       10
<PAGE>   12
 
     The Trustee will send to the holders of the Exchangeable Shares the notice
of each meeting at which the Pioneer Stockholders are entitled to vote, together
with the related meeting materials and a statement as to the manner in which the
holder may instruct the Trustee to exercise the votes attaching to the Voting
Share, at the same time as Pioneer sends such notice and materials to the
Pioneer Stockholders. The Trustee will also send to the holders copies of all
information statements, interim and annual financial statements, reports and
other materials sent by Pioneer to the Pioneer Stockholders at the same time as
such materials are sent to the Pioneer Stockholders. To the extent such
materials are provided to the Trustee by Pioneer, the Trustee will also send to
the holders all materials sent by third parties to Pioneer Stockholders,
including dissident proxy circulars and tender and exchange offer circulars, as
soon as possible after such materials are first sent to Pioneer Stockholders.
 
     All rights of a holder of Exchangeable Shares to exercise votes attached to
the Voting Share will cease upon the exchange of all such holder's Exchangeable
Shares for shares of Pioneer Common Stock.
 
     Exchange Rights. Under the Voting and Exchange Trust Agreement, Pioneer
will grant the Exchange Rights (as defined below) to the Trustee for the benefit
of the holders of the Exchangeable Shares. "Exchange Rights" means the Automatic
Exchange Rights and the optional exchange right granted to the Trustee for the
use and benefit of the holders of the Exchangeable Shares pursuant to the Voting
and Exchange Trust Agreement to require Pioneer to purchase Exchangeable Shares
from the holders thereof in exchange for shares of Pioneer Common Stock upon the
occurrence of a Pioneer Canada Insolvency Event.
 
     Optional Exchange Right. Upon the occurrence and during the continuance of
a Pioneer Canada Insolvency Event, a holder of Exchangeable Shares will be
entitled to instruct the Trustee to exercise the optional Exchange Right with
respect to any or all of the Exchangeable Shares held by such holder, thereby
requiring Pioneer to purchase such Exchangeable Shares from the holder.
Immediately upon the occurrence of a Pioneer Canada Insolvency Event or any
event which may with the passage of time or the giving of notice become a
Pioneer Canada Insolvency Event, Pioneer Canada and Pioneer will give written
notice thereof to the Trustee. As soon as practicable thereafter, the Trustee
will notify each holder of Exchangeable Shares of such event or potential event
and will advise the holder of its rights with respect to the optional Exchange
Right. "Pioneer Canada Insolvency Event" means any insolvency or bankruptcy
proceeding instituted by or against Pioneer Canada, including any such
proceeding under the Companies' Creditors Arrangement Act (Canada) and the
Bankruptcy and Insolvency Act (Canada) and the admission in writing by Pioneer
Canada of its inability to pay its debts generally as they become due and the
inability of Pioneer Canada, as a result of solvency requirements of applicable
law, to redeem any Exchangeable Shares tendered for retraction.
 
     The consideration for each Exchangeable Share to be acquired under the
optional Exchange Right will be one share of Pioneer Common Stock plus an
additional amount equivalent to the full amount of all dividends declared and
unpaid on the Exchangeable Share.
 
     If, as a result of liquidity or solvency provisions of applicable law,
Pioneer Canada is unable to redeem all of the Exchangeable Shares tendered for
retraction by a holder in accordance with the Exchangeable Share Provisions, the
holder will be deemed to have exercised the optional Exchange Right with respect
to the unredeemed Exchangeable Shares and Pioneer will be required to purchase
such shares from the holder in the manner set forth above.
 
     Automatic Exchange Right. In the event of a Pioneer Liquidation Event,
Pioneer will be required to acquire each outstanding Exchangeable Share by
exchanging one share of Pioneer Common Stock for each such Exchangeable Share,
plus an additional amount equivalent to the full amount of all declared and
unpaid dividends on the Exchangeable Shares. "Pioneer Liquidation Event" means
(i) any determination by the Pioneer Board to institute voluntary liquidation,
dissolution, or winding-up proceedings with respect to Pioneer or to effect any
other distribution of assets of Pioneer among its stockholders for the purpose
of winding up its affairs; or (ii) immediately upon the earlier of (A) receipt
by Pioneer of notice of, and (B) Pioneer becoming aware of any threatened or
instituted claim, suit, petition or other proceedings with respect to the
involuntary liquidation, dissolution or winding-up of Pioneer or to effect any
other distribution of assets of Pioneer among its stockholders for the purpose
of winding-up its affairs.
 
                                       11
<PAGE>   13
 
DELIVERY OF PIONEER COMMON STOCK
 
     Pioneer will ensure that all shares of Pioneer Common Stock to be delivered
by it under the Support Agreement or on the exercise of the Exchange Rights
under the Voting and Exchange Trust Agreement are duly registered, qualified or
approved under applicable Canadian and United States securities laws, if
required so that such shares may be freely traded by the holder thereof (other
than any restriction on transfer by reason of a holder being a "control person"
of Pioneer for purposes of Canadian law or an "affiliate" of Pioneer for
purposes of United States law). In addition, Pioneer will take all actions
necessary to cause all such shares of Pioneer Common Stock to be listed or
quoted for trading on all stock exchanges or quotation systems on which
outstanding shares of Pioneer Common Stock are then listed or quoted for
trading.
 
CALL RIGHTS
 
   
     The following description of (i) the right of Pioneer, in the event of a
proposed liquidation, dissolution or winding-up of Pioneer Canada, to purchase
all of the outstanding Exchangeable Shares from the holders thereof on the
effective date of any such liquidation, dissolution or winding-up in exchange
for shares of Pioneer Common Stock pursuant to the Plan of Arrangement (the
"Liquidation Call Right"), (ii) the right of Pioneer to purchase all of the
outstanding Exchangeable Shares from the holders thereof on the Automatic
Redemption Date in exchange for shares of Pioneer Common Stock pursuant to the
Plan of Arrangement (the "Redemption Call Right") and (iii) the overriding right
of Pioneer, in the event of a proposed retraction of Exchangeable Shares by a
holder thereof, to purchase from such holder on the Retraction Date (as defined
herein) the Exchangeable Shares tendered for retraction in exchange for shares
of Pioneer Common Stock pursuant to the Exchangeable Share Provisions (the
"Retraction Call Right", and, together with the Liquidation Call Right and the
Redemption Call Right, the "Call Rights"), is qualified in its entirety by
reference to the full text of the Plan of Arrangement and the Exchangeable Share
Provisions, which are attached as Exhibits 2.3. and 3.4, respectively, to the
Registration Statement of which this Prospectus is a part.
    
 
   
     In the circumstances described below, Pioneer will have certain overriding
rights to acquire Exchangeable Shares from holders thereof for one share of
Pioneer Common Stock for each Exchangeable Share acquired, plus an amount
equivalent to the full amount of all declared and unpaid dividends on the
Exchangeable Shares. Different Canadian federal income tax consequences to a
holder of Exchangeable Shares may arise depending upon whether the Call Rights
are exercised by Pioneer or whether the relevant Exchangeable Shares are
redeemed by Pioneer Canada pursuant to the Exchangeable Share Provisions in the
absence of the exercise by Pioneer of the Call Rights. See "Income Tax
Considerations -- Canadian Federal Income Tax Considerations."
    
 
     Retraction Call Right. Pursuant to the Exchangeable Share Provisions, a
holder requesting Pioneer Canada to redeem the Exchangeable Shares will be
deemed to offer such shares to Pioneer, and Pioneer will have an overriding
Retraction Call Right to acquire all but not less than all of the Exchangeable
Shares that the holder has requested Pioneer Canada to redeem in exchange for
one share of Pioneer Common Stock for each Exchangeable Share, plus an
additional amount equivalent to the full amount of all declared and unpaid
dividends thereon.
 
     At the time of a Retraction Request by a holder of Exchangeable Shares,
Pioneer Canada will immediately notify Pioneer. Pioneer must then advise Pioneer
Canada within two business days as to whether Pioneer will exercise the
Retraction Call Right. If Pioneer does not advise Pioneer Canada within such two
business day period, Pioneer Canada will notify the holder as soon as possible
thereafter that Pioneer will not exercise the Retraction Call Right. A holder
may revoke his or her Retraction Request, at any time prior to the close of
business on the business day preceding the Retraction Date in which case the
holder's Exchangeable Shares will neither be purchased by Pioneer nor redeemed
by Pioneer Canada. If the holder does not revoke his or her Retraction Request,
on the Retraction Date the Exchangeable Shares that the holder has requested
Pioneer Canada to redeem will be acquired by Pioneer (assuming Pioneer exercises
its Retraction Call Right) or redeemed by Pioneer Canada, as the case may be, in
each case for one share of Pioneer Common Stock for each Exchangeable Share plus
an additional amount equivalent to the full amount of all declared and unpaid
dividends on the Exchangeable Shares.
 
                                       12
<PAGE>   14
 
     Liquidation Call Right. Pursuant to the Plan of Arrangement, Pioneer will
be granted an overriding Liquidation Call Right, in the event of and
notwithstanding a proposed Pioneer Canada Insolvency Event, to acquire all but
not less than all of the Exchangeable Shares then outstanding in exchange for
Pioneer Common Stock and, upon the exercise by Pioneer of the Liquidation Call
Right, the holders thereof will be obligated to transfer such shares to Pioneer.
The acquisition by Pioneer of all of the outstanding Exchangeable Shares upon
the exercise of the Liquidation Call Right will occur on the effective date of
the voluntary or involuntary liquidation, dissolution or winding-up of Pioneer
Canada.
 
     Redemption Call Right. Pursuant to the Plan of Arrangement, Pioneer will be
granted an overriding Redemption Call Right, notwithstanding the proposed
automatic redemption of the Exchangeable Shares by Pioneer Canada pursuant to
the Exchangeable Share Provisions, to acquire on the Automatic Redemption Date
all but not less than all of the Exchangeable Shares then outstanding in
exchange for Pioneer Common Stock plus an additional amount equivalent to the
full amount of all declared and unpaid dividends on the Exchangeable Shares and,
upon the exercise by Pioneer of the Redemption Call Right, the holders thereof
will be obligated to transfer such shares to Pioneer.
 
   
     Effect of Call Right Exercise. If Pioneer exercises one or more of its Call
Rights, it will directly issue shares of Pioneer Common Stock to holders of
Exchangeable Shares and will become the holder of such Exchangeable Shares.
Pioneer will not be entitled to exercise any voting rights attached to the
Exchangeable Shares it so acquires. If Pioneer declines to exercise its Call
Rights when applicable, it will be required, pursuant to the Support Agreement,
to issue shares of Pioneer Common Stock to Pioneer Canada which will, in turn,
transfer such stock to the holders of Exchangeable Shares in consideration for
the return and cancellation of such Exchangeable Shares. The tax consequences
resulting from Pioneer's exercise of one or more of the Call Rights are
discussed in "Income Tax Considerations -- Canadian Federal Income Tax
Considerations," which includes a discussion of deemed dividends and Part VI.1
tax.
    
 
                              PLAN OF DISTRIBUTION
 
EXCHANGEABLE SHARES
 
     Pursuant to the terms of the Combination Agreement, Pioneer Canada will
acquire all the outstanding Chauvco Common Shares, other than Chauvco Common
Shares held by holders who properly exercised their rights of dissent and are
ultimately entitled to be paid fair value for their shares, in an arrangement
subject to the approval by the Court of Queen's Bench of Alberta pursuant to
Part 15 of the Business Corporations Act (Alberta). At the Effective Time, each
existing holder of Chauvco Common Shares will automatically transfer each
Chauvco Common Share such holder holds to Pioneer Canada in consideration for
(i) a number of shares of Pioneer Common Stock or Exchangeable Shares, or a
combination of both, determined in accordance with an agreed-upon exchange ratio
(the "Exchange Ratio") and, in certain cases, based upon such holder's election,
and (ii) one common share ("CRI Share") of Chauvco Resources International Ltd.
("CRI"), a wholly-owned subsidiary of Chauvco which will have its principal
properties, operations and oil reserves located in Gabon, central west Africa.
Only holders resident in Canada may elect to receive Exchangeable Shares, and
such holders may elect to have their Chauvco Common Shares transferred for a
combination of shares of Pioneer Common Stock and Exchangeable Shares. In
addition, each existing holder of options to purchase Chauvco Common Shares who
has not exercised its right of dissent in accordance with Article 3 of the Plan
of Arrangement will automatically transfer to Pioneer Canada each Chauvco option
such holder holds in consideration for (i) one CRI Share and (ii) a number of
shares of Pioneer Common Stock determined in accordance with the Exchange Ratio
and in accordance with the holder's election of whether or not to pay the
exercise price of such Chauvco option in cash. In certain circumstances, Pioneer
has the right to cause Pioneer Canada to deliver fewer shares of Pioneer Common
Stock and Exchangeable Shares and to pay cash to the Chauvco shareholders and to
Chauvco optionholders. As a result of the Arrangement, Chauvco will become a
wholly-owned subsidiary of Pioneer Canada, and CRI will be transferred to
Chauvco's shareholders and optionholders.
 
                                       13
<PAGE>   15
 
     Pioneer Common Stock may be issued to holders of Exchangeable Shares as
follows: (i) holders of Exchangeable Shares may require at any time that such
shares be exchanged for an equivalent number of shares of Pioneer Common Stock
plus an additional amount equivalent to all declared and unpaid dividends on
such Exchangeable Shares (see "-- Procedures for Issuance of Pioneer Common
Stock -- Exchange and Call Right"); (ii) subject to applicable law and the
Redemption Call Rights of Pioneer described below, Pioneer Canada must redeem
such Exchangeable Shares on the Automatic Redemption Date by exchanging therefor
an equal number of shares of Pioneer Common Stock plus an additional amount
equivalent to the full amount of all declared and unpaid dividends on such
Exchangeable Shares (see "-- Procedures for Issuance of Pioneer Common
Stock -- Exchange and Call Right"); (iii) upon the occurrence of a Pioneer
Canada Insolvency Event, holders of Exchangeable Shares may elect to, or, in the
case of Pioneer's exercise of the Liquidation Call Right, will be obligated to,
exchange such Exchangeable Shares for shares of Pioneer Common Stock (see
"-- Procedures for Issuance of Pioneer Common Stock -- Liquidation Rights of
Holders of Exchangeable Shares"); and (iv) upon the occurrence of a Pioneer
Liquidation Event, Pioneer will be required to acquire each outstanding
Exchangeable Share by exchanging one share of Pioneer Common Stock for each such
Exchangeable Share, plus an additional amount equivalent to the full amount of
all declared and unpaid dividends on the Exchangeable Shares (see "-- Procedures
for Issuance of Pioneer Common Stock -- Liquidation Rights of Holders of
Exchangeable Shares").
 
PROCEDURES FOR ISSUANCE OF PIONEER COMMON STOCK
 
     Exchange and Call Right. Holders of the Exchangeable Shares will be
entitled at any time following the Effective Time to retract (i.e., require
Pioneer Canada to redeem) any or all such Exchangeable Shares owned by them and
to receive an equivalent number of shares of Pioneer Common Stock plus an
additional amount equivalent to all declared and unpaid dividends on such
Exchangeable Shares. Holders of the Exchangeable Shares may effect such
retraction by presenting a certificate or certificates to Pioneer Canada or its
transfer agent representing the number of Exchangeable Shares the holder desires
to retract together with a duly executed statement in the form of Exhibit A to
the Exchangeable Share Provisions or in such other form as may be acceptable to
Pioneer Canada (the "Retraction Request") specifying the number of Exchangeable
Shares the holder wishes to retract and the date upon which the holder desires
to receive the Pioneer Common Stock, which must be between three and 10 business
days after the request is received by Pioneer Canada (the "Retraction Date"),
and such other documents as may be required to effect the retraction of the
Exchangeable Shares.
 
     Upon receipt of the Exchangeable Shares, the Retraction Request and other
required documentation from the holder thereof, Pioneer Canada must immediately
notify Pioneer of such Retraction Request. Pioneer will thereafter have two
business days in which to exercise the Retraction Call Right to purchase all of
the Exchangeable Shares submitted by the holder thereof by the delivery of an
equivalent number of shares of Pioneer Common Stock plus an additional amount
equivalent to the full amount of all declared and unpaid dividends on the
Exchangeable Shares to the transfer agent for delivery to such holder on the
Retraction Date. In the event Pioneer determines not to exercise its Retraction
Call Right and provided that the Retraction Request is not revoked in accordance
with the Exchangeable Share Provisions, Pioneer Canada is obligated to deliver
to the holder the number of shares of Pioneer Common Stock equal to the number
of Exchangeable Shares submitted by the holder for retraction and payment of an
additional amount equivalent to the full amount of all declared and unpaid
dividends on such Exchangeable Shares by the Retraction Date.
 
     Subject to applicable law and the Redemption Call Rights of Pioneer
described below, on the Automatic Redemption Date, Pioneer Canada must redeem
all but not less than all of the then outstanding Exchangeable Shares in
exchange for an equal number of shares of Pioneer Common Stock, plus an
additional amount equivalent to the full amount of all declared and unpaid
dividends on such Exchangeable Shares. Notwithstanding any proposed redemption
of the Exchangeable Shares of Pioneer Canada, Pioneer will have the overriding
right to purchase on the Automatic Redemption Date all but not less than all of
the then outstanding Exchangeable Shares in exchange for one share of Pioneer
Common Stock for each such Exchangeable Share, plus an additional amount
equivalent to the full amount of all declared and unpaid dividends on such
Exchangeable Share. Pioneer Canada shall, at least 120 days before the Automatic
 
                                       14
<PAGE>   16
 
Redemption Date, provide the registered holders of Exchangeable Shares with
written notice of the proposed redemption or purchase of the Exchangeable Shares
by Pioneer Canada or Pioneer, as the case may be. For a more detailed
description of the Exchange Rights and the Call Rights in connection with the
Exchangeable Shares, see "Description of Capital Stock -- Pioneer Canada Share
Capital -- Exchangeable Shares of Pioneer Canada," "-- Voting and Exchange Trust
Agreement -- Exchange Rights" and "-- Call Rights."
 
     Effect of Call Right Exercise. If Pioneer exercises one or more of its Call
Rights, it will directly issue shares of Pioneer Common Stock to holders of
Exchangeable Shares and will become the holder of such Exchangeable Shares.
Pioneer will not be entitled to exercise any voting rights attached to the
Exchangeable Shares it so acquires. If Pioneer declines to exercise its Call
Rights when applicable, it will be required, pursuant to the Support Agreement,
to issue Pioneer Common Stock to Pioneer Canada which will, in turn, transfer
such stock to the holders of Exchangeable Shares in consideration for the return
and cancellation of such Exchangeable Shares. The tax consequences resulting
from Pioneer's exercise of one or more of the Call Rights are discussed in
"Income Tax Considerations -- Canadian Federal Income Tax Considerations," which
includes a discussion of deemed dividends and Part VI.1 tax.
 
     Upon the occurrence of a Pioneer Canada Insolvency Event, holders of the
Exchangeable Shares will have preferential rights to receive from Pioneer one
share of Pioneer Common Stock for each Exchangeable Share they hold, plus an
additional amount equivalent to the full amount of any declared and unpaid
dividends on each such Exchangeable Share. In the event of a proposed Pioneer
Canada Insolvency Event, Pioneer will have the right to purchase all of the
outstanding Exchangeable Shares from the holders thereof at the effective time
of any such liquidation, dissolution, or winding up in exchange for one share of
Pioneer Common Stock for each such Exchangeable Share, plus an additional amount
equivalent to the full amount of all declared and unpaid dividends on such
Exchangeable Share.
 
     Upon the occurrence of a Pioneer Liquidation Event, in order for the
holders of the Exchangeable Shares to participate on a pro rata basis with the
holders of Pioneer Common Stock, each holder of Exchangeable Shares will
automatically receive in exchange therefor an equivalent number of shares of
Pioneer Common Stock, plus an additional amount equivalent to the full amount of
any declared and unpaid dividends on such Exchangeable Shares. For a more
detailed description of the Exchange Rights and the Call Rights in connection
with the Exchangeable Shares, see "Description of Capital Stock -- Voting and
Exchange Trust Agreement."
 
                           INCOME TAX CONSIDERATIONS
 
CANADIAN FEDERAL INCOME TAX CONSIDERATIONS
 
     In the opinion of MacKimmie Matthews, Canadian tax counsel for Pioneer
Canada, the following is a summary of the principal Canadian federal income tax
considerations generally applicable to the holders of Exchangeable Shares (the
"Pioneer Canada Shareholders") who, for purposes of the Canadian Tax Act, will
hold their Exchangeable Shares and shares of Pioneer Common Stock as capital
property and deal at arm's length with Pioneer and Pioneer Canada. This summary
does not apply to a holder with respect to whom Pioneer is a foreign affiliate
within the meaning of the Canadian Tax Act.
 
     All Pioneer Canada Shareholders should consult their own tax advisors as to
whether, as a matter of fact, they hold their Exchangeable Shares and shares of
Pioneer Common Stock as capital property for the purposes of the Canadian Tax
Act. In addition, the mark-to-market rules contained in the Canadian Tax Act
relating to financial institutions (including certain financial institutions,
registered securities dealers and corporations controlled by one or more of the
foregoing) will deem such financial institutions not to hold their Exchangeable
Shares and shares of Pioneer Common Stock as capital property for purposes of
the Canadian Tax Act. Pioneer Canada Shareholders that are financial
institutions should consult their own tax advisors to determine the tax
consequences to them of the application of the mark-to-market rules.
 
     This summary is based on the current provisions of the Canadian Tax Act,
the regulations thereunder, the current provisions of the Canada-United States
Income Tax Convention, 1980, as amended (the "Tax
 
                                       15
<PAGE>   17
 
Treaty"), and Canadian tax counsel's understanding of the current administrative
practices of Revenue Canada, Customs, Excise and Taxation ("Revenue Canada").
This summary takes into account the amendments to the Canadian Tax Act and
regulations publicly announced by the Minister of Finance prior to the date
hereof (the "Proposed Amendments") and assumes that all Proposed Amendments will
be enacted in their present form. However, no assurances can be given that the
Proposed Amendments will be enacted in the form proposed, or at all.
 
     Except for the foregoing, this summary does not take into account or
anticipate any changes in law, whether by legislative, administrative or
judicial decision or action, nor does it take into account provincial,
territorial or foreign income tax legislation or considerations, which may
differ from the Canadian federal income tax considerations described herein.
 
     WHILE THIS SUMMARY IS INTENDED TO ADDRESS ALL PRINCIPAL CANADIAN FEDERAL
INCOME TAX CONSIDERATIONS, IT IS OF A GENERAL NATURE ONLY AND IS NOT INTENDED TO
BE, NOR SHOULD IT BE CONSTRUED TO BE, LEGAL OR TAX ADVICE TO ANY PARTICULAR
PIONEER CANADA SHAREHOLDER. THEREFORE, EACH PIONEER CANADA SHAREHOLDER IS URGED
TO CONSULT WITH SUCH SHAREHOLDER'S OWN TAX ADVISORS WITH RESPECT TO SUCH
SHAREHOLDER'S PARTICULAR CIRCUMSTANCES. NO ADVANCE INCOME TAX RULING HAS BEEN
OBTAINED FROM REVENUE CANADA TO CONFIRM THE TAX CONSEQUENCES OF ANY OF THE
TRANSACTIONS DESCRIBED HEREIN.
 
     For purposes of the Canadian Tax Act, all amounts relating to the
acquisition, holding or disposition of shares of Pioneer Common Stock, including
dividends, adjusted cost base and proceeds of disposition must be converted into
Canadian dollars based on the prevailing United States dollar exchange rate at
the time such amounts arise.
 
  Pioneer Canada Shareholders Resident in Canada
 
     The following portion of the summary is applicable to Pioneer Canada
Shareholders who, for purposes of the Canadian Tax Act, are resident or deemed
to be resident in Canada.
 
     Dividends Paid on Exchangeable Shares. A shareholder who is an individual
will include dividends received or deemed to be received on the Exchangeable
Shares in computing the shareholder's income, and will be subject to the
gross-up and dividend tax credit rules normally applicable to taxable dividends
received from taxable Canadian corporations.
 
     Subject to the special rules described below, in the case of a shareholder
that is a corporation, dividends received or deemed to be received on the
Exchangeable Shares will normally be deductible in computing its taxable income.
A shareholder that is a "private corporation" (as defined in the Canadian Tax
Act), or any other corporation resident in Canada and controlled or deemed to be
controlled by or for the benefit of an individual or a related group of
individuals, may be liable under Part IV of the Canadian Tax Act to pay a
refundable tax of 33 1/3% on dividends received or deemed to be received on the
Exchangeable Shares to the extent that such dividends are deductible in
computing the shareholder's taxable income.
 
     Special rules will apply to certain corporations that receive dividends on
the Exchangeable Shares:
 
          (a) In the case of a shareholder that is a "specified financial
     institution" under the Canadian Tax Act, a dividend received on its
     Exchangeable Shares will be deductible in computing its taxable income only
     if either:
 
             (i) the specified financial institution did not acquire the
        Exchangeable Shares in the ordinary course of the business carried on by
        such institution; or
 
             (ii) at the time of the receipt of the dividend by the specified
        financial institution, the Exchangeable Shares are listed on a
        prescribed stock exchange in Canada (which currently includes the TSE)
        and the specified financial institution, either alone or together with
        persons with whom it
 
                                       16
<PAGE>   18
 
        does not deal at arm's length, does not receive (or is not deemed to
        receive) dividends in respect of more than 10% of the issued and
        outstanding Exchangeable Shares.
 
          (b) If Pioneer Canada or any person with whom Pioneer Canada does not
     deal at arm's length is a specified financial institution when a dividend
     is paid on an Exchangeable Share, then dividends received or deemed to be
     received by a shareholder that is a corporation may not be deductible in
     computing taxable income but, instead, may be fully included in taxable
     income under Part I of the Canadian Tax Act. Pioneer Canada has informed
     Canadian tax counsel that it is of the view that neither it nor any person
     with whom it does not deal at arm's length is a specified financial
     institution at the current time, but there can be no assurances that this
     status will not change prior to any dividend which is received or deemed to
     be received by a corporate shareholder.
 
     The Exchangeable Shares will be "taxable preferred shares" and "short-term
preferred shares" for purposes of the Canadian Tax Act. Accordingly, Pioneer
Canada will be subject to a 66 2/3% tax under Part VI.1 of the Canadian Tax Act
on dividends, other than excluded dividends, paid or deemed to be paid on the
Exchangeable Shares. Dividends received or deemed to be received on the
Exchangeable Shares will not be subject to the 10% tax under Part IV.1 of the
Canadian Tax Act applicable to certain corporations.
 
     In the event the United States Internal Revenue Service (the "IRS") imposes
U.S. withholding tax with respect to dividends paid on the Exchangeable Shares
as set out in "-- United States Federal Income Tax Considerations -- Non-U.S.
Holders -- Dividends," it is unlikely that a Canadian resident Pioneer Canada
Shareholder will be entitled to a foreign tax credit in Canada with respect to
such U.S. withholding tax.
 
     Redemption or Exchange of Exchangeable Shares. The disposition of an
Exchangeable Share for a share of Pioneer Common Stock will be a taxable
transaction. A disposition can occur as a redemption by Pioneer Canada or as an
acquisition by Pioneer. The Canadian federal income tax consequences of a
redemption differ from those of an acquisition. A holder exercising the right of
retraction in respect of an Exchangeable Share cannot control whether the share
will be acquired by Pioneer under the Retraction Call Right or redeemed by
Pioneer Canada if the Retraction Call Right is not exercised; however, a holder
who exercises the right of retraction will be notified if the Retraction Call
Right will not be exercised by Pioneer and the holder may cancel the Retraction
Request and retain the Exchangeable Share.
 
     The following explains the tax treatment to a holder of an Exchangeable
Share on a redemption by Pioneer Canada or an acquisition by Pioneer.
 
     (a) Redemption of Exchangeable Shares by Pioneer Canada. On a redemption
(including a retraction) of an Exchangeable Share by Pioneer Canada, a portion
of the redemption proceeds may be deemed to be a dividend received by the
holder. The deemed dividend is calculated as the amount, if any, by which the
redemption proceeds exceeds the paid-up capital at that time of the Exchangeable
Share. The deemed dividend is subject to the tax treatment accorded to dividends
described above. Pioneer Canada is required to calculate the deemed dividend and
report the amount to the holder of the Exchangeable Share. The holder of the
Exchangeable Share may also have a capital gain or capital loss as a result of
the redemption to the extent that the proceeds of disposition of the
Exchangeable Share, less any reasonable cost of disposition, exceed (or are less
than) the adjusted cost base to the holder of the Exchangeable Share. The holder
of the Exchangeable Share is considered to have disposed of the Exchangeable
Share for proceeds of disposition equal to the redemption proceeds less the
amount of the deemed dividend. In the case of a shareholder that is a
corporation, in some circumstances the amount of any deemed dividend may be
treated as proceeds of disposition, and not as a dividend, for purposes of
calculating a capital gain. Three-quarters of any capital gain (referred to in
the Canadian Tax Act as a "taxable capital gain") must be included in income and
three-quarters of any capital loss (referred to in the Canadian Tax Act as an
"allowable capital loss") may generally be used to offset taxable capital gains
in the year the loss arises, in any of the three prior years or in any
subsequent year.
 
     A Pioneer Canada Shareholder that is throughout the relevant taxation year
a "Canadian-controlled private corporation" (as defined in the Canadian Tax Act)
may be liable to pay an additional refundable tax of
 
                                       17
<PAGE>   19
 
6 2/3% on its "aggregate investment income" for the year, which is defined to
include taxable capital gains (but not dividends or deemed dividends deductible
in computing taxable income).
 
     If a Pioneer Canada Shareholder is a corporation, the amount of any capital
loss arising from a disposition of Exchangeable Shares may be reduced by the
amount of dividends received or deemed to have been received by it on the
Exchangeable Shares previously owned by such holder, to the extent and under
circumstances prescribed by the Canadian Tax Act. Similar rules may apply where
a corporation is a member of a partnership or a beneficiary of a trust that owns
Exchangeable Shares or where a trust or partnership of which a corporation is a
beneficiary or a member is a member of a partnership or a beneficiary of a trust
that owns Exchangeable Shares.
 
     Capital gains realized by a Pioneer Canada Shareholder who is an individual
may be subject to an alternative minimum tax. The Canadian Tax Act provides that
the tax payable by individuals (other than certain trusts) is the greater of the
tax otherwise determined and an alternative minimum tax. For purposes of
computing the alternative minimum tax, one-quarter of all capital gains are
added back to the individual's income as otherwise determined.
 
     (b) Acquisition of Exchangeable Shares by Pioneer. On the acquisition of an
Exchangeable Share by Pioneer, the holder will in general realize a capital gain
(or a capital loss) equal to the amount by which the proceeds of disposition of
the Exchangeable Share, less any reasonable cost of disposition, exceed (or are
less than) the adjusted cost base to the holder of the Exchangeable Share. A
holder will be required to include in income any such capital gain or capital
loss in the manner and subject to the rules described above. The acquisition of
an Exchangeable Share by Pioneer will not result in a deemed dividend to the
holder of the Exchangeable Share.
 
     The redemption proceeds in the case of a redemption by Pioneer Canada and
the proceeds of disposition in the case of an acquisition by Pioneer will be the
fair market value of a share of Pioneer Common Stock at the time of the
disposition plus the amount of all declared but unpaid dividends on the
Exchangeable Share.
 
     The cost base of a share of Pioneer Common Stock received on the
retraction, redemption or exchange of an Exchangeable Share will be equal to the
fair market value of the share of Pioneer Common Stock at the time of such
event.
 
     The following example illustrates the different treatment for a redemption
by Pioneer Canada and an acquisition by Pioneer, assuming hypothetically that
the fair market value of the Pioneer Common Stock at the time of an exchange is
$50.00, the Exchangeable Shares have a paid-up capital at that time of $5.00 per
share and the adjusted cost base to the holder of the Exchangeable Share is
$10.00.
 
<TABLE>
<CAPTION>
                                                              REDEMPTION    ACQUISITION
                                                              ----------    -----------
<S>                                                           <C>           <C>
Exchange Consideration......................................    $50.00        $50.00
Deemed Dividend(a)..........................................     45.00           (b)
Proceeds of Disposition for capital gains purposes..........      5.00         50.00
Adjusted Cost Base..........................................     10.00         10.00
                                                                ------        ------
Capital Gain (Capital Loss)(a)..............................    $(5.00)       $40.00
                                                                ======        ======
</TABLE>
 
- ---------------
 
(a) These line items (being the deemed dividend and the capital gain (or capital
    loss)) must be reported by the former holder of the Exchangeable Share.
 
(b) There is no deemed dividend in the case of an acquisition of the
    Exchangeable Shares by a person other than Pioneer Canada.
 
     Dividends on Pioneer Common Stock. Dividends paid on shares of Pioneer
Common Stock will be included in the recipient's income for the purposes of the
Canadian Tax Act. Such dividends received by an individual shareholder will not
be subject to the gross-up and dividend tax credit rules in the Canadian Tax
Act. A corporation which is a shareholder will include such dividends in
computing its taxable income. United States withholding tax on such dividends
will be eligible for foreign tax credit or deduction treatment where
 
                                       18
<PAGE>   20
 
applicable under the Canadian Tax Act. Special considerations may also apply to
the receipt of dividends by a Canadian resident corporate shareholder at a time
when Pioneer is a "foreign affiliate" of the shareholder within the meaning of
the Canadian Tax Act.
 
     Disposition of Pioneer Common Stock. A disposition or deemed disposition of
a share of Pioneer Common Stock by a holder will generally result in a capital
gain (or capital loss) equal to the amount by which the proceeds of disposition,
net of any reasonable costs of disposition, exceed (or are less than) the
adjusted cost base to the holder of the share. Such holders will be required to
include in income any such capital gain or capital loss in the manner and
subject to the rules described above.
 
  Eligibility for Investment
 
     Qualified Investments. Provided the Exchangeable Shares and the shares of
Pioneer Common Stock are listed on a prescribed stock exchange (which currently
includes the TSE and the NYSE), such securities will be qualified investments
under the Canadian Tax Act for trusts governed by registered retirement savings
plans, registered retirement income funds and deferred profit sharing plans. The
Voting Rights and Exchange Rights will not be qualified investments under the
Canadian Tax Act. However, Pioneer Canada is of the view that the fair market
value of these rights is nominal.
 
     Foreign Property. Provided the Exchangeable Shares are listed on a
prescribed stock exchange in Canada (which currently includes the TSE) and
Pioneer Canada continues to maintain a substantial presence in Canada, the
Exchangeable Shares will not be foreign property under the Canadian Tax Act for
trusts governed by registered pension plans, registered retirement savings
plans, registered retirement income funds and deferred profit sharing plans or
for certain other tax-exempt persons. Pioneer Canada will be considered to have
a substantial presence in Canada if it satisfies certain asset tests or if it
maintains an office in Canada and Pioneer Canada, or a corporation controlled by
it, employs more than five employees in Canada full time in the active conduct
of a business, other than an investment activity or a business carried on
through a partnership of which the corporation is not a majority interest
partner. Pioneer Canada has informed Canadian tax counsel that following the
Arrangement, Pioneer Canada will satisfy this substantial presence test and
expects that it will continue to satisfy the test.
 
     The shares of Pioneer Common Stock will be foreign property under the
Canadian Tax Act as will the Voting Rights and Exchange Rights.
 
  Pioneer Canada Shareholders Not Resident in Canada
 
     The following portion of the summary is applicable to Pioneer Canada
Shareholders who, for purposes of the Canadian Tax Act, have not been and will
not be resident or deemed to be resident in Canada at any time while they have
held Exchangeable Shares or will hold shares of Pioneer Common Stock and to whom
such shares are not "taxable Canadian property" and, in the case of a
non-resident of Canada who carries on an insurance business in Canada and
elsewhere, the shares are not effectively connected with its Canadian insurance
business.
 
     Pioneer Canada Shareholders not resident in Canada are not entitled to
receive Exchangeable Shares pursuant to the Plan of Arrangement, and it is not
expected that a Pioneer Canada Shareholder not resident in Canada would, in
general, wish to acquire Exchangeable Shares on the trading markets after their
original issuance. Accordingly, this portion of the summary is expected to have
limited applicability.
 
     A holder of Exchangeable Shares will not be subject to tax under the
Canadian Tax Act on the exchange of an Exchangeable Share for a share of Pioneer
Common Stock, except to the extent the exchange gives rise to a deemed dividend
(as described above for Canadian residents), or on the sale or other disposition
of an Exchangeable Share or a share of Pioneer Common Stock, unless the shares
constitute "taxable Canadian property" to the holder (within the meaning of the
Canadian Tax Act). Generally, the Exchangeable Shares and shares of Pioneer
Common Stock will not be taxable Canadian property provided that such shares are
listed on a prescribed stock exchange (which currently includes the TSE and the
NYSE), the holder does not use or hold, and is not deemed to use or hold, the
Exchangeable Shares or the shares of Pioneer Common
 
                                       19
<PAGE>   21
 
Stock, as applicable, in connection with carrying on a business in Canada and
the holder, persons with whom such holder does not deal at arm's length, or the
holder and such persons, has not owned (or had under option) 25% or more of the
issued shares of any class or series of the capital stock of Pioneer Canada or
Pioneer at any time within five years preceding the date in question. In the
case of Pioneer, even if the holder exceeds the 25% threshold with respect to
shares of Pioneer Common Stock, any gain on the sale or other disposition of
such shares may not be taxable in Canada, and such holders should consult their
own tax advisors. The Exchangeable Shares have been conditionally approved for
listing on the TSE, and Pioneer has indicated that it intends to use its best
efforts to cause Pioneer Canada to maintain such listing.
 
     Any dividends paid on the Exchangeable Shares will be subject to
non-resident withholding tax under the Canadian Tax Act at the rate of 25%,
although such rate may be reduced under the provisions of an applicable income
tax treaty. For example, under the Tax Treaty, the rate is generally reduced to
15% for dividends paid to a person who is resident in the United States for
purposes of the Tax Treaty.
 
UNITED STATES FEDERAL INCOME TAX CONSIDERATIONS
 
     This section summarizes the material United States federal income tax
considerations generally applicable to Pioneer Canada Shareholders of owning and
disposing of Exchangeable Shares (and Pioneer Common Stock received in exchange
therefor), but it does not purport to be a complete analysis of all of the
potential tax effects thereof.
 
     This summary is based on United States federal income tax law currently in
effect. The legal conclusions contained herein represent the opinion of Vinson &
Elkins L.L.P., United States counsel to Pioneer Canada. No statutory, judicial
or administrative authority exists which directly addresses certain of the
United States federal income tax consequences of the ownership of instruments
comparable to the Exchangeable Shares. Consequently, some aspects of the United
States federal income tax treatment of the holding and exchange of Exchangeable
Shares for shares of Pioneer Common Stock are not certain. No advance income tax
ruling has been sought or obtained from the United States Internal Revenue
Service (the "IRS") regarding the tax consequences of the transactions described
herein, and there can be no assurance that the IRS will not successfully
challenge certain of the conclusions reached in this discussion, particularly
those as to which uncertainty is indicated.
 
     Except as specifically stated otherwise, the discussion does not address
state, local, or foreign taxes or federal tax aspects other than United States
federal income taxation under the Internal Revenue Code of 1986, as amended (the
"U.S. Code"), nor does it address all aspects of such tax that may be applicable
to the specific circumstances of a particular Pioneer Canada Shareholder. The
discussion may not apply to Pioneer Canada Shareholders that are subject to
special provisions of the U.S. Code, such as tax-exempt organizations, financial
institutions, insurance companies, broker-dealers, persons having a "functional
currency" other than the United States dollar, and Pioneer Canada Shareholders
who hold Exchangeable Shares as part of a straddle, wash sale, hedging or
conversion transaction (other than by virtue of their participation in the
Arrangement).
 
     WHILE THIS SUMMARY IS INTENDED TO ADDRESS ALL PRINCIPAL UNITED STATES
FEDERAL INCOME TAX CONSIDERATIONS, IT IS OF A GENERAL NATURE ONLY AND IS NOT
INTENDED TO BE, NOR SHOULD IT BE CONSTRUED AS, LEGAL OR TAX ADVICE TO ANY
PARTICULAR PIONEER CANADA SHAREHOLDER. THEREFORE, EACH PIONEER CANADA
SHAREHOLDER IS URGED TO CONSULT WITH A TAX ADVISOR WITH RESPECT TO ITS
PARTICULAR CIRCUMSTANCES AS SUCH MAY AFFECT THE UNITED STATES FEDERAL, STATE AND
LOCAL TAX CONSEQUENCES AND THE FOREIGN TAX CONSEQUENCES OF THE ACQUISITION,
OWNERSHIP AND DISPOSITION OF EXCHANGEABLE SHARES AND PIONEER COMMON STOCK
RECEIVED IN EXCHANGE THEREFOR. NO ADVANCE INCOME TAX RULING HAS BEEN OBTAINED
FROM THE IRS TO CONFIRM THE TAX CONSEQUENCES OF ANY OF THE TRANSACTIONS
DESCRIBED HEREIN.
 
                                       20
<PAGE>   22
 
  Non-U.S. Holders
 
     General. This subsection is addressed to Pioneer Canada Shareholders that
(i) are Non-U.S. Holders (as defined below), and (ii) will hold Exchangeable
Shares (and Pioneer Common Stock received in exchange therefor) as capital
assets. For purposes of the foregoing, the term "Non-U.S. Holder" means a
nonresident alien individual, or a foreign corporation, a foreign partnership or
a foreign estate or trust which is other than a U.S. Holder (as defined below).
An individual may, subject to certain exceptions, be deemed to be a resident
alien (as opposed to a nonresident alien) by virtue of being present in the
United States for at least 31 days in the calendar year and for an aggregate of
at least 183 days during a three-year period ending in the current calendar year
(counting for such purposes all of the days present in the calendar year,
one-third of the days present in the immediately preceding year, and one-sixth
of the days present in the second preceding year). Resident aliens are subject
to tax as if they were U.S. citizens.
 
     It is expected that most Pioneer Canada Shareholders who, for purposes of
the Canadian Tax Act, are resident or deemed to be resident in Canada will be
Non-U.S. Holders. A Non-U.S. Holder seeking benefits under an applicable tax
treaty or an exemption from U.S. withholding tax for "effectively connected
income," as described below, may be required to comply with additional
certification and other requirements in order to establish his entitlement to
such benefits or exemption. This discussion does not consider U.S. tax
consequences that may be relevant to a Non-U.S. Holder who is a United States
expatriate or who has other special facts and circumstances.
 
     Dividends. Neither Pioneer Canada nor Pioneer intends to withhold any
amount for U.S. withholding tax on dividends paid to Non-U.S. Holders of
Exchangeable Shares. There is some possibility, however, that the IRS may
successfully assert that U.S. withholding tax is payable with respect to such
dividends.
 
     Dividends received by a Non-U.S. Holder with respect to Pioneer Common
Stock that are not effectively connected with the conduct of a trade or business
in the United States will generally be subject to U.S. withholding tax. The U.S.
withholding tax rate is 30% unless reduced by treaty. Under the Tax Treaty, the
rate currently is 15%, generally, on dividends paid to residents of Canada.
 
     Gain on Dispositions. A Non-U.S. Holder generally will not be subject to
U.S. tax on any gain recognized on the receipt, sale or other disposition of the
Exchangeable Shares, the exchange of the Exchangeable Shares for Pioneer Common
Stock, or the sale or other disposition of such Pioneer Common Stock, unless (a)
such gain is effectively connected with a trade or business in the United
States, or, if a tax treaty applies, is attributable to a permanent
establishment maintained by the Non-U.S. Holder in the United States, or (b) the
Non-U.S. Holder is an individual who is present in the United States for 183
days or more in the taxable year of disposition, and certain other conditions
are satisfied. In this connection, any Non-U.S. Holder who at any time actually
and constructively owns more than 5% of either the Exchangeable Shares or the
Pioneer Common Stock should be aware that Pioneer is probably a "United States
real property holding corporation" within the meaning of section 897 of the U.S.
Code, and therefore such Non-U.S. Holder should seek advice whether his gain or
loss would be deemed to be effectively connected with a U.S. business.
 
     U.S. Estate Tax. Pioneer Common Stock held by an individual Non-U.S. Holder
at the time of death will be deemed to be a U.S. situs asset for purposes of
U.S. estate tax law and therefore will generally be subject to the U.S. estate
tax, except as may otherwise be provided by an applicable tax or estate tax
treaty with the United States. No opinion is expressed as to the situs of
Exchangeable Shares for purposes of U.S. estate tax law.
 
  U.S. Holders
 
     General. This subsection is addressed to Pioneer Canada Shareholders that
(i) are U.S. Holders (as defined below) and (ii) hold Exchangeable Shares (and
Pioneer Common Stock received in exchange therefor) as capital assets. For
purposes of the foregoing, the term "U.S. Holder" means (a) a citizen or
individual resident of the United States, (b) a corporation or partnership
created or organized in or under the laws of the United States or any state
thereof, (c) an estate the income of which is includible in its gross income for
United States federal income tax purposes without regard to its source, or (d) a
trust as to which a
 
                                       21
<PAGE>   23
 
court in the United States is able to exercise primary supervision over the
administration of the trust and one or more United States fiduciaries have the
authority to control all substantial decisions of the trust.
 
     U.S. Holders of Chauvco Stock are not entitled to receive Exchangeable
Shares pursuant to the Plan of Arrangement, and it is not expected that a U.S.
Holder would, in general, wish to acquire Exchangeable Shares on the trading
markets after their original issuance. Accordingly, this subsection is expected
to have limited applicability.
 
     Ownership and Disposition of Exchangeable Shares. Assuming that Pioneer
Canada is not a PFIC (as defined in the following paragraphs of this
subsection), dividends paid to a U.S. Holder with respect to Exchangeable Shares
generally will constitute ordinary dividend income to the extent paid out of
Pioneer Canada's earnings and profits as determined under U.S. tax principles.
The amount of a dividend paid in non-U.S. currency will be calculated by
reference to the exchange rate in effect on the date the dividend is
distributed. Foreign currency gain or loss may be realized on any currency
received that is not converted into United States dollars on such date.
Dividends will generally be treated as foreign source passive income for foreign
tax credit limitation purposes. Any foreign tax imposed on such dividends will
generally be allowable either as a credit against the holder's United States
federal income tax liability (subject to applicable limitations) or as a
deduction in computing U.S. taxable income. Gain or loss recognized on the
disposition of Exchangeable Shares will generally be treated as U.S. source for
foreign tax credit limitation purposes.
 
     Exchange of Exchangeable Shares. A U.S. Holder that exercises such holder's
right to exchange its Exchangeable Shares for shares of Pioneer Common Stock
will generally recognize gain or loss on such exchange, subject to the
discussion below. Such gain or loss will be equal to the difference between the
fair market value of the shares of Pioneer Common Stock received, as determined
at the time of the exchange, and the U.S. Holder's tax basis in the Exchangeable
Shares surrendered. The gain or loss will generally be capital gain or loss,
except that, with respect to any declared but unpaid dividends on the
Exchangeable Shares, ordinary income may be recognized. A U.S. Holder will
generally, subject to the discussion below, have a tax basis in the shares of
Pioneer Common Stock received equal to the fair market value of such shares at
the time of the exchange. The holding period for such shares will generally,
subject to the discussion below, begin on the day after the exchange. The IRS
could assert, however, that the Exchangeable Shares and certain of the rights
associated therewith constitute "offsetting positions" for purposes of the
straddle rules set forth in Section 1092 of the U.S. Code. In such case, the
holding period of the Exchangeable Shares would not increase while held by a
U.S. Holder.
 
     It is possible, however, that a U.S. Holder who holds Exchangeable Shares
not acquired in the Arrangement would not be permitted to recognize loss on an
exchange of such Exchangeable Shares for Pioneer Common Stock. In that case, the
U.S. Holder will have a tax basis in the shares of Pioneer Common Stock received
equal to the tax basis of the Exchangeable Shares exchanged therefor, and the
U.S. Holder's holding period for the Pioneer Common Stock received will include
the U.S. Holder's holding period in the Exchangeable Shares exchanged therefor.
 
     Under the Taxpayer Relief Act of 1997 (the "1997 Act"), the maximum capital
gains rate for individuals has been reduced to 20% for capital assets held for
more than 18 months before disposition, but the rate applicable to gain realized
on the exchange of Exchangeable Shares by a U.S. Holder will depend upon the
holding period of such Exchangeable Shares, the holder's other taxable income,
and other factors.
 
     For U.S. foreign tax credit purposes, any gain realized by a U.S. Holder
will generally be treated as U.S. source, except that, if under the Tax Treaty
any Canadian tax is imposed on the exchange, the gain could be treated as
Canadian source at the election of the U.S. Holder. As a result of such
election, the Canadian tax would generally be available as a credit (subject to
applicable limitations) against the U.S. tax attributable to such gain. A U.S.
Holder that is ineligible for a foreign tax credit with respect to any tax paid
may be entitled to a deduction therefor in computing United States taxable
income.
 
     Under certain limited circumstances, an exchange by a U.S. Holder of
Exchangeable Shares for shares of Pioneer Common Stock may be characterized as a
tax-free exchange. In order to be so characterized, however, certain
requirements must be satisfied as of the date of the exchange. Whether such
requirements
 
                                       22
<PAGE>   24
 
can be satisfied will depend on facts and circumstances existing at that time
and which cannot be accurately predicted as of the date of this Prospectus.
Moreover, there is no direct authority as to the proper treatment for United
States federal income tax purposes of the exchange of Exchangeable Shares for
shares of Pioneer Common Stock. For these reasons, there can be no assurance
that an exchange of Exchangeable Shares for Pioneer Common Stock should properly
be treated as tax free under any circumstances. If, however, such exchange did
qualify as a tax-free exchange, a U.S. Holder's tax basis in the shares of
Pioneer Common Stock received would be equal to such holder's tax basis in the
Exchangeable Shares exchanged therefor, and the holding period of the shares of
Pioneer Common Stock received by such U.S. Holder should include the holding
period of the Exchangeable Share exchanged therefor.
 
     Passive Foreign Investment Company Considerations. For United States
federal income tax purposes, Pioneer Canada generally will be treated as a
passive foreign investment company (a "PFIC") for any taxable year ending after
the consummation of the Arrangement during which either (i) 75 percent or more
of its gross income was passive income (as defined for United States federal
income tax purposes) or (ii) on average for such taxable year, 50 percent or
more of its assets (measured as described below) produced or were held for the
production of passive income. For purposes of applying the foregoing tests, the
assets and gross income of Pioneer Canada's significant subsidiaries will be
attributed to Pioneer Canada.
 
     For purposes of applying the 50% assets test, and in the absence of
regulations affording an exception to securities listed for trading on the TSE,
Pioneer Canada's assets must be measured by their adjusted tax bases (as
calculated in order to compute earnings and profits for United States federal
income tax purposes), subject to certain adjustments. As a result, it is
possible that Pioneer Canada could be a PFIC even though less than 50% of
Pioneer Canada's assets (measured by the fair market value of such assets) do
not constitute passive assets.
 
     Pioneer and Pioneer Canada intend to endeavor to cause Pioneer Canada to
avoid PFIC status in the future, although there can be no assurance that they
will be able to do so or that their intent will not change. Moreover, in
connection with the transactions contemplated herein, no opinion will be deemed
given regarding Pioneer Canada's status as a PFIC. After the Arrangement,
Pioneer Canada will endeavor to notify U.S. Holders of Exchangeable Shares if it
believes that Pioneer Canada was a PFIC for that taxable year.
 
     If a corporation is a PFIC at any time in which a U.S. Holder owns its
shares and has not elected ("QEF Election") to treat the corporation as a
qualified electing fund, the gain recognized on the disposition of such shares
is taxed as ordinary income at rates that might be higher than those otherwise
applicable, and an interest charge is imposed on such gain. The general effect
of a QEF election is to require the U.S. Holder's pro rata share of the
corporation's income for each taxable year in which the corporation is
classified as a PFIC to be included in his taxable income, even if no dividend
distributions are received. The holder, however, can also elect to defer payment
of the taxes on such income, subject to an interest charge, until the shares are
transferred or a distribution on such shares is received. In addition, it is
possible that under the 1997 Act a U.S. Holder will be permitted, beginning in
1998, to make a mark-to-market election with respect to shares of a PFIC in lieu
of incurring the special tax and interest charge on disposition of those shares.
 
     The foregoing summary of the possible application of the PFIC rules to
Pioneer Canada and the U.S. Holders of Exchangeable Shares is only a summary of
certain material aspects of those rules. Because the United States federal
income tax consequences to such U.S. Holders under the PFIC provisions are
significant and complex, such U.S. Holders are urged to discuss these
consequences with their tax advisors.
 
                                 LEGAL MATTERS
 
   
     Certain legal matters in connection with the shares of Pioneer Common Stock
offered hereby will be passed upon for Pioneer by Vinson & Elkins L.L.P.,
Dallas, Texas. Certain federal Canadian and U.S. tax matters will be passed upon
for Pioneer by MacKimmie Matthews, Calgary, Canada, and Vinson & Elkins L.L.P.,
Dallas, Texas, as set forth under "Income Tax Considerations." Michael D.
Wortley, a partner in the law firm of Vinson & Elkins L.L.P., is a director of
Pioneer and beneficially owns 6,623 shares of Pioneer Common Stock.
    
 
                                       23
<PAGE>   25
 
                                    EXPERTS
 
   
     The Consolidated Financial Statements of Pioneer (successor to Parker &
Parsley and subsidiaries) have been incorporated by reference in this
Registration Statement in reliance upon the report of KPMG Peat Marwick LLP,
independent certified public accountants, and upon the authority of said firm as
experts in accounting and auditing. The report of KPMG Peat Marwick LLP refers
to a change in the method of accounting for the impairment of long-lived assets
and for long-lived assets to be disposed of in 1995 and a change in the method
of accounting for income taxes in 1993.
    
 
     The Consolidated Financial Statements of Mesa incorporated by reference in
this Registration Statement have been audited by Arthur Andersen LLP,
independent public accountants, as indicated in their report with respect
thereto, and are incorporated by reference herein in reliance upon the authority
of said firm as experts in giving said report.
 
   
     The Financial Statements of Greenhill Petroleum Corporation incorporated by
reference in this Registration Statement, have been incorporated herein in
reliance on the report of Coopers & Lybrand L.L.P., independent accountants,
given on authority of that firm as experts in accounting and auditing.
    
 
     The Consolidated Financial Statements of Chauvco incorporated by reference
in this Registration Statement have been audited by Price Waterhouse, chartered
accountants, as indicated in their report with respect thereto, and are
incorporated by reference herein in reliance upon the authority of said firm as
experts in giving said report.
 
   
     The estimates of proved reserves of Pioneer (successor to Parker & Parsley
and subsidiaries) as of December 31, 1996, incorporated by reference in this
Registration Statement are based upon a reserve report prepared by Pioneer and
audited by Netherland, Sewell & Associates, Inc., independent petroleum
consultants, and are incorporated by reference herein upon the authority of such
firm as experts with respect to such matters covered by such report.
    
 
   
     The estimates of Mesa's proved reserves as of December 31, 1996,
incorporated by reference in this Registration Statement with respect to its
Hugoton and West Panhandle field properties are based upon a reserve report
prepared by Williamson Petroleum Consultants, Inc., independent petroleum
consultants, and are incorporated by reference herein upon the authority of such
firm as experts with respect to such matters covered by such report. The
estimates of Greenhill Petroleum Corporation's proved reserves as of December
31, 1996, incorporated by reference in this Registration Statement pursuant to
items 9 and 14 under the section entitled "Incorporation of Certain Documents By
Reference" in this Registration Statement, are based upon a reserve report
prepared by Miller and Lents, Ltd., independent petroleum consultants, and are
incorporated by reference herein upon the authority of such firm as experts with
respect to such matters covered by such report.
    
 
     The estimates of Chauvco's proved reserves as of December 31, 1996,
incorporated by reference in this Registration Statement are based upon reserve
reports prepared by Gilbert Lausten Jung Associates, Ltd. and Martin Petroleum
and Associates, independent petroleum consultants, and are incorporated by
reference herein upon the authority of such firm as experts with respect to such
matters covered by such report.
 
                                       24
<PAGE>   26
 
             ======================================================
 
     NO PERSON HAS BEEN AUTHORIZED TO GIVE ANY INFORMATION OR TO MAKE ANY
REPRESENTATIONS OTHER THAN THOSE CONTAINED IN THIS PROSPECTUS OR IN A PROSPECTUS
SUPPLEMENT, AND IF GIVEN OR MADE, SUCH INFORMATION OR REPRESENTATIONS MUST NOT
BE RELIED UPON AS HAVING BEEN AUTHORIZED BY PIONEER OR ANY OTHER PERSON. NEITHER
THIS PROSPECTUS NOR ANY PROSPECTUS SUPPLEMENT CONSTITUTES AN OFFER TO SELL OR A
SOLICITATION OF ANY OFFER TO BUY ANY SECURITIES OTHER THAN THE SECURITIES TO
WHICH IT RELATES, NOR DOES IT CONSTITUTE AN OFFER TO SELL OR A SOLICITATION OF
ANY OFFER TO BUY THE SECURITIES IN ANY JURISDICTION IN WHICH THE OFFER OR
SOLICITATION IS NOT AUTHORIZED, OR IN WHICH THE PERSON MAKING THE OFFER OR
SOLICITATION IS NOT QUALIFIED TO DO SO, OR TO ANY PERSON TO WHOM IT IS UNLAWFUL
TO MAKE THE OFFER OR SOLICITATION. NEITHER THE DELIVERY OF THIS PROSPECTUS NOR
ANY PROSPECTUS SUPPLEMENT NOR ANY SALE MADE HEREUNDER OR THEREUNDER SHALL, UNDER
ANY CIRCUMSTANCES, CREATE ANY IMPLICATION THAT ANY INFORMATION CONTAINED HEREIN
OR THEREIN IS CORRECT AS OF ANY TIME SUBSEQUENT TO THE DATE HEREOF.
 
                             ---------------------
 
                               TABLE OF CONTENTS
 
   
<TABLE>
<CAPTION>
                                         PAGE
                                         ----
<S>                                      <C>
Available Information..................    2
Incorporation of Certain Documents by
  Reference............................    2
Risk Factors...........................    3
Pioneer................................    4
Use of Proceeds........................    4
Description of Capital Stock...........    4
Plan of Distribution...................   13
Income Tax Considerations..............   15
Legal Matters..........................   23
Experts................................   24
</TABLE>
    
 
             ======================================================
 
             ======================================================
 
   
                               18,004,289 SHARES
    
 
                                PIONEER NATURAL
                               RESOURCES COMPANY
 
                                  COMMON STOCK
                               ------------------
 
                                   PROSPECTUS
                               ------------------
                               DECEMBER   , 1997
 
             ======================================================
<PAGE>   27
 
                                    PART II
 
                     INFORMATION NOT REQUIRED IN PROSPECTUS
 
ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.
 
     The following table sets forth those expenses to be incurred by the
Registrant, Pioneer Natural Resources Company (the "Company"), in connection
with the issuance and distribution of the securities being registered. Except
for the Securities and Exchange Commission registration fee, all amounts shown
are estimates.
 
   
<TABLE>
<S>                                                           <C>
Securities and Exchange Commission registration fee.........  $      0
Accounting fees and expenses................................     5,000
Legal fees and expenses.....................................    15,000
Transfer agent's fees and expenses..........................    10,000
Blue Sky fees and expenses, including counsel fees..........     1,000
Fees of rating agencies.....................................         0
Listing fees................................................   140,755
Printing and engraving expenses.............................    60,000
Miscellaneous...............................................    10,000
                                                              --------
          Total.............................................  $241,755
                                                              ========
</TABLE>
    
 
ITEM 15. INDEMNIFICATION OF DIRECTORS AND OFFICERS.
 
   
     Article Twelfth of the Restated Certificate of Incorporation of the
Registrant provides that the Registrant must indemnify its officers and
directors to the extent allowed by the Delaware General Corporation Law.
Pursuant to Section 145 of the Delaware General Corporation Law, the Registrant
generally has the power to indemnify its present and former directors and
officers against expenses and liabilities incurred by them in connection with
any suit to which they are, or are threatened to be made, a party by reason of
their serving in those positions so long as they acted in good faith and in a
manner they reasonably believed to be in, or not opposed to, the best interests
of the Registrant, and with respect to any criminal action, they had no
reasonable cause to believe their conduct was unlawful. With respect to suits by
or in the right of the Registrant, however, indemnification is generally limited
to attorneys' fees and other expenses and is not available if the person is
adjudged to be liable to the Registrant unless the court determines that
indemnification is appropriate. The statute expressly provides that the power to
indemnify authorized thereby is not exclusive of any rights granted under any
by-law, agreement, vote of stockholders or disinterested directors, or
otherwise. The Registrant also has the power to purchase and maintain insurance
for its directors and officers. Additionally, Article Twelfth of the Restated
Certificate of Incorporation provides that, in the event that an officer or
director files suit against the Registrant seeking indemnification of
liabilities or expenses incurred, the burden will be on the Registrant to prove
that the indemnification would not be permitted under the Delaware General
Corporation Law.
    
 
   
     The Registrant has entered into Indemnification Agreements with each of its
directors and officers. These agreements provide that the Registrant must
indemnify an officer or director for liabilities incurred to the fullest extent
permitted by the Delaware General Corporation Law. The Registrant must, within
ten days of a request, indemnify an officer or director for expenses incurred in
the defense of a claim or other proceeding. The obligation of the Registrant to
provide the indemnification does not apply if, before the date on which the
Registrant must provide the indemnification, the Registrant's Board of
Directors, or a representative chosen by the Board of Directors, concludes that
indemnification would be improper under the Delaware General Corporation Law.
    
 
     The preceding discussion of the Registrant's Restated Certificate of
Incorporation, Section 145 of the Delaware General Corporation Law, and the
Indemnification Agreements is not intended to be exhaustive and is qualified in
its entirety by the Restated Certificate of Incorporation, Section 145 of the
Delaware General Corporation Law, and the Indemnification Agreements.
 
                                      II-1
<PAGE>   28
 
ITEM 16. EXHIBITS AND FINANCIAL STATEMENT SCHEDULES.
 
     There are filed with the Registration Statement the following exhibits:
 
   
<TABLE>
<CAPTION>
        EXHIBIT
         NUMBER                                  DESCRIPTION
        -------                                  -----------
<C>                      <S>
          2.1            -- Amended and Restated Agreement and Plan of Merger, dated
                            as of April 6, 1997, by and among Mesa, Mesa Operating
                            Co. ("MOC"), MXP Reincorporation Corp. and Parker &
                            Parsley (incorporated by reference to Exhibit 2.1 to
                            Pioneer's Registration Statement on Form S-4, dated June
                            27, 1997, Registration No. 333-26951).
          2.2            -- Combination Agreement, dated as of September 3, 1997,
                            between Pioneer and Chauvco (incorporated by reference to
                            Exhibit 2.2 to Pioneer's Registration Statement on Form
                            S-3, Registration No. 333-42315, filed with the SEC on
                            December 15, 1997).
          2.3            -- Plan of Arrangement under Section 186 of the Business
                            Corporations Act (Alberta) (incorporated by reference to
                            Annex E to the Definitive Joint Management Information
                            Circular and Proxy Statement of Pioneer and Chauvco, File
                            No. 001-13245, filed with the SEC on November 17, 1997).
          2.4            -- Form of Support Agreement between Pioneer and Pioneer
                            Canada (incorporated by reference to Annex H to the
                            Definitive Joint Management Information Circular and
                            Proxy Statement of Pioneer and Chauvco, File No.
                            001-13245, filed with the SEC on November 17, 1997).
          2.5            -- Form of Voting and Exchange Trust Agreement among
                            Pioneer, Pioneer Canada and Montreal Trust Company of
                            Canada, as Trustee (incorporated by reference to Annex I
                            to the Definitive Joint Management Information Circular
                            and Proxy Statement of Pioneer and Chauvco, File No.
                            001-13245, filed with the SEC on November 17, 1997).
          2.6            -- Amended and Restated Shareholders Agreement, dated as of
                            September 3, 1997, by and among Pioneer and Guy J.
                            Turcotte (incorporated by reference to Exhibit 2.6 to
                            Pioneer's Registration Statement on Form S-3,
                            Registration No. 333-42315, filed with the SEC on
                            December 15, 1997).
          2.7            -- Shareholders Agreement, dated as of September 3, 1997, by
                            and among Pioneer, Chauvco, DNR-MESA Holdings, L.P.
                            ("DNR"), Scott D. Sheffield and I. Jon Brumley
                            (incorporated by reference to Exhibit 2.3 to Pioneer's
                            Current Report on Form 8-K, File No. 001-13245, filed
                            with the SEC on October 2, 1997).
          2.8            -- Shareholders Agreement, dated as of September 3, 1997, by
                            and among Pioneer, Trimac Corporation and Gendis Inc.
                            (incorporated by reference to Exhibit 2.4 to Pioneer's
                            Current Report on Form 8-K, File No. 001-13245, filed
                            with the SEC on October 2, 1997).
          3.1            -- Amended and Restated Certificate of Incorporation of
                            Pioneer (incorporated by reference to Exhibit 3.1 to
                            Pioneer's Registration Statement on Form S-4,
                            Registration No. 333-26951).
          3.2            -- Restated Bylaws of Pioneer (incorporated by reference to
                            Exhibit 3.2 to Pioneer's Registration Statement on Form
                            S-4, Registration No. 333-26951).
          3.3            -- Certificate of Designations of Special Preferred Voting
                            Stock (incorporated by reference to Exhibit 3.3 to
                            Pioneer's Registration Statement on Form S-3,
                            Registration No. 333-42315, filed with the SEC on
                            December 15, 1997).
          3.4            -- Terms and Conditions of Exchangeable Shares (incorporated
                            by reference to Annex F to the Definitive Joint
                            Management Information Circular and Proxy Statement of
                            Pioneer and Chauvco, File No. 001-13245, filed with the
                            SEC on November 17, 1997).
</TABLE>
    
 
                                      II-2
<PAGE>   29
   
<TABLE>
<CAPTION>
        EXHIBIT
         NUMBER                                  DESCRIPTION
        -------                                  -----------
<C>                      <S>
          4.1            -- Form of Certificate of Common Stock, par value $.01 per
                            share, of Pioneer (incorporated by reference to Exhibit
                            4.1 to Pioneer's Registration Statement on Form S-4,
                            Registration No. 333-26951).
          5.1**          -- Opinion of Vinson & Elkins L.L.P. as to the legality of
                            the securities to be registered.
          8.1**          -- Opinion of MacKimmie Matthews regarding tax matters.
          8.2**          -- Opinion of Vinson & Elkins L.L.P. regarding tax matters.
          9.1            -- Shareholder Agreement, dated as of April 6, 1997, between
                            Mesa, Boone Pickens and Parker & Parsley (incorporated by
                            reference to Exhibit 2.4 of Mesa's Form 8-K filed April
                            8, 1997).
          9.2            -- Shareholders Agreement, dated as of April 6, 1997,
                            between DNR and Mesa (incorporated by reference to
                            Exhibit 2.2 of Mesa's Form 8-K filed April 8, 1997).
         10.1            -- Indenture, dated July 2, 1996, among Pioneer Natural
                            Resources USA, Inc. ("Pioneer USA") (formerly MOC), as
                            Issuer, Pioneer (Mesa's successor), as Guarantor, and
                            Harris Trust and Savings Bank, as Trustee, relating to
                            the 11 5/8% Senior Subordinated Discount Notes Due 2006
                            (incorporated by reference to Exhibit 4.17 of Mesa's Form
                            10-Q dated August 13, 1996).
         10.2            -- First Supplemental Indenture, dated as of April 15, 1997,
                            among Pioneer USA (formerly MOC), as Issuer, Mesa, the
                            subsidiary guarantors named therein, Pioneer, and Harris
                            Trust and Savings Bank, as Trustee, with respect to the
                            indenture identified above as Exhibit 10.1 (incorporated
                            by reference to Exhibit 10.1 to Pioneer's Quarterly
                            Report on Form 10-Q for the period ended September 30,
                            1997, File No. 001-13245).
         10.3            -- Second Supplemental Indenture, dated as of August 7,
                            1997, among Pioneer USA (formerly MOC), as Issuer, Mesa,
                            the subsidiary guarantors named therein, Pioneer, and
                            Harris Trust and Savings Bank, as Trustee, with respect
                            to the indenture identified above as Exhibit 10.1
                            (incorporated by reference to Exhibit 10.2 to Pioneer's
                            Quarterly Report on Form 10-Q for the period ended
                            September 30, 1997, File No. 001-13245).
         10.4            -- Indenture, dated July 2, 1996, among Pioneer USA
                            (formerly MOC), as Issuer, Pioneer (Mesa's successor), as
                            Guarantor, and Harris Trust and Savings Bank, as Trustee,
                            relating to 10 5/8% Senior Subordinated Notes Due 2006
                            (incorporated by reference to Exhibit 4.18 of Mesa's Form
                            10-Q, dated August 13, 1996).
         10.5            -- First Supplemental Indenture, dated as of April 15, 1997,
                            among Pioneer USA (formerly MOC), as Issuer, Mesa, the
                            subsidiary guarantors named therein, Pioneer, and Harris
                            Trust and Savings Bank, as Trustee, with respect to the
                            indenture identified above as Exhibit 10.4 (incorporated
                            by reference to Exhibit 10.3 to Pioneer's Quarterly
                            Report on Form 10-Q for the period ended September 30,
                            1997, File No. 001-13245).
         10.6            -- Second Supplemental Indenture, dated as of August 7,
                            1997, among Pioneer USA (formerly MOC), as Issuer, Mesa,
                            the subsidiary guarantors named therein, Pioneer, and
                            Harris Trust and Savings Bank, as Trustee, with respect
                            to the indenture identified above as Exhibit 10.4
                            (incorporated by reference to Exhibit 10.4 to Pioneer's
                            Quarterly Report on Form 10-Q for the period ended
                            September 30, 1997, File No. 001-13245).
</TABLE>
    
 
                                      II-3
<PAGE>   30
   
<TABLE>
<CAPTION>
        EXHIBIT
         NUMBER                                  DESCRIPTION
        -------                                  -----------
<C>                      <S>
         10.7            -- Indentures relating to $50,000,000 principal amount of
                            8 1/2% Convertible Subordinated Debentures due 2005 of
                            Dorchester Master Limited Partnership ($3,762,000
                            principal amount of which were outstanding and held by
                            non affiliates at December 31, 1996) and $100,000,000
                            principal amount of 9 1/2% Senior Notes due 2000 of
                            Bridge Oil (U.S.A.) Inc. ($2,063,000 principal amount of
                            which were outstanding at December 31, 1996) have been
                            omitted pursuant to Item 601(b)(4)(iii)(A) of Regulation
                            S-K. Pioneer hereby agrees to furnish a copy of the
                            indentures to the Securities and Exchange Commission upon
                            request (incorporated by reference to Parker & Parsley's
                            Form 10-K, dated December 31, 1996).
         10.8            -- Indenture, dated April 12, 1995, between Pioneer USA
                            (successor to Parker & Parsley), and The Chase Manhattan
                            Bank (National Association), as Trustee (incorporated by
                            reference to Exhibit 4.1 to Parker & Parsley's Current
                            Report on Form 8-K, dated April 12, 1995, File No.
                            1-10695).
         10.9            -- First Supplemental Indenture, dated as of August 7, 1997,
                            among Parker & Parsley, The Chase Manhattan Bank, as
                            Trustee, and Pioneer USA, with respect to the indenture
                            identified above as Exhibit 10.8 (incorporated by
                            reference to Exhibit 10.5 to Pioneer's Quarterly Report
                            on Form 10-Q for the period ended September 30, 1997,
                            File No. 001-13245).
         10.10           -- Form of 8 7/8% Senior Notes Due 2005, dated as of April
                            12, 1995, in the aggregate principal amount of
                            $150,000,000, together with Officers' Certificate dated
                            April 12, 1995, establishing the terms of the 8 7/8%
                            Senior Notes Due 2005 pursuant to the indenture
                            identified above as Exhibit 10.8 (incorporated by
                            reference to Exhibit 4.2 to Parker & Parsley's Quarterly
                            Report on Form 10-Q for the period ended June 30, 1995,
                            File No. 1-10695).
         10.11           -- Form of 8 1/4% Senior Notes due 2007, dated as of August
                            22, 1995, in the aggregate principal amount of
                            $150,000,000, together with Officers' Certificate, dated
                            August 22, 1995, establishing the terms of the 8 1/4%
                            Senior Notes due 2007 pursuant to the indenture
                            identified above as Exhibit 10.8 (incorporated by
                            reference to Exhibit 1.2 to Parker & Parsley's Current
                            Report on Form 8-K, dated August 17, 1995, File No.
                            1-10695).
         10.12           -- Agreement of Sale between Pioneer Corporation and Cabot
                            Corporation, dated August 29, 1984 (incorporated by
                            reference to Exhibit 10.5 to Pioneer Corporation's Form
                            10-K, dated December 31, 1985).
         10.13           -- Settlement Agreement, dated March 15, 1989, by and among
                            Mesa Operating Limited Partnership and Mesa Limited
                            Partnership, et al., Energas Company and the City of
                            Amarillo (incorporated by reference to Exhibit 10(k) to
                            Mesa Limited Partnership's Form 10-K, dated December 31,
                            1990).
         10.14           -- Gas Purchase Agreement, dated December 1, 1989, between
                            Williams Natural Gas Company and Mesa Operating Limited
                            Partnership acting on behalf of itself and as agent for
                            Mesa Midcontinent Limited Partnership (incorporated by
                            reference to Exhibit 10.1 to Registration Statement of
                            Mesa Limited Partnership on Form S-3, Registration No.
                            33-32978).
         10.15           -- "B" Contract Production Allocation Agreement, dated July
                            29, 1991, and effective as of January 1, 1991, between
                            Colorado Interstate Gas Company and Mesa Operating
                            Limited Partnership (incorporated by reference to Exhibit
                            10(r) to Mesa's Form 10-K, dated December 31, 1991).
         10.16           -- Amendment to "B" Contract Production Allocation Agreement
                            effective as of January 1, 1993, between Colorado
                            Interstate Gas Company and Mesa Operating Limited
                            Partnership (incorporated by reference to Exhibit 10.24
                            to Mesa's Registration Statement on Form S-1,
                            Registration No. 033-51909).
</TABLE>
    
 
                                      II-4
<PAGE>   31
   
<TABLE>
<CAPTION>
        EXHIBIT
         NUMBER                                  DESCRIPTION
        -------                                  -----------
<C>                      <S>
         10.17           -- Amended Peak Day Gas Purchase Agreement, dated effective
                            June 19, 1991, between Colorado Interstate Gas Company
                            and Mesa Operating Limited Partnership (incorporated by
                            reference to Exhibit 10(t) to Mesa's Form 10-K, dated
                            December 31, 1991).
         10.18           -- Omnibus Amendment to Collateral Instruments to
                            Supplemental Stipulation and Agreement, dated June 19,
                            1991, between Colorado Interstate Gas Company and Mesa
                            Operating Limited Partnership (incorporated by reference
                            to Exhibit 10(u) to Mesa's Form 10-K, dated December 31,
                            1991).
         10.19           -- Amarillo Supply Agreement between Mesa Operating Limited
                            Partnership, Seller, and Energas Company, a division of
                            Atmos Energy Corporation, Buyer, dated effective January
                            2, 1993 (incorporated by reference to Exhibit 10.14 to
                            Mesa's Form 10-K, dated December 31, 1995).
         10.20           -- Gas Supply Agreement, dated May 11, 1994, between MOC, as
                            successor to Mesa Operating Limited Partnership, acting
                            on behalf of itself and as agent for Hugoton Capital
                            Limited Partnership, and Williams Gas Marketing Company,
                            and Gas Supply Guarantee, dated May 11, 1994
                            (incorporated by reference to Exhibit 10.16 to Mesa's
                            Form 10-K, dated December 31, 1995).
         10.21           -- Gas Transportation Agreement, dated June 14, 1994,
                            between Western Resources, Inc. and MOC, acting on behalf
                            of itself and as agent for Hugoton Capital Limited
                            Partnership (incorporated by reference to Exhibit 10.24
                            to Mesa's Form 10-K, dated December 31, 1994).
         10.22           -- 1991 Stock Option Plan of Mesa (incorporated by reference
                            to Exhibit 10(v) to Mesa's Form 10-K, dated December 31,
                            1991).
         10.23           -- Interruptible Gas Transportation and Sales Agreement,
                            dated January 1, 1991, between Mesa Operating Limited
                            Partnership and Energas Company and Amendment, dated
                            January 1, 1995 (incorporated by reference to Exhibit
                            10.22 to Mesa's Form 10-K, dated December 31, 1995).
         10.24           -- "B" Contract Operating Agreement, dated January 1, 1988,
                            between Mesa Operating Limited Partnership and Colorado
                            Interstate Gas Company (incorporated by reference to
                            Exhibit 10.23 to Mesa's Form 10-K, dated December 31,
                            1995).
         10.25           -- Gathering Agreement, dated May 29, 1987, between Mesa
                            Operating Limited Partnership and Colorado Interstate Gas
                            Company (incorporated by reference to Exhibit 10.15 to
                            Pioneer's Quarterly Report on Form 10-Q for the period
                            ended September 30, 1997, File No. 001-13245).
         10.26           -- Amendment to Gathering Agreement, dated July 15, 1990,
                            between Colorado Interstate Gas Company and Mesa
                            Operating Limited Partnership (incorporated by reference
                            to Exhibit 10.24 to Mesa's Form 10-K, dated December 31,
                            1995).
         10.27           -- Amendment to 1990 Gathering Agreement Amendment, dated
                            September 1, 1997, between Colorado Interstate Gas
                            Company and Pioneer USA (incorporated by reference to
                            Exhibit 10.6 to Pioneer's Quarterly Report on Form 10-Q
                            for the period ended September 30, 1997, File No.
                            001-13245).
         10.28           -- Gas Purchase Agreement, dated January 1, 1996, between
                            MOC, as Seller, and KN Marketing L.P., as Buyer, and
                            Amendment, dated August 1, 1995 (incorporated by
                            reference to Exhibit 10.25 to Mesa's Form 10-K, dated
                            December 31, 1995).
         10.29           -- Employment Agreement, dated as of August 21, 1996,
                            between Mesa and I. Jon Brumley (incorporated by
                            reference to Exhibit 10.26 of Mesa's Form 10-K, dated
                            December 31, 1996).
</TABLE>
    
 
                                      II-5
<PAGE>   32
   
<TABLE>
<CAPTION>
        EXHIBIT
         NUMBER                                  DESCRIPTION
        -------                                  -----------
<C>                      <S>
         10.30           -- Stock Purchase Agreement, dated April 26, 1996, between
                            Mesa and DNR (incorporated by reference to Exhibit No. 10
                            to Mesa's Form 8-K filed on April 29, 1996).
         10.31           -- 1996 Incentive Plan of Mesa (incorporated by reference to
                            Exhibit 10.28 to Pioneer's Registration Statement on Form
                            S-4, dated June 27, 1997, Registration No. 333-26951).
         10.32           -- Mesa Management Severance Plan, dated April 4, 1997,
                            including a Schedule of Participants on Schedule A for
                            the purpose of defining the payment of certain benefits
                            upon the termination of the officer's employment under
                            certain circumstances (incorporated by reference to
                            Exhibit 10.29 to Pioneer's Registration Statement on Form
                            S-4, dated June 27, 1997, Registration No. 333-26951).
         10.33           -- Parker & Parsley Petroleum Company Long-Term Incentive
                            Plan, dated February 19, 1991, (incorporated by reference
                            to Exhibit 4.1 to Parker & Parsley's Registration
                            Statement on Form S-8, Registration No. 33-38971).
         10.34           -- First Amendment to the Parker & Parsley Petroleum Company
                            Long-Term Incentive Plan, dated August 23, 1991
                            (incorporated by reference to Exhibit 10.2 to Parker &
                            Parsley's Registration Statement on Form S-1, dated
                            February 28, 1992, Registration No. 33-46082).
         10.35           -- Agreement of Partnership of P&P Employees 89-B Conv.,
                            L.P. (formerly P&P Employees 89-B GP), dated October 31,
                            1989, among Parker & Parsley, Ltd. and the Investor
                            Partners (as defined therein, which includes individuals
                            who are directors and executive officers of Parker &
                            Parsley), together with a schedule identifying
                            substantially identical documents and setting forth the
                            material details in which those documents differ from the
                            foregoing document (incorporated by reference to Exhibit
                            10.50 to Parker & Parsley's Registration Statement on
                            Form S-4, dated December 31, 1990, Registration No.
                            33-38436).
         10.36           -- Amendment to Agreement of Partnership of P&P Employees
                            89-B GP, dated May 31, 1990, among Parker & Parsley, Ltd.
                            and the Investor Partners (as defined therein, which
                            includes individuals who are directors and executives
                            officers of Parker & Parsley), together with a schedule
                            identifying substantially identical documents and setting
                            forth the material details in which those documents
                            differ form the foregoing document (incorporated by
                            reference to Exhibit 10.51 to Parker & Parsley's
                            Registration Statement on Form S-4, dated December 31,
                            1990, Registration No. 33-38436).
         10.37           -- Schedule identifying additional documents substantially
                            identical to the Amendment to Agreement of Partnership of
                            P&P Employees 89-B GP included as Exhibit 10.5 and
                            setting forth the material details in which those
                            documents differ from that document (incorporated by
                            reference to Exhibit 10.52 to Parker & Parsley's
                            Registration Statement on Form S-1, dated February 28,
                            1992, Registration No. 33-46082).
         10.38           -- Agreement of Partnership of P&P Employees 90 Spraberry
                            Private Development GP, dated October 16, 1990, among
                            Parker & Parsley, Ltd., James D. Moring, and the General
                            Partners (as defined therein, which includes individuals
                            who are directors and executive officers of Parker &
                            Parsley), and form of Amendment to Agreement of
                            Partnership of P&P Employees 90 Spraberry Private
                            Development GP, together with a schedule identifying
                            substantially identical documents and setting forth the
                            material details in which those documents differ from the
                            foregoing document (incorporated by reference to Exhibit
                            10.52 to Parker & Parsley's Registration Statement on
                            Form S-4, dated December 31, 1990, Registration No.
                            33-38436).
</TABLE>
    
 
                                      II-6
<PAGE>   33
   
<TABLE>
<CAPTION>
        EXHIBIT
         NUMBER                                  DESCRIPTION
        -------                                  -----------
<C>                      <S>
         10.39           -- Amendment to Agreement of Partnership of Parker & Parsley
                            90-A GP, dated February 19, 1991, among Parker & Parsley
                            Development Company and the Investor Partners (as defined
                            therein, which includes individuals who are directors and
                            executive officers of Parker & Parsley), together with a
                            schedule identifying substantially identical documents
                            and setting forth the material details in which those
                            documents differ from the foregoing document
                            (incorporated by reference to Exhibit 10.58 to Parker &
                            Parsley's Registration Statement on Form S-1, dated
                            February 28, 1992, Registration No. 33-46082).
         10.40           -- Agreement of Partnership of P&P Employees 91-A, GP, dated
                            September 30, 1991, among Parker & Parsley Development
                            Company, James D. Moring, and the General Partners (as
                            defined therein, which includes individuals who are
                            directors and executive officers of Parker & Parsley),
                            together with a schedule identifying substantially
                            identical documents and setting forth the material
                            details in which those documents differ from the
                            foregoing document (incorporated by reference to Exhibit
                            10.61 to Parker & Parsley's Registration Statement on
                            Form S-1, dated February 28, 1992, Registration No.
                            33-46082).
         10.41           -- Development Drilling Program Agreement of Parker &
                            Parsley 91-A Development Drilling Program, dated
                            September 30, 1991, among Parker & Parsley Development
                            Company, the P&P Employee Participants (as defined
                            therein, which includes individuals who are directors and
                            executive officers of Parker & Parsley), P&P Employees
                            91-A, GP, and Parker & Parsley 91-A, L.P., together with
                            a schedule identifying substantially identical documents
                            and setting forth the material details in which those
                            documents differ from the foregoing document
                            (incorporated by reference to Exhibit 10.63 to Parker &
                            Parsley's Registration Statement on Form S-1, dated
                            February 28, 1992, Registration No. 33-46082).
         10.42           -- Development Drilling Program Agreement, dated August 1,
                            1989, among Parker & Parsley, Ltd., Parker & Parsley
                            Development Partners L.P., certain key employees of
                            Parker & Parsley, Ltd. (which includes individuals who
                            are directors and executive officers of Parker &
                            Parsley), and related persons, P&P Employees 89-A GP, and
                            Parker & Parsley 89- A, GP, and Parker & Parsley 89-A,
                            L.P., together with a schedule identifying substantially
                            identical documents and setting forth the material
                            details in which those documents differ from the
                            foregoing document (incorporated by reference to Exhibit
                            10.56 to Parker & Parsley's Registration Statement on
                            Form S-4, dated December 31, 1990, Registration No.
                            33-38436).
         10.43           -- Amendment to Development Drilling Program Agreement,
                            dated February 19, 1991, amending the Development
                            Drilling Program Agreement included in Exhibit 10.11,
                            together with a schedule identifying substantially
                            identical documents and setting forth the material
                            details in which those documents differ from the
                            foregoing document (incorporated by reference to Exhibit
                            10.66 to Parker & Parsley's Registration Statement on
                            Form S-1, dated February 28, 1992, Registration No.
                            33-46082).
         10.44           -- Amendment to Agreement of Partnership of P&P Employees 90
                            Spraberry Private Development GP, dated April 22, 1991,
                            among the Partners (as defined therein, which includes
                            individuals who are directors and executive officers of
                            Parker & Parsley) (incorporated by reference to Exhibit
                            10.67 to Parker & Parsley's Registration Statement on
                            Form S-1, dated February 28, 1992, Registration No.
                            33-46082).
</TABLE>
    
 
                                      II-7
<PAGE>   34
   
<TABLE>
<CAPTION>
        EXHIBIT
         NUMBER                                  DESCRIPTION
        -------                                  -----------
<C>                      <S>
         10.45           -- Agreement of Limited Partnership of Parker & Parsley 1992
                            Direct Investment Program, Ltd., dated as of July 24,
                            1992, among Parker & Parsley Development Company, as
                            managing general partner, and certain key employees of
                            Parker & Parsley (including individuals who are directors
                            and executive officers of Parker & Parsley), as
                            non-managing general partners and limited partners
                            (incorporated by reference to Exhibit 10.57 to Parker
                            Parsley's Annual Report on Form 10-K for the year ended
                            December 31, 1993, Commission File No. 1-10695).
         10.46           -- Agreement of Limited Partnership of Parker & Parsley 1993
                            Direct Investment Program, Ltd., dated as of January 1,
                            1993, among Parker & Parsley Development Company, as
                            managing general partner, and certain key employees of
                            Parker & Parsley (including individuals who are directors
                            and executive officers of Parker & Parsley), as
                            non-managing general partners and limited partners
                            (incorporated by reference to Exhibit 10.49 to Parker &
                            Parsley's Annual Report on Form 10-K for the year ended
                            December 31, 1993, Commission File No. 1-10695).
         10.47           -- Agreement of Limited Partnership of Parker & Parsley 1994
                            Direct Investment Program, Ltd., dated as of January 1,
                            1994, among Parker & Parsley Development Company, as
                            managing general partner, and certain key employees of
                            Parker & Parsley (including individuals who are directors
                            and executive officers of Parker & Parsley), as
                            non-managing general partners and limited partners
                            (incorporated by reference to Exhibit 10.20 to Parker &
                            Parsley's Annual Report on Form 10-K for the year ended
                            December 31, 1994, Commission File No. 1-10695).
         10.48**         -- Stock Acquisition Loan Agreement entered into as of June
                            15, 1995, between Parker & Parsley and Scott D.
                            Sheffield, together with Schedule I identifying executive
                            officers with substantially identical agreements
                            providing for Parker & Parsley's loans to such officers
                            in order to acquire shares of Parker & Parsley's Common
                            Stock, par value $0.01 per share.
         10.49           -- Omnibus Amendment to Nonstatutory Stock Option
                            Agreements, included as part of the Long-Term Incentive
                            Plan, dated as of November 16, 1995, between Parker &
                            Parsley and Named Executive Officers identified on
                            Schedule 1 setting forth additional details relating to
                            the Long-Term Incentive Plan (incorporated by reference
                            to Parker & Parsley's Annual Report on Form 10-K for the
                            year ended December 31, 1995, Commission File No.
                            1-10695).
         10.50           -- Severance Agreement, dated as of August 8, 1997, between
                            Pioneer and Scott D. Sheffield, together with a schedule
                            identifying substantially identical agreements between
                            Pioneer and each of the other named executive officers
                            identified on Schedule I for the purpose of defining the
                            payment of certain benefits upon the termination of the
                            officer's employment under certain circumstances
                            (incorporated by reference to Exhibit 10.7 to Pioneer's
                            Quarterly Report on Form 10-Q for the period ended
                            September 30, 1997, File No. 001-13245).
         10.51           -- Indemnification Agreement, dated as of August 8, 1997,
                            between Pioneer and Scott D. Sheffield, together with a
                            schedule identifying substantially identical agreements
                            between Pioneer and each of Pioneer's other directors and
                            named executive officers identified on Schedule I
                            (incorporated by reference to Exhibit 10.8 to Pioneer's
                            Quarterly Report on Form 10-Q for the period ended
                            September 30, 1997, File No. 001-13245).
         10.52           -- Pioneer Natural Resources Company Long-Term Incentive
                            Plan (incorporated by reference to Exhibit 4.1 to
                            Pioneer's Registration Statement on Form S-8,
                            Registration No. 333-35087).
         10.53           -- Pioneer Natural Resources Company Employee Stock Purchase
                            Plan (incorporated by reference to Exhibit 4.1 to
                            Pioneer's Registration Statement on Form S-8,
                            Registration No. 333-35165).
</TABLE>
    
 
                                      II-8
<PAGE>   35
   
<TABLE>
<CAPTION>
        EXHIBIT
         NUMBER                                  DESCRIPTION
        -------                                  -----------
<C>                      <S>
         10.54           -- Pioneer Natural Resources Company Deferred Compensation
                            Retirement Plan (incorporated by reference to Exhibit 4.1
                            to Pioneer's Registration Statement on Form S-8,
                            Registration No. 333-39153).
         10.55           -- Pioneer Natural Resources USA, Inc. 401(k) Plan
                            (incorporated by reference to Exhibit 4.1 to Pioneer's
                            Registration Statement on Form S-8, Registration No.
                            333-39249).
         10.56           -- Credit Facility Agreement (Primary Facility), dated as of
                            August 7, 1997, between Pioneer USA, as Borrower, and
                            NationsBank of Texas, N.A., as Administrative Agent, CIBC
                            Inc., as Documentation Agent, Morgan Guaranty Trust
                            Company of New York, as Documentation Agent, and The
                            Chase Manhattan Bank, as Syndication Agent; Bank of
                            America Natural Trust and Savings Association, The Bank
                            of New York, The Bank of Nova Scotia, Royal Bank of
                            Canada, Union Bank of California, N.A., The Fuji Bank,
                            Limited-Houston Agency and Wells Fargo Bank, N.A., as
                            Co-Agents; and certain other lenders (incorporated by
                            reference to Exhibit 10.1 to Pioneer's Form 8-K, dated
                            August 7, 1997, File No. 333-26951).
         10.57           -- Credit Facility Agreement (364 Day Facility), dated as of
                            August 7, 1997, between Pioneer USA, as Borrower, and
                            NationsBank of Texas, N.A., as Administrative Agent, CIBC
                            Inc., as Documentation Agent, Morgan Guaranty Trust
                            Company of New York, as Documentation Agent, and The
                            Chase Manhattan Bank, as Syndication Agent; Bank of
                            America Natural Trust and Savings Association, The Bank
                            of New York, The Bank of Nova Scotia, Royal Bank of
                            Canada, Union Bank of California, N.A., The Fuji Bank,
                            Limited-Houston Agency and Wells Fargo Bank, N.A., as
                            Co-Agents; and certain other lenders (incorporated by
                            reference to Exhibit 10.2 to Pioneer's Form 8-K, dated
                            August 7, 1997, File No. 333-26951).
         23.1**          -- Consent of KPMG Peat Marwick LLP.
         23.2**          -- Consent of Arthur Andersen LLP.
         23.3**          -- Consent of Price Waterhouse, chartered accountants.
         23.4**          -- Consent of Coopers & Lybrand L.L.P.
         23.5**          -- Consent of Netherland, Sewell & Associates, Inc.
         23.6**          -- Consent of Williamson Petroleum Consultants, Inc.
         23.7**          -- Consent of Miller and Lents, Ltd.
         23.8**          -- Consent of Gilbert Lausten Jung Associates, Ltd.
         23.9**          -- Consent of Martin Petroleum and Associates.
         23.10**         -- Consent of Vinson & Elkins L.L.P. (included in the
                            opinion filed as Exhibit 5.1 to this Registration
                            Statement).
         23.11**         -- Consent of MacKimmie Matthews (included in the opinion
                            filed as Exhibit 8.1 to this Registration Statement).
         23.12**         -- Consent of Vinson & Elkins L.L.P. (included in the
                            opinion filed as Exhibit 8.2 to this Registration
                            Statement).
         24.1*           -- Powers of Attorney of directors and officers of Pioneer.
</TABLE>
    
 
- ---------------
 
   
*  Previously filed.
    
 
   
** Filed herewith.
    
 
                                      II-9
<PAGE>   36
 
ITEM 17. UNDERTAKINGS
 
     The undersigned Registrant hereby undertakes:
 
          (a) To file, during any period in which offers or sales are being
     made, a post-effective amendment to this Registration Statement.
 
             (1) To include any prospectus required by Section 10(a)(3) of the
        Securities Act of 1933 (the "Securities Act");
 
             (2) To reflect in the prospectus any facts or events arising after
        the effective date of this Registration Statement (or the most recent
        post-effective amendment thereof) which, individually or in the
        aggregate, represent a fundamental change in the information set forth
        in this Registration Statement; and
 
             (3) To include any material information with respect to the plan of
        distribution not previously disclosed in this Registration Statement or
        any material change to the information in this Registration Statement;
 
             Provided, however, that clauses (1) and (2) above do not apply if
        the information required to be included in a post-effective amendment by
        those clauses is contained in periodic reports filed by the Registrant
        pursuant to Section 13 or Section 15(d) of the Exchange Act that are
        incorporated by reference into this Registration Statement;
 
          (b) That, for the purpose of determining any liability under the
     Securities Act, each such post-effective amendment shall be deemed to be a
     new registration statement relating to the securities offered therein, and
     the offering of the securities at that time shall be deemed to be the
     initial bona fide offering thereof; and
 
          (c) To remove from registration by means of a post-effective amendment
     any of the securities being registered that remain unsold at the
     termination of the offering.
 
     The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
Registrant's annual report pursuant to Section 13(a) or 15(d) of the Exchange
Act that is incorporated by reference in this Registration Statement shall be
deemed to be a new registration statement relating to the securities offered
therein, and the offering of the securities at that time shall be deemed to be
the initial bona fide offering thereof.
 
     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 in Item 15 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, or
controlling person of the Registrant in the successful defense of any action,
suit, or proceeding) is asserted by the director, 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 the issue.
 
     For purposes of determining any liability under the Securities Act of 1933,
the information omitted from the form of prospectus filed as part of this
registration statement in reliance upon Rule 430A 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 Registration
Statement as of the time it was declared effective.
 
     For the purpose of determining any liability under the Securities Act of
1933, 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.
 
                                      II-10
<PAGE>   37
 
                                   SIGNATURES
 
   
     Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this First Amendment to
Registration Statement to be signed on its behalf by the undersigned, thereunto
duly authorized, in Irving, Texas, on December 17, 1997.
    
 
                                          PIONEER NATURAL RESOURCES COMPANY
 
                                          By:    /s/ SCOTT D. SHEFFIELD
                                            ------------------------------------
                                                    Scott D. Sheffield,
                                               President and Chief Executive
                                                           Officer
 
   
     Pursuant to the requirements of the Securities Act, this First Amendment to
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated. Each person whose signature appears below
hereby authorizes and appoints Scott D. Sheffield, Mark L. Withrow and M.
Garrett Smith, or any of them, as his attorney-in-fact to sign on his behalf
individually and in the capacity stated below all amendments and post-effective
amendments to this Registration Statement (including any additional registration
statement filed pursuant to Rule 462 of the Securities Act with respect to this
Registration Statement) as that attorney-in-fact may deem necessary or
appropriate.
    
 
   
<TABLE>
<CAPTION>
                      SIGNATURE                                   TITLE                    DATE
                      ---------                                   -----                    ----
<C>                                                    <S>                           <C>
                 /s/ I. JON BRUMLEY                    Chairman of the Board and     December 17, 1997
- -----------------------------------------------------    Director
                   I. Jon Brumley
 
               /s/ SCOTT D. SHEFFIELD                  Director, President and       December 17, 1997
- -----------------------------------------------------    Chief Executive Officer
                 Scott D. Sheffield                      (principal executive
                                                         officer)
 
                /s/ M. GARRETT SMITH                   Executive Vice President and  December 17, 1997
- -----------------------------------------------------    Chief Financial Officer
                  M. Garrett Smith                       (principal financial and
                                                         accounting officer)
 
                          *                            Director                      December 17, 1997
- -----------------------------------------------------
                 R. Hartwell Gardner
 
                /s/ KENNETH A. HERSH                   Director                      December 17, 1997
- -----------------------------------------------------
                  Kenneth A. Hersh
 
                /s/ JAMES L. HOUGHTON                  Director                      December 17, 1997
- -----------------------------------------------------
                  James L. Houghton
 
                 /s/ JERRY P. JONES                    Director                      December 17, 1997
- -----------------------------------------------------
                   Jerry P. Jones
 
             /s/ CHARLES E. RAMSEY, JR.                Director                      December 17, 1997
- -----------------------------------------------------
               Charles E. Ramsey, Jr.
 
                          *                            Director                      December 17, 1997
- -----------------------------------------------------
                   Arthur L. Smith
</TABLE>
    
 
                                      II-11
<PAGE>   38
   
<TABLE>
<CAPTION>
                      SIGNATURE                                   TITLE                    DATE
                      ---------                                   -----                    ----
<C>                                                    <S>                           <C>
 
                 /s/ PHILIP B. SMITH                   Director                      December 17, 1997
- -----------------------------------------------------
                   Philip B. Smith
 
               /s/ ROBERT L. STILLWELL                 Director                      December 17, 1997
- -----------------------------------------------------
                 Robert L. Stillwell
 
              *By: /s/ M. GARRETT SMITH
  ------------------------------------------------
                  M. Garrett Smith,
                  Attorney-in-fact
</TABLE>
    
 
                                      II-12
<PAGE>   39
 
                               INDEX TO EXHIBITS
 
   
<TABLE>
<CAPTION>
        EXHIBIT
         NUMBER                                  DESCRIPTION
        -------                                  -----------
<C>                      <S>
          5.1            -- Opinion of Vinson & Elkins L.L.P. as to the legality of
                            the securities to be registered.
          8.1            -- Opinion of MacKimmie Matthews regarding tax matters.
          8.2            -- Opinion of Vinson & Elkins L.L.P. regarding tax matters.
         23.1            -- Consent of KPMG Peat Marwick LLP.
         23.2            -- Consent of Arthur Andersen LLP.
         23.3            -- Consent of Price Waterhouse, chartered accountants.
         23.4            -- Consent of Coopers & Lybrand L.L.P.
         23.5            -- Consent of Netherland, Sewell & Associates, Inc.
         23.6            -- Consent of Williamson Petroleum Consultants, Inc.
         23.7            -- Consent of Miller and Lents, Ltd.
         23.8            -- Consent of Gilbert Lausten Jung Associates, Ltd.
         23.9            -- Consent of Martin Petroleum and Associates.
</TABLE>
    

<PAGE>   1
                                                                    EXHIBIT 5.1




                      [VINSON & ELKINS L.L.P. LETTERHEAD]




                                December 17, 1997




Pioneer Natural Resources Company
1400 Williams Square West
5205 North O'Connor Blvd.
Irving, Texas  75039

Ladies and Gentlemen:

   
         We have acted as counsel for Pioneer Natural Resources Company, a
Delaware corporation (the "Company"), in connection with the Company's
registration under the Securities Act of 1933 (the "Act") of the offer and
issuance of 18,004,289 shares of common stock, par value $.01 per share (the
"Pioneer Common Stock"), of the Company pursuant to the Company's Registration
Statement on Form S-3 originally filed with the Securities and Exchange
Commission (the "Commission ") on November 3, 1997, and subsequently amended
(the "Registration Statement").
    

         In reaching the opinions set forth herein, we have examined and are
familiar with the originals or copies, certified or otherwise, of such
documents and records of the Company and such statutes, regulations, and other
instructions as we have deemed necessary or advisable for purposes of this
opinion, including (i) the Registration Statement, (ii) the Amended and
Restated Certificate of Incorporation of the Company, and (iii) the Amended and
Restated By-Laws of the Company.

         We have assumed that (i) all information contained in all documents
reviewed by us is true, correct, and complete, (ii) all signatures on all
documents reviewed by us are genuine, (iii) all documents submitted to us as
originals are true and complete, (iv) all documents submitted to us as copies
are true and compete copies of the originals thereof, and (v) all persons
executing and delivering originals or copies of documents examined by us were
competent to execute and deliver such documents. In addition, we have assumed
that: (i) the shares of Pioneer Common Stock that have yet to be issued will be
issued in accordance with the Combination Agreement in the form previously
reviewed by us (the "Combination Agreement"), (ii) the full consideration for
each share of Pioneer Common Stock that has yet to be issued shall be received
by the Company and in no event shall be less than the par value of such share
of Pioneer Common Stock, and (iii) certificates evidencing the shares of
Pioneer Common Stock that have yet to be issued shall be properly executed and
delivered by the Company in accordance with the Delaware General Corporation
Law ("DGCL").



<PAGE>   2


Pioneer Natural Resources Company
December 17, 1997
Page 2

         Based on the foregoing and having due regard for the legal
considerations we deem relevant, we are of the opinion that the shares of
Pioneer Common Stock, when issued in accordance with the Combination Agreement,
will be validly issued by the Company, fully paid, and non-assessable.

         This opinion is limited in all respects to the DGCL and the federal
laws of the United States of America. You should be aware that we are not
admitted to the practice of law in the State of Delaware.

         This opinion letter may be filed as an exhibit to the Registration
Statement. Consent is also given to the reference to us under the caption
"Legal Matters" in the Registration Statement and in the Prospectus included in
the Registration Statement as having passed on the validly of the shares of
Pioneer Common Stock. In giving this consent, however, we do not thereby admit
that we fall within the category of persons whose consent is required under
Section 7 of the Act or the rules and regulations of the Commission promulgated
thereunder.

                                           Very truly yours,

                                           /s/  VINSON & ELKINS L.L.P.



<PAGE>   1
                                                                    EXHIBIT 8.1




                       [MacKIMMIE MATTHEWS LETTERHEAD]



                                                               December 17, 1997





Pioneer Natural Resources Company
1400 Williams Square West
5205 North O'Connor Blvd.
Irving, Texas  75039

                       REGISTRATION STATEMENT ON FORM S-3
   
We have acted as counsel to Pioneer Natural Resources Company, a Delaware
corporation ("Pioneer"), and Pioneer Natural Resources (Canada) Ltd., a British
Columbia corporation ("Pioneer Canada"), in connection with the offer of
18,004,289 shares of Pioneer Common Stock issuable upon the redemption by
Pioneer Canada or the acquisition by Pioneer of Exchangeable Shares, pursuant to
a Registration Statement on Form S-3, No. 333-39381 (the "Registration
Statement") originally filed with the Securities and Exchange Commission under
the United States Securities Act of 1933 on November 3, 1997, and amended on
December 17, 1997. In connection therewith, we have participated in the
preparation of the discussion set forth under the subcaption "Income Tax
Considerations -- Canadian Federal Income Tax Considerations" (the "Discussion")
in the Registration Statement. Capitalized terms used and not otherwise defined
herein are used as defined in the Registration Statement.
    

The Discussion, subject to the qualifications stated therein, reflects our
opinion as to the material Canadian federal income tax considerations for a
Pioneer Canada Shareholder in regard to the ownership of Pioneer Canada
Exchangeable Shares and the receipt of shares of Pioneer Common Stock upon the
redemption of a Pioneer Canada Shareholder's Exchangeable Shares by Pioneer
Canada or the acquisition by Pioneer of such Exchangeable Shares.

We hereby consent to the use of our name in the Registration Statement and to
the filing of this opinion as an exhibit to the Registration Statement. However,
this consent does not constitute an admission that we are "experts" within the
meaning of such term as used in the Securities Act of 1933.




   
                                                MacKIMMIE MATTHEWS 

                                                /s/ MacKIMMIE MATTHEWS
    


<PAGE>   1
                                                                    EXHIBIT 8.2




                      [VINSON & ELKINS L.L.P. LETTERHEAD]



(214) 220-7700                                                    (214) 220-7716

                               December 17, 1997




Pioneer Natural Resources Company
1400 Williams Square West
5205 North O'Connor Blvd.
Irving, Texas  75039

         Re:  Registration Statement on Form S-3

Ladies and Gentlemen:

   
         We have acted as counsel to Pioneer Natural Resources Company, a
Delaware corporation, in connection with the offer of 18,004,289 shares of
Pioneer Common Stock issuable upon the redemption by Pioneer Canada or the
acquisition by Pioneer of Exchangeable Shares, pursuant to a Registration
Statement on Form S-3, No. 333-39381 (the "Registration Statement") originally
filed with the Securities and Exchange Commission under the Securities Act of
1933 on November 3, 1997, and amended on December 17, 1997. In connection
therewith, we have participated in the preparation of the discussion set forth
under the subcaption "Income Tax Considerations -- United States Federal Income
Tax Considerations" (the "Discussion") in the Registration Statement.
Capitalized terms used and not otherwise defined herein are used as defined in
the Registration Statement.
    

         The Discussion, subject to the qualifications stated therein, reflects
our opinion as to the material United States federal income tax considerations
for a Pioneer Canada Shareholder in regard to the ownership of Pioneer Canada
Exchangeable Shares and the receipt of shares of Pioneer Common Stock upon the
redemption of a Pioneer Canada Shareholder's Exchangeable Shares by Pioneer
Canada or the acquisition by Pioneer of such Exchangeable Shares.

         We hereby consent to the use of our name in the Registration Statement
and to the filing of this opinion as an exhibit to the Registration Statement.
However, this consent does not constitute an admission that we are "experts"
within the meaning of such term as used in the Securities Act of 1933.

                                                Respectfully submitted,



                                                /s/  VINSON & ELKINS L.L.P.




<PAGE>   1
                                                                   EXHIBIT 10.48

                        STOCK ACQUISITION LOAN AGREEMENT

     This Stock Acquisition Loan Agreement ("Agreement") is made and entered in
to as of June 15, 1995, between Parker & Parsley Petroleum Company, a Delaware 
corporation (the "Company") and Scott D. Sheffield (the "Executive").

                                   RECITALS:

     A. WHEREAS, the Board of Directors of Company (the "Board") deem it to be
in the best interest of the Company for the officers of the Company and its
Subsidiaries to accumulate and maintain significant stock ownership in the
Company;

     B. WHEREAS, Executive is an officer of Company or one of its Subsidiaries
and an employee of a Subsidiary of Company, and is desirous of accumulating
ownership of additional shares of the Company's common stock ("Shares"); and

     C. WHEREAS, the Board desires to provide Executive the means to acquire
Company's common stock;

     NOW, THEREFORE, in consideration of the foregoing and of the respective
covenants and agreements set forth below, the parties hereto agree as follows:

     1. Loan. In consideration of Executive's continued employment with any
Subsidiary of Company and other value received, Company agrees, from and after
the date hereof until the earlier to occur of (a) Executive's termination of
employment with any Subsidiary of the Company, or (b) June 22, 1995, to make
advances up to, but not in excess of $125,419 (the "Advances"), to Executive 
for the purchase of Shares. In consideration of the Advances and other value 
received, Executive promises to pay to Company the aggregate amount of all 
Advances, together with interest thereon at a rate per annum equal to 7.0625%, 
calculated on the basis of actual days elapsed, but computed as if each 
calendar year consisted of 365 or 366 days, as the case may be, as provided 
herein.

     2. Use of Advances. The Advances shall be used by Executive exclusively to
purchase Shares not later than June 22, 1995. Advances shall be made only upon
Executive's delivery to Company of written evidence satisfactory to Company of
Executive's purchase of Shares.

     3. Repayment of Loan. Advances shall be due and payable in three (3) equal
installments on each of June 15, 1996, 1997 and 1998 (each a "Payment Date"),
together with all interest then accrued but unpaid; provided, however,
Advances, together with all interest then accrued but unpaid shall be
immediately due and payable upon the termination of Executive's employment
prior to a Change in Control (as hereinafter defined) for a reason other than a
reason set forth in clause (c) of this paragraph; and provided further,
however, that amounts due under this Agreement on a Payment Date shall be

                                       1

<PAGE>   2



automatically forgiven by Company, without further obligation or liability
therefor by Executive, (a) in the event a Change in Control has occurred prior
to such Payment Date; (b) if Executive is then employed by Company or any
Subsidiary of Company (the "Employer"); or (c) if not employed by Employer on
such Payment Date and Executive's employment was terminated:

                  (i)      as the result of Executive's death;

                  (ii)     by Employer:

                           (A)      as a result of Executive's absence from his
                                    employment duties on a full time basis (1)
                                    for 180 consecutive calendar days due to
                                    Executive's incapacity arising from
                                    physical or mental impairment or illness;
                                    and (2) following Company's written notice
                                    of termination of employment (whether
                                    before or after the end of such 180 day
                                    period); or

                           (B)      for any reason other than for Cause; or

                  (iii)    by Employee (1) for Good Reason; or (2) if his
                           health should become impaired to an extent that
                           makes the continued performance of his employment
                           duties hazardous to his physical or mental health or
                           his life.

         4. Certain Definitions. As used in this Agreement, each of the
following terms or phrases has the meaning stated:

                  (a)      "Acquiring Person" means any Person other than
                           Company, any of Company's Subsidiaries, any employee
                           benefit plan of Company or of a Subsidiary of
                           Company or of a corporation owned directly or
                           indirectly by the stockholders of Company in
                           substantially the same proportions as their
                           ownership of stock of Company, or any trustee or
                           other fiduciary holding securities under an employee
                           benefit plan of Company or of a Subsidiary of
                           Company or of a corporation owned directly or
                           indirectly by the stockholders of Company in
                           substantially the same proportions as their
                           ownership of stock of Company.

                  (b)      "Across-the-Board Salary Reduction" shall mean a
                           reduction in the Base Salary that is a part of, and
                           is at a rate consistent with, a reduction in the
                           base salaries paid to all executive officers of the
                           Company or its Subsidiaries.

                  (c)      "Affiliate" means, with respect to any Person, any
                           other Person that directly or indirectly controls,
                           is controlled by, or is under common control with
                           the Person in question. As used in this definition
                           of

                                       2

<PAGE>   3



                           "Affiliate," the term "control" means the possession,
                           directly or indirectly, of the power to direct or
                           cause the direction of the management and policies
                           of a Person, whether through ownership of Voting
                           Securities, by contract, or otherwise.

                  (d)      "Base Salary" shall mean compensation paid by
                           Company to Executive for Executive's services in a
                           specified amount per annum. The Base Salary shall be
                           payable in substantially equal bi-monthly
                           installments. The Compensation Committee of the
                           Board may review the Base Salary periodically and
                           may grant such increases, or effect such reductions,
                           in the Base Salary as the Compensation Committee
                           considers appropriate in accordance with such
                           compensation guidelines and policies as it may
                           establish from time to time.

                  (e)      "Cause" shall mean (i) Executive's continued failure
                           to substantially perform his duties and
                           responsibilities (other than any such failure
                           resulting from Executive's incapacity due to
                           physical or mental impairment or illness) after
                           written demand for substantial performance is
                           delivered by [IN THE CASE OF THE CHIEF EXECUTIVE
                           OFFICER: the Board] [IN THE CASE OF THE OTHER
                           EXECUTIVE OFFICERS: the Board or the Chief Executive
                           Officer] specifically identifying the manner in
                           which [IN THE CASE OF THE CHIEF EXECUTIVE OFFICER:
                           the Board] [IN THE CASE OF THE OTHER EXECUTIVE
                           OFFICERS: the Board or the Chief Executive Officer,
                           as the case may be,] believes Executive has not
                           substantially performed such duties and
                           responsibilities; (ii) Executive's engaging in
                           misconduct that is materially injurious to the
                           Company or an Affiliate of the Company, monetarily
                           or otherwise; or (iii) disclosure of Nonpublic
                           Information as further defined. For purposes of
                           clause (ii) of this paragraph, an act, or failure to
                           act, on Executive's part shall be considered
                           "misconduct" if done, or omitted, by Executive not
                           in good faith and without reasonable belief that
                           such act, or failure to act, was in the best
                           interests of the Company.

                  (f)      "Change in Control" shall have the meaning given
                           that term in Section 1.6 of Company's Long-term
                           Incentive Plan dated February 19, 1991, as amended
                           from time to time.

                  (g)      "Excessive Salary Reduction" shall mean (i) a
                           reduction in the Base Salary that is not an
                           Across-the-Board Salary Reduction and that, when
                           combined with the net effect of all prior increases
                           and reductions in the Base Salary (other than prior
                           reductions that were Across-the-Board Salary
                           Reductions), results in the Base Salary being less
                           than 80% of the highest Base Salary to which
                           Executive

                                       3

<PAGE>   4



                           has ever been subject; or (ii) a reduction in the
                           Base Salary (whether or not an Across-the-Board
                           Salary Reduction) that, when combined with the net
                           effect of all prior increases and reductions in the
                           Base Salary (whether or not Across-the-Board Salary
                           Reductions), results in the Base Salary being less
                           than 65% of the highest Base Salary to which
                           Executive has ever been subject.

                  (h)      "Good Reason" shall mean the occurrence, subsequent
                           to the date hereof, of (i) a material reduction in
                           Executive's authorities, powers, functions, duties
                           or responsibilities; (ii) an Excessive Salary
                           Reduction; or (iii) a material failure by Company or
                           its Subsidiaries to comply with any written
                           agreement among Executive and Company or its
                           Subsidiary.

                  (j)      "Incumbent Board" means all individuals who, as of
                           the date hereof, constitute the board of directors
                           of Company and any other individual who becomes a
                           director of Company after that date and whose
                           election or appointment by the board of directors or
                           nomination for election by Company's stockholders
                           was approved by a vote of at least a majority of the
                           directors then comprising the Incumbent Board.

                  (k)      "Nonpublic Information" means various information
                           regarding the Company and its business, operations
                           and affairs which Executive, in connection with his
                           employment with the Company, has received, and will
                           continue to receive, and which is not publicly
                           available other than as a result of disclosure by
                           Executive in violation of any written agreement
                           between Executive and Company or any of its
                           Subsidiaries, is referred to herein as "Nonpublic
                           Information."

                  (l)      "Person" means any person or entity of any nature
                           whatsoever, specifically including an individual, a
                           firm, a company, a corporation, a partnership, a
                           trust or other entity. A Person, together with that
                           Person's Affiliates and Associates (as those terms
                           are defined in Rule 12b-2 under the Securities
                           Exchange Act of 1934, as amended ("Exchange Act")),
                           and any Persons acting as a partnership, limited
                           partnership, joint venture, association, syndicate
                           or other group (whether or not formally organized),
                           or otherwise acting jointly or in concert or in a
                           coordinated or consciously parallel manner (whether
                           or not pursuant to any express agreement), for the
                           purpose of acquiring, holding voting or disposing of
                           securities of Company with such Person, shall be
                           deemed a single "Person."
                           

                  (m)      "Subsidiary" means any corporation or other entity
                           of which a majority of the voting power of the
                           Voting Securities is owned, directly or indirectly,
                           by such Person.

                                       4

<PAGE>   5



                  (n)      "Voting Securities" means any securities or equity
                           interests that vote generally in the election of
                           directors, in the admission of general partners, or
                           in the selection of any other similar governing
                           body.

         5.       Miscellaneous.

                  (a)      Usury Savings Clause. Regardless of any provision
                           contained herein, Company shall never be entitled to
                           receive, collect, or apply, as interest, any amount
                           in excess of the maximum rate permitted by law
                           ("Highest Lawful Rate"), and, in the event the
                           Company ever receives, collects, or applies as
                           interest, any such excess, such amount which would
                           be excessive interest shall be deemed a partial
                           prepayment of advances and treated hereunder as
                           such; and, if all Advances have been paid or
                           forgiven in full, any remaining excess shall
                           forthwith be paid to Executive. In determining
                           whether or not the interest paid or payable, under
                           any specific contingency, exceeds the Highest Lawful
                           Rate, Executive and the Company shall, to the
                           maximum extent permitted under applicable law, (a)
                           treat all Advances as but a single extension of
                           credit, (b) characterize any nonprincipal payment as
                           an expense, fee, or premium rather than an interest,
                           (c) exclude voluntary prepayments and the effects
                           thereof, and (d) spread the total amount of interest
                           throughout the entire contemplated term hereof;
                           provided that if all Advances have been paid or
                           forgiven in full prior to the end of the full
                           contemplated term hereof, and if the interest
                           received for the actual period of existence thereof
                           exceeds the Highest Lawful Rate, the Company shall
                           refund to Executive the amount of such excess, and,
                           in such event, the Company shall not be subject to
                           any penalties provided by any law for contracting
                           for, charging, taking, reserving, or receiving
                           interest in excess of the Highest Lawful Rate.

                  (b)      Successors. This Agreement shall be binding upon,
                           and inure to the benefit of Company and Executive
                           and their respective successors, assigns, personal
                           and legal representatives, executors,
                           administrators, heirs, distributees, devisees, and
                           legatees, as applicable.

                  (c)      Notices. For purposes of this Agreement, notices and
                           all other communications provided for in the
                           Agreement shall be in writing and shall be deemed to
                           have been duly given when (i) delivered personally;
                           (ii) sent by telecopy or similar electronic device
                           and confirmed; (iii) delivered by overnight mail; or
                           (iv) sent by registered or certified mail, postage
                           prepaid, addressed as follows:

                                If to Executive:

                                       5

<PAGE>   6



                         (marked "Personal and Confidential")
                               Scott D. Sheffield
                               303 W. Wall, Suite 101
                               Midland, Texas 79701

                         If to Company:

                               Parker & Parsley Petroleum Company
                               303 W. Wall, Suite 101
                               Midland, Texas  79701
                               Attention:        President, and
                                                 General Counsel

                           or to such other address as any party may have
                           furnished to the other in writing in accordance
                           herewith, except that notices of change of address
                           shall be effective only upon receipt.

                  (d)      Waivers. No provision of this Agreement may be
                           modified, waived, or discharged unless such waiver,
                           modification, or discharge is agreed to in writing
                           signed by Executive and Company. No waiver by either
                           party hereto of, or compliance with, any condition
                           or provision of this Agreement to be performed by
                           such other party shall be deemed a waiver of similar
                           or dissimilar provisions or conditions at the same
                           or at any prior or subsequent time. No agreements or
                           representations, oral or otherwise, express or
                           implied, with respect to the subject matter hereof
                           have been made by either party which are not set
                           forth expressly in this Agreement.

                  (e)      Governing Law. THIS AGREEMENT IS BEING MADE AND
                           EXECUTED IN, AND IS INTENDED TO BE PERFORMED, IN THE
                           STATE OF TEXAS, AND SHALL BE GOVERNED, CONSTRUED,
                           INTERPRETED, AND ENFORCED IN ACCORDANCE WITH THE
                           SUBSTANTIVE LAWS OF THE STATE OF TEXAS.

                  (f)      Attorney Fees. All legal fees and costs incurred by
                           Executive in connection with resolution of any
                           dispute or controversy under or in connection with
                           this Agreement shall be reimbursed promptly by
                           Company to the Executive upon presentation by
                           Executive to Company, after the dispute or
                           controversy is resolved in favor of Executive.

                  (g)      Validity. The invalidity or unenforceability of any
                           provision or provisions of this Agreement shall not
                           affect the validity or enforceability of any other
                           provision of this Agreement, which shall remain in
                           full force and effect.

                                       6

<PAGE>   7



                  (h)      Counterparts. This Agreement may be executed in
                           several counterparts, each of which shall be deemed
                           to be an original, but all of which together will
                           constitute one and the same agreement.

                  (i)      Offset. Without limiting any remedy available to the
                           Company at law or in equity for breach by Executive
                           of this Agreement, Company may offset any amounts
                           owed, or benefits payable, by Company to Executive
                           under any other agreement or benefit plans against
                           amounts due and payable from Executive to Company
                           under this Agreement.









          [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK]

                                       7

<PAGE>   8



         IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date and year first above written.

                                   PARKER & PARSLEY PETROLEUM COMPANY


                                   By: /s/  Scott D. Sheffield
                                       -----------------------------------------
                                       Scott D. Sheffield, President

                                   EXECUTIVE:


                                       /s/  Scott D. Sheffield
                                   ---------------------------------------------










                                       8

<PAGE>   9



                                   SCHEDULE I

<TABLE>
<CAPTION>
Name of Officer                              Maximum Loan Agreement
- ---------------                              ----------------------

<S>                                          <C>
Buddy J. Knight                                    $ 41,806
Denny B. Bullard                                   $ 45,151
David E. Sanders                                   $ 40,134
Danny Kellum                                       $ 40,134
Howard Bradford                                    $ 33,445
Hermann E. Eben                                    $ 40,134
Lon C. Kile                                        $ 43,478
Larry N. Paulsen                                   $ 40,134
Timothy L. Dove                                    $ 46,823
W.T. Howard                                        $ 43,478
Mark L. Withrow                                    $ 55,184
</TABLE>


                                       9

<PAGE>   1
                                                                   EXHIBIT 23.1


                        CONSENT OF KPMG PEAT MARWICK LLP

The Board of Directors and Stockholders
Pioneer Natural Resources Company

We consent to the use of our reports incorporated herein by reference and to
the reference to our firm under the heading "Experts" in the registration
statement (no. 333-39381) and in the first amendment to the registration
statement. Our reports refer to a change in the method of accounting for the
impairment of long-lived assets and for long-lived assets to be disposed of
and a change in the method of accounting for income taxes.



                                           /s/ KPMG PEAT MARWICK LLP

                                           KPMG PEAT MARWICK LLP

Midland, Texas
December 15, 1997

<PAGE>   1
                                                                EXHIBIT 23.2

                   CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS


As independent public accountants, we hereby consent to the use of our reports
and to all references to our Firm included in or made a part of this 
Registration Statement (no. 333-39381) and in the first amendment to this
Registration Statement.

                                                     /s/ Arthur Andersen LLP

                                                         Arthur Andersen LLP



Dallas, Texas
December 15, 1997



<PAGE>   1
                                                                    EXHIBIT 23.3

                       CONSENT OF INDEPENDENT ACCOUNTANTS

We consent to the incorporation by reference in this Form S-3 registration
statement (no. 333-39381) of Pioneer Natural Resources Company and its first
amendment of our report dated February 14, 1997 except for Notes 12(b) and (c)
which are as of September 3, 1997 and October 29, 1997, respectively, on our
audit of the consolidated financial statements of Chauvco Resources Ltd. as at
December 31, 1996 and 1995 and for each of the years in the three year period
ended December 31, 1996, and to all references to Price Waterhouse included in
or made a part of this registration statement and its first amendment.


                                              /s/  Price Waterhouse
                                              Chartered Accountants


Calgary, Alberta                                                
December 15, 1997

<PAGE>   1
                                                                    EXHIBIT 23.4



                       CONSENT OF INDEPENDENT ACCOUNTANTS


We consent to the incorporation by reference in this Form S-3 registration
statement (no. 333-39381) of Pioneer Natural Resources Company and its first
amendment of our report dated July 26, 1996, on our audit of the financial
statements of Greenhill Petroleum Corporation as of June 30, 1996, and for the
year ended. We also consent to the reference to our firm under the caption
"Expert" in the registration statement and its first amendment.


                                                   /s/ Coopers & Lybrand L.L.P.

                                                   Coopers & Lybrand L.L.P.

Houston, Texas
December 15, 1997

<PAGE>   1
                                                                    EXHIBIT 23.5



           CONSENT OF INDEPENDENT PETROLEUM ENGINEERS AND GEOLOGISTS

         We hereby consent to the use of our audit letter prepared for Parker & 
Parsley Petroleum Company, effective December 31, 1996, and to all references
to our firm included in or made a part of this Registration Statement on Form
S-3 (No. 333-39381) of Pioneer Natural Resources Company and its first
amendment.

                                           NETHERLAND, SEWELL & ASSOCIATES, INC.



                                           By:  /s/ Frederic D. Sewell
                                                --------------------------
                                                Frederic D. Sewell
                                                President

Dallas, Texas
December 16, 1997

<PAGE>   1
                                                                    EXHIBIT 23.6

                CONSENT OF INDEPENDENT ENGINEERS AND GEOLOGISTS

   
Williamson Petroleum Consultants, Inc. (Williamson) hereby consents to the
incorporation by reference to our report entitled "Evaluation and Review of
Oil and Gas Reserves to the Interests of Mesa Inc. in the Hugoton Area,
Various Counties, Kansas and West Panhandle Area, Various Counties, Texas,
Effective December 31, 1996, for Disclosure to the Securities and Exchange
Commission, Williamson Project 6.8421" dated March 17, 1997, with respect to
Mesa Inc. and to all references to our firm included in or made a part of the
Pioneer Natural Resources Company Registration Statement on Form S-3 filed with
the Securities and Exchange Commission on November 3, 1997 and to Amendment 1
to Form S-3 to be filed on December 17, 1997.
    


                                      /s/ Williamson Petroleum Consultants, Inc.
                                      ------------------------------------------
                                          WILLIAMSON PETROLEUM CONSULTANTS, INC.

   
Houston, Texas
December 16, 1997
    

<PAGE>   1
                                                                    EXHIBIT 23.7




                      [MILLER AND LENTS, LTD. LETTERHEAD]


                               December 15, 1997

Pioneer Natural Resources Company
1400 Williams Square West
5205 N. O'Connor Blvd.
Irving, Texas 75039-3746

                           Re:  Consent of Independent Petroleum Engineers
                                and Geology

Gentlemen:

       We hereby consent to the incorporation by reference of our report dated
February 24, 1997 entitled "Reserve and Net Revenue Forecast as of December 31,
1996, SEC Price Case" with respect to Greenhill Petroleum Corporation and Mesa
Inc.  and to all references to our firm included in or made a part of the
Registration Statement on Form S-3 (No. 333-39381) of Pioneer Natural Resources
Company and its first amendment.

                                                Very truly yours,
                                                  
                                                MILLER AND LENTS, LTD.



                                                By       /s/ P.G. Von Tungeln
                                                         P.G. Von Tungeln
                                                         Chairman

<PAGE>   1
                                                                EXHIBIT 23.8

              [GILBERT LAUSTSEN JUNG ASSOCIATES LTD. LETTERHEAD]


               CONSENT OF INDEPENDENT ENGINEERS AND GEOLOGISTS

We hereby consent to the incorporation by reference of our reports entitled:

    o Chauvco Resources Ltd. (Tidal Properties), Reserve Determination and
      Economic Analysis, effective January 1, 1997, dated February 14, 1997.

    o Chauvco Resources (Gabon) S.A., Remboue Field, Reserve Determination
      and Economic Analysis, effective December 31, 1996, dated February 14, 
      1997.

    o Chauvco Resources (Gabon) S.A., Remboue Field, Reserve Determination
      and Economic Analysis, effective August 1, 1997, dated August 19, 1997.

    o Chauvco Resources (Argentina) S.A., Reserve Determination and
      Economic Analysis, effective December 31, 1996, dated January 21, 1997.

and to all references to our firm included in or made a part of this
Registration Statement on Form S-3 (no. 333-39381) of Pioneer Natural 
Resources Company and its first amendment.

                                        Yours very truly,

                                        GILBERT LAUSTSEN JUNG
                                        ASSOCIATES LTD.


                                  per:  /s/ Wayne Chow

                                        Wayne W. Chow, P. Eng.
                                        Vice-President

Calgary, Alberta
Dated: December 15, 1997





<PAGE>   1
                                                                   EXHIBIT 23.9


               CONSENT OF INDEPENDENT ENGINEERS AND GEOLOGISTS


        We hereby consent to the incorporation by reference of our report
entitled "Audit of Evaluation of Oil and Gas Reserves of Chauvco Resources
Ltd." prepared for Chauvco Resources Ltd. dated February 8, 1997 and to all
references to our firm included in or made a part of this Registration
Statement on Form S-3 (no. 333-39381) of Pioneer Natural Resources Company and
its first amendment.


                                        MARTIN PETROLEUM & ASSOCIATES



                                  By:   /s/ John M. Hewitt
                                        -----------------------------
                                        John M. Hewitt, M.A., P.Eng.
                                        Associate Partner


Calgary, Alberta
December 15, 1997


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