Document:

<PAGE>

                                                         Exhibit 10.13
                           CONFIDENTIALITY AGREEMENT

        THIS AGREEMENT entered into by and between ________________ having an
address at _____________ ("RECIPIENT") and Edwards Lifescience Corporation, a
Delaware corporation,  having an address at _____________, Irvine, California,
92614 ("EDWARDS").

        EDWARDS is the owner of certain proprietary information related to
_____________ ("INFORMATION") with all right, title and interest vested therein
and has authority to disclose said INFORMATION.  EDWARDS agrees to disclose such
INFORMATION, and RECIPIENT agrees to accept such disclosure to allow RECIPIENT
to evaluate such INFORMATION for the purpose of _____________.

        In consideration of the opportunity to evaluate EDWARDS'S INFORMATION,
RECIPIENT agrees to receive that INFORMATION from EDWARDS and to hold the
INFORMATION in confidence and not to disclose it to any third parties, nor to
use it for any purpose other than to evaluate it for the purpose described
above, for a period of five (5) years from the date of disclosure, unless or
until:

        1) said INFORMATION shall become a matter of public knowledge in a
single publication through no fault of RECIPIENT, or

        2) said INFORMATION shall have in its entirety been known to RECIPIENT
prior to its receipt from EDWARDS, as exhibited by written records to EDWARDS
within ten (10) days of the disclosure, or

        3) said INFORMATION shall be disclosed to RECIPIENT in its entirety by a
third party who is not under any obligation of confidentiality to EDWARDS, or

        4) said INFORMATION is approved in writing by EDWARDS for release by
RECIPIENT.

        It is understood that acceptance of the INFORMATION by RECIPIENT shall
not give the right to use the INFORMATION for other than evaluation purposes
unless or until a formal written agreement for the specified purpose has been
entered into by the parties.
<PAGE>

IN WITNESS WHEREOF, the parties have caused this instrument to be executed by
their duly authorized representatives.

EDWARDS LIFESCIENCES CORPORATION

By:                                        By:
      ----------------------------               ----------------------------

Title:                                     Title:
      ----------------------------               ----------------------------

Date:                                      Date:
      ----------------------------               ----------------------------<PAGE>   1

                                                                    EXHIBIT 10.1

                        VOTING AND SHAREHOLDERS AGREEMENT

         This VOTING AND SHAREHOLDERS AGREEMENT (this "AGREEMENT"), is made and
entered into as of the 8th day of February, 2000, by and among Prize Energy
Corp. (formerly known as Vista Energy Resources, Inc.), a Delaware corporation
(the "COMPANY"); Prize Natural Resources, Inc. (formerly known as Prize Energy
Corp.), a Delaware corporation ("OLD PRIZE"); the individuals and trusts listed
as Management Owners on the execution pages hereof (the "MANAGEMENT OWNERS");
the individuals listed as Employee Owners on the execution pages hereof
(including any additional persons who are employed by the Company and
subsequently become parties to this Agreement, the "EMPLOYEE OWNERS"); the
individuals and trusts listed as Other Owners on the execution pages hereof (the
"OTHER OWNERS"); Natural Gas Partners II, L.P., a Delaware limited partnership
("NGP II"), Natural Gas Partners III, L.P., a Delaware limited partnership ("NGP
III"), and Natural Gas Partners V, L.P., a Delaware limited partnership ("NGP V"
and, together with NGP II and NGP III, "NGP"); and Pioneer Natural Resources
USA, Inc., a Delaware corporation ("PIONEER").

                              W I T N E S S E T H:

         WHEREAS, Old Prize and certain of the Owners (as defined in Section 3
of this Agreement) entered into an Amended and Restated Voting and Shareholders
Agreement dated as of June 29, 1999 (the "PRIZE AGREEMENT"); and

         WHEREAS, Old Prize, the Company and a newly-formed wholly-owned
subsidiary of the Company ("MERGER SUB") entered into an Agreement and Plan of
Merger dated as of October 8, 1999 (the "MERGER AGREEMENT"), under which, on the
date hereof, Merger Sub was merged with and into Old Prize and Old Prize became
a wholly-owned subsidiary of the Company; and

         WHEREAS, under the terms of the Merger Agreement, upon consummation of
the Merger, the Prize Agreement is to be terminated and the parties hereto are
to enter into this Agreement; and

         WHEREAS, certain terms are defined in Section 3 of this Agreement;

         NOW, THEREFORE, in consideration of the premises and the mutual terms,
covenants and conditions contained herein, the parties hereto agree as follows:

         Section 1. Voting Agreement.

         (a) General Rights. From and after the date hereof and until the
provisions of this Section 1 shall terminate as provided in Section 1(b) below,
the Owners, NGP II and NGP III agree to vote all of their Owner Shares and any
other voting securities of the Company over which they have voting control, and
the Company will take all reasonable actions within its control, that may be
necessary in order to cause:

             (i) the nomination and election to the Board of one representative
         designated by a majority in interest of the Management Owners;

             (ii) the nomination and election to the Board of (A) three
         representatives designated by NGP before the Pioneer Board Reduction
         Event, and (B) four representatives designated by NGP on and after the
         Pioneer Board Reduction Event;

             (iii) the nomination and election to the Board of (A) two
         representatives designated by Pioneer before the Pioneer Board
         Reduction Event, and (B) one representative designated by Pioneer on
         and after the Pioneer Board Reduction Event;

             (iv) the removal from the Board (with or without cause) of any or
         all of the representatives designated by the Management Owners
         hereunder at the written request of a majority in interest of the
         Management Owners (but only upon such written request an under no other
         circumstances);

<PAGE>   2

             (v) the removal from the Board (with or without cause) of any or
         all of the representatives designated by NGP hereunder at the written
         request of NG (but only upon such written request and under no other
         circumstances);

             (vi) the removal from the Board (with or without cause) of any or
         all of the representatives designated by Pioneer hereunder at the
         written request o Pioneer (but only upon such written request and under
         no other circumstances);

             (vii) in the event that any representative designated by the
         Management Owners hereunder for any reason ceases to serve as a member
         of the Board during his term of office, the resulting vacancy on the
         Board to be filled by a representative designated by a majority in
         interest of the Management Owners;

             (viii) in the event that any representative designated by NGP
         hereunder for any reason ceases to serve as a member of the Board
         during his term of office the resulting vacancy on the Board to be
         filled by a representative designated by an authorized representative
         of NGP;

             (ix) in the event that any representative designated by Pioneer
         hereunder for any reason ceases to serve as a member of the Board
         during his term of office, the resulting vacancy on the Board to be
         filled by a representative designated by an authorized representative
         of Pioneer; and

             (x) the provisions of Section 3 of Article Four of the Amended and
         Restated Bylaws (together with related definitions) to be preserved and
         effective without amendment, elimination, or superseding (whether
         directly, by charter amendment, or otherwise) from the form attached as
         Exhibit A hereto unless Pioneer previously consents in writing to such
         amendment elimination, or superseding.

         (b) Termination.

             (i) The provisions of this Section 1 shall terminate with respect
         to any single Management Owner on the first to occur of the following:

                 (A) the date such Management Owner's employment by the Company
             shall have terminated;

                 (B) the date of the death of such Management Owner; or

                 (C) the date of any Transfer or purported Transfer of Owner
             Shares by such Management Owner in violation of the terms of this
             Agreement.

             (ii) The provisions of this Section 1 shall terminate as to Pioneer
         on the first date on which Pioneer no longer owns shares of Common
         Stock and Underlying Common Stock that constitute at least 14.03% of
         the total issued and outstanding shares of Common Stock (Underlying
         Common Stock shall be deemed issued and outstanding for the purpose of
         such computation) (whether such reduction of percentage ownership
         occurs as a result of Pioneer's sale or transfer of such shares, the
         Company's issuance of additional shares or the merger, consolidation or
         other transaction involving the Company, or otherwise). Upon the
         reduction of Pioneer's percentage ownership below 14.03%, as described
         in the preceding sentence, the provisions of this Section 1 shall also
         terminate as to all other Owners, NGP II and NGP III.

             (iii) The provisions of this Section 1 shall terminate as to all
         Owners, NGP II and NGP III on June 29, 2009, unless extended by the
         parties hereto.

         Section 2. Transfer Restrictions.

         (a) General Rights. In addition to any restrictions on the Transfer of
Owner Shares that are imposed under the Securities Act or other applicable
securities laws, no Restricted Owner shall Transfer or Pledge all or any part of
such Restricted Owner's Owner Shares without the prior written consent of the
other Restricted Owners or in accordance with this Section 2.

<PAGE>   3
         (b) Pro-Rata Sale Rights after an IPO. No Restricted Owner shall have
the right to effect any Transfer of any of such Restricted Owner's Owner Shares
(other than as provided in Section 2(c) below) unless such Restricted Owner
proposing to Transfer such Owner Shares (a "SELLING OWNER") complies with the
tag-along requirements of clause (i) or (ii) below:

             (i) Public Sales Through a Broker. If a proposed Transfer is to be
         effectuated through an established brokerage firm and utilizing the
         public securities markets (e.g., the New York Stock Exchange, the
         American Stock Exchange or The Nasdaq Stock Market):

                 (A) Such Selling Owner shall provide prior written notice of
             the following (a "PUBLIC SALE NOTICE") to all other Restricted
             Owners ("TAG-ALONG OWNER(S)"): (I) the number of shares proposed to
             be made available for sale (the "INITIAL SHARES"); (II) the pricing
             and other instructions pursuant to which the selected broker will
             be operating; (III) the name, address, telephone number and
             facsimile number of the selected broker and the name of the
             selected registered representative at the broker; and (IV)
             confirmation that the selected broker and selected registered
             representative have been advised that the Tag-Along Owners may
             desire to elect to participate in the proposed Transfer.

                 (B) After receiving a Public Sale Notice, a Tag-Along Owner
             shall be entitled to elect to participate, up to such Tag-Along
             Owner's Proportionate Share, in the proposed Transfer through the
             selected broker, subject to such Tag-Along Owner's (I) meeting all
             of the broker's requirements to establish a customer account and
             execute transactions for such Tag-Along Owner, (II) meeting all of
             the requirements imposed by applicable securities laws in order to
             execute such transaction (including without limitation Rule 144
             under the Securities Act), and (III) responding in writing within
             three business days after receipt of such Public Sale Notice to
             such Selling Owner, to the Tag-Along Owners and to the selected
             broker. If the designated broker is unable to sell an amount at
             least equal to the number of Initial Shares, the aggregate number
             of shares which the designated broker is able to sell shall be
             allocated on a Proportionate Share basis among such Selling Owner
             and all Tag-Along Owners electing to participate in the proposed
             Transfer.

             (ii) Non-Broker Transactions. In the case of a proposed Transfer
         that does not fall within the provisions of clause (i) of this Section
         2(b), such Selling Owner shall cause the Person or group that proposes
         to acquire Owner Shares from such Selling Owner (the "PROPOSED
         PURCHASER") to offer in writing (the "PURCHASE OFFER") to the Tag-Along
         Owners, to purchase a Proportionate Share of the Owner Shares of the
         Tag-Along Owners. Any such purchase shall be made in accordance with
         the following:

                 (A) the purchase from each Tag-Along Owner shall be made at the
             highest price per share and on such other terms and conditions as
             the Proposed Purchaser has offered to purchase Owner Shares from
             such Selling Owner;

                 (B) each Tag-Along Owner shall have no more than 20 days from
             the receipt of the Purchase Offer in which to accept such Purchase
             Offer, in whole or in part;

                 (C) to the extent that a Tag-Along Owner accepts such Purchase
             Offer, the number of Owner Shares to be sold to the Proposed
             Purchaser by such Selling Owner shall be proportionately reduced to
             the extent necessary to comply with this Section 2(b)(ii); and

                 (D) the closing of such purchase shall occur within 30 days
             after such acceptance or at such other time as such Selling Owner,
             the Tag-Along Owners and the Proposed Purchaser may agree.

         (c) Excluded Transfers. The provisions of Section 2(b) above do not
apply to any Transfer by a Restricted Owner of such Restricted Owner's Owner
Shares in an Excluded Affiliate Transfer. The provisions of Section 2(b) above
do not apply to any Transfer by a Restricted Owner of such Restricted Owner's
Owner Shares pursuant to (i) a registration statement filed under the Securities
Act, or (ii) a sale effected through a national securities exchange or the
Nasdaq Stock Market.

<PAGE>   4

         (d) Termination. This Section 2 shall terminate upon the earlier to
occur of the following: (i) the first date on which the Restricted Owners
collectively do not own at least 20% of the outstanding common equity securities
of the Company or of any successor or assignee resulting from the consolidation,
merger or sale of all or substantially all of the assets of the Company; (ii)
the adjudication of the Company as a bankrupt, the execution by the Company of
an assignment for the benefit of creditors or the appointment of a receiver of
the Company; (iii) the voluntary or involuntary dissolution of the Company; (iv)
when there is otherwise only one surviving Restricted Owner as a party to this
Agreement; (v) June 29, 2009, unless extended by agreement of Restricted Owners
holding at least 80% of the Owner Shares held by Restricted Owners outstanding
on such date; or (vi) the written agreement of all the Restricted Owners. The
Company and the Restricted Owners each covenant and agree to enter into and to
use their best efforts to cause any successor or assignee of the Company
described in clause (i) of this Section 2(d) to enter into an agreement
substantially similar to this Agreement if the Restricted Owners collectively
own at least 20% of the equity interests in such successor or assignee.

         Section 3. Definitions. As used in this Agreement, the following terms
shall have the meanings assigned to them in this Section 3:

         "AGREEMENT" means this Voting and Shareholders Agreement, as the same
may be amended from time to time.

         "BOARD" means the Board of Directors of the Company.

         "COMMON STOCK" means the Company's common stock, par value $.01 per
share

         "COMPANY" has the meaning set forth in the introductory paragraph of
this Agreement.

         "EMPLOYEE OWNERS" has the meaning set forth in the introductory
paragraph of this Agreement.

         "EXCLUDED AFFILIATE TRANSFER" means (i) any transfer of Owner Shares by
a Restricted Owner, other than a Management Owner or an Employee Owner (whether
voluntarily or by operation of law) to a partner or other affiliate or a legal
successor of such Restricted Owner; (ii) any transfer of Owner Shares by a
Restricted Owner who is an individual to a member of such Restricted Owner's
family or to a revocable trust for estate planning purposes, but only if such
Restricted Owner retains the right to vote such Owner Shares following such
transfer; (iii) any transfer upon the death of a Restricted Owner who is an
individual; and (iv) any transfer of Owner Shares by a Restricted Owner which is
a trust to the principal beneficiary of that trust; provided that, in the case
of any Transfer described in clause (i), (ii), (iii) or (iv) above, such
transferee agrees to be bound by the terms of this Agreement and evidences same
by executing a copy of this Agreement promptly upon receiving the assignment of
such Owner Shares.

         "INITIAL SHARES" has the meaning set forth in Section 2(b)(i)(A)
hereof.

         "MANAGEMENT OWNERS" has the meaning set forth in the introductory
paragraph of this Agreement.

         "MERGER AGREEMENT" has the meaning set forth in the recitals to this
Agreement.

         "MERGER SUB" has the meaning set forth in the recitals to this
Agreement.

         "NGP," "NGP II," "NGP III" and "NGP V" have the meanings set forth in
the introductory paragraph of this Agreement.

         "OLD PRIZE" has the meaning set forth in the introductory paragraph of
this Agreement.

         "OTHER OWNERS" has the meaning set forth in the introductory paragraph
of this Agreement.

         "OWNER(S)" means the Management Owners, the Employee Owners, the Other
Owners, NGP V and Pioneer.

         "OWNER SHARES" means, with respect to any Owner, NGP II or NGP III: (i)
a shares of Common Stock or Series A Preferred currently held or held in the
future by such Person (including, without limitation, shares acquired upon the
exercise of any option, warrant or other right); provided that, in order to
participate in any Purchase Offer, such conversion must occur no later than the
deadline for acceptance of such offer as set forth in Section 2(b)(ii)(B)
hereof;

<PAGE>   5
(ii) any equity securities issued or issuable directly or indirectly to such
Person with respect to the Common Stock or Series A Preferred referred to in
clause (i) above by way of stock dividend or stock split or in connection with a
combination of shares, conversion, recapitalization, merger, consolidation or
other reorganization; and (iii) any other shares of any class or series of
voting security of the Company currently held or held in the future by such
Person. As to any particular shares constituting Owner Shares, such shares will
cease to be Owner Shares when they have been transferred to any person who is
not a current or future party to this Agreement, other than pursuant to an
Excluded Affiliate Transfer. In determining the percentage ownership of any
Person or Persons or their participation in a transaction, the Series A
Preferred shall be counted on an as-converted basis with the Common Stock.

         "PERMITTED ASSIGN" means an assignee of Owner Shares who acquires such
shares in an Excluded Affiliate Transfer.

         "PERSON" means any individual, corporation, partnership, limited
liability company, trust, unincorporated association, business, or other legal
entity, and any governmental agency or political subdivision thereof.

         "PIONEER" has the meaning set forth in the introductory paragraph of
this Agreement.

         "PIONEER BOARD REDUCTION EVENT" means the first date on which Pioneer
no longer owns Common Stock and Underlying Common Stock that constitutes at
least 25.2% of the total issued and outstanding shares of Common Stock (whether
as a result of Pioneer's sale or transfer of such shares, the Company's issuance
of additional shares, a merger, consolidation or other fundamental change
involving the Company, or otherwise).

         "PLEDGE" means any pledge of an interest in, or other encumbrance
placed upon, Owner Shares as security for indebtedness or for other purposes.

         "PRIZE AGREEMENT" has the meaning set forth in the recitals to this
Agreement.

         "PROPORTIONATE SHARE" means:

             (i) for purposes of Section 2(b)(i) hereof, the number of Owner
         Shares equal to the product of (A) the Initial Shares, or such lesser
         number of shares that the designated broker has been able to sell
         pursuant Section 2(b)(i) hereof, times (B) the fraction which is equal
         to the total number of Owner Shares which a Tag-Along Owner electing to
         participate in a Transfer owns, over the aggregate number of Owner
         Shares owned by the Selling Owner and all Tag-Along Owners electing to
         participate in a Transfer; and

             (ii) for purposes of Section 2(b)(ii) hereof, the number of Owner
         Shares equal to the product of (A) the total number of Owner Shares
         which a Proposed Purchaser has offered to purchase from the Selling
         Owner, times (B) the fraction which is equal to the total number of
         Owner Shares which a Tag-Along Owner owns, over the aggregate number of
         Owner Shares owned by the Selling Owner and all Tag-Along Owners who
         have elected participate in a Transfer.

         "PROPOSED PURCHASER" has the meaning set forth in Section 2(b)(ii)
hereof.

         "PUBLIC SALE NOTICE" has the meaning set forth in Section 2(b)(i)(A)
hereof.

         "PURCHASE OFFER" has the meaning set forth in Section 2(b)(ii) hereof.

         "RESTRICTED OWNER(S)" means the Management Owners, the Employee Owners
an NGP V and their successors and assigns.

         "SECURITIES ACT" means the Securities Act of 1933, as amended.

         "SELLING OWNER" has the meaning set forth in Section 2(b) hereof.

         "SERIES A PREFERRED" means the Company's Series A 6% convertible
preferred stock, par value $.01 per share.

         "TAG-ALONG OWNER(S)" has the meaning set forth in Section 2(b)(i)(A)
hereof.

<PAGE>   6
         "TRANSFER" means any sale, assignment or other disposition of Owner
Shares, other than a Pledge.

         "UNDERLYING COMMON STOCK" means at any time, with respect to any share
of Series A Preferred, the aggregate number of shares of Common Stock into which
such share is then convertible.

         Section 4. Enforcement; Legends. No Owner Shares shall be transferred
on the books of the Company, nor shall any Transfer be effective, unless and
until the terms and provisions of this Agreement are first complied with and, in
case of violation of this Agreement by the attempted Transfer of Owner Shares
without compliance with the terms and provisions hereof, such Transfer shall be
invalid and of no effect. The Owners will cause the Company to imprint a legend
on any certificates evidencing Owner Shares which are subject to this Agreement
referring to the voting rights and the restrictions on Transfer of the Owner
Shares imposed hereunder. Any such legend shall be removed from the certificates
evidencing any shares which cease to be Owner Shares, as set forth in the
definition of such term in Section 3 hereof.

         Section 5. Termination. This Agreement shall terminate upon the
termination of Section 1 and Section 2 hereof.

         Section 6. Miscellaneous.

         (a) Benefit. This Agreement will only bind and inure to the benefit
of, and will only be enforceable by and against, the original parties
hereto and any Permitted Assigns under this Agreement.

         (b) Notices. Whenever in this Agreement, notice is required as
permitted to be given it shall be given in writing, and if such notice is given
by registered United States mail it shall be deemed to have been received on the
second business day after the date such notice is posted. All notices hereunder
to the Company shall be mailed to it at the address of its principal place of
business and all notices to the Owners shall be mailed to them at their last
known address as shown on the books and records of the Company. Any party may
change such party's mailing address by giving written notice of such change to
all other parties.

         (c) GOVERNING LAW. THIS AGREEMENT AND THE RIGHTS AND DUTIES OF THE
PARTIES HEREUNDER SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS
OF THE STATE OF DELAWARE.

         (d) Number. Words in the singular shall be construed to include the
plural and vice versa, unless the context otherwise requires.

         (e) Headings. The headings appearing in this Agreement are inserted
only for convenience of reference and in no way shall be construed to define,
limit or describe the scope or intent of any provision of this Agreement.

         (f) Severability. Every provision in this Agreement is intended to be
severable. In the event that any provision in this Agreement shall be held
invalid, the same shall not affect in any respect whatsoever the validity of the
remaining provisions of this Agreement; provided, however, that if any such
provision may be made enforceable by limitation thereof, then such provision
shall be deemed to be so limited and shall be enforceable to the maximum extent
permitted by applicable law.

         (g) Joinder. The spouse, if any, of each Management Owner and Employee
Owner who resides in a state subject to community property laws joins in the
execution and delivery of this Agreement for the express purpose of binding his
or her community property interests, if any, in the Owner Shares.

         (h) Counterparts. This Agreement may be executed in one or more
counterparts, each of which shall be deemed an original and all of which shall
constitute but one and the same instrument.

         (i) Entirety and Modification. This Agreement constitutes the entire
agreement of the parties hereto with respect to the subject matter hereof and
supercedes all prior understandings and agreements regarding such matters,
including without limitation the Prize Agreement, which is hereby terminated.
This Agreement may not be modified, supplemented or amended in any respect
except by written instrument executed by all parties hereto.

<PAGE>   7

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

Prize Energy Corp.                           Prize Natural Resources, Inc.
(formerly known as                           (formerly known as
Vista Energy Resources, Inc.)                Prize Energy Corp.)

By: /s/ Philip B. Smith                      By: /s/ Lon C. Kile
    ------------------------------               -------------------------------
    Philip B. Smith, Chairman and                Lon C. Kile
    Chief Executive Officer                      President

<PAGE>   8

                                MANAGEMENT OWNERS

Initial Number of
Owner Shares:
356,257 shares of                /s/ Philip B. Smith
Common Stock                     -----------------------------------------------
                                 Philip B. Smith, Trustee of The Philip B. Smith
                                 Revocable Trust Dated July 25, 1994

Initial Number of
Owner Shares:
84,134 shares of                 /s/ Philip B. Smith
Common Stock                     -----------------------------------------------
                                 Philip B. Smith, Trustee of The Scott C. Smith
                                 Irrevocable Trust Dated January 15, 1996

Initial Number of
Owner Shares:
84,134 shares of                 /s/ Philip B. Smith
Common Stock                     -----------------------------------------------
                                 Philip B. Smith, Trustee of The Laura E. Smith
                                 Irrevocable Trust Dated January 15, 1996

Initial Number of
Owner Shares:
41,630 shares of                 /s/ Lon C. Kile
Common Stock                     -----------------------------------------------
                                 Lon C. Kile

                                       NGP

                                 NATURAL GAS PARTNERS II, L.P.

                                 By: G.F.W. Energy II, L.P., General Partner
                                 By: GFW II, L.L.C., General Partner

Initial Number of
Owner Shares:
511,709 shares of                By: /s/ Kenneth A. Hersh
Common Stock                        --------------------------------------------
                                    Kenneth A. Hersh, Authorized Member

                                 NATURAL GAS PARTNERS III, L.P.

                                 By: Rainwater Energy Investors, L.P.
                                     General Partner
                                 By: GFW III, L.L.C., General Partner
Initial Number of
Owner Shares:
699,390 shares of                By: /s/ Kenneth A. Hersh
Common Stock                         -------------------------------------------
                                     Kenneth A. Hersh, Authorized Member

<PAGE>   9

                                 NATURAL GAS PARTNERS V, L.P.

                                 By: G.F.W. Energy V, L.P., General Partner
                                 By: GFW V, L.L.C., General Partner
Initial Number of
Owner Shares:
7,326,821 shares of              By: /s/ Kenneth A. Hersh
Common Stock                         -------------------------------------------
                                     Kenneth A. Hersh, Authorized Member

                                     PIONEER

                                 PIONEER NATURAL RESOURCES USA, INC.

Initial Number of
Owner Shares:                    By: /s/ Mark L. Withrow
3,984,197 shares of                  -------------------------------------------
Series A Preferred                   Mark L. Withrow
                                     Executive Vice President

                                 EMPLOYEE OWNERS

Initial Number of
Owner Shares:
42,068 shares of                 /s/ Monica Griffin
Common Stock                     -------------------------------------------
                                 Monica Griffin

                                  OTHER OWNERS

Initial Number of
Owner Shares:
356,257 shares of                /s/ Kathlyn C. Smith
Common Stock                     -------------------------------------------
                                 Kathlyn C. Smith, Trustee of The Kathlyn C.
                                 Smith Revocable Trust Dated July 25, 1994

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