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                                                                    EXHIBIT 10.2

                            STOCK PURCHASE AGREEMENT



         STOCK PURCHASE AGREEMENT (this "AGREEMENT"), dated as of April 1, 2003,
by and between Plains Resources Inc., a Delaware corporation (the "COMPANY"),
and EnCap Investments L.L.C., a Delaware limited liability company (the
"ENCAP").

         WHEREAS, the Company desires to repurchase 400,000 shares (the
"SHARES") of its common stock, par value $.10 per share (the "COMMON STOCK") at
a price of $10.00 per share from certain entities controlled by EnCap
(collectively, the "ENCAP AFFILIATES") under the terms and conditions set forth
herein, and to cancel and retire the Shares after such repurchase.

         NOW, THEREFORE, in consideration of the premises and the mutual
promises herein made, and in consideration of the representations, warranties,
agreements and covenants herein, the parties hereto hereby agree as follows:

     1.  Sale and Purchase.

         (a) On the Closing Date (as hereinafter defined), EnCap shall cause the
EnCap Affiliates listed on Schedule I hereto to sell, assign and transfer to the
Company the number of Shares set forth opposite such EnCap Affiliate's name on
Schedule I hereto free and clear of all security interests, pledges, mortgages,
liens, charges, encumbrances, adverse claims, restrictions, or other burdens or
encumbrances of any kind ("Encumbrances"), other than those restrictions arising
from applicable federal and state securities laws.

         (b) At the Closing (as hereinafter defined), the Company shall pay to
EnCap, on behalf of the EnCap Affiliates, as the purchase price of the Shares,
$10.00 per share in cash multiplied by the number of Shares sold, assigned and
transferred by the EnCap Affiliates, by wire transfer of immediately available
funds to EnCap's bank account set forth on Schedule I.

         (c) At the Closing, EnCap shall cause the EnCap Affiliates to deliver
to the Company a certificate or certificates representing such EnCap Affiliate's
Shares registered in such EnCap Affiliate's name, together with a duly executed
stock power endorsed to the Company with signatures guaranteed by a national
bank or trust company or a member firm of the New York Stock Exchange or such
other assignments or instruments of conveyance and transfer, in form and
substance satisfactory to the Company and its counsel, as shall be effective to
vest in the Company all of such EnCap Affiliate's right, title and interest in
and to the Shares.

     2.  Representations and Warranties of EnCap. EnCap represents and warrants
to the Company that:

         (a) Each EnCap Affiliate is the sole record owner and is the beneficial
owner of such EnCap Affiliate's Shares; other than EnCap, no person has a right
to acquire or direct the disposition, or holds a proxy or other right to vote or
direct the vote, of the Shares; and each EnCap Affiliate has good and valid
title to the Shares, free and clear of any Encumbrances

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except restrictions arising from applicable federal and state securities laws.
Other than this Agreement, there is no option, warrant, right, call, proxy,
agreement, commitment or understanding of any nature whatsoever, fixed or
contingent, that directly or indirectly (i) calls for the sale, pledge or other
transfer or disposition of any of the Shares, any interest therein or any rights
with respect thereto, or relates to the voting, disposition, exercise,
conversion or control of the Shares, or (ii) obligates EnCap or any EnCap
Affiliate to grant, offer or enter into any of the foregoing.

         (b) The sale by the EnCap Affiliates of the Shares and the delivery of
the certificates representing the Shares to the Company against receipt of
payment therefor pursuant hereto will transfer to the Company good and valid
title to the Shares, free and clear of all Encumbrances except restrictions
arising from applicable federal and state securities laws.

         (c) EnCap has all necessary power and authority to enter into this
Agreement, to carry out its obligations hereunder, and to consummate the
transactions contemplated hereby. The execution and delivery of this Agreement
by EnCap, the performance by EnCap of its obligations hereunder, and the
consummation by EnCap of the transactions contemplated hereby have been duly
authorized by all requisite action on the part of EnCap and no other proceedings
on the part of EnCap are necessary to authorize this Agreement or the
consummation of the transactions contemplated hereby. This Agreement has been
duly executed and delivered by EnCap and constitutes a legal, valid and binding
obligation of EnCap enforceable against EnCap in accordance with its terms.

         (d) The execution, delivery and performance of this Agreement by EnCap
does not and will not (a) violate or conflict with or result in a breach of any
provision of the organizational documents of EnCap, (b) violate or conflict with
any law or order, writ, judgment, injunction, decree, stipulation,
determination, or award (a "GOVERNMENTAL ORDER") entered into by or with any
federal, state, local, or any foreign government, governmental, regulatory, or
administrative authority, agency or commission or any court (a "GOVERNMENTAL
AUTHORITY") applicable to EnCap or any of its assets and properties, or (c)
conflict with, result in any violation or breach of or constitute a default (or
an event which, with the giving of notice or lapse of time, or both, would
become a default) under, require any notice or consent under, or give to others
any rights of termination, amendment, acceleration or cancellation of, or result
in any loss of any benefit, the triggering of any payment by, or the increase in
other obligation of, EnCap or the creation of any Encumbrance on any assets or
properties of EnCap pursuant to any material contract, license, permit,
franchise or other instrument or arrangement to which EnCap is a party or by
which it, or any of such assets or properties is bound or affected, except for,
in the case of clauses (b) and (c), such conflicts, violations, breaches,
defaults or other occurrences which would not (i) impair, in any material
respect, the ability of EnCap to perform its obligations under this Agreement or
(ii) prevent or materially delay the consummation of any of the transactions
contemplated hereby. The execution, delivery and performance of this Agreement
by EnCap do not and will not require any consent, waiver, approval,
authorization or other order of, action by, filing with or notification to, any
Governmental Authority, except (a) the requirements of the Securities Exchange
Act of 1934, as amended (the "EXCHANGE ACT"), and (b) any other consent,
approval, authorization, filing or notice the failure of which to make or obtain
would not (i) impair, in any material respect, the ability of EnCap to perform
its



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obligations under this Agreement, or (ii) prevent or materially delay the
consummation of any of the transactions contemplated hereby.

         (e) EnCap has not entered into any agreement, arrangement or
understanding with any person which will result in the obligation of the Company
to pay any finder's fees, brokerage or agent's commissions or other like
payments in connection with the negotiations leading to this Agreement of the
consummation of the transactions contemplated hereby.

     3.  Representations and Warranties of the Company. The Company represents
and warrants to EnCap, for itself and for the benefit of the EnCap Affiliates,
that:

         (a) The Company has all necessary power and authority to enter into
this Agreement, to carry out its obligations hereunder, and to consummate the
transactions contemplated hereby. The execution and delivery of this Agreement
by the Company, the performance by the Company of its obligations hereunder, and
the consummation by the Company of the transactions contemplated hereby have
been duly authorized by all requisite action on the part of the Company and no
other proceedings on the part of the Company are necessary to authorize this
Agreement or the consummation of the transactions contemplated hereby. This
Agreement has been duly executed and delivered by the Company and constitutes a
legal, valid and binding obligation of the Company enforceable against it in
accordance with its terms.

         (b) The execution, delivery and performance of this Agreement by the
Company does not and will not (a) violate or conflict with or result in a breach
of any provision of the organizational documents of the Company, (b) violate or
conflict with any law or Governmental Order applicable to the Company or any of
its assets and properties, or (c) conflict with, result in any violation or
breach of or constitute a default (or an event which, with the giving of notice
or lapse of time, or both, would become a default) under, require any notice or
consent under, or give to others any rights of termination, amendment,
acceleration or cancellation of, or result in any loss of any benefit, the
triggering of any payment by, or the increase in other obligation of, the
Company or the creation of any Encumbrance on any assets or properties of the
Company pursuant to any material contract, license, permit, franchise or other
instrument or arrangement to which the Company is a party or by which it, or any
of its assets or properties is bound or affected, except for, in the case of
clauses (b) and (c), such conflicts, violations, breaches, defaults or other
occurrences which would not (i) impair, in any material respect, the ability of
the Company to perform its obligations under this Agreement or (ii) prevent or
materially delay the consummation of any of the transactions contemplated
hereby. The execution, delivery and performance of this Agreement by the Company
do not and will not require any consent, waiver, approval, authorization or
other order of, action by, filing with or notification to, any Governmental
Authority, except (a) the requirements of the Exchange Act, and (b) any other
consent, approval, authorization, filing or notice the failure of which to make
or obtain would not (i) impair, in any material respect, the ability of the
Company to perform its obligations under this Agreement, or (ii) prevent or
materially delay the consummation of any of the transactions contemplated
hereby.

     4.  Covenants. EnCap hereby covenants and agrees that neither it nor any
EnCap Affiliate will enter into any transaction, take any action, or by inaction
permit any event to occur,


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that would result in any of EnCap's representations or warranties herein
contained not being true and correct as of (a) the time immediately after the
occurrence of such transaction, action or event and (b) the Closing Date. The
Company hereby covenants and agrees that it will not enter into any transaction,
take any action, or by inaction permit any event to occur, that would result in
any of the Company's representations or warranties herein contained not being
true and correct as of (a) the time immediately after the occurrence of such
transaction, action or event and (b) the Closing Date.

     5.  Adjustments Upon Changes in Capitalization. In the event of any
reorganization, recapitalization, split, merger, stock split, stock dividend,
combination or exchange of shares, issuance of other securities in exchange for
Common Stock or any other change in the outstanding securities of the Company
that results in a change in the number and the kind of shares of Common Stock or
securities convertible into Common Stock, the terms "Shares" shall be deemed to
refer to and include the Shares as well as all such dividends and distributions,
and EnCap shall cause the EnCap Affiliates to deliver the Shares and all such
dividends and distributions to the Company at the Closing and the amount to be
paid per share by the Company shall be adjusted so that the total amount to be
paid by the Company hereunder remains unchanged.

     6.  Closing; Conditions to Closing. The consummation of the transactions
contemplated by this Agreement (the "CLOSING") shall take place at the offices
of the Company, 500 Dallas Street, Suite 700, Houston, Texas 77002, on the
business day after satisfaction or waiver of the conditions set forth in this
Section 6. This Agreement shall terminate if the conditions set forth in this
Section 6 have not been satisfied or waived by April 11, 2003. The date on which
the Closing occurs is referred to in this Agreement as the "CLOSING DATE." The
obligations of the Company to consummate the transactions contemplated by this
Agreement are subject to the satisfaction of the following conditions, unless
waived by the Company in writing: (a) each of the representations and warranties
made by EnCap in this Agreement being true and correct as of the date hereof and
the Closing; (b) EnCap has performed in all respects each and every covenant and
agreement contained in this Agreement required to be performed by EnCap on or
before the Closing; and (c) the Company and EnCap shall have obtained all
approvals and consents necessary or required for the consummation of the
transactions contemplated by this Agreement.

     7.  Validity and Severability. If any provision of this Agreement shall be
held to be illegal, invalid or unenforceable under any applicable law, then such
contravention or invalidity shall not invalidate the entire Agreement. Such
provision shall be deemed to be modified to the extent necessary to render it
legal, valid and enforceable, and if no such modification shall render it legal,
valid and enforceable, then this Agreement shall be construed as if not
containing the provision held to be illegal, invalid or unenforceable and the
rights and obligations of the parties hereto shall be construed and enforced
accordingly.

     8.  Governing Law. This Agreement shall be governed by, and construed in
accordance with, the laws of the State of Texas applicable to contracts executed
in and to be performed in that state and without regard to any applicable
conflicts of law.


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     9.  Expenses. Whether or not the transactions contemplated by this
Agreement are consummated, each of the parties hereto shall pay their respective
fees and expenses incurred in connection herewith.

     10. Entire Agreement. This Agreement represents the entire agreement of the
parties with respect to the subject matter hereof and shall supersede any and
all previous contracts, arrangements or understandings between the parties
hereto, with respect to the subject matter hereof.

     11. Amendment. This Agreement may not be amended or modified except by an
instrument in writing signed by each of, or on behalf of each of, the parties.
This Agreement may be amended or supplemented in writing by the parties hereto
with respect to any of the terms contained in this Agreement.

     12. Assignment; Binding Effect. Neither party shall assign any of its
respective rights, interests or obligations hereunder without the prior written
consent of the other party, which consent may be granted or withheld in such
other party's sole discretion. Subject to the preceding sentence, this Agreement
shall be binding upon and inure to the benefit of the parties hereto and their
respective successors and assigns. Nothing in this Agreement, express or
implied, is intended to confer on any person other than the parties hereto or
their respective successors and assigns any rights, remedies, obligations or
liabilities under or by reason of this Agreement.

     13. Notices. All notices or communications hereunder shall be in writing
(including facsimile or similar writing) addressed as follows:

     (a) To EnCap:

         EnCap Investments L.L.C.
         1100 Louisiana, Suite 3150
         Houston, Texas 77002
         Attention: Mr. Jason DeLorenzo

         with a copy to:

         Thompson & Knight LLP
         333 Clay St., Suite 3300
         Houston, TX 77002
         Telecopy: (713) 654-1871
         Attention: Timothy T. Samson

     (b) To the Company:

         Plains Resources Inc.
         500 Dallas Street, Suite 700
         Houston, Texas 77002
         Telecopy: (713) 654-1523
         Attention: Franklin R. Bay


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         with a copy to:

         Akin, Gump, Strauss, Hauer & Feld, L.L.P.
         300 West 6th Street, Suite 2100
         Austin, Texas  78701
         Telecopy: 512-703-1112
         Attention: Michael F. Meskill

Any such notice or communication shall be deemed given (i) when made, if made by
hand delivery, and upon confirmation of receipt, if made by facsimile, (ii) one
business day after being deposited with a next day courier, postage prepaid, or
(iii) three business days after being sent certified or registered mail, return
receipt requested, postage prepaid, in each case addressed as above (or to such
other address as such party may designate in writing from time to time).

     14. Counterparts. This Agreement may be executed and delivered (including
by facsimile transmission) in one or more counterparts, and by the different
parties hereto in separate counterparts, each of which when executed and
delivered shall be deemed to be an original, but all of which taken together
shall constitute one and the same agreement.

     15. Survival of Representations, Warranties, Covenants and Agreements. The
representations, warranties, covenants and agreements contained in this
Agreement shall survive the Closing Date.

     16. Transactions. This Agreement may be terminated and the transactions
contemplated hereby may be abandoned at any time prior to the Closing (a) by the
mutual written consent of the Company and EnCap or (b) by either party if there
has been a breach by the other party of any representation, warranty, covenant
or agreement of such other party in this Agreement which could prevent or
materially delay the consummation of the transactions contemplated hereby.

                            [SIGNATURE PAGE FOLLOWS]



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         IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
executed as of the date first written above by their respective representatives
thereunto duly authorized.

                                        PLAINS RESOURCES INC.



                                        By:
                                           --------------------------------
                                        Name:  Franklin R. Bay
                                        Title: Senior Vice President, Chief
                                               Legal Officer and Secretary


                                        ENCAP INVESTMENTS L.L.C



                                        By:
                                           --------------------------------
                                        Name:  D. Martin Phillips
                                        Title: Managing Director



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                                   SCHEDULE I
                                ENCAP AFFILIATES

<Table>
<Caption>


             ENCAP AFFILIATE                    NUMBER OF SHARES           PURCHASE PRICE
             ---------------                    ----------------           --------------
<S>                                             <C>                        <C>
   EnCap Energy Capital Fund III, L.P.              199,918                  $ 1,999,180

  EnCap Energy Capital Fund III-B, L.P.             151,169                  $ 1,511,690

       BOCP Energy Partners, L.P.                    48,913                  $   489,130

                                     TOTAL:         400,000                  $ 4,000,000
</Table>