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                                                                    EXHIBIT 10.5

                      ASSIGNMENT AND ASSUMPTION AGREEMENT

         This Assignment and Assumption Agreement (this "Agreement") is made
effective as of 7:00 a.m., June 1, 2000, by and among Crosby Acquisition LLC, a
Delaware limited liability company ("Crosby"), and Argosy Energy, Inc., a
Delaware corporation ("Argosy Energy").

                                 R E C I T A L

         WHEREAS, pursuant to that certain Loan, Settlement and Acquisition
Agreement dated as of May 31, 2000 (the "Purchase Agreement"), by and among
Lender, and Aviva Petroleum, Inc., a Texas corporation ("Parent"), Aviva
America, Inc., a Delaware corporation ("Aviva America"), Aviva Operating, Inc.,
a Delaware corporation ("Aviva Operating"), Aviva Overseas, Inc., a Delaware
corporation ("Aviva Overseas"), Neo Energy, Inc., a Texas corporation ("Neo"),
Garnet Resources Corp., a Delaware corporation ("Garnet"), Argosy Energy, and
Argosy Energy International, a Utah limited partnership ("Argosy International")
(Parent, Aviva America, Aviva Operating, Aviva Overseas, Neo, Garnet, Argosy
Energy and Argosy International are referred to collectively herein as the
"Aviva Parties"), Argosy Energy has agreed to sell, grant, convey, transfer,
assign and deliver to Lender all its right, title and interest in the
Partnership Interests in Argosy International.

                               A G R E E M E N T

         NOW, THEREFORE, in consideration of the promises herein and in the
Purchase Agreement, and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Argosy Energy and Crosby hereby
agree as follows:

         1.  Transfer of Partnership Interest. Argosy Energy hereby conveys,
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transfers, assigns, grants, sells and delivers, to Crosby, and Crosby acquires,
accepts and purchases, all of Argosy Energy's right, title and interest in the
Partnership Interests, which the parties acknowledge is a 44.6403% general
partnership interest (the "General Partnership Interest").

         2.  Assumption. Crosby hereby assumes and accepts the General
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Partnership Interest.

         3.  Further Assurances. Argosy Energy shall execute and deliver to
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Crosby such further documents and instruments, and take such other action, that
may be reasonably requested by Crosby to evidence this conveyance, transfer and
assignment of the General Partnership Interest.

         4.  Definitions. All capitalized terms not otherwise defined herein
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shall have the meaning ascribed to them in the Purchase Agreement.

         5.  Inurement. The conveyance, assignment and transfer herein shall be
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effective as of the date hereof, and shall inure to the benefit of and be
binding upon the parties hereto and their successors or permitted assigns.
However, nothing in this Agreement, express or implied, shall give any other
person any benefit or any legal or equitable right or remedy with respect
hereto.
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         6.  Conveyance Subject to the Purchase Agreement. This conveyance is
             --------------------------------------------
made pursuant to the Purchase Agreement and is subject to the terms thereof.

         7.  Counterparts. This Agreement may be separately executed in
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counterparts, each of which when so executed shall be deemed to constitute one
and the same Agreement. Such execution and delivery may be accomplished by
facsimile transmission.

         8.  Governing Law. This Agreement shall be governed by and construed in
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accordance with the laws of the State of Texas, without regard to principles of
conflicts of law thereof.



                           [SIGNATURES ON NEXT PAGE]

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         IN WITNESS WHEREOF, each of the undersigned has caused this Agreement
to be executed as of the day and year first written above.


                                        ARGOSY ENERGY, INC.



                                        By: /s/ R. Suttil
                                            -------------
                                            President

                                        CROSBY ACQUISITION, LLC

                                        By:  CROSBY CAPITAL, LLC, as sole member


                                        By: /s/ Jay A. Chaffee
                                            ------------------
                                            Jay A. Chaffee
                                            President

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