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                                                                    EXHIBIT 10.4

                      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 Capital LLC, a
Texas limited liability company ("Lender"), and Neo Energy, Inc., a Texas
corporation ("Neo").

                                 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, Garnet Resources Corp., a Delaware
corporation ("Garnet"), Argosy Energy , Inc., a Delaware corporation ("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"), Neo 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, Neo and Lender hereby agree as
follows:

          1.   Transfer of Partnership Interest. Neo hereby conveys, transfers,
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assigns, grants, sells and delivers, to Lender, and Lender acquires, accepts and
purchases, all of Neo's right, title and interest in the Partnership Interests,
which the parties acknowledge is a 49.9054% limited partnership interest (the
"Neo Partnership Interest").

          2.   Assumption. Lender hereby assumes and accepts the Neo Partnership
               ----------
Interest.

          3.   Further Assurances. Neo shall execute and deliver to Lender such
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further documents and instruments, and take such other action, that may be
reasonably requested by Lender to evidence this conveyance, transfer and
assignment of the Neo 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
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be 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
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in 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.


                                       NEO ENERGY, INC.



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

                                       CROSBY CAPITAL, LLC


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

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