Document:

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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,
             --------------------------------
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
             ----------
Partnership Interest.

         3.  Further Assurances. Argosy Energy shall execute and deliver to
             ------------------
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
             -----------
shall have the meaning ascribed to them in the Purchase Agreement.

         5.  Inurement. The conveyance, assignment and transfer herein shall 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.
<PAGE>

         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
             ------------
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
             -------------
accordance with the laws of the State of Texas, without regard to principles of
conflicts of law thereof.

                           [SIGNATURES ON NEXT PAGE]

                                       2
<PAGE>

         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

                                       3<PAGE>

                                                                    EXHIBIT 10.6

                      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 Garnet Resources Corp., a
Delaware corporation ("Garnet").

                                 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, 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"), Garnet 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, Garnet and Lender hereby agree as
follows:

     1.   Transfer of Partnership Interest. Garnet hereby conveys, transfers,
          --------------------------------
assigns, grants, sells and delivers, to Lender, and Lender acquires, accepts and
purchases, all of Garnet's right, title and interest in the Partnership
Interests, which the parties acknowledge is a 5.0738% limited partnership
interest (the "Garnet Partnership Interest").

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

     3.   Further Assurances. Garnet shall execute and deliver to Lender such
          ------------------
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 Garnet Partnership Interest.

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

     5.   Inurement. The conveyance, assignment and transfer herein shall 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.
<PAGE>

     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
          ------------
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
          -------------
accordance with the laws of the State of Texas, without regard to principles of
conflicts of law thereof.

                           [SIGNATURES ON NEXT PAGE]

                                       2
<PAGE>

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

                                    GARNET RESOURCES CORP.

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

                                    CROSBY ACQUISITION, LLC

                                    By:  CROSBY CAPITAL, LLC, as sole member

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

                                       3<PAGE>

                                                                    EXHIBIT 10.7

                      ASSIGNMENT AND ASSUMPTION AGREEMENT

     This Assignment and Assumption Agreement (this "Agreement") is made
effective as of 7:30 a.m., June 1, 2000, by and among Crosby Capital, LLC, a
Texas limited liability company ("Lender") and Aviva Overseas, Inc., a Delaware
corporation ("Aviva Overseas").

                                 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, Neo Energy, Inc., a
Texas corporation ("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"), Lender has agreed to sell, grant, convey, transfer, assign and
deliver to Aviva Overseas all its right, title and interest in a 22.1196%
limited partnership interest in Argosy International (the "LP Interest").

                               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, Aviva Overseas and Lender hereby
agree as follows:

     1.   Transfer of Partnership Interest. Lender hereby conveys, transfers,
          --------------------------------
assigns, grants, sells and delivers, to Aviva Overseas, and Aviva Overseas
acquires, accepts and purchases, all of Lender's right, title and interest in
the LP Interest.

     2.   Assumption. Aviva Overseas hereby assumes and accepts the LP Interest.
          ----------

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

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

     5.   Inurement. The conveyance, assignment and transfer herein shall 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.

     6.   Conveyance Subject to the Purchase Agreement. This conveyance is made
          --------------------------------------------
pursuant to the Purchase Agreement and is subject to the terms thereof.
<PAGE>

     7.   Counterparts. This Agreement may be separately executed in
          ------------
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
          -------------
accordance with the laws of the State of Texas, without regard to principles of
conflicts of law thereof.

                           [SIGNATURES ON NEXT PAGE]
<PAGE>

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

                                                     AVIVA OVERSEAS, INC.

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

                                                     CROSBY CAPITAL, LLC

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

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