Document:

<PAGE>

                                                                    EXHIBIT 10.3

                                   GUARANTY
                                   --------
                               (Aviva Overseas, Inc.)

     THIS GUARANTY is made as of May 31, 2000, by Aviva Overseas, Inc., a
Delaware corporation ("Guarantor"), in favor of Crosby Capital, LLC (the
"Lender").

                                   RECITALS:

     1.   Aviva Petroleum Inc., a Texas corporation ("Parent"), Aviva America,
Inc., a Delaware corporation, ("Aviva America"), Guarantor, Aviva Operating,
Inc., a Nevada corporation ("Aviva Operating"), Argosy Energy Incorporated
("Argosy Energy"), Neo Energy, Inc., a Texas corporation ("Neo" or "Borrower"),
Garnet Resources Corporation, a Delaware corporation ("Garnet") and Lender are
parties to a Loan, Settlement and Acquisition Agreement (the "Loan Agreement")
of even date herewith, pursuant to which Lender has agreed to renew and extend
credit to Neo.

     2.   Neo has executed in favor of Lender that certain promissory note of
even date herewith, payable to the order of Lender in the principal amount of
$2,750,000 (such promissory note, as from time to time amended, and all
promissory notes given in substitution, renewal or extension therefor or
thereof, in whole or in part, being herein collectively called the "Amended and
Restated Neo Note").

     3.   Aviva Operating, Garnet, Argosy Energy, Garnet PNG Corporation, a
Delaware corporation ("Garnet PNG"), Aviva America, Aviva Delaware Inc., a
Delaware corporation, Argosy Energy International, a Utah limited partnership
("Argosy International"), Neo, Parent, ING (U.S.) Capital Corporation, a
Delaware corporation ("ING"), Chase Bank of Texas, National Association
("Chase"), the Overseas Private Investment Corporation, an agency of the United
States of America ("OPIC") and ING, as Lender for the creditors ("Collateral
Agent") are parties to a Joint Finance and Intercreditor Agreement (herein, as
from time to time amended, supplemented or restated, called the "Intercreditor
Agreement"), pursuant to which ING has agreed to act as Collateral Agent for the
benefit of creditors.

     4.   Pursuant to the Loan Agreement, Guarantor has agreed to execute and
deliver to Lender a satisfactory guaranty of all of the indebtedness of Neo
under the Amended and Restated Neo Note and the Loan Documents (as defined in
the Intercreditor Agreement).

     5.   Parent owns directly, or indirectly through one or more subsidiaries,
one-hundred percent (100%) of the outstanding shares of common stock of
Guarantor and one-hundred percent (100%) of the outstanding shares of common
stock of Neo.

     6.   Parent, Neo, Guarantor, and the other direct and indirect subsidiaries
of Parent are mutually dependent on each other in the conduct of their
respective businesses under a holding company structure, with the credit needed
from time to time by each often being provided by another or by means of
financing obtained by one such affiliate with the support of the others for
their mutual benefit and the ability of each to obtain such financing being
dependent on the successful operations of the others.
<PAGE>

     7.   The board of directors of Guarantor has determined that Guarantor's
execution, delivery and performance of this Guaranty may reasonably be expected
to benefit Guarantor, directly or indirectly, is in the best interests of
Guarantor and is necessary or convenient to the conduct, promotion or attainment
of the business of Guarantor.

     NOW, THEREFORE, in consideration of the premises, of the benefits which
will inure to Guarantor from Lender's advances of funds to Neo, and other good
and valuable consideration, the receipt and sufficiency of all of which are
hereby acknowledged, and in order to induce Lender to enter the Loan Agreement,
Guarantor hereby agrees with Lender as follows:

                                  AGREEMENTS

     Section 1. Definitions. Reference is hereby made to the Loan Agreement and
                -----------
the Intercreditor Agreement for all purposes. All terms used in this Guaranty
which are defined in the Loan Agreement and the Intercreditor Agreement and not
otherwise defined herein shall have the same meanings when used herein. All
references herein to any Loan Document or other document or instrument refer to
the same as from time to time amended, supplemented or restated. As used herein
the following terms shall have the following meanings:

     "Borrower" means Neo.
      --------

     "Lender" means Crosby Capital, LLC, its successors and any respective
      ------
assignees under any of the Loan Documents.

     "Note" means the Amended and Restated Neo Note.
      ----

     "Obligations" means collectively all of the indebtedness, obligations, and
      -----------
undertakings which are guaranteed by Guarantor and described in subsections (a),
(b) and (c) of Section 2, subject to Section 2(g).

     "Obligors" means Borrower, Guarantor and any other endorsers, guarantors or
      --------
obligors, primary or secondary, of any or all of the Obligations.

     "Security" means any rights, properties, or interests of Lender, under the
      --------
Security Documents or otherwise, which provide recourse or other benefits to
Lender in connection with the Obligations or the non-payment or non-performance
thereof, including Collateral (whether real or personal, tangible or intangible)
in which Lender have rights under or pursuant to any Loan Documents, guarantees
of the payment or performance of any Obligation, bonds, surety agreements, keep-
well agreements, letters of credit, rights of subrogation, rights of offset, and
rights pursuant to which other claims are subordinated to the Obligations.

     Section 2. Guaranty.
                --------

     (a)  Guarantor hereby irrevocably, absolutely, and unconditionally
guarantees Lender the prompt, complete, and full payment when due, and no matter
how the same shall become due, of:

                                       2
<PAGE>

          (i)  the Amended and Restated Neo Note, including all principal, all
     interest thereon and all other sums payable thereunder; and

          (ii) All other sums payable under the other Loan Documents, whether
     for principal, interest, reimbursements for drawings under Letters of
     Credit, fees (whether commitment fees, letter of credit fees, or
     otherwise), or otherwise.

Without limiting the generality of the foregoing, Guarantor's liability
hereunder shall extend to and include all post-petition interest, expenses, and
other duties and liabilities described above in this subsection (a), or below in
the following subsection (b), which would be owed but for the fact that they are
unenforceable or not allowable due to the existence of a bankruptcy,
reorganization, or similar proceeding.

     (b)  Guarantor hereby irrevocably, absolutely, and unconditionally
guarantees to Lender the prompt, complete and full performance, when due, and no
matter how the same shall become due, of all obligations and undertakings of any
Aviva Party under, by reason of, or pursuant to any of the Loan Documents.

     (c)  Guarantor hereby irrevocably, absolutely and unconditionally
guarantees to Lender the prompt, complete and full performance, when due, and no
matter how the same shall become due, of all obligations and undertakings of any
Aviva Party under, by reason of, or pursuant to the Loan Agreement.

     (d)  If Borrower or any Obligors shall for any reason fail to pay any
Obligation, as and when such Obligation shall become due and payable, whether at
its stated maturity, as a result of the exercise of any power to accelerate, or
otherwise, Guarantor will, forthwith upon demand by Lender, pay such Obligation
in full to the Lender for the benefit of the Lender to whom such Obligation is
owed. If Borrower or Obligors shall for any reason fail to perform promptly any
Obligation, Guarantor will, forthwith upon demand by Lender, cause such
Obligation to be performed or, if specified by Lender, provide sufficient funds,
in such amount and manner as Lender shall in good faith determine, for the
prompt, full and faithful performance of such Obligation by Lender or such other
Person as Lender shall designate.

     (e)  If any of Borrower, Obligors or Guarantor fail to pay or perform
any Obligation as described in the immediately preceding subsections (a), (b),
or (c) Guarantor will incur the additional obligation to pay to Lender, and
Guarantor will forthwith upon demand by Lender pay to Lender, the amount of any
and all expenses, including reasonable fees and disbursements of Lender's
counsel and of any experts or agents retained by Lender, which Lender may incur
as a result of such failure.

     (f)  As between Guarantor and Lender, Guarantor shall be primarily liable
hereunder for the payment and performance of the Obligations.

     (g)  Upon transfer and assignment of the Amended and Restated Neo Note to
Parent in accordance with the Loan Agreement, the obligations set forth in
Sections 2(a) and (b) hereof shall no longer constitute Obligations guaranteed
hereunder.

     Section 3. Unconditional Guaranty.
                ----------------------

                                       3
<PAGE>

     (a)  To the fullest extent permitted by applicable law, no action which
Lender may take or omit to take in connection with any of the Loan Documents,
any of the Obligations (or any other indebtedness owing by Borrower to Lender),
or any Security, and no course of dealing of Lender with any Obligor or any
other Person, shall release or diminish Guarantor's obligations, liabilities,
agreements or duties hereunder, affect this Guaranty in any way, or afford
Guarantor any recourse against Lender, regardless of whether any such action or
inaction may increase any risks to or liabilities of Lender or any Obligor or
increase any risk to or diminish any safeguard of any Security. Without limiting
the foregoing, Guarantor hereby expressly agrees that Lender may, from time to
time, without notice to or the consent of Guarantor, do any or all of the
following, to the fullest extent permitted by applicable law:

          (i)   Amend, change or modify, in whole or in part, any one or more of
     the Loan Documents and give or refuse to give any waivers or other
     indulgences with respect thereto.

          (ii)  Neglect, delay, fail, or refuse to take or prosecute any action
     for the collection or enforcement of any of the Obligations, to foreclose
     or take or prosecute any action in connection with any Security or Loan
     Document, to bring suit against any Obligor or any other Person, or to take
     any other action concerning the Obligations or the Loan Documents.

          (iii) Accelerate, change, rearrange, extend, or renew the time, rate,
     terms, or manner for payment or performance of any one or more of the
     Obligations (whether for principal, interest, fees, expenses,
     indemnifications, affirmative or negative covenants, or otherwise).

          (iv)  Compromise or settle any unpaid or unperformed Obligation or any
     other obligation or amount due or owing, or claimed to be due or owing,
     under any one or more of the Loan Documents.

          (v)   Take, exchange, amend, eliminate, surrender, release, or
     subordinate any or all Security for any or all of the Obligations, accept
     additional or substituted Security therefor, and perfect or fail to perfect
     Lender's rights in any or all Security.

          (vi)  Discharge, release, substitute or add Obligors.

          (vii) Apply all monies received from Obligors or others, or from any
     Security for any of the Obligations, to payment of the Obligations as
     Lender may determine to be in its best interest, without in any way being
     required to marshall Security or assets or to apply all or any part of such
     monies upon any particular Obligations.

     (b)  No action or inaction of any Obligor or any other Person, and no
change of law or circumstances, shall release or diminish Guarantor's
obligations, liabilities, agreements, or duties hereunder, affect this Guaranty
in any way, or afford Guarantor any recourse against Lender. Without limiting
the foregoing, the obligations, liabilities, agreements, and duties of Guarantor
under this Guaranty shall not be released, diminished, impaired, reduced, or
affected by the occurrence of any or all of the following from time to time,
even if occurring without notice to or without the consent of Guarantor:

                                       4
<PAGE>

          (i)   Any voluntary or involuntary liquidation, dissolution, sale of
     all or substantially all assets, marshalling of assets or liabilities,
     receivership, conservatorship, assignment for the benefit of creditors,
     insolvency, bankruptcy, reorganization, arrangement, or composition of any
     Obligor or any other proceedings involving any Obligor or any of the assets
     of any Obligor under laws for the protection of debtors, or stay of actions
     or lien enforcement proceedings against, any Obligor, any properties of any
     Obligor, or the estate in bankruptcy of any Obligor in the course of or
     resulting from any such proceedings.

          (ii)  The failure by Lender to file or enforce a claim in any
     proceeding described in the immediately preceding subsection (i) or to take
     any other action in any proceeding to which any Obligor is a party.

          (iii) The release by operation of law of any Obligor from any of the
     Obligations or any other obligations to Lender.

          (iv)  The invalidity, deficiency, illegality, or unenforceability of
     any of the Obligations or the Loan Documents, in whole or in part, any bar
     by any statute of limitations or other law of recovery on any of the
     Obligations, or any defense or excuse for failure to perform on account of
     force majeure, act of God, casualty, impossibility, impracticability, or
     other defense or excuse whatsoever.

          (v)   The failure of any Obligor or any other Person to sign any
     guaranty or other instrument or agreement within the contemplation of any
     Obligor or Lender.

          (vi)  The fact that Guarantor may have incurred directly part of the
     Obligations or is otherwise primarily liable therefor.

          (vii) Without limiting any of the foregoing, any fact or event
     (whether or not similar to any of the foregoing) which in the absence of
     this provision would or might constitute or afford a legal or equitable
     discharge or release of or defense to a guarantor or surety other than the
     actual payment and performance by Guarantor under this Guaranty.

     (c)  Lender may invoke the benefits of this Guaranty before pursuing any
remedies against any Obligor or any other Person and before proceeding against
any Security now or hereafter existing for the payment or performance of any of
the Obligations. Lender may maintain an action against Guarantor on this
Guaranty without joining any other Obligor therein and without bringing separate
action against any other Obligor.

     (d)  If any payment to Lender by any Obligor is held to constitute a
preference or avoidable transfer under applicable state or federal laws, or if
for any reason Lender is required to refund such payment to the payor thereof or
to pay the amount thereof to any other Person, such payment to Lender shall not
constitute a release of Guarantor from any liability hereunder, and Guarantor
agrees to pay such amount to Lender on demand and agrees and acknowledges that
this Guaranty shall continue to be effective or shall be reinstated, as the case
may be, to the extent of any such payment or payments. Any transfer by
subrogation which is made as contemplated in Section 6 prior to any such payment
or payments shall (regardless of the terms

                                       5
<PAGE>

of such transfer) be automatically voided upon the making of any such payment or
payments, and all rights so transferred shall thereupon revert to and be vested
in the Lender.

     (e)  This is a continuing guaranty and shall apply to and cover all
Obligations and renewals and extensions thereof and substitutions therefor from
time to time.

     Section 4. Waiver. Guarantor hereby waives, with respect to the
                ------
Obligations, this Guaranty, and the other Loan Documents:

     (a)  notice of the incurrence of any Obligation by Borrower, and notice
of any kind concerning the assets, liabilities, financial condition,
creditworthiness, businesses, prospects, or other affairs of Borrower (it being
understood and agreed that (i) Guarantor shall take full responsibility for
informing itself of such matters, (ii) Lender shall not have responsibility of
any kind to inform Guarantor of such matters, and (iii) Lender is hereby
authorized to assume that Guarantor, by virtue of its relationships with
Borrower which are independent of this Guaranty, has full and complete knowledge
of such matters at each time when Lender extends credit to Borrower or takes any
other action which may change or increase Guarantor's liabilities or losses
hereunder).

     (b)  notice that Lender, any Obligor, or any other Person has taken or
omitted to take any action under any Loan Document or any other agreement or
instrument relating thereto or relating to any Obligation.

     (c)  notice of acceptance of this Guaranty and all rights of Guarantor
under  ss.34.02 of the Texas  Business and  Commerce  Code or any law of similar
import in the state of New York.

     (d)  demand, presentment for payment, and notice of demand, dishonor,
nonpayment, or nonperformance.

     (e)  notice of intention to accelerate, notice of acceleration, protest,
notice of protest, notice of any exercise of remedies (as described in the
following Section 5 or otherwise), and all other notices of any kind whatsoever.

     Section 5. Exercise of Remedies. Lender shall have the right to
                --------------------
enforce, from time to time, in any order and at such Lender's sole discretion,
any rights, powers and remedies which Lender may have under the Loan Documents
or otherwise, including judicial foreclosure, the exercise of rights of power of
sale, the taking of a deed or assignment in lieu of foreclosure, the appointment
of a receiver to collect rents, issues and profits, the exercise of remedies
against personal property, or the enforcement of any assignment of leases,
rentals, oil or gas production, or other properties or rights, whether real or
personal, tangible or intangible; and Guarantor shall be liable to Lender
hereunder for any deficiency resulting from the exercise by Lender of any such
right or remedy even though any rights which Guarantor may have against Borrower
or others may be destroyed or diminished by exercise of any such right or
remedy. No failure on the part of Lender to exercise, and no delay in
exercising, any right hereunder or under any other Loan Document shall operate
as a waiver thereof; nor shall any single or partial exercise of any right
preclude any other or further exercise thereof or the exercise of any other
right. The rights, powers and remedies of Lender provided herein and in the
other Loan Documents are cumulative and are in addition to, and not exclusive
of, any other rights, powers or remedies provided by law

                                       6
<PAGE>

or in equity. The rights of Lender hereunder are not conditional or contingent
on any attempt by Lender to exercise any of its rights under any other Loan
Document against any Obligor or any other Person.

     Section 6. No Subrogation. No payment or distribution to Lender pursuant to
                --------------
the provisions of this Guaranty shall entitle Guarantor to exercise any rights
of subrogation in respect thereof.

     Section 7. Successors and Assigns. Guarantor's rights or obligations
                ----------------------
hereunder may not be assigned or delegated, but this Guaranty and such
obligations shall pass to and be fully binding upon the successors of Guarantor,
as well as Guarantor. This Guaranty shall apply to and inure to the benefit of
Lender and their successors or assigns. Without limiting the generality of the
immediately preceding sentence, Lender may assign, grant a participation in, or
otherwise transfer any Obligation held by it or any portion thereof, and Lender
may assign or otherwise transfer its rights or any portion thereof under any
Loan Document, to any other Person, and such other Person shall thereupon become
vested with all of the benefits in respect thereof granted to Lender hereunder
unless otherwise expressly provided by Lender in connection with such assignment
or transfer.

     Section 8. Subordination and Offset. Other than as expressly provided in
                ------------------------
the Subordination Agreement, Guarantor hereby grants to Lender a right of offset
to secure the payment of the Obligations and Guarantor's obligations and
liabilities hereunder, which right of offset shall be upon any and all monies,
securities and other property (and the proceeds therefrom) of Guarantor now or
hereafter held or received by or in transit to Lender from or for the account of
Guarantor, whether for safekeeping, custody, pledge, transmission, collection or
otherwise, and also upon any and all deposits (general or special), credits and
claims of Guarantor at any time existing against Lender. Upon the occurrence of
any Default or Event of Default Lender is hereby authorized at any time and from
time to time, without notice to Guarantor, to offset, appropriate and apply any
and all items herein above referred to against the Obligations and Guarantor's
obligations and liabilities hereunder irrespective of whether or not Lender
shall have made any demand under this Guaranty and although such obligations and
liabilities may be contingent or unmatured. Lender agrees to promptly notify
Guarantor after any such offset and application made by Lender, provided that
the failure to give such notice shall not affect the validity of such offset and
application. The rights of Lender under this section are in addition to, and
shall not be limited by, any other rights and remedies (including other rights
of offset) which lender may have.

     Section 9. Representations and Warranties. Guarantor hereby represents and
                ------------------------------
warrants to Lender as follows:

     (a)  The Recitals at the beginning of this Guaranty are true and correct
in all respects.

     (b)  All representations and warranties made in the Intercreditor Agreement
and the Loan Agreement are true and correct in all respects.

     (c)  Guarantor is a corporation, organized, existing and in good
standing under the laws of its state of organization, having all corporate
powers required to carry on its business and

                                       7
<PAGE>

enter into and carry out the obligations contemplated hereby. Guarantor is duly
qualified, in good standing, and authorized to do business in all other
jurisdictions within the United States wherein the character of the properties
owned or held by it or the nature of the business transacted by it makes such
qualification necessary or required by law.

     (d)  Guarantor has duly taken all corporate action necessary to authorize
the execution and delivery by it of this Agreement and to authorize the
consummation of the transactions contemplated thereby and the performance of its
obligations hereunder.

     (e)  The execution and delivery by Guarantor of this Agreement, the
performance by each of its obligations hereunder, and the consummation of the
transactions contemplated hereby, do not and will not (i) conflict with any
provision of (l) any domestic or foreign law, statute, rule or regulation, (2)
the articles or certificate of incorporation, bylaws, or charter of Guarantor,
(3) any material agreement, judgment, license, order or permit applicable to or
binding upon Guarantor, (ii) result in the acceleration of any Debt owed by
Guarantor, or (iii) result in or require the creation of any Lien upon any
assets or properties of Guarantor except as expressly contemplated herein or in
the Loan Documents. No consent, approval, authorization or order of, and no
notice to or filing with, any court or governmental authority or third party is
required in connection with the execution, delivery or performance by Guarantor
hereunder or to consummate any transactions contemplated hereby.

     (f)  The direct or indirect value of the consideration received and to be
received by Guarantor in connection herewith is reasonably worth at least as
much as the liability and obligations of Guarantor hereunder, and the incurrence
of such liability and obligations in return for such consideration may
reasonably be expected to benefit Guarantor, directly or indirectly.

     (g)  Guarantor is not "insolvent" on the date hereof (that is, the sum of
Guarantor's absolute and contingent liabilities, including the Obligations, does
not exceed the fair market value of Guarantor's assets). Guarantor's capital is
adequate for the businesses in which Guarantor is engaged and intends to be
engaged. Guarantor has not hereby incurred, nor does Guarantor intend to incur
or believe that it will incur, debts which will be beyond its ability to pay as
such debts mature.

     Section 10. No Oral Change. No amendment of any provision of this Guaranty
                 --------------
shall be effective unless it is in writing and signed by Guarantor and Lender,
and no waiver of any provision of this Guaranty, and no consent to any departure
by Guarantor therefrom, shall be effective unless it is in writing and signed by
Lender, and then such waiver or consent shall be effective only in the specific
instance and for the specific purpose for which given.

     Section 11. Invalidity of Particular Provisions. If any term or provision
                 -----------------------------------
of this Guaranty shall be determined to be illegal or unenforceable all other
terms and provisions hereof shall nevertheless remain effective and shall be
enforced to the fullest extent permitted by applicable law.

     Section 12. Headings and References. The headings used herein are for
                 -----------------------
purposes of convenience only and shall not be used in construing the provisions
hereof. The words "this Guaranty," "this instrument," "herein," "hereof,"
"hereby" and words of similar import refer to

                                       8
<PAGE>

this Guaranty as a whole and not to any particular subdivision unless expressly
so limited. The phrases "this section" and "this subsection" and similar phrases
refer only to the subdivisions hereof in which such phrases occur. The word "or"
is not exclusive, and the word "including" (in its various forms) means
"including without limitation". Pronouns in masculine, feminine and neuter
genders shall be construed to include any other gender, and words in the
singular form shall be construed to include the plural and vice versa, unless
the context otherwise requires.

     Section 13. Term. This Guaranty shall be irrevocable until all of the
                 ----
Obligations have been completely and finally paid and performed, and Lender
shall not have any obligation to make any loans or other advances to Borrower,
and all obligations and undertakings of Borrower under, by reason of, or
pursuant to the Loan Documents have been completely performed, and this Guaranty
is thereafter subject to reinstatement as provided in Section 3(d). All
extensions of credit and financial accommodations heretofore or hereafter made
to Borrower shall be conclusively presumed to have been made in acceptance
hereof and in reliance hereon.

     Section 14. Notices. Any notice or communication required or permitted
                 -------
hereunder shall be given as provided in the Loan Agreement.

     Section 15. Limitation on Interest. Lender and Guarantor intend to contract
                 ----------------------
in strict compliance with applicable usury law from time to time in effect, and
the provisions of the Intercreditor Agreement limiting the interest for which
Guarantor is obligated are expressly incorporated herein by reference.

     Section 16. Not a Loan Document. This Guaranty does not constitute a Loan
                 -------------------
Document, as defined in the Intercreditor Agreement.

     Section 17. Counterparts. This Guaranty may be executed in any number of
                 ------------
counterparts, each of which when so executed shall be deemed to constitute one
and the same Guaranty.

     SECTION 18. GOVERNING LAW; SUBMISSION TO PROCESS. THIS GUARANTY SHALL
                 ------------------------------------
BE DEEMED A CONTRACT AND INSTRUMENT MADE UNDER THE LAWS OF THE STATE OF TEXAS
AND SHALL BE CONSTRUED AND ENFORCED IN ACCORDANCE WITH AND GOVERNED BY THE LAWS
OF THE STATE OF TEXAS AND THE LAWS OF THE UNITED STATES OF AMERICA, WITHOUT
REGARD TO PRINCIPLES OF CONFLICTS OF LAW. GUARANTOR HEREBY AGREES THAT ANY LEGAL
ACTION OR PROCEEDING AGAINST IT WITH RESPECT TO THIS GUARANTY MAY BE BROUGHT IN
THE COURTS OF THE STATE OF TEXAS OR (TO THE EXTENT THEY HAVE SUBJECT MATTER
JURISDICTION) OF THE UNITED STATES OF AMERICA FOR THE NORTHERN DISTRICT OF TEXAS
AS LENDER MAY ELECT, AND, BY EXECUTION AND DELIVERY HEREOF, GUARANTOR ACCEPTS
AND CONSENTS FOR ITSELF AND IN RESPECT TO ITS PROPERTY, GENERALLY AND
UNCONDITIONALLY, TO THE JURISDICTION OF THE AFORESAID COURTS AND AGREES THAT
SUCH JURISDICTION SHALL BE EXCLUSIVE, UNLESS WAIVED BY LENDER IN WRITING, WITH
RESPECT TO ANY ACTION OR PROCEEDING BROUGHT BY IT AGAINST LENDER AND ANY
QUESTIONS RELATING TO USURY. GUARANTOR

                                       9
<PAGE>

WAIVES ANY RIGHT TO STAY OR TO DISMISS ANY ACTION OR PROCEEDING BROUGHT BEFORE
SAID COURTS ON THE BASIS OF FORUM NON CONVENIENS. NOTHING HEREIN SHALL AFFECT
THE RIGHT OF LENDER TO SERVE PROCESS IN ANY MANNER PERMITTED BY LAW OR SHALL
LIMIT THE RIGHT OF LENDER TO BRING PROCEEDINGS AGAINST ANY OBLIGOR IN THE COURTS
OF ANY OTHER JURISDICTION.

     IN WITNESS WHEREOF, Guarantor has executed and delivered this Guaranty as
of the date first written above.

                                      AVIVA OVERSEAS, INC.

                                      By: /s/ R. Suttill
                                          --------------
                                          Name: Ron Suttill
                                          Title: President

                                       10<PAGE>

                                                                    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,
               --------------------------------
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
               ------------------
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
               -----------
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.

                                       NEO ENERGY, INC.

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

                                       CROSBY CAPITAL, LLC

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

                                       3

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