<PAGE>

                                                                    Exhibit 10.1

                                 First Amendment

                                       TO

                                Credit Agreement

                                      Among

                         Mission Resources Corporation,
                                  as Borrower,

                            The Chase Manhattan Bank,
                            as Administrative Agent,

                                  BNP Paribas,
                              as Syndication Agent,

                            First Union National Bank
                                       AND
                              Fleet National Bank,
                           as Co-Documentation Agents,

                                       and

                          The Lenders Signatory Hereto

                          Effective as of May 29, 2001

<PAGE>

                       First Amendment to Credit Agreement

     This First Amendment to Credit Agreement (this "First Amendment") executed
effective as of the 29th of May, 2001 (the "Effective Date") is among Mission
Resources Corporation, a corporation formed under the laws of the State of
Delaware (the "Borrower"); each of the undersigned guarantors (the "Guarantors",
and together with the Borrower, the "Obligors"); each of the lenders that is a
signatory hereto (collectively, the "Lenders"); The Chase Manhattan Bank, as
administrative agent for the Lenders (in such capacity, together with its
successors, the "Administrative Agent"), BNP Paribas, as syndication agent for
the Lenders (in such capacity, together with its successors, the "Syndication
Agent"); and First Union National Bank and Fleet National Bank, as
co-documentation agents for the Lenders (in such capacity, together with its
successors, the "Co-Documentation Agents").

                                    Recitals

     A. The Borrower, the Agents and the Lenders are parties to that certain
Credit Agreement dated as of May 16, 2001 (the "Credit Agreement"), pursuant
to which the Lenders have made certain credit available to and on behalf of the
Borrower.

     B. The Borrower has requested and the Agents and the Lenders have agreed to
amend certain provisions of the Credit Agreement.

     C. NOW, THEREFORE, in consideration of the premises and the mutual
covenants herein contained, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto agree as
follows:

     Section 1. Defined Terms. Each capitalized term which is defined in the
Credit Agreement, but which is not defined in this First Amendment, shall have
the meaning ascribed such term in the Credit Agreement. Unless otherwise
indicated, all section references in this First Amendment refer to the Credit
Agreement.

     Section 2. Amendments to Credit Agreement.

     2.1  Amendments to Section 1.01.

     (a)  The definition of "Agreement" is hereby amended to read as follows;

          "Agreement" shall mean this Credit Agreement, as amended by the First
     Amendment, and as further amended from time to time.

     (b)  The following definitions of "First Amendment" and "First Amendment
Effective Date" are hereby added where alphabetically appropriate:

          "First Amendment" shall mean that certain First Amendment to Credit
     Agreement dated as of May 29, 2001 among the Borrower, the Guarantors, the
     Agents and the Lenders.

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          "First Amendment Effective Date" shall mean the "Effective Date" as
     such term is defined in the First Amendment.

     (c)  The definition of "Senior Subordinated Notes" is hereby amended to
read as follows:

          "Senior Subordinated Notes" shall mean the Borrower's 10-7/8% senior
     subordinated notes due April 2007, provided that the aggregate principal
     amount of all such notes shall not exceed $225,000,000.

     2.2  Section 2.08. Section 2.08(a) is hereby amended by deleting such
section in its entirety and inserting in lieu thereof the following:

     (a)  Amount. For the period from and including the Closing Date to but not
     including the first Redetermination Date, the amount of the Borrowing Base
     shall be the applicable amount based upon the amount of Senior Subordinated
     Notes outstanding as set forth below:

<TABLE>
<CAPTION>
     ------------------------------------------------------------------------------------------
                          Tier 1         Tier 2         Tier 3         Tier 4         Tier 5
     ------------------------------------------------------------------------------------------
     <S>               <C>            <C>            <C>            <C>            <C>
     to 7/31/01        $200,000,000   $185,000,000   $177,500,000   $170,000,000   $162,500,000
     ------------------------------------------------------------------------------------------
     8/01/01 until     $185,000,000   $170,000,000   $162,500,000   $155,000,000   $147,500,000
     Redetermination
     ------------------------------------------------------------------------------------------
</TABLE>

          Notwithstanding the foregoing, the Borrowing Base will be subject to
     further adjustments pursuant to either Section 8.08(c) or Section 9.13. The
     Borrowing Base may not exceed the Aggregate Maximum Credit Amounts. So long
     as any of the Commitments are in effect or any LC Exposure or Loans are
     outstanding hereunder, the amount of the Aggregate Commitments shall be
     governed by the then effective Borrowing Base. For purposes of this Section
     2.08(a), (i) "Tier 1" shall be in effect when the aggregate principal
     amount of the Senior Subordinated Notes is less than or equal to $100
     million; (ii) "Tier 2" shall be in effect when the aggregate principal
     amount of the Senior Subordinated Notes is greater than $100 million and
     less than or equal to $150 million; (iii) "Tier 3" shall be in effect when
     the aggregate principal amount of the Senior Subordinated Notes is greater
     than $150 million and less than or equal to $175 million; (iv) "Tier 4"
     shall be in effect when the aggregate principal amount of the Senior
     Subordinated Notes is greater than $175 million and less than or equal to
     $200 million; and (v) "Tier 5" shall be in effect when the aggregate
     principal amount of the Senior Subordinated Notes is greater than $200
     million. For purposes of this Section 2.08(a), the "aggregate principal
     amount" shall mean the stated face amount of the Senior Subordinated Notes
     without giving effect to any original issue discount.

     2.3 Section 9.02. Section 9.02(h) is hereby amended by deleting such
section in its entirety and inserting in lieu thereof the following:

<PAGE>

          (h) Debt (i) under the Senior Subordinated Notes and any guarantees
     thereof and (ii) which constitutes Permitted Refinancing Debt and any
     guarantees thereof, and

     Section 3. Conditions Precedent. The effectiveness of this First Amendment
is subject to the receipt by the Administrative Agent of the following documents
and satisfaction of the other conditions provided in this Section 3, each of
which shall be reasonably satisfactory to the Administrative Agent in form and
substance:

     3.1 Loan Documents. The Agent shall have received multiple counterparts as
requested of the this First Amendment from each Lender.

     3.2 No Default. No Default or Event of Default shall have occurred and be
continuing as of the Effective Date.

     Section 4. Representation and Warranties; Etc. Each Obligor hereby affirms:
(a) that as of the date of execution and delivery of this First Amendment, all
of the representations and warranties contained in each Loan Document to which
such Obligor is a parry are true and correct in all material respects as though
made on and as of the Effective Date (unless made as of a specific earlier date,
in which case, was true as of such date); and (b) that after giving effect to
this First Amendment and to the transactions contemplated hereby, no Defaults
exist under the Loan Documents or will exist under the Loan Documents.

     Section 5. Miscellaneous.

     5.1 Confirmation. The provisions of the Credit Agreement (as amended by
this First Amendment) shall remain in full force and effect in accordance with
its terms following the effectiveness of this First Amendment.

     5.2 Ratification and Affirmation of Obligors. Each of the Obligors hereby
expressly (i) acknowledges the terms of this First Amendment, (ii) ratifies and
affirms its obligations under the Guaranty Agreement and the other Security
Instruments to which it is a party, (iii) acknowledges, renews and extends its
continued liability under the Guaranty Agreement and the other Security
Instruments to which it is a party and agrees that its guarantee under the
Guaranty Agreement and the other Security Instruments to which it is a party
remains in full force and effect with respect to the Indebtedness as amended
hereby.

     5.3 Counterparts. This First Amendment may be executed by one or more of
the parties hereto in any number of separate counterparts, and all of such
counterparts taken together shall be deemed to constitute one and the same
instrument.

     5.4 No Oral Agreement. This Written First Amendment, the Credit Agreement
and the Other Loan Documents executed in connection herewith and therewith
represent the final, agreement between the parties and may not be contradicted
by evidence of prior, contemporaneous, or unwritten oral agreements of the
parties. There are no subsequent oral agreements between the parties.

<PAGE>

     5.5 GOVERNING LAW. This First Amendment (including, but not limited to,
the validity and enforceability hereof) shall be governed by, and construed in
accordance with, the laws of the State of Texas.

                          [SIGNATURES BEGIN NEXT PAGE]

<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this First Amendment to
be duly executed effective as of the date first written above.

BORROWER:                           MISSION RESOURCES CORPORATION


                                    By:
                                       -----------------------------------------
                                       Ann Kaesermann
                                       Vice-President - Chief Accounting Officer


GUARANTORS:                         BARGO PETROLEUM CORPORATION


                                    By:
                                       -----------------------------------------
                                       Ann Kaesermann
                                       Vice-President - Chief Accounting Officer


                                    BELLWETHER INTERNATIONAL, INC.


                                    By:
                                       -----------------------------------------
                                       Ann Kaesermann
                                       Vice-President - Chief Accounting Officer


                                    PAN AMERICAN ENERGY FINANCE CORP.


                                    By:
                                       -----------------------------------------
                                       Ann Kaesermann
                                       Vice-President - Chief Accounting Officer


                                    PETROBELL, INC.


                                    By:
                                       -----------------------------------------
                                       Ann Kaesermann
                                       Vice-President - Chief Accounting Officer

<PAGE>

ADMINISTRATIVE AGENT:               THE CHASE MANHATTAN BANK,
                                    as Administrative Agent


                                    By:
                                       -----------------------------------------
                                       Robert Traband
                                       Vice-President


SYNDICATION AGENT:                  BNP PARIBAS,
                                    as Syndication Agent


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:


DOCUMENTATION AGENT:                FIRST UNION NATIONAL BANK,
                                    as Documentation Agent


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:


 DOCUMENTATION AGENT:               FLEET NATIONAL BANK,
                                    as Documentation Agent


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:

<PAGE>

LENDERS:                            THE CHASE MANHATTAN BANK


                                    By.
                                       -----------------------------------------
                                       Robert Traband
                                       Vice-President


                                    BNP PARIBAS


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:


                                    FIRST UNION NATIONAL BANK


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:


                                    FLEET NATIONAL BANK


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Titlte:


                                    U.S. BANK NATIONAL ASSOCIATION


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:

<PAGE>

                                    UNION BANK OF CALIFORNIA, N.A.


                                    BY:
                                       -----------------------------------------
                                    Name:
                                    Title:


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:


                                    WELLS FARGO BANK TEXAS, N.A.


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:


                                    CIBC INC.


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:


                                    HIBERNIA NATIONAL BANK


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:


                                    SOUTHWEST BANK OF TEXAS, N.A.


                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title: