Document:

<PAGE>

                                                                    EXHIBIT 10.1

                      FOURTH AMENDMENT TO CREDIT AGREEMENT

                                  BY AND AMONG

                           ATP OIL & GAS CORPORATION,
                                  as Borrower,

                                  BNP PARIBAS,
                            as Administrative Agent,

                        UNION BANK OF CALIFORNIA, N.A.,
                             as Syndication Agent,

                                      and

                          THE LENDERS SIGNATORY HERETO

                        Effective as of October 1, 2001
<PAGE>

                               TABLE OF CONTENTS
                               -----------------
<TABLE>
<CAPTION>
                                                                            Page
                                                                            ----
<S>                                                                          <C>
ARTICLE I        DEFINITIONS................................................  1
 Section 1.01    Terms Defined Above........................................  1
 Section 1.02    Terms Defined in Credit Agreement..........................  1
 Section 1.03    Other Definitional Provisions..............................  1

ARTICLE II       AMENDMENTS TO CREDIT AGREEMENT.............................  2
 Section 2.01    Amendments and Supplements to Definitions..................  2
 Section 2.02    Amendments to Article II...................................  2
 Section 2.03    Amendments to Article III..................................  3
 Section 2.04    Amendments to Article VIII.................................  3
 Section 2.05    Amendments to Article IX...................................  4

ARTICLE III      CONDITIONS.................................................  4
 Section 3.01    Loan Documents.............................................  4
 Section 3.02    Corporate Proceedings of Loan Parties......................  4
 Section 3.03    Representations and Warranties.............................  4
 Section 3.04    No Default.................................................  4
 Section 3.05    No Change..................................................  5
 Section 3.06    Security Instruments.......................................  5
 Section 3.07    Amendment Fee..............................................  5
 Section 3.08    Legal Fees of Administrative Agent's Counsel...............  5
 Section 3.09    Other Instruments or Documents.............................  5

ARTICLE IV       MISCELLANEOUS..............................................  5
 Section 4.01    Adoption, Ratification and Confirmation of Credit Agreement  5
 Section 4.02    Successors and Assigns.....................................  5
 Section 4.03    Counterparts...............................................  5
 Section 4.04    Number and Gender..........................................  5
 Section 4.05    Entire Agreement...........................................  6
 Section 4.06    Invalidity.................................................  6
 Section 4.07    Titles of Articles, Sections and Subsections...............  6
 Section 4.08    Use of Unscheduled Redetermination.........................  6
 Section 4.09    Governing Law..............................................  6
</TABLE>

                                       i

<PAGE>

                      FOURTH AMENDMENT TO CREDIT AGREEMENT
                      ------------------------------------

     This FOURTH AMENDMENT TO CREDIT AGREEMENT (this "Fourth Amendment")
                                                      ----------------
executed effective as of the 1st day of October, 2001 (the "Effective Date"), is
                                                            --------------
by and among ATP OIL & GAS CORPORATION, a corporation formed under the laws of
the State of Texas (the "Borrower"); each of the lenders that is a signatory
                         --------
hereto or which becomes a signatory hereto and to the hereinafter described
Credit Agreement as provided in Section 12.06 of the Credit Agreement
(individually, together with its successors and assigns, a "Lender" and,
                                                            ------
collectively, the "Lenders"); BNP PARIBAS, a financial institution formed under
                   -------
the laws of France (in its individual capacity, "Paribas"), as agent for the
                                                 -------
Lenders (in such capacity, together with its successors in such capacity, the

"Administrative Agent"); and UNION BANK OF CALIFORNIA, N.A., as syndication
---------------------
agent.

                                R E C I T A L S
                                ---------------

     A. The Borrower, Administrative Agent and Lenders are parties to that
certain Credit Agreement dated as of April 27, 2001 as amended and supplemented
by that certain (i) First Amendment to Credit Agreement dated as of June 29,
2001, (ii) Second Amendment to Credit Agreement dated as of June 29, 2001 and
(iii) Third Amendment to Credit Agreement dated as of June 30, 2001, (the
"Credit Agreement"), pursuant to which the Lenders agreed to make loans to and
 ----------------
extensions of credit on behalf of the Borrower.

     B. The Borrower and the Lenders desire to amend the Credit Agreement in
the particulars hereinafter provided.

     NOW, THEREFORE, in consideration of the premises and the mutual covenants
herein contained, the parties hereto agree as follows:

                                   ARTICLE I

                                  DEFINITIONS

     Section 1.01  Terms Defined Above.  As used in this Fourth Amendment, each
                   -------------------
of the terms defined in the opening paragraph and the Recitals above shall have
the meaning assigned to such terms therein.

     Section 1.02  Terms Defined in Credit Agreement.  Each term defined in the
                   ---------------------------------
Credit Agreement and used herein without definition shall have the meaning
assigned to such term in the Credit Agreement, unless expressly provided to the
contrary.

     Section 1.03  Other Definitional Provisions.
                   -----------------------------

     (a) The words "hereby", "herein", "hereinafter", "hereof", "hereto" and
"hereunder" when used in this Fourth Amendment shall refer to this Fourth
Amendment as a whole and not to any particular Article, Section, subsection or
provision of this Fourth Amendment.

                                       1
<PAGE>

     (b) Section, subsection and Exhibit references herein are to such Sections,
subsections and Exhibits to this Fourth Amendment unless otherwise specified.

                                   ARTICLE II

                         AMENDMENTS TO CREDIT AGREEMENT

     The Borrower, the Administrative Agent, and the Lenders agree that the
Credit Agreement is hereby amended, effective as of the Effective Date, in the
following particulars.

     Section 2.01  Amendments and Supplements to Definitions.
                   -----------------------------------------

     (a) The following term, which is defined in Section 1.02 of the Credit
Agreement, is hereby amended in its entirety to read as follows:

              "Agreement" shall mean this Credit Agreement, as amended and
               ---------
     supplemented by the First Amendment, the Second Amendment, the Third
     Amendment and the Fourth Amendment and as the same may from time to time be
     further amended or supplemented.

     (b) Section 1.02 of the Credit Agreement is hereby further amended and
supplemented by adding the following new definitions  where alphabetically
appropriate, which read in their entirety as follows:

              "Fourth Amendment" shall mean that certain Fourth Amendment to
               ----------------
     Credit Agreement dated as of October 1, 2001, by and among the Borrower,
     the Administrative Agent and the Lenders.

          Section 2.02  Amendments to Article II.
                        ------------------------

     (a) Section 2.01(c) of the Credit Agreement is hereby amended by adding as
the last sentence thereof the following sentence:

              "Notwithstanding anything contained in this Agreement to the
     contrary, from and after the effective date of the Fourth Amendment, no
     LIBOR Loans shall be permitted to remain outstanding or be made until the
     Administrative Agent sends written notice to the Borrower that LIBOR Loans
     shall be permitted."

          (b) Section 2.08(a) of the Credit Agreement is hereby amended in its
entirety to read as follows:

              "(a) Generally. The Borrowing Base and the Monthly Reduction
                   ---------
     Amount shall be determined in accordance with Section 2.08(b) by the
     Administrative Agent with the concurrence of the Lenders and is subject to
     redetermination in accordance with Section 2.08(d). Upon any
     redetermination of the Borrowing Base or the Monthly Reduction Amount, such
     redetermination

                                       2
<PAGE>

          shall remain in effect until the next successive Redetermination Date;
          provided, however, the then effective Borrowing Base shall reduce on
          --------  -------
          the first day of each month by the then effective Monthly Reduction
          Amount. So long as any of the Commitments are in effect or any LC
          Exposure or Loans are outstanding hereunder, this facility shall be
          governed by the then effective Borrowing Base and Monthly Reduction
          Amount. During the period from and after the effective date of the
          Fourth Amendment until the next redetermination thereof pursuant to
          Section 2.08(d) or adjustment pursuant to Section 8.08(c), the amount
          of the Borrowing Base shall be $50,000,000, as reduced on a cumulative
          basis on the first day of each month following such effective date by
          the applicable Monthly Reduction Amount. The Monthly Reduction Amount
          shall be $4,000,000 (or the outstanding principal balance of the
          Loans, if less than $4,000,000) each month until the next
          redetermination thereof pursuant to Section 2.08(d). Schedule 2.08(a)
                                                               ----------------
          attached to the Fourth Amendment sets forth, on a month by month
          basis, the Monthly Reduction Amount for each month and the resulting
          then effective Borrowing Base. Schedule 2.08(a) shall be amended, and
                                         ----------------
          a revised Schedule 2.08(a) distributed by the Administrative Agent to
                    ----------------
          the Borrower and each Lender, upon any redetermination of the
          Borrowing Base or Monthly Reduction Amount that necessitates a revised
          Schedule 2.08(a). No delay for any reason whatsoever in a
          ----------------
          redetermination of the Monthly Reduction Amount shall affect the
          Borrower's obligations under Section 2.07(b)(iii)."

          Section 2.03  Amendments to Article III. Section 3.02(c) of the Credit
                        -------------------------
Agreement is hereby amended in its entirety to read as follows:

                   "(c) Due Dates. Accrued interest on Base Rate Loans shall be
                        ---------
          payable monthly on the first day of each month commencing October 1,
          2001, and accrued interest on each LIBOR Loan shall be payable on the
          last day of the Interest Period therefor and, if such Interest Period
          is longer than three months, at three-month intervals following the
          first day of such Interest Period, except that interest payable at the
          Post-Default Rate shall be payable from time to time on demand and
          interest on any LIBOR Loan that is converted into a Base Rate Loan
          (pursuant to Section 5.04) shall be payable on the date of conversion
          (but only to the extent so converted). Any accrued and unpaid interest
          on the Loans on the Termination Date shall be paid on such date."

          Section 2.04  Amendments to Article VIII.  Article VIII of the Credit
                        --------------------------
Agreement is hereby amended by adding thereto new Section 8.12 to read in its
entirety as follows:

          "Section 8.11 Hedging Agreements. The Borrower shall maintain all
                        ------------------
          Hedging Agreements in effect on the effective date of the Fourth
          Amendment and in addition thereto, the Borrower shall enter into and
          maintain Hedging Agreements or other contractual agreements, in form
          and substance reasonably satisfactory to the Majority Lenders,
          pursuant to which at least 5,000,000 cubic feet per day of gas
          production from the Borrower's Oil and Gas Properties is hedged for
          the

                                       3
<PAGE>

          period of November 1, 2001 through February 29, 2004 for a net
          realized price of not less than $3.00 per mcf."

          Section 2.05 Amendments to Article IX.  Section 9.14 of the Credit
                       ------------------------
Agreement is hereby amended by replacing the first sentence thereof with the
following sentence:

          "The Borrower will not permit its Debt Ratio as of the end of any
          fiscal quarter of the Borrower (calculated quarterly at the end of
          each fiscal quarter) to be greater than 3.0 to 1.0."

                                  ARTICLE III

                                   CONDITIONS

          The enforceability of this Fourth Amendment against the Administrative
Agent and the Lenders is subject to the satisfaction of the following conditions
precedent:

          Section 3.01 Loan Documents. The Administrative Agent shall have
                       --------------
received multiple original counterparts, as requested by the Administrative
Agent, of this Fourth Amendment executed and delivered by a duly authorized
officer of the Borrower, the Administrative Agent, each Lender.

          Section 3.02 Corporate Proceedings of Loan Parties. The Administrative
                       -------------------------------------
Agent shall have received multiple copies, as requested by the Administrative
Agent, of the resolutions, in form and substance reasonably satisfactory to the
Administrative Agent, of the Board of Directors of the Borrower, authorizing the
execution, delivery and performance of this Fourth Amendment and the other Loan
Documents referenced in Section 3.01 hereof, each such copy being attached to an
original certificate of the Secretary or an Assistant Secretary of the Borrower,
dated as of the date of execution of this Fourth Amendment, certifying (i) that
the resolutions attached thereto are true, correct and complete copies of
resolutions duly adopted by written consent or at a meeting of the Board of
Directors, (ii) that such resolutions constitute all resolutions adopted with
respect to the transactions contemplated hereby, (iii) that such resolutions
have not been amended, modified, revoked or rescinded as of the date of
execution of this Fourth Amendment, (iv) that the articles of incorporation and
bylaws of the Borrower have not been amended or otherwise modified since the
effective date of the Credit Agreement, except pursuant to any amendments
attached thereto, and (v) as to the incumbency and signature of the officer of
the Borrower executing this Fourth Amendment.

          Section 3.03 Representations and Warranties. Except as affected by the
                       ------------------------------
transactions contemplated in the Credit Agreement and this Fourth Amendment,
each of the representations and warranties made by the Borrower in or pursuant
to the Loan Documents shall be true and correct in all material respects as of
the Effective Date, as if made on and as of such date.

          Section 3.04 No Default.  No Default or Event of Default shall have
                       ----------
occurred and be continuing as of the Effective Date, except for any such Default
or Event of Default arising from a failure by the Borrower to be in compliance
with financial covenants as of September 30, 2001,

                                       4
<PAGE>

which compliance with such covenants cannot be determined until the books of the
Borrower are closed for the period ending September 30, 2001.

     Section 3.05  No Change.  No event shall have occurred since December 31,
                   ---------
2000, which, in the reasonable opinion of the Lenders, could have a Material
Adverse Effect.

     Section 3.06  Security Instruments.  All of the Security Instruments shall
                   --------------------
be in full force and effect and provide to the Administrative Agent and the
Lenders the security intended thereby to secure the Obligations.

     Section 3.07  Amendment Fee.  The Borrower shall have paid to the
                   -------------
Administrative Agent for the benefit of the Lenders an amendment fee of
$100,000.

     Section 3.08  Legal Fees of Administrative Agent's Counsel.  The Borrower
                   --------------------------------------------
shall have paid all reasonable fees and expenses of the Administrative Agent's
legal counsel and other consultants pursuant to all invoices presented for
payment on or prior to the Effective Date.

     Section 3.09  Other Instruments or Documents.  The Administrative Agent or
                   ------------------------------
any Lender or counsel to the Administrative Agent shall receive such other
instruments or documents as they may reasonably request.

                                   ARTICLE IV

                                 MISCELLANEOUS

     Section 4.01  Adoption, Ratification and Confirmation of Credit Agreement.
                   -----------------------------------------------------------
Each of the Borrower, the Administrative Agent, and the Lenders does hereby
adopt, ratify and confirm the Credit Agreement, as amended hereby, and
acknowledges and agrees that the Credit Agreement, as amended hereby, is and
remains in full force and effect.

     Section 4.02  Successors and Assigns.  This Fourth Amendment shall be
                   ----------------------
binding upon and inure to the benefit of the parties hereto and their respective
successors and assigns permitted pursuant to the Credit Agreement.

     Section 4.03  Counterparts.  This Fourth 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 and shall be enforceable as of the Effective Date upon the execution
of one or more counterparts hereof by the Borrower, the Administrative Agent and
the Lenders.  In this regard, each of the parties hereto acknowledges that a
counterpart of this Fourth Amendment containing a set of counterpart execution
pages reflecting the execution of each party hereto shall be sufficient to
reflect the execution of this Fourth Amendment by each necessary party hereto
and shall constitute one instrument.

     Section 4.04  Number and Gender.  Whenever the context requires, reference
                   -----------------
herein made to the single number shall be understood to include the plural; and
likewise, the plural shall be understood to include the singular.  Words
denoting sex shall be construed to include the masculine, feminine and neuter,
when such construction is appropriate; and specific enumeration

                                       5
<PAGE>

shall not exclude the general but shall be construed as cumulative. Definitions
of terms defined in the singular or plural shall be equally applicable to the
plural or singular, as the case may be, unless otherwise indicated.

     Section 4.05  Entire Agreement.  This Fourth Amendment constitutes the
                   ----------------
entire agreement among the parties hereto with respect to the subject hereof.
All prior understandings, statements and agreements, whether written or oral,
relating to the subject hereof are superseded by this Fourth Amendment.

     Section 4.06  Invalidity.  In the event that any one or more of the
                   ----------
provisions contained in this Fourth Amendment shall for any reason be held
invalid, illegal or unenforceable in any respect, such invalidity, illegality or
unenforceability shall not affect any other provision of this Fourth Amendment.

     Section 4.07  Titles of Articles, Sections and Subsections.  All titles or
                   --------------------------------------------
headings to Articles, Sections, subsections or other divisions of this Fourth
Amendment or the exhibits hereto, if any, are only for the convenience of the
parties and shall not be construed to have any effect or meaning with respect to
the other content of such Articles, Sections, subsections, other divisions or
exhibits, such other content being controlling as the agreement among the
parties hereto.

     Section 4.08  Use of Unscheduled Redetermination.  The Borrowing Base and
                   ----------------------------------
Monthly Reduction Amounts set forth in Section 2.02 of this Fourth Amendment are
based on the unscheduled redetermination of the Borrowing Base and Monthly
Reduction Amount requested by the Borrower and conducted by the Administrative
Agent and the Lenders.  The Administrative Agent hereby waives payment of the
$10,0000 engineering fee that was due in connection with such unscheduled
redetermination conditioned upon the execution of this Fourth Amendment and if
payment of all fees and expenses required to be paid pursuant to the terms and
conditions of this Fourth Amendment are paid concurrent with the execution of
this Fourth Amendment.  The Administrative Agent and the Lenders shall not be
required at Borrower's request to conduct another unscheduled redetermination of
the Borrowing Base or the Monthly Reduction Amount during calendar year 2001.
In lieu of the redetermination of the Borrowing Base and Monthly Reduction
Amount scheduled, pursuant to Section 2.08(d), to occur on or around the first
Business Day of October, 2001, the Lenders shall redetermine the amount of the
Borrowing Base and the Monthly Reduction Amount in accordance with Section
2.08(b) on or around the last Business Day of October, 2001.

     Section 4.09  Governing Law.  This Fourth Amendment shall be deemed to be a
                   -------------
contract made under and shall be governed by and construed in accordance with
the internal laws of the State of Texas.

     This Fourth Amendment, the Credit Agreement, as amended hereby, the Notes,
     and the other Loan Documents represent the final agreement between the
     parties and may not be contradicted by evidence of prior, contemporaneous,
     or subsequent oral agreements of the parties.

                                       6
<PAGE>

There are no unwritten oral agreements between the parties.

                        [SIGNATURES BEGIN ON NEXT PAGE]

                                       7
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Fourth Amendment to
be duly executed and delivered by their proper and duly authorized officers as
of the Effective Date.

BORROWER:                                         ATP OIL & GAS CORPORATION

                                                  By: /s/ T. Paul Bulmahn
                                                     -------------------------
                                                      T. Paul Bulmahn
                                                      President

                                      S-1
<PAGE>

LENDER AND ADMINISTRATIVE           BNP PARIBAS, individually and
AGENT:                              as Administrative Agent

                                    By:    /s/ Brian M. Malone
                                       ------------------------
                                    Name:  Brian M. Malone
                                    Title: Managing Director

                                    By:    /s/ Betsy Jocher
                                       ------------------------
                                    Name:  Betsy Jocher
                                    Title: Vice President

                                      S-2

<PAGE>

LENDER AND SYNDICATION              UNION BANK OF CALIFORNIA, N.A.,
AGENT:                              individually and as Syndication Agent

                                    By:    /s/ Sean Murphy
                                       -----------------------------
                                    Name:  Sean Murphy
                                    Title: Assistant Vice President

                                    By:    /s/ Ali Ahmed
                                       -----------------------------
                                    Name:  Ali Ahmed
                                    Title: Assistant Vice President

                                      S-3

<PAGE>

                                Schedule 2.08(a)
                                ----------------

                  Borrowing Base and Monthly Reduction Amount

<TABLE>
<CAPTION>
                Date                     Monthly Reduction          Borrowing Base
                ----                     -----------------          --------------
                                              Amount
                                              ------
            <S>                          <C>                            <C>
              10-01-01                       None                  $50,000,000
              11-01-01                    $4,000,000               $46,000,000
              12-01-01                    $4,000,000               $42,000,000
              01-01-02                    $4,000,000               $38,000,000
              02-01-02                    $4,000,000               $34,000,000
              03-01-02                    $4,000,000               $30,000,000
              04-01-02                    $4,000,000               $26,000,000
              05-01-02                    $4,000,000               $22,000,000
              06-01-02                    $4,000,000               $18,000,000
              07-01-02                    $4,000,000               $14,000,000
              08-01-02                    $4,000,000               $10,000,000
              09-01-02                    $4,000,000               $ 6,000,000
              10-01-02                    $4,000,000                 2,000,000
              11-01-02                    $2,000,000                   --0--
</TABLE><PAGE>

                                                                    EXHIBIT 10.2

                      FIFTH AMENDMENT TO CREDIT AGREEMENT

     This FIFTH AMENDMENT TO CREDIT AGREEMENT (this "Fifth Amendment") executed
effective as of the 5th day of November 2001 (the "Fifth Amendment Effective
Date"), is by and among ATP OIL & GAS CORPORATION, a corporation formed under
the laws of the State of Texas (the "Borrower"); each of the lenders that is a
signatory hereto or which becomes a signatory hereto and to the hereinafter
described Credit Agreement as provided in Section 12.06 of the Credit Agreement
(individually, together with its successors and assigns, a "Lender" and
collectively, the "Lenders"); and UNION BANK OF CALIFORNIA, N.A., a national
banking association (in its individual capacity, "UBOC") as agent for the
Lenders (in such capacity, together with any successors in such capacity, the
"Administrative Agent").

                                R E C I T A L S
                                ---------------

     A.  The Borrower, Administrative Agent and Lenders are parties to that
certain Credit Agreement dated as of April 27, 2001 as amended and supplemented
by that certain (i) First Amendment to Credit Agreement dated as of June 29,
2001, (ii) Second Amendment to Credit Agreement dated as of June 29, 2001, (iii)
Third Amendment to Credit Agreement dated as of June 30, 2001, and (iv) Fourth
Amendment to Credit Agreement dated as of October 1, 2001 (as so amended, the
"Credit Agreement"), pursuant to which the Lenders agreed to make loans to and
extensions of credit on behalf of the Borrower.

     B.  The Borrower, the Lenders and the Administrative Agent desire to amend
the Credit Agreement further as provided herein.

     NOW THEREFORE, in consideration of the premises and the mutual covenants
herein contained, the parties hereto agree as follows:

                                  ARTICLE I.
                                  DEFINITIONS

     SECTION 1.01 TERMS DEFINED ABOVE. As used in this Fifth Amendment, each of
the terms defined in the opening paragraph and the Recitals above shall have the
meanings assigned to such terms therein.

     SECTION 1.02 TERMS DEFINED IN THE CREDIT AGREEMENT. Each term defined in
the Credit Agreement and used herein without definition shall have the meaning
assigned to such term in the Credit Agreement, unless expressly provided to the
contrary.

     SECTION 1.03   OTHER DEFINITIONAL PROVISIONS.

     (a) The words "hereby", "herein", "hereinafter", "hereof", "hereto" and
"hereunder" when used in this Fifth Amendment shall refer to this Fifth
Amendment as a

                                       1
<PAGE>

whole and not to any particular Article, Section, subsection or provision of
this Fifth Amendment.

     (b) Section, subsection and Exhibit references herein are to such Sections,
subsections and Exhibits to this Fifth Amendment unless otherwise specified.

                                  ARTICLE II.
                         AMENDMENTS TO CREDIT AGREEMENT

     The Borrower, the Administrative Agent and the Lenders agree that the
Credit Agreement is hereby amended, effective as of the Fifth Amendment
Effective Date, in the following particulars.

     SECTION 2.01  AMENDMENTS AND SUPPLEMENTS TO DEFINITIONS.

     (a) The following terms, which are defined in Section 1.02 of the Credit
Agreement, are hereby amended in their entirety to read as follows:

     "Agreement" shall mean this Credit Agreement, as amended and supplemented
     by the First Amendment, the Second Amendment, the Third Amendment, the
     Fourth Amendment, and the Fifth Amendment, and as the same may from time to
     time be further amended or supplemented.

     "Aggregate Maximum Credit Amounts" at any time shall equal the sum of the
     Maximum Credit Amounts of the Lenders, as the same may be reduced  pursuant
     to Section 2.03(b).  As of the Fifth Amendment Effective Date, the
     Aggregate Maximum Credit Amount is $100,000,000.

     "Applicable Margin" shall mean, at any time except as otherwise
     specifically set out in this definition, the applicable per annum
     percentage set forth at the appropriate intersection in the table shown
     below, based on the Borrowing Base Utilization as in effect from time to
     time:

BORROWING BASE UTILIZATION                              APPLICABLE MARGIN
-------------------------------------------------------------------------------
                                                 LIBOR Loans    Base Rate Loans
-------------------------------------------------------------------------------
Less than 25%                                      2.25%            0.25%
-------------------------------------------------------------------------------
Greater than or equal to 25% but less than 50%     2.50%            0.50%
-------------------------------------------------------------------------------
Greater than or equal to 50% but less than 75%     2.875%           0.75%
-------------------------------------------------------------------------------
Greater than or equal to 75%                       3.125%           1.00%
-------------------------------------------------------------------------------

     ; provided, however, at any time that a Borrowing Base Deficiency exists,
     the "Applicable Margin" shall be increased by an additional 2.00% in excess
     of the Applicable Margin that would otherwise apply if no Borrowing Base
     Deficiency existed.

                                       2
<PAGE>

     "Debt Ratio" shall mean, as of the end of each calendar month, the ratio of
     (i) Debt of the Borrower and its Consolidated Subsidiaries as of the end of
     such calendar month, less cash on hand exceeding $10,000,000 to (ii) EBITDA
     (less interest income) for the immediately preceding twelve calendar months
     then ended.

     "Fee Letter" shall mean that certain letter agreement between the
     Administrative Agent and the Borrower, dated as of the Fifth Amendment
     Effective Date, concerning certain fees in connection with this Agreement
     and the Fifth Amendment and any agreements or instruments executed in
     connection therewith, as the same may be amended or replaced from time to
     time.

     "Interest Coverage Ratio" shall mean, as of the end of any calendar month,
     the ratio of (i) EBITDA for the immediately preceding twelve calendar month
     period then ended to (ii) cash interest payments made by the Borrower and
     its Consolidated Subsidiaries for such twelve calendar month period.

     "LC Commitment" shall mean, at any time, an amount equal to $10,000,000.

     "Termination Date" shall mean the earlier to occur of (i) November 5, 2003
     and (ii) the date that the Commitments are sooner terminated pursuant to
     Sections 2.03(b) or 10.02.

     (b) Section 1.02 of the Credit Agreement is hereby further amended and
supplemented by adding the following new definitions where alphabetically
appropriate, which read in their entirety as follows:

     "Fifth Amendment" shall mean that certain Fifth Amendment to Credit
     Agreement dated as of November 5, 2001, by and among the Borrower, the
     Administrative Agent and the Lenders.

     "Fifth Amendment Effective Date" shall mean November 5, 2001.

     "Proven Reserves" shall mean, at any particular time, the estimated
     quantities of Hydrocarbons which geological and engineering data
     demonstrate with reasonable certainty to be recoverable in future years
     from known reservoirs attributable to Oil and Gas Properties included or to
     be included in the Borrowing Base under then existing economic and
     operating conditions (i.e., prices and costs as of the date the estimate is
     made).  The prices used may include consideration of changes in existing
     prices provided only by contractual arrangements, but not on escalations
     based upon future conditions.

     (c) Section 1.02 of the Credit Agreement is hereby further amended by
deleting the definition of "Senior Unsecured Debt" in its entirety.

                                       3
<PAGE>

     SECTION 2.02  AMENDMENTS TO ARTICLE II.

     (a) Section 2.01(c) of the Credit Agreement is hereby amended by deleting
the last sentence thereof in its entirety which was added pursuant to the terms
of the Fourth Amendment.

     (b) Section 2.03(c) of the Credit Agreement is hereby amended in its
entirety to read as follows:

         "(c)   Intentionally omitted."

     (c) Clause (iv) of Section 2.07(b) of the Credit Agreement is hereby
deleted in its entirety.

     (d) Section 2.08(a) of the Credit Agreement is hereby amended in its
entirety to read as follows:

         "(a)  Generally.  The Borrowing Base and the Monthly Reduction Amount
     shall be determined in accordance with Section 2.08(b) by the
     Administrative Agent with the concurrence of the Majority Lenders (except
     that any increase in the Borrowing Base shall be approved by all the
     Lenders) and is subject to redetermination in accordance with Section
     2.08(d).  Upon any redetermination of the Borrowing Base or the Monthly
     Reduction Amount, such redetermination shall remain in effect until the
     next successive Redetermination Date; provided, however, the then effective
     Borrowing Base shall reduce on the first day of each month by the then
     effective Monthly Reduction Amount.  So long as any of the Commitments are
     in effect or any LC Exposure or Loans are outstanding hereunder, this
     facility shall be governed by the then effective Borrowing Base and Monthly
     Reduction Amount.  During the period from and after the Fifth Amendment
     Effective Date until the next redetermination thereof pursuant to Section
     2.08(d) or adjustment pursuant to Section 8.08(c), the amount of the
     Borrowing Base shall be $70,000,000, as reduced on a cumulative basis on
     the 27th day of each month following such effective date by the applicable
     Monthly Reduction Amount.  As of the Fifth Amendment Effective Date, the
     Monthly Reduction Amount shall be $2,000,000 each month (with the first
     such reduction occurring on December 27, 2001) until the next
     redetermination thereof pursuant to Section 2.08(d).  Schedule 2.08(a)
     attached to the Fifth Amendment sets forth, on a month by month basis, the
     Monthly Reduction Amount for each month and the resulting then effective
     Borrowing Base for the period from the Fifth Amendment Effective Date until
     the next scheduled redetermination of the Borrowing Base and the Monthly
     Reduction Amount.  Schedule 2.08(a) shall be amended, and a revised
     Schedule 2.08(a) shall be distributed by the Administrative Agent to the
     Borrower and each Lender, upon any redetermination (whether scheduled or
     unscheduled) of the Borrowing Base or Monthly Reduction Amount that

                                       4
<PAGE>

     necessitates a revised Schedule 2.08(a).  No delay for any reason
     whatsoever in a redetermination of the Monthly Reduction Amount shall
     affect the Borrower's obligations under Section 2.07(b)(iii)."

     (e) Section 2.08(d) of the Credit Agreement is hereby amended in its
entirety to read as follows:

         "(d)  Redetermination Dates.  So long as any of the Commitments are in
     effect and until payment in full of all loans hereunder, on or around the
     first Business Day of each January, April, July and October (each being a
     "Scheduled Redetermination Date"), the Majority Lenders shall redetermine
     the amount of the Borrowing Base and the Monthly Reduction Amount in
     accordance with Section 2.08(b); provided that any increase in the
     Borrowing Base shall require approval of all the Lenders.  In addition (i)
     the Borrower may initiate a redetermination of the Borrowing Base and the
     Monthly Reduction Amount at any other time as it so elects by specifying in
     writing to the Administrative Agent (who will promptly notify the Lenders)
     the date by which the Borrower will furnish to the Administrative Agent and
     the Lenders a Reserve Report in accordance with Section 8.07(b) and the
     date by which such redetermination is requested to occur; provided,
     however, that the Borrower may initiate only one such unscheduled
     redetermination per calendar year and (ii) the Administrative Agent may
     initiate a redetermination of the Borrowing Base and the Monthly Reduction
     Amount at any other time as it so elects by specifying in writing to the
     Borrower the date by which the Borrower is to furnish a Reserve Report in
     accordance with Section 8.07(b) and the date on which such redetermination
     is to occur; provided, however, that the Administrative Agent may initiate
     only one such unscheduled redetermination between each Scheduled
     Redetermination Date.  Concurrently with the delivery of each Reserve
     Report delivered in connection with each Borrowing Base redetermination
     (whether scheduled or unscheduled), the Borrower shall pay an engineering
     fee to the Administrative Agent, for its own account, of $10,000.

      (f) Section 2.08(f) of the Credit Agreement is hereby deleted in its
entirety.

     SECTION 2.03  AMENDMENTS TO ARTICLE VIII.

     (a) Section 8.01 of the Credit Agreement is hereby amended by (i) replacing
existing subsection (j) with the following new subsection (j), and (ii) adding
new subsection (l) as follows:

          "(j)  Monthly Financial Statements.  As soon as available and in any
     event within 25 days after the end of each calendar month, consolidating
     statements of income of the Borrower and its Consolidated

                                       5
<PAGE>

     Subsidiaries for the immediately preceding calendar month, and the related
     consolidating balance sheets as of the end of such period and including an
     accounts receivable aging report, accompanied by the certificate of a
     Responsible Officer, which certificate shall state that said financial
     statements fairly present the consolidated and consolidating financial
     condition and results of operations of the Borrower and its Consolidated
     Subsidiaries in accordance with GAAP, as at the end of, and for, such
     period (subject to normal year-end audit adjustments).

          (l)   Capital Expenditures Budget.  As soon as available and in any
     event no later than 30 days prior to each calendar quarter, (i) a budget
     ("Capital Expenditures Budget") of the Borrower's projected capital
     expenditures on a line item basis (categorized by project and including an
     overall amount for contingencies) for the six-month period beginning on the
     first day of such calendar quarter, and otherwise in a form and with
     amounts satisfactory to the Administrative Agent and the Majority Lenders
     (except that the first such report shall cover the eight-month period
     occurring from November 1, 2001 through June 30, 2002 and shall be
     delivered on the Fifth Amendment Effective Date), and (ii) a certificate of
     a Responsible Officer of the Borrower, which certificate shall state that
     such Capital Expenditures Budget was prepared in good faith and based on
     reasonable assumptions regarding the Borrower's capital expenditure
     requirements for such period."

      (b) Section 8.03(a) of the Credit Agreement is hereby amended by deleting
the last sentence thereof in its entirety and replacing it with the following:

          "The Borrower shall, within five Business Days following the Fifth
     Amendment Effective Date, obtain endorsements to such insurance policies
     naming "Union Bank of California, N.A., as Administrative Agent for the
     Beneficiaries" as joint loss payee and containing provisions that such
     policies will not be canceled without 30 days prior written notice having
     been given by the insurance company to the Administrative Agent."

      (c) Section 8.07(a) of the Credit Agreement is hereby amended in its
entirety to read as follows:

          "(a)    As soon as available but in any event on or before June 1,
     September 1 and December 1 of each year, the Borrower shall furnish to the
     Administrative Agent and the Lenders a Reserve Report dated effective as of
     the immediately preceding April 1, July 1 or October 1, respectively, and
     as soon as available but in any event on or before March 15 of each year,
     the Borrower shall furnish to the Administrative Agent and the Lenders a
     Reserve Report dated effective as of the immediately preceding January 1.
     The Reserve Report dated effective as of January 1 of each year shall be
     prepared by Ryder Scott Company, S.A. Holditch &

                                       6
<PAGE>

     Associates, Inc., or other certified independent petroleum engineers or
     other independent petroleum consultant(s) reasonably acceptable to the
     Administrative Agent and each of the Reserve Reports dated effective as of
     April 1, July 1 or October 1 shall be prepared by or under the supervision
     of the chief engineer of the Borrower who shall certify such Reserve Report
     to be true and accurate and to have been prepared in accordance with the
     procedures used in the immediately preceding Reserve Report dated effective
     as of January 1."

     (d) Section 8.11 of the Credit Agreement is hereby amended in its entirety
to read as follows:

          "Section 8.11   Intentionally omitted."

     (e) Section 8.12 of the Credit Agreement is hereby amended in its entirety
to read as follows:

          "Section 8.12  Hedging Agreements. The Borrower shall not enter into
     or maintain commodity hedging transactions which (a) are longer in duration
     than two years or (b) cover notional volumes of more than 80% of the
     anticipated production volumes attributable to Proven Reserves of the
     Borrower and its Subsidiaries during any such two-year period following the
     date such hedges are entered into.

     (f) Article VIII of the Credit Agreement is hereby further amended by
adding thereto a new Section 8.13 to read in its entirety as follows:

          "Section 8.13.  Payment of Trade Debt.  The Borrower shall, and shall
     cause each Subsidiary to, within thirty (30) days after the same becomes
     due (or, with respect to any vendor, within such longer period as is
     acceptable to such vendor) pay all Debt (other than the Obligations) owed
     by it and maintain appropriate accruals and reserves for all of the
     foregoing Debt in accordance with GAAP."

     SECTION 2.04  AMENDMENT TO ARTICLE IX.

     (a) Section 9.01(f) of the Credit Agreement is hereby amended in its
entirety to read as follows:

          "(f)  Intentionally Omitted."

     (b) Section 9.03 of the Credit Agreement is hereby amended in its entirety
to read as follows:

          "Section 9.03  Investments, Loans and Advances.  Neither the Borrower
     nor any Subsidiary will make or permit to remain outstanding any loans or
     advance to or investments in any Person (including without limitation, any
     acquisition of assets or stock of such Person or

                                       7
<PAGE>

     commitment to consummate such a transaction), except that the foregoing
     restriction shall not apply to:

          "(a) investments, loans or advances reflected in the Financial
               Statements or which are disclosed to the Lenders in Schedule
               9.03;

          (b)  accounts receivable arising in the ordinary course of business;

          (c)  direct obligations of the United States or any agency thereof, or
               obligations guaranteed by the United States or any agency
               thereof, in each case maturing within one year from the date of
               creation thereof;

          (d)  commercial paper maturing within one year from the date of
               creation thereof rated in the highest grade by Standard & Poor's
               Corporation or Moody's Investors Service, Inc.;

          (e)  deposits maturing within one year from the date of creation
               thereof with, including certificates of deposit issued by, any
               Lender or any office located in the United States of any other
               bank or trust company which is organized under the laws of the
               United States or any state thereof, has capital, surplus and
               undivided profits aggregating at least $500,000,000 (as of the
               date of such Lender's or bank or trust company's most recent
               financial reports) and has a short term deposit rating of no
               lower than A2 or P2, as such rating is set forth from time to
               time, by Standard & Poor's Corporation or Moody's Investors
               Service, Inc., respectively;

          (f)  deposits in money market funds investing exclusively in
               investments described in Section 9.03(c), 9.03(d) or 9.03(e);

          (g)  investments, loans or advances made by the Borrower in or to ATP
               (UK) in an aggregate amount not to exceed $10,000,000 at any time
               which are evidenced by promissory notes issued by ATP (UK)
               payable to the order of the Borrower and collaterally assigned
               to, and held in the possession of, the Administrative Agent;"

          (h)  investments by the Borrower in direct ownership interests in
               additional Oil and Gas Properties and gas gathering systems and
               other facilities related thereto; provided that, (i) no such
               investments under this clause (h) may, unless the Administrative
               Agent and the Lenders otherwise consent in writing, be made
               before January 1, 2002, and (ii) the amount of any such
               investments permitted by this clause (h) may not exceed
               $1,000,000 during any six-month period between scheduled
               Borrowing Base redeterminations (except that such $1,000,000
               limit may be exceeded in connection with any proposed investment

                                       8
<PAGE>

               to the extent of any unused availability that exists under the
               Borrowing Base on the date such proposed investment is to be
               made); and

          (i)  other investments, loans or advances not otherwise permitted by
               this Section 9.03 in an aggregate amount not to exceed $250,000
               at any time."

     (c) Section 9.13 of the Credit Agreement is hereby amended in its entirety
to read as follows:

          "Section 9.13   Current Ratio.  The Borrower will not permit its ratio
     of (i) consolidated current assets as of the end of any calendar month
     commencing with the calendar month ending December 31, 2001 and continuing
     monthly thereafter to (ii) consolidated current liabilities (excluding
     current maturities of the Notes) as of the end of the relevant calendar
     month to be less than 1.0 to 1.0."

     (d) Section 9.14 of the Credit Agreement is hereby amended in its entirety
to read as follows:

          "Section 9.14   Debt Coverage Ratio.  The Borrower will not permit its
     Debt Ratio as of the end of any calendar month (calculated monthly at the
     end of each calendar month) to be greater than 3.0 to 1.0."

     (e) Section 9.15 of the Credit Agreement is hereby amended in its entirety
to read as follows:

          "Section 9.15   Interest Coverage Ratio.  The Borrower will not permit
     its Interest Coverage Ratio as of the end of any calendar month (calculated
     monthly at the end of each month) to be less than 3.0 to 1.0."

     (f) New Sections 9.23 and 9.24 are hereby added to the Credit Agreement as
follows:

          "Section 9.23  Capital Expenditures.  The Borrower shall not, as of
     the end of any calendar quarter, permit its consolidated capital
     expenditures during the six-month period (or, if applicable, eight-month
     period from November 1, 2001 through June 30, 2002) then ended to exceed
     (a) with respect to any project or capital expenditure line item described
     in the applicable Capital Expenditures Budget for such period, an amount
     equal to the amount budgeted for such line item; provided that, the
     Borrower may for purposes of determining compliance with this Section
     9.23(a), allocate to such line items all or a portion of the amount set
     forth in the such Capital Expenditures Budget for contingencies plus an
     amount equal to $1,000,000, and (b) with respect to the aggregate amount
     budgeted for all such line items collectively, an amount equal to the sum
     of (i) the amount budgeted for all such line items in the aggregate as set

                                       9
<PAGE>

     forth in the applicable Capital Expenditures Budget plus (ii) the amount of
     any contingencies set forth in the Capital Expenditures Budget, plus (iii)
     $1,000,000.

          Section 9.24  Working Capital Deficit.  The Borrower shall not permit
     its consolidated current liabilities (excluding current maturities of the
     Notes) as of November 30, 2001 minus its consolidated current assets
     (including any unused amounts under the Borrowing Base) as of such month
     end to be greater than $10,000,000."

     SECTION 2.05 AMENDMENT TO ARTICLE X. Section 10.01 of the Credit Agreement
is hereby amended by adding thereto a new subsection (o) to read in its entirety
as follows:

          "(o) this Credit Agreement and the other Loan Documents have not be
     amended and restated in their entirety to the reasonable satisfaction of
     the Administrative Agent and the Lenders on or before January 31, 2002;
     provided that neither the Administrative Agent nor any Lender shall be
     entitled to require any change in the economic terms of this Agreement or
     any Loan Document in connection with any such amendment and restatement
     other than those changes to the economic terms which are permitted in
     connection with the syndication of the credit facilities evidenced by the
     Credit Agreement in accordance with the Letter Agreement dated as of
     October 5, 2001 between Union Bank of California, N.A. and the Borrower."

     SECTION 2.06 AMENDMENT TO SCHEDULES. Schedules 2.08(a), 7.02, 7.03, 7.09,
7.10, 7.14, 7.17, 7.19, 7.20, 7.22, 7.23, 9.01, 9.02, 9.03 of the Credit
Agreement are hereby replaced in their entirety with the corresponding Schedules
attached to this Fifth Amendment.

     SECTION 2.07 AMENDMENT TO EXHIBIT. Exhibit A to the Credit Agreement is
hereby replaced in its entirety with the Exhibit A attached to this Fifth
Amendment.

                                 ARTICLE III.
                         REPRESENTATIONS AND WARRANTIES

     SECTION 3.01    REPRESENTATIONS AND WARRANTIES. The Borrower represents and
warrants to the Administrative Agent and the Lenders that:

     (a) Except for such which are made only as of a prior date, the
representations and warranties set forth in the Credit Agreement and in the
other Loan Documents are true and correct in all material respects as of the
Fifth Amendment Effective Date as if made on and as of such date.

     (b) The execution, delivery and performance of this Fifth Amendment are
within the corporate power and authority of the Borrower and have been duly
authorized by appropriate proceedings.

                                       10
<PAGE>

     (c) This Fifth Amendment constitutes a legal, valid, and binding obligation
of the Borrower enforceable in accordance with its terms, except as limited by
applicable bankruptcy, insolvency, reorganization, moratorium, or similar laws
affecting the rights of creditors generally and general principles of equity.

     (d) All of the Liens created pursuant to the Security Instruments are valid
and subsisting and constitute valid and enforceable security interests in, and
Liens on, all of the Mortgaged Property subject thereto and subject to no other
Liens (other than Excepted Liens and other Liens not prohibited by Section 9.02
of the Credit Agreement).

                                  ARTICLE IV.
                                  CONDITIONS

     This Fifth Amendment shall become effective and enforceable against the
parties hereto and the Credit Agreement shall be amended as provided herein upon
the occurrence of the following conditions precedent on or before the Fifth
Amendment Effective Date:

     SECTION 4.01 LOAN DOCUMENTS. The Administrative Agent shall have received
multiple original counterparts, as requested by the Administrative Agent, of
this Fifth Amendment duly and validly executed and delivered by duly authorized
officers of the Borrower, the Administrative Agent and each Lender.

     SECTION 4.02 CORPORATE PROCEEDINGS OF LOAN PARTIES. The Administrative
Agent shall have received multiple copies, as requested by the Administrative
Agent, of the resolutions, in form and substance reasonably satisfactory to the
Administrative Agent, of the Board of Directors of the Borrower, authorizing the
execution, delivery and performance of this Fifth Amendment and the other Loan
Documents and the transactions contemplated hereby and thereby, each such copy
being attached to an original certificate of the Secretary or an Assistant
Secretary of the Borrower, dated the Fifth Amendment Effective Date, certifying
(a) that the resolutions attached thereto are true, correct and complete copies
of resolutions duly adopted by written consent or at a meeting of the Board of
Directors of the Borrower, (b) that such resolutions constitute all resolutions
adopted with respect to the transactions contemplated hereby, (c) that such
resolutions have not been amended, modified, revoked or rescinded as of the
Fifth Amendment Effective Date, (d) that the articles of incorporation and
bylaws attached thereto are the true, correct and complete copies of such
articles and bylaws and that such articles and bylaws have not been amended,
supplemented or otherwise modified except pursuant to any amendments attached
thereto, and (e) as to the incumbency and specimen signature of the officers of
the Borrower executing any Loan Documents (including, without limitation, this
Fifth Amendment).

     SECTION 4.03 NO DEFAULT. After giving effect to the Waiver of Defaults and
Ratification of Intercreditor and Subordination Agreement dated as of the Fifth
Amendment Effective Date among the Borrower, Aquila Energy Capital Corporation,
UBOC and the Administrative Agent, no Default or Event of Default shall have
occurred and be continuing as of the Fifth Amendment Effective Date.

                                       11
<PAGE>

     SECTION 4.04 MATERIAL ADVERSE EFFECT. No event shall have occurred or
circumstance shall have arisen since December 31, 2000, which, in the reasonable
opinion of the Lenders, could have a Material Adverse Effect.

     SECTION 4.05 SECURITY INSTRUMENTS. All of the Security Instruments,
including the Security Instruments listed on Schedule 1 attached hereto, shall
be in full force and effect and provide to the Administrative Agent and the
Lenders the security intended thereby to secure the Obligations.

     SECTION 4.06 LEGAL FEES OF ADMINISTRATIVE AGENT'S COUNSEL. The Borrower
shall have paid all fees and expenses of the Administrative Agent's outside
legal counsel and other consultants pursuant to all invoices presented for
payment on or prior to the Fifth Amendment Effective Date.

     SECTION 4.07 OTHER INSTRUMENTS OR DOCUMENTS. The Administrative Agent or
any Lender or counsel to the Administrative Agent shall have received such other
instruments or documents as any of them may reasonably request.

     SECTION 4.08 OPINION OF COUNSEL. The Administrative Agent shall have
received a duly executed original opinion issued by outside counsel to the
Borrower, in form and substance reasonably satisfactory to the Administrative
Agent and its counsel;

     SECTION 4.09 LIENS. The Administrative Agent shall have received the
results of lien, tax and judgment searches of the UCC Records of the Secretary
of State of the States of Texas and Louisiana from a source acceptable to the
Administrative Agent and reflecting no Liens against any of the Mortgaged
Properties as to which perfection of a Lien is accomplished by the filing of a
financing statement other than Liens not prohibited under Section 9.02 of the
Credit Agreement and Liens (if any) which are being released concurrently with
the effectiveness of this Fifth Amendment.

     SECTION 4.10 TITLE DUE DILIGENCE. The Administrative Agent shall be
satisfied in its sole discretion with its due diligence analysis and review of
the status of the Borrower's title to its Oil and Gas Properties and to the
other Mortgaged Properties and which review should reflect no Liens, other than
Liens not prohibited under Section 9.02 of the Credit Agreement, against any of
the Mortgaged Properties and Liens (if any) which are being released
concurrently with the effectiveness of this Fifth Amendment.

     SECTION 4.11 CONSENT. The Administrative Agent shall have received true and
complete copies of all governmental or other third party consents, licenses and
approvals required in connection with the execution, delivery, performance,
validity and enforceability of this Fifth Amendment and the other Loan
Documents, including, without limitation, the Waiver of Defaults and
Ratification of Intercreditor and Subordination Agreement dated as of November
5, 2001 among the Borrower, the Administrative Agent, the Lenders, and Aquila
Energy Capital Corporation.

     SECTION 4.12 RELIANCE LETTER. The Administrative Agent shall have received
a letter from Jackson Walker L.L.P. permitting the Administrative Agent and the
Lenders to rely on certain title opinions issued by Jackson Walker L.L.P.,
counsel to the

                                       12
<PAGE>

Borrower. Failure of the Borrower to comply with this Section 4.12 shall
constitute an Event of Default under the Credit Agreement and the other Loan
Documents.

                                  ARTICLE V.
                                 MISCELLANEOUS

     SECTION 5.01 FINANCING STATEMENTS. The Borrower hereby authorizes the
Administrative Agent to file any financing statements without the signature of
the Borrower to the extent permitted by applicable law in order to perfect or
maintain the perfection of any security interest granted under the Security
Instruments or the other Loan Documents.

     SECTION 5.02 LEGAL OPINION. On or before November 19, 2001, the Borrower
shall cause the Subordinated Creditor to deliver the opinion of Porter & Hedges,
L.L.P., counsel to the Subordinated Creditor, as required under Section 3.09 of
the Subordination Agreement. Failure of the Borrower to comply with this Section
5.02 shall constitute an Event of Default under the Credit Agreement and the
other Loan Documents.

     SECTION 5.03 USE OF UNSCHEDULED REDETERMINATION. The establishment of the
Borrowing Base and Monthly Reduction Amounts set forth in the Credit Agreement
as revised by Section 2.02(b) of this Fifth Amendment is in lieu of the
redetermination of the Borrowing Base and Monthly Reduction Amount scheduled for
the last Business Day of October, 2001 pursuant to Section 4.08 of the Fourth
Amendment. As set forth in Section 4.08 of the Fourth Amendment, the
Administrative Agent and the Lenders shall not be required at the Borrower's
request to conduct an unscheduled redetermination of the Borrowing Base or the
Monthly Reduction Amount during the balance of calendar year 2001.

     SECTION 5.04 EFFECT ON LOAN DOCUMENTS.

     (a) Each of the Borrower, the Administrative Agent and the Lenders does
hereby adopt, ratify, and confirm the Credit Agreement, as amended hereby, and
acknowledges and agrees that the Credit Agreement, as amended hereby, is and
remains in full force and effect. Nothing herein shall act as a waiver of any of
the Administrative Agent's or Lender's rights under the Loan Documents, as
amended, including the waiver of any Default or Event of Default, however
denominated. From and after the Fifth Amendment Effective Date, all references
to the Credit Agreement and the Loan Documents shall mean such Credit Agreement
and such Loan Documents as amended by this Fifth Amendment.

     (b) This Fifth Amendment is a Loan Document for the purposes of the
provisions of the other Loan Documents. Without limiting the foregoing, any
breach of representations, warranties, and covenants under this Fifth Amendment
shall be a Default or Event of Default, as applicable, under the Credit
Agreement.

     SECTION 5.05 COUNTERPARTS. This Fifth Amendment may be signed in any number
of counterparts, each of which shall be an original and all of which, taken

                                       13
<PAGE>

together, constitute a single instrument. This Fifth Amendment may be executed
by facsimile signature and all such signatures shall be effective as originals.

     SECTION 5.06 SUCCESSORS AND ASSIGNS. This Fifth Amendment shall be binding
upon and inure to the benefit of the parties hereto and their respective
successors and assigns permitted pursuant to the Credit Agreement.

     SECTION 5.07 NUMBER AND GENDER. Whenever the context requires, reference
herein made to the single number shall be understood to include the plural; and
likewise, the plural shall be understood to include the singular. Words denoting
sex shall be construed to include the masculine, feminine and neuter, when such
construction is appropriate; and specific enumeration shall not exclude the
general but shall be construed as cumulative. Definitions of terms defined in
the singular or plural shall be equally applicable to the plural or singular, as
the case may be, unless otherwise indicated.

     SECTION 5.08 ENTIRE AGREEMENT. This Fifth Amendment constitutes the entire
agreement among the parties hereto with respect to the subject hereof. All prior
understandings, statements and agreements, whether written or oral, relating to
the subject hereof are superseded by this Fifth Amendment.

     SECTION 5.09 INVALIDITY. In the event that any one or more of the
provisions contained in this Fifth Amendment shall for any reason be held
invalid, illegal or unenforceable in any respect, such invalidity, illegality or
unenforceability shall not affect any other provision of this Fifth Amendment.

     SECTION 5.10 TITLES OF ARTICLES, SECTIONS AND SUBSECTIONS. All titles or
headings to Articles, Sections, subsections or other divisions of this Fifth
Amendment or the exhibits hereto, if any, are only for the convenience of the
parties and shall not be construed to have any effect or meaning with respect to
the other content of such Articles, Sections, subsections, other divisions or
exhibits, such other content being controlling as the agreement among the
parties hereto.

     SECTION 5.11 GOVERNING LAW. This Fifth Amendment shall be deemed to be a
contract made under and shall be governed by and construed in accordance with
the internal laws of the State of Texas.

THIS FIFTH AMENDMENT, THE CREDIT AGREEMENT, AS AMENDED HEREBY, THE NOTES, AND
THE OTHER LOAN DOCUMENTS REPRESENT THE FINAL AGREEMENT BETWEEN THE PARTIES AND
MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT
ORAL AGREEMENTS OF THE PARTIES.

THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

                        [SIGNATURES BEGIN ON NEXT PAGE]

                                       14
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Fifth
Amendment to be duly executed and delivered by their proper and duly authorized
officers as of the Fifth Amendment Effective Date.

BORROWER:                        ATP OIL & GAS CORPORATION

                                 By:  /s/T. Paul Bulmahn
                                      --------------------------------
                                      T. Paul Bulmahn
                                      President

                                       15
<PAGE>

LENDER AND ADMINISTRATIVE        UNION BANK OF CALIFORNIA, N.A.
AGENT                            individually and as Administrative Agent

                                 By:    /s/Damien Meiburger
                                        ----------------------------------
                                 Name:  Damien Meiburger
                                 Title: Senior Vice President

                                 By:    /s/Ali Ahmed
                                        ----------------------------------
                                 Name:  Ali Ahmed
                                 Title: Assistant Vice President

                                       16
<PAGE>

                                   EXHIBIT A

                                  FORM OF NOTE
                                  ------------

$___________________________                   _______________________, 200_

          FOR VALUE RECEIVED, ATP OIL & GAS CORPORATION, a ________________
corporation (the "Borrower") hereby promises to pay to the order of
_______________________ (the "Lender"), at the Principal Office of Union Bank of
California, N.A. (the "Administrative Agent") at Lincoln Plaza, 500 N. Akard
Street, Suite 4200, Dallas, Texas 75204, the principal sum of __________________
Dollars ($___________) (or such lesser amount as shall equal the aggregate
unpaid principal amount of the Loans made by the Lender to the Borrower under
the Credit Agreement, as hereinafter defined), in lawful money of the United
States of America and in immediately available funds, on the dates and in the
principal amounts provided in the Credit Agreement, and to pay interest on the
unpaid principal amount of such Loan at such office, in like money and funds,
for the period commencing on the date of such Loan until such Loan shall be paid
in full, at the rates per annum and on the dates provided in the Credit
Agreement.

          The date, amount, Type, interest rate, Interest Period and maturity of
each Loan made by the Lender to the Borrower, and each payment made on account
of the principal thereof, shall be recorded by the Lender on its books and,
prior to any transfer of this Note, endorsed by the Lender on the schedules
attached hereto or any continuation thereof.

          This Note is one of the Notes referred to in the Credit Agreement
dated as of April 27, 2001 among the Borrower, the Lenders which are or become
parties hereto (including the Lender) and the Administrative Agent (as the same
has been and may be further amended or supplemented from time to time, the
"Credit Agreement"), and evidences Loans made by the Lender thereunder.
Capitalized terms used in this Note have the respective meanings assigned to
them in the Credit Agreement.

          This Note is issued pursuant to the Credit Agreement and is entitled
to the benefits provided for in the Credit Agreement and the Security
Instruments.  The Credit Agreement provides for the acceleration of the maturity
of this Note upon the occurrence of certain events, for prepayments of Loans
upon the terms and conditions specified therein and other provisions relevant to
this Note.

          THIS NOTE SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE
LAWS OF THE STATE OF TEXAS.

                                 ATP OIL & GAS CORPORATION

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

                                       17
<PAGE>

                                Schedule 2.08(a)
                                ----------------

 Monthly Reduction Amount and resulting Borrowing Base after Monthly Reduction

  Date           Monthly Reduction Amount          Borrowing Base
-------------------------------------------------------------------------------
12-27-01               $2,000,000                     $68,000,000
-------------------------------------------------------------------------------
 1-27-02               $2,000,000*                    $66,000,000*
-------------------------------------------------------------------------------

*These amounts are subject to possible change as a result of  either (a) the
redetermination of the Borrowing Base and the Monthly Reduction Amount scheduled
on or around the first Business Day of January, 2002 contemplated by Section
2.08(d) or (b) any unscheduled redetermination of the Borrowing Base and Monthly
Reduction Amount pursuant to such Section 2.08(d).

                                       18

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