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                                                                   EXHIBIT 10.20

                      SIXTH AMENDMENT TO CREDIT AGREEMENT

     This SIXTH AMENDMENT TO CREDIT AGREEMENT (this "Amendment") executed
effective as of the 31st day of January, 2002 (the "Sixth 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 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, the Administrative Agent and the 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, (iv)
Fourth Amendment to Credit Agreement dated as of October 1, 2001, and (v) Fifth
Amendment to Credit Agreement dated as of November 5, 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.  Subject to the terms and conditions of this Amendment, the Borrower,
the Administrative Agent and the Lenders wish to amend the Credit Agreement and
to provide for a guaranty by ATP Energy, Inc. (the "ATP Energy").

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

     SECTION 1. DEFINITIONS. As used in this Amendment, each of the terms
defined in the opening paragraph and the Recitals above shall have the meanings
assigned to such terms therein. 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. The words
"hereby", "herein", "hereinafter", "hereof", "hereto" and "hereunder" when used
in this Amendment shall refer to this Amendment as a whole and not to any
particular Article, Section, subsection or provision of this Amendment. Section,
subsection and Exhibit references herein are to such Sections, subsections and
Exhibits to this Amendment unless otherwise specified. 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 and
feminine, 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.

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     SECTION 2. AMENDMENTS. The Borrower, the Administrative Agent and the
Lenders agree that the Credit Agreement is hereby amended, effective as of the
Sixth Amendment Effective Date, in the following particulars:

        (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, the Fifth Amendment, the Sixth Amendment, and as the same
     may from time to time be further amended or supplemented.

     "Loan Documents"  shall mean this Agreement, the Notes, the Fee Letter, all
     Letters of Credit, all Letter of Credit Agreements, the Security
     Instruments, the Guaranty, and any other document expressly indicating
     therein that such document is a "Loan Document" under this Agreement.

        (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:

     "ATP Energy" means ATP Energy, Inc., a Texas corporation.

     "Guaranty" means the Guaranty dated as of the Sixth Amendment Effective
     Date made by ATP Energy in favor of the Administrative Agent for the
     benefit of the Credit Parties (as therein defined) as the same may be
     amended, supplemented or otherwise modified from time to time in the
     future.

     "Sixth Amendment" shall mean that certain Sixth Amendment to Credit
     Agreement dated as of January 31, 2002, by and among the Borrower, the
     Administrative Agent and the Lenders.

     "Sixth Amendment Effective Date" shall mean January 31, 2002.

        (c) Section 10.01(o) of the Credit Agreement is hereby amended is hereby
amended in its entirety to read as follows:

          "(o)  this Credit Agreement and the other Loan Documents shall not be
     amended and restated in their entirety, to the reasonable satisfaction of
     the Administrative Agent and the Lenders, on or before February 28, 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;
     or".

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        (d) Section 10.01 of the Credit Agreement is hereby amended by adding a
new Section 10.01(p) to read as follows:

        "(p) any of the provisions in the Guaranty shall for any reason cease
     to be in full force and effect, valid and binding on ATP Energy, or ATP
     Energy shall so state in writing."

     SECTION 3. 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
Sixth Amendment Effective Date as if made on and as of such date.

        (b) The execution, delivery and performance of this Amendment are within
the corporate power and authority of the Borrower and have been duly authorized
by appropriate corporate proceedings, and the execution, delivery and
performance of the Guaranty dated as of the date hereof by ATP Energy (the
"Guaranty") are within the corporate power and authority of ATP Energy and have
been duly authorized by appropriate corporate proceedings.

        (c) This 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) No consent, order, authorization, or approval or other action by,
and no notice to or filing with, any Governmental Authority or any other Person
is required for the due execution, delivery, and performance by the Borrower of
this Amendment or the other Loan Documents to which the Borrower is a party or
by ATP Energy of the Guaranty or the other Loan Documents to which it is a party
or the consummation of the transactions contemplated thereby.

     SECTION 4. CONDITIONS TO EFFECTIVENESS: This 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 Sixth Amendment Effective Date:

        (a) Loan Documents. The Administrative Agent shall have received
multiple original counterparts, as requested by the Administrative Agent, of (i)
this Amendment duly and validly executed and delivered by duly authorized
officers of the Borrower, the Administrative Agent and each Lender and (ii) the
Guaranty duly and validly executed and delivered by a duly authorized officer of
ATP Energy.

        (b) 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 ATP Energy, authorizing the
execution, delivery and performance of the Guaranty and the

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transaction contemplated thereby, each such copy being attached to an original
certificate of the Secretary or an Assistant Secretary of ATP Energy, dated the
Sixth Amendment Effective Date, 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 of ATP Energy, (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 Sixth Amendment Effective
Date, (iv) 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 (v) as to the incumbency
and specimen signature of the officer of ATP Energy executing any Loan Documents
(including, without limitation, the Guaranty).

        (c) No Default. No Default or Event of Default shall have occurred and
be continuing as of the Sixth Amendment Effective Date.

        (d) Material Adverse Effect. No event shall have occurred or
circumstance shall have arisen since September 30, 2001, which, in the
reasonable opinion of the Lenders, could have a Material Adverse Effect.

        (e) 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 Sixth Amendment Effective Date.

        (f) 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 5.  MISCELLANEOUS.

        (a) Effect on Loan Documents. 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 Sixth 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
Amendment. This 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 Amendment shall be a
Default or Event of Default, as applicable, under the Credit Agreement.

        (b) Counterparts. This Amendment may be signed in any number of
counterparts, each of which shall be an original and all of which, taken
together, constitute a single instrument.

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This Amendment may be executed by facsimile signature and all such signatures
shall be effective as originals.

        (c) Successors and Assigns. This 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.

        (d) Entire Agreement. This 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 Amendment.

        (e) Invalidity. In the event that any one or more of the provisions
contained in this 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 Amendment.

        (f) Titles of Articles, Sections and Subsections. All titles or headings
to Articles, Sections, subsections or other divisions of this 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.

        (g) Governing Law. This 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 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.

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


BORROWER:                        ATP OIL & GAS CORPORATION



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

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



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



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

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