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                                                                   EXHIBIT 10.23

                                 FIRST AMENDMENT
                                       TO
                             NOTE PURCHASE AGREEMENT
                               DATED JUNE 29, 2001
                    BY AND BETWEEN ATP OIL & GAS CORPORATION
                      AND AQUILA ENERGY CAPITAL CORPORATION

         This First Amendment ("First Amendment") to the Note Purchase Agreement
dated June 29, 2001, by and between ATP OIL & GAS CORPORATION, a Texas
corporation (the "Issuer") and AQUILA ENERGY CAPITAL CORPORATION, a Delaware
corporation (the "Purchaser"), is entered into on this 26th day of March 2002.

                                  WITNESSETH:

         A.       Issuer and Purchaser heretofore entered into a Note Purchase
Agreement dated June 29, 2001 (the "Note Purchase Agreement").

         B.       Issuer and Purchaser hereby desire to amend the Note Purchase
Agreement, subject to the terms and conditions contained herein.

         NOW, THEREFORE, in consideration of the mutual promises herein
contained, and for other good and valuable consideration, the receipt and
sufficiency of which are acknowledged by Issuer and Purchaser, and each
intending to be legally bound hereby, Issuer and Purchaser agree as follows:

         I.       Specific Amendments to Note Purchase Agreement
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         Article I of the Note Purchase Agreement is hereby amended by adding
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the following definition thereto:

                  "First Amendment" means that certain First Amendment to the
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         Note Purchase Agreement executed by Purchaser and Issuer effective on
         March 27, 2002.

         Section 7.1(v) of the Note Purchase Agreement is hereby amended by
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replacing the text of that Section in its entirety with:

                  As of the end of each fiscal quarter beginning with the
                  quarter ending June 30, 2002, Issuer shall have a positive
                  working capital, as defined by GAAP, and no time during any
                  fiscal quarter after March 31, 2002, shall the Issuer's
                  working capital be less than a negative $5 million; provided
                  that the working capital calculation shall include unused
                  portions of the credit availability under the Senior Debt and
                  exclude, for future periods, mark to market amounts under any
                  Swap Agreements and current maturities of the Senior Debt. Any
                  amount relating to litigation, either an asset or a liability
                  of the Issuer, will be considered current at such time as
                  either: (a) a judgement entered by a court of competent
                  jurisdiction awarding such amount to the prevailing party has
                  become final (a "Final Judgment"), or (b) Issuer and the other
                  party(ies) to such
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                  litigation have entered into a binding agreement providing
                  for the payment of such amount to or by Issuer; provided that
                  if any Final Judgment is appealed by either party and
                  execution of such Final Judgment is stayed by appellant's
                  posting of a supersedeas bond, then so long as execution of
                  such Final Judgment is stayed the amount of such judgement
                  shall not be deemed a current asset or current liability.
                  Notwithstanding the foregoing, the Issuer may have a negative
                  working capital not in excess of $10 million at December 31,
                  2001 and $5 million at March 31, 2002 calculated as above.

         II.      Reaffirmation of Representations and Warranties. To induce
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Purchaser to enter into this First Amendment, Issuer hereby reaffirms as of the
date hereof its representations and warranties contained in Article IV of the
Note Purchase Agreement and in all other documents executed pursuant thereto,
and additionally represents and warrants as follows:

                  A.     The execution and delivery of this First Amendment and
         the performance by Issuer of its obligations under this First Amendment
         are within Issuer's power, have been duly authorized by all necessary
         corporate action, have received all necessary governmental approval (if
         any shall be required), and do not and will not contravene or conflict
         with any provision of law or of the Articles of Incorporation or Bylaws
         of Issuer or of any agreement binding upon Issuer.

                  B.     The Note Purchase Agreement as amended by this First
         Amendment, represents the legal, valid and binding obligations of
         Issuer, enforceable against Issuer in accordance with its terms,
         subject as to enforcement only to bankruptcy, insolvency,
         reorganization, moratorium or other similar laws affecting the
         enforcement of creditors' rights generally.

                  C.     No Event of Default has occurred and is continuing as
         of the date hereof.

         III.     Defined Terms. Except as amended hereby, terms used herein
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that are defined in the Note Purchase Agreement shall have the same meanings
herein.

         IV.      Reaffirmation of Loan Agreement. This First Amendment shall be
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deemed to be an amendment to the Note Purchase Agreement, and the Note Purchase
Agreement, as further amended hereby, is hereby ratified, approved and confirmed
in each and every respect. All references to the Note Purchase Agreement herein
and in any other document, instrument, agreement or writing shall hereafter be
deemed to refer to the Note Purchase Agreement as amended hereby.

         V.       Entire Agreement. The Note Purchase Agreement, as hereby
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amended, embodies the entire agreement between Issuer and Purchaser and
supersedes all prior proposals, agreements and understandings relating to the
subject matter hereof. Issuer certifies that it is relying on no representation,
warranty, covenant or agreement except for those set forth in the Note Purchase
Agreement as hereby further amended and the other documents previously executed
or executed of even date herewith.

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         VI.      Governing Law. THIS FIRST AMENDMENT SHALL BE GOVERNED BY AND
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CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS AND THE APPLICABLE
LAWS OF THE UNITED STATES OF AMERICA. This First Amendment has been entered into
in Harris County, Texas and shall be performable for all purposes in Harris
County, Texas. Courts within the State of Texas shall have jurisdiction over any
and all disputes between Issuer and Purchaser, whether in law or equity,
including, but not limited to, any and all disputes arising out of or relating
to this First Amendment or any other Transaction Document.

         VII.     Severability. Whenever possible, each provision of this First
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Amendment shall be interpreted in such manner as to be effective and valid under
applicable law, but if any provision of this First Amendment shall be prohibited
by or invalid under applicable law, such provision shall be ineffective to the
extent of such prohibition or invalidity, without invalidating the remainder of
such provision or the remaining provisions of this First Amendment.

         VIII.    Section Captions. Section captions used in this First
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Amendment are for convenience of reference only, and shall not affect the
construction of this First Amendment.

         IX.      Successors and Assigns. This First Amendment shall be binding
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upon Issuer and Purchaser and their respective successors and assigns, and shall
inure to the benefit of Issuer and Purchaser, and the respective successors and
assigns of Purchaser.

         X.       Non-Application of Chapter 346 of Texas Finance Code. In no
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event shall Chapter 346 of the Texas Finance Code (which regulates certain
revolving loan accounts and revolving tri-party accounts) apply to this Note
Purchase Agreement as hereby further amended or any other Transaction Documents
or the transactions contemplated hereby.

         XI.      NOTICE. THIS FIRST AMENDMENT, TOGETHER WITH THE NOTE PURCHASE
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AGREEMENT AND THE OTHER TRANSACTION 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.

                           [Signature page follows.]

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

PURCHASER                                    ISSUER

AQUILA ENERGY CAPITAL                        ATP OIL & GAS CORPORATION,
CORPORATION, a Delaware corporation          a Texas corporation

By:      /s/ Kenneth F. Wyatt             By:       /s/ T. Paul Bulmahn
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         Kenneth F. Wyatt                             T. Paul Bulmahn
         Vice President                               President

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