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                                                                    EXHIBIT 10.2

                     FIFTH SUPPLEMENT TO PURCHASE AGREEMENT

     THIS FIFTH SUPPLEMENT TO PURCHASE AGREEMENT dated as of July 9, 2001 (this
"Supplement"), is made by:

     o    TransTexas Gas Corporation, a Delaware corporation (herein called
          "Grantor"),

     o    Mirant Americas Energy Capital, LP, formerly named Southern Producer
          Services, L.P. ("SPS"),

     o    TCW DR VI Investment Partnership, L.P. ("Fund VI"), acting through its
          agent, TCW Asset Management Company (SPS and Fund VI are herein
          collectively called, "Grantee"), and

     o    TCW Asset Management Company, as Agent on behalf of Fund VI (in such
          capacity, "Funds Agent").

                                    RECITALS

     1. Grantor, SPS, TCW Portfolio No. 1555 DR V Sub-Custody Partnership, L.P.
("Fund V"), Fund VI and Funds Agent have heretofore entered into that certain
Purchase Agreement dated as of March 14, 2000 (herein, as heretofore amended or
supplemented, called the "Original Purchase Agreement"), and, as contemplated
therein, Grantor has, by means of a Production Payment Conveyance dated as of
the same date (herein, as heretofore amended or supplemented, called the
"Original Conveyance"), conveyed to SPS, Fund V and Fund VI the "Production
Payment" as therein defined, burdening interests of Grantor in certain oil and
gas properties.

     2. Effective as of December 1, 2000, Fund V assigned all of its right,
title, and interest under the Original Conveyance to SPS pursuant to that
certain Conveyance of Interest in Production Payment dated as of February 7,
2001.

     3. As contemplated in the Original Purchase Agreement, Grantor now desires
to supplement and amend the Original Conveyance in consideration of the payment
by SPS of an additional Purchase Price Payment, as more fully described below.

     NOW, THEREFORE, in consideration of the premises and the mutual covenants
and agreements contained herein, Grantor, SPS, Fund VI, and Funds Agent hereby
agree as follows:

                     ARTICLE I - Definitions and References

     Section 1.1. Defined Terms and References. As used herein, the terms
"Original Conveyance", "Original Purchase Agreement", "Grantor", "SPS", "Fund
V", "Fund VI",

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"Grantee" and "Funds Agent" have the meanings given them above. Reference is
also made to the Original Purchase Agreement and to the Original Conveyance for
the meaning of various terms defined therein, all of which shall when used
herein (unless otherwise expressly defined herein) have the same meanings. For
purposes of this Supplement, unless the context otherwise requires, the
following additional terms shall have the following meanings:

     "Current Closing Date" has the meaning given such term in Section 2.3.

     "Current Conveyance Supplement" means the Fifth Supplement to Production
Payment Conveyance executed by Grantor and Grantee substantially in the form of
Exhibit A hereto.

     "Current Purchase Price Payment" means a Purchase Price Payment in the
amount of $15,000,000.

     "Current Supplement Documents" means this Supplement, the Current
Conveyance Supplement, and all other Production Payment Documents delivered
substantially contemporaneously herewith.

     "Production Payment Conveyance" means the Original Conveyance as amended
and supplemented by the Current Conveyance Supplement.

     "Purchase Agreement" means the Original Purchase Agreement as supplemented
and amended hereby.

     Section 1.2. Rules of Construction. All references in this Supplement to
articles, sections, subsections and other subdivisions refer to corresponding
articles, sections, subsections and other subdivisions of this Supplement unless
expressly provided otherwise. Titles appearing at the beginning of any of such
subdivisions are for convenience only and shall not constitute part of such
subdivisions and shall be disregarded in construing the language contained in
such subdivisions. The words "this Supplement, "this instrument", "herein",
"hereof", "hereby", "hereunder" and words of similar import refer to this
Supplement as a whole and not to any particular subdivision unless expressly so
limited. Unless the context otherwise requires: "including" (and its grammatical
variations) means "including without limitation"; "or" is not exclusive; words
in the singular form shall be construed to include the plural and vice versa;
words in any gender include all other genders; references herein to any
instrument or agreement refer to such instrument or agreement as it may be from
time to time supplemented or amended; and references herein to any Person
include such Person's successors and assigns. All references in this Supplement
to exhibits and schedules refer to the exhibits and schedules to this Supplement
unless expressly provided otherwise, and all such exhibits and schedules are
hereby incorporated herein by reference and made a part hereof for all purposes.

                             ARTICLE II - Amendment

     Section 2.1. Agreement to Amend. Upon the terms and conditions of this
Supplement and the Purchase Agreement, Grantor and Grantee agree to amend and
supplement the Original Conveyance by executing and delivering the Current
Conveyance Supplement.

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     Section 2.2. Closing. On the Current Closing Date, Grantor shall deliver
the Current Conveyance Supplement to Grantee, and Grantor shall satisfy all of
the conditions set out in Sections 3.4 and 3.5 of the Purchase Agreement (other
than the conditions in Section 3.4(e) and (f) of the Purchase Agreement, which
are hereby waived without prejudice to the rights of SPS and Fund VI to request
and receive the same in connection with any subsequent supplement to the
Purchase Agreement). Thereupon, SPS shall pay all of the Current Purchase Price
Payment to Grantor in accordance with the terms and procedures of the Purchase
Agreement.

     Section 2.3. Time and Place of Closing. The closing for the consummation of
the transactions contemplated herein shall take place on July 9, 2001 or on such
other date as may be agreed to by Grantor and Grantee (herein called the
"Current Closing Date").

                   ARTICLE III - Representations and Covenants

     Section 3.1. Representations and Warranties of Grantor. To induce Grantee
to enter into the Current Supplement Documents and to pay the Current Purchase
Price Payment, Grantor hereby represents and warrants and covenants to Grantee
that:

     (a) All representations and warranties made by Grantor or any other
TransTexas Company in any Production Payment Document now or previously
delivered are true and correct as of the Current Closing Date (unless such
representations and warranties are expressly limited to an earlier date, in
which case such representations and warranties are true and correct as of such
earlier date), provided that the representations and warranties confirmed in the
Omnibus Certificates heretofore delivered are true and correct as such
Certificates are amended and updated through the date hereof. Without limitation
of the foregoing, no Senior Notes Event of Default exists.

     (b) Each TransTexas Company has performed all agreements, covenants, and
conditions which it is required by any Production Payment Document to perform on
or prior to the Current Closing Date.

     (c) The consummation on the Current Closing Date of the transactions
contemplated in the Current Supplement Documents: (i) is not prohibited by any
law or any regulation or order of any court or governmental agency or authority
applicable to Grantor or any other TransTexas Company, and (ii) does not subject
any of them to any penalty or other onerous condition under or pursuant to any
such law, regulation or order.

     (d) Grantor shall use the Current Purchase Price Payment solely to pay
accounts payable for Subject Wells.

     Section 3.2. Covenants. To induce Grantee to enter into the Current
Supplement Documents and to pay the Current Purchase Price Payment, Grantor
hereby covenants with Grantee that Grantor will perform all of its covenants and
duties under the Production Payment Documents, all as fully as if they were set
out in full herein.

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     Section 3.3. Representations and Warranties of Grantee. Each Person
included within Grantee hereby represents and warrants to Grantor that: (a) such
Person has incurred no obligation or liability, contingent or otherwise, for
broker's or finder's fees in respect of any of the matters provided for in this
Supplement for which fees Grantor might be liable; (b) the Purchase Agreement
constitutes the legal, valid and binding act and obligation of such Person,
enforceable against such Person in accordance with its terms except as such
enforcement may be limited by bankruptcy, insolvency, moratorium and other
similar laws applicable to creditors' rights generally or by general principles
of equity; (c) no bankruptcy or insolvency proceeding is presently pending (or,
to such Person's best knowledge, threatened) by or against such Person under any
applicable bankruptcy, insolvency or other similar law of any jurisdiction; (d)
such Person has not made a general assignment for the benefit of creditors; (e)
such Person is acquiring the Production Payment for its own account and not with
any intention to transfer all or any part of the Production Payment to others in
violation of the Securities Act of 1933, as amended, or any other applicable
securities laws, and (f) such Person is a "United States person", within the
meaning of Section 7701 of the Internal Revenue Code of 1986, as amended. Fund
VI further represents and warrants that in connection with the transactions
contemplated herein and in the Purchase Agreement (i) it is represented by Funds
Agent, an investment manager that qualifies as a "qualified professional asset
manager" as defined in Department of Labor Prohibited Transaction Exemption
84-14 (the "QPAM Exemption") and (ii) each of the conditions of the QPAM
Exemption are satisfied and will, throughout the term of the Purchase Agreement
(as hereby supplemented and amended), be satisfied.

                           ARTICLE IV - Miscellaneous

     Section 4.1. Ratification of Production Payment Documents. The Original
Purchase Agreement as hereby supplemented and amended is hereby ratified and
confirmed in all respects. The other Production Payments Documents, as they may
be supplemented, amended or affected by the Current Supplement Documents, are
hereby ratified and confirmed in all respects. Any reference to the Purchase
Agreement in any Production Payment Document shall be deemed to refer to this
Supplement also, and any reference in any Production Payment Document to any
other document or instrument amended, renewed, extended or otherwise affected by
any Current Supplement Document shall also refer to such Current Supplement
Document. The execution, delivery and effectiveness of this Supplement and the
other Current Supplement Documents shall not, except as expressly provided
herein or therein, operate as a waiver of any right, power or remedy of Grantee
or Funds Agent under the Purchase Agreement or any other Production Payment
Document nor constitute a waiver of any provision of the Purchase Agreement or
any other Production Payment Document.

     Section 4.2. Survival of Agreements. All representations, warranties,
covenants and agreements of Grantor herein shall survive the execution and
delivery of this Supplement and the other Current Supplement Documents and shall
further survive until terminated in accordance with the Purchase Agreement.

     Section 4.3. Production Payment Documents. This Supplement and the other
Current Supplement Documents are Production Payment Documents, and all
provisions in the Purchase

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Agreement pertaining to Production Payment Documents (including the arbitration
provisions of Section 6.9 of the Purchase Agreement) apply hereto and thereto.

     THIS WRITTEN AGREEMENT AND THE OTHER PRODUCTION PAYMENT 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.

     Section 4.4. Governing Law. This Supplement shall be deemed a contract and
instrument made under the laws of the State of Texas and shall be construed and
enforced in accordance with and governed by the laws of the State of Texas and
the laws of the United States of America, without regard to principles of
conflicts of law.

     Section 4.5. Counterparts. This Supplement may be separately executed in
counterparts and by the different parties hereto in separate counterparts, each
of which when so executed shall be deemed to constitute one and the same
Supplement.

            [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK]

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     IN WITNESS WHEREOF, this Supplement is executed as of the date first
written above.

                                       TRANSTEXAS GAS CORPORATION

                                       By:
                                           -------------------------------------
                                           Ward Simon
                                           Vice President and Treasurer


                                       MIRANT AMERICAS ENERGY CAPITAL, LP

                                       BY: Mirant Americas Development, Inc.,
                                           its general partner

                                           By:
                                               ---------------------------------
                                               John A. Rigas
                                               Attorney-in-Fact


                                       TCW DR VI INVESTMENT PARTNERSHIP, L.P.

                                       By: TCW ASSET MANAGEMENT COMPANY, as
                                           their Agent

                                           By:
                                               ---------------------------------
                                               Kurt A. Talbot
                                               Senior Vice President

                                       TCW ASSET MANAGEMENT COMPANY, as Agent

                                       By:
                                           -------------------------------------
                                           Kurt A. Talbot
                                           Senior Vice President

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                                 ACKNOWLEDGMENT

     To induce Grantee to enter into the above Supplement, the undersigned
Galveston Bay Pipeline Company and Galveston Bay Processing Company each hereby
ratify and confirm their various agreements made in connection with the
Production Payment Documents referred to above and acknowledge and agree that
such agreements remain in full force and effect after taking into account the
Current Supplement Documents referred to above.

EXECUTED as of July 9, 2001.

GALVESTON BAY PIPELINE COMPANY

By:
    ---------------------------------
    Simon Ward
    Vice President and Treasurer


GALVESTON BAY PROCESSING COMPANY

By:
    ---------------------------------
    Simon Ward
    Vice President and Treasurer

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                                    EXHIBIT A

                         FIFTH SUPPLEMENT TO PRODUCTION
                               PAYMENT CONVEYANCE