Document:

<PAGE>   1
                                                                    EXHIBIT 10.6

                    SEVENTH SUPPLEMENT TO PURCHASE AGREEMENT

     THIS SEVENTH SUPPLEMENT TO PURCHASE AGREEMENT dated as of September 10,
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,

     o    TCW Global Project Fund Ltd. ("GPF"), acting through its agent, TCW
          London International, Limited (SPS, Fund VI, and GPF are herein
          collectively called, "Grantee"), and

     o    TCW London International, Limited ("TCW London"), as Agent on behalf
          of GPF, and TCW Asset Management Company ("Tamco"), as Agent on behalf
          of Fund VI (TCW London and Tamco are in such capacities herein
          collectively called "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. Effective as of September 10, 2001, SPS assigned to GPF an undivided
18.54493% interest in the Production Payment pursuant to that certain Partial
Conveyance of Production Payment and Seventh Supplement to Production Payment
Conveyance dated as of September 10, 2001 (the "GPF Assignment").

<PAGE>   2

     NOW, THEREFORE, in consideration of the premises and the mutual covenants
and agreements contained herein, Grantor, SPS, Fund VI, GPF, 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", "GPF", "Grantee", "Tamco", "TCW London", 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 2.6.

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

     "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

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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 - Amendments

     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.

     Section 2.2. Grantee. In order to take into account the GPF Assignment, (a)
all references to "Grantee" in the Original Purchase Agreement, as supplemented
hereby, shall refer to SPS, Fund VI, and GPF with respect to matters occurring
after the "Effective Time", as defined in the Current Conveyance Supplement, and
(b) all references to "Fund V" in the Original Purchase Agreement, as
supplemented hereby, shall refer to GPF with respect to matters occurring after
the "Effective Time", as defined in the Current Conveyance Supplement.

     Section 2.3. Funds Agent. Subsections (a), (b), (h) and (i) of Section 6.10
of the Original Purchase Agreement are hereby amended and restated in their
entirety to read as follows:

          (a) Funds Agent. Fund VI, for itself and for its successors and
     assigns as owner of the Production Payment, has appointed Tamco as its
     agent to act for and on behalf of Fund VI under and pursuant to this
     Agreement and the other Production Payment Documents, and Tamco has
     accepted such appointment. GPF has, by means of separate agreements,
     appointed TCW London International, Limited ("TCW London") as its agent to
     act for and on behalf of GPF under and pursuant to this Agreement and the
     other Production Payment Documents, and TCW London has accepted such
     appointment. Accordingly, as used herein, "Funds Agent" refers to Tamco,
     acting on behalf of Fund VI, and to TCW London, acting on behalf of GPF,
     provided that in subsections (b), (c), (d), (e) and (f) of this Section
     6.10, "Funds Agent" refers only to Tamco acting on behalf of Fund VI. Funds
     Agent is authorized to act on behalf of GPF and Fund VI in (i) exercising
     rights and remedies with respect to any matter under any of the Production
     Payment Documents, (ii) giving notices or instructions to Grantor or SPS,
     (iii) receiving information from or notices by Grantor or SPS, (iv)
     communicating to Grantor or SPS determinations required or permitted to be
     made under this Agreement or any other Production Payment Document, and (v)
     agreeing to, and executing and delivering, all Conveyance Supplements and
     Purchase Agreement Supplements, the Production Sales Agreements and any
     other agreements for the sale of PP Hydrocarbons, all other Production
     Payment Documents, and all amendments, supplements, waivers or consents to,
     of or under any Production Payment Documents. Funds Agent may, on behalf of
     GPF and Fund VI, take any other action which GPF or Fund VI is entitled to
     take hereunder or under any of the Production Payment Documents. Grantor
     and SPS may rely on any action of Funds Agent as binding upon GPF and Fund
     VI. Such appointment of Tamco as Funds Agent shall not, however, impair or
     modify any rights, obligations or duties which Tamco or any of its
     Affiliates otherwise has with respect to Fund VI. In its administration of
     this Agreement and the other Production Payment Documents, except to the
     extent to which another standard applies to Tamco by reason of any Fund
     Governing

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     Document or other document or relationship between Tamco and any Person
     making up Fund VI, Tamco will exercise the same care that it exercises in
     the administration or handling of transactions for its own account.

          (b) Definitions. As used in this Section 6.10:

               "Fund Governing Documents" means all documents and instruments
          (other than the Production Payment Documents) under which Tamco and
          its Affiliates have undertaken to act for any of the TCW
          Beneficiaries.

               "Holders" means Fund VI and each of its successors or assigns at
          any time owning an interest in the Production Payment.

               "Requisite Holders" means, at any time, Fund VI or its successors
          and assigns owning at least two-thirds of the total Percentage Shares
          in the Production Payment previously held by Fund VI.

          (h) Reliance by Grantor and SPS. GPF and Fund VI and each other Holder
     agree that, prior to the delivery to Grantor or SPS of a notice of the
     removal or termination of Tamco or TCW London (or any subsequent Funds
     Agent) as Funds Agent as set forth below, Grantor or SPS, as the case may
     be, shall: (i) be entitled to rely on Tamco's (or any successor Funds
     Agent's) authority to act on behalf of Fund VI and on TCW London's (or any
     subsequent Funds Agent's) authority to act on behalf of GPF in all dealings
     with Tamco or TCW London (or any such subsequent Funds Agent) with respect
     to the Production Payment Documents; (ii) be protected in relying on
     actions, communications, notices and terminations relating thereto or
     required or permitted thereunder by Funds Agent; and (iii) discharge its
     obligations under this Agreement and the Production Payment Documents by
     delivering payments, notices and other information to Funds Agent or to
     such bank accounts and addresses as Funds Agent may from time to time
     specify. In the event of the removal of Funds Agent and the appointment of
     a successor Funds Agent, neither Grantor nor SPS shall be required to
     recognize any such removal or appointment unless and until it shall have
     received a writing setting forth such removal and appointment executed by
     the Person or Person's from whom the removed Funds Agent was acting, as
     well as the acceptance of such appointment by such successor Funds Agent,
     and Grantor and SPS shall be entitled to rely on such writing as being
     genuine and what it purports to be without any necessity of any
     investigation whatsoever.

          (i) ACKNOWLEDGMENT, WAIVER AND RELEASE BY GRANTOR AND SPS. GRANTOR AND
     SPS HAVE BEEN INFORMED, AND HEREBY ACKNOWLEDGE AND AGREE, THAT ALL ACTS BY
     TAMCO AND TCW LONDON AS FUNDS AGENT IN CONNECTION WITH THE PRODUCTION
     PAYMENT DOCUMENTS ARE DONE ON BEHALF OF GPF AND FUND VI AND THE TCW
     BENEFICIARIES, AND THAT TAMCO AND TCW LONDON AND THEIR AFFILIATES
     (EXCLUDING GPF AND FUND VI)

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     AND ITS AND THEIR SHAREHOLDERS, REPRESENTATIVES, TRUSTEES, AGENTS,
     EMPLOYEES, OFFICERS, DIRECTORS, AND ATTORNEYS (COLLECTIVELY, THE "TCW
     ENTITIES"), EXCLUDING GPF AND FUND VI, SHALL NOT BE PERSONALLY LIABLE TO
     ANY PERSON (OTHER THAN THE TCW BENEFICIARIES) WITH RESPECT TO ANY ACTIONS
     TAKEN (OR NOT TAKEN) BY TAMCO OR TCW LONDON IN THEIR CAPACITIES AS FUNDS
     AGENT UNDER THIS AGREEMENT AND THE OTHER PRODUCTION PAYMENT DOCUMENTS. IN
     FURTHERANCE OF THE FOREGOING, EACH OF GRANTOR AND SPS HEREBY WAIVES AND
     RELEASES (FOR ITSELF AND ON BEHALF OF ITS SHAREHOLDERS, AFFILIATES,
     REPRESENTATIVES, TRUSTEES, AGENTS, EMPLOYEES, OFFICERS, DIRECTORS, AND
     ATTORNEYS) THE TCW ENTITIES FROM ANY AND ALL SUCH LIABILITIES. THIS
     SUBSECTION (I) IN NO WAY (A) RELEASES OR DIMINISHES ANY LIABILITY THAT GPF
     AND FUND VI MAY HAVE FOR ANY ACTION TAKEN (OR NOT TAKEN) BY TAMCO OR TCW
     LONDON AS FUNDS AGENT OR (B) REDUCES OR DIMINISHES THE BENEFIT TO GPF AND
     FUND VI OF ANY WAIVERS, RELEASES AND INDEMNITIES IN OTHER PROVISIONS OF THE
     PRODUCTION PAYMENT DOCUMENTS THAT OTHERWISE BENEFIT GPF AND FUND VI."

     Section 4. Notices. All notices, requests, consents, demands and other
communications which are required or permitted under any Production Payment
Document shall be given to GPF pursuant to Section 6.3 of the Original Purchase
Agreement at its address specified below:

                          TCW Global Project Fund Ltd.
                          c/o Conyers, Dill & Pearman
                          Clarendon House
                          Church Street
                          Hamilton, Bermuda
                          Attention: Kevin Butler
                          Telephone: 441/299-4993
                          Telecopy: 441/292-4720

       with a copy to:    TCW London International, Limited
                          865 South Figueroa
                          Los Angeles, California 90017
                          Attention: Arthur Carlson
                          Telephone: 213/244-0053
                          Telecopy: 213/244-0604

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     Section 2.5. Closing. On the Current Closing Date, Grantor shall deliver
the Current Conveyance Supplement to Grantee, to take effect as and when
provided therein.

     Section 2.6. Time and Place of Closing. The closing for the consummation of
the transactions contemplated herein shall take place on September 10, 2001 or
on such other date as may be agreed to by Grantor and Grantee (herein called the
"Current Closing Date").

     Section 2.7. Winnie Guaranty. Grantor hereby confirms that GPF, as a
transferee and assign of SPS, is a "Guaranteed Party", as such term is defined
in the Winnie Guaranty, and the Winnie Guaranty shall apply to and inure to the
benefit of GPF.

                   ARTICLE III - Representations and Covenants

     Section 3.1. Representations and Warranties of Grantor. To induce Grantee
to enter into the Current Supplement Documents, 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.

     Section 3.2. Covenants. To induce Grantee to enter into the Current
Supplement Documents, 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.

     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

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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; and
(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. Fund VI further represents and warrants that in
connection with the transactions contemplated herein and in the Purchase
Agreement (i) it is represented by Tamco, 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 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.

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     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:
                                           -------------------------------------
                                           Simon Ward
                                           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 its
                                           Agent

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

                                       TCW GLOBAL PROJECT FUND LTD.

                                       By: TCW LONDON INTERNATIONAL, LIMITED, as
                                           Investment Advisor

                                           By:
                                               ---------------------------------
                                               Randall S. Wade
                                               Vice President

                                           By:
                                               ---------------------------------
                                               George R. Hutchinson
                                               Managing Director

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                                       TCW ASSET MANAGEMENT COMPANY, as Agent

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

                                       10
<PAGE>   11

                                 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 the date of the above supplement.

GALVESTON BAY PIPELINE COMPANY

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

GALVESTON BAY PROCESSING COMPANY

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

                                       11

<PAGE>   12

                                    EXHIBIT A

                    PARTIAL CONVEYANCE OF PRODUCTION PAYMENT
                                       AND
               SEVENTH SUPPLEMENT TO PRODUCTION PAYMENT CONVEYANCE<PAGE>   1
                                                                    EXHIBIT 10.7

                    PARTIAL CONVEYANCE OF PRODUCTION PAYMENT
                                       AND
               SEVENTH SUPPLEMENT TO PRODUCTION PAYMENT CONVEYANCE

     THIS PARTIAL CONVEYANCE OF PRODUCTION PAYMENT AND SEVENTH SUPPLEMENT TO
PRODUCTION PAYMENT CONVEYANCE (this "Supplement"), dated as of the date set out
at the end hereof, is made by TransTexas Gas Corporation, a Delaware corporation
("Grantor"), Mirant Americas Energy Capital, LP, formerly named Southern
Producer Services, L.P. ("SPS"), TCW DR VI Investment Partnership, L.P. ("Fund
VI"), and TCW Global Project Fund Ltd. ("GPF"). SPS, Fund VI, and GPF are herein
collectively called "Grantee".

                                    RECITALS:

A.   Effective as of March 1, 2000, Grantor executed in favor of TCW Portfolio
     No. 1555 DR V Sub-Custody Partnership, L.P. ("Fund V"), Fund VI, and SPS
     that certain Production Payment Conveyance dated as of March 14, 2000,
     which has been supplemented and amended by various supplements given prior
     to September 10, 2001 (as so supplemented and amended, the "Original
     Conveyance"). The Original Conveyance, including all such supplements, has
     been recorded as described on Schedule 1 to the Sixth Supplement to
     Production Payment Conveyance that is referred to below and attached
     hereto.

B.   Effective as of December 1, 2000, Fund V assigned all of its right, title,
     and interest under the PP Conveyance to SPS pursuant to a Conveyance of
     Interest in Production Payment.

C.   Effective as of the date hereof, Grantor, Fund VI, and SPS executed and
     delivered that certain Sixth Supplement to Production Payment Conveyance
     (the "Sixth Supplement"), a copy of which is attached hereto as Exhibit A
     and made a part hereof for all purposes. The Original Conveyance, as
     supplemented and amended by the Sixth Supplement and by this Supplement, is
     herein called the "PP Conveyance". By means of the Original Conveyance and
     the Sixth Supplement, a production payment (the "Production Payment") is
     now owned by Fund VI and SPS in undivided interests.

D.   SPS desires to convey to GPF an undivided 18.54493% interest in the
     Production Payment, upon and subject to the provisions hereof, in order
     that SPS, Fund VI, and GPF shall own the Production Payment in the
     percentages described below.

                                   CONVEYANCE:

     1. For a good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, SPS does hereby convey to GPF, 18.54493% of the
entire 8/8ths interest in the Production Payment, such conveyance to be
effective as of the Effective Time, as hereafter defined, but subject to the
reservations and exceptions set out below (the "GPF Assignment").

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<PAGE>   2

     2. As a result of this conveyance, and taking into account the percentage
interest in the Production Payment previously owned by SPS and Fund VI, SPS,
Fund VI, and GPF now own the following undivided percentage interests in the
Production Payment, and their "Percentage Shares", as defined in the PP
Conveyance, shall be deemed to be as follows:

            Fund VI  31.64025%
            SPS      49.81482%
            GPF      18.54493%

     3. SPS does hereby further assign to GPF such undivided interests in SPS's
accounts receivable from the sale of PP Hydrocarbons (as defined in the Original
Conveyance) - to the extent such accounts receivable exist and are unpaid at the
Effective Time and arise from the sale of PP Hydrocarbons before the Effective
Time - as are necessary in order to cause SPS and GPF to share in all
collections of such accounts receivable after the Effective Time in proportion
to their respective amended Percentage Shares as set out above.

     4. SPS expressly reserves to itself, and is not assigning to GPF, all of
its rights with respect to the Surviving Duties. As used herein, "Surviving
Duties" means (a) all indemnification obligations, reimbursement obligations and
other obligations of Grantor that are, as provided in the PP Conveyance (and/or
in any related documents or instruments), to survive any termination of the PP
Conveyance, and (b) without limitation of the foregoing, (i) the indemnification
obligations of Grantor under Section 4.1 of the PP Conveyance, (ii) Grantor's
obligations to pay all severance taxes in connection with all PP Hydrocarbons at
any time purchased by Grantor, and (iii) Grantor's obligations to pay all
royalties in connection with all PP Hydrocarbons at any time produced.

     5. Grantor hereby ratifies and confirms all of the Surviving Duties owed to
SPS and hereby agrees to pay and perform the same. Grantor further confirms that
it independently owes the Surviving Duties to Fund VI, as well as SPS, due to
Fund VI being a beneficiary of the PP Conveyance prior to this Conveyance from
SPS to Fund VI, and Grantor hereby ratifies and confirms the Surviving Duties
owed to Fund VI and agrees to pay and perform the same.

     6. SPS hereby represents and warrants to GPF that the interests assigned
herein have not been conveyed to any other person. Except for the foregoing
sentence, herein, this Partial Conveyance of Production Payment is given without
representation or warranty of any kind, express, implied or otherwise.

                           SUPPLEMENTS AND AGREEMENTS:

     7. In order to take into account the GPF Assignment, (a) all references to
"Grantee" in the Original Conveyance, as supplemented hereby, shall refer to
SPS, Fund VI, and GPF with respect to matters occurring after the date hereof
and (b) all references to "Fund V" in the Original Conveyance, as supplemented
hereby, shall refer to GPF with respect to matters occurring after the date
hereof.

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<PAGE>   3

     8. The definition of "GPF", "Tamco", and "TCW London" are hereby added to
Section 1.1 of the Original Conveyance in appropriate alphabetical order to read
as follows:

          " 'GPF' means TCW Global Project Fund Ltd., a Bermuda-domiciled
     special purpose company, and, unless the context in which used shall
     otherwise require, such term shall also include any successor to it as
     owner at the time in question of any or all of the Production Payment."

          " 'Tamco' means TCW Asset Management Company, a California
     corporation."

          " 'TCW London' means TCW London International, Limited, a California
     corporation."

     9. The definitions of "Affiliate", "Funds Agent", and "TCW Beneficiary" in
Section 1.1 of the Original Conveyance are hereby amended in their entirety to
read as follows:

          " 'Affiliate' means, with respect to any Person: (a) any other Person
     directly or indirectly owning, controlling or holding with power to vote
     10% or more of the outstanding voting securities of such Person, (b) any
     other Person 10% or more of whose outstanding voting securities are
     directly or indirectly owned, controlled or held with power to vote by such
     Person, and (c) any other Person directly or indirectly controlling,
     controlled by or under common control with such Person; provided that,
     "Affiliate" also means, with respect to GPF, Fund V and Fund VI, (i) any of
     the TCW Beneficiaries, (ii) any trustee, general partner, investment
     manager, custodian, custodial agent, or other fiduciary of or for GPF, Fund
     V, Fund VI, or any TCW Beneficiary, (iii) Trust Company of the West, a
     California trust company, and (iv) Tamco and TCW London, in each case
     whether acting as Funds Agent or in any other capacity."

          " 'Funds Agent' means the Persons named as Funds Agent in Section 8.7,
     acting in such capacity, together with their successors and assigns in such
     capacity."

          " 'TCW Beneficiary' means (a) any Person that at any time is a general
     or limited partner in Fund V or in Fund VI or any Person for whom such a
     general or limited partner is acting as investment manager, custodian or
     custodial agent or in a similar capacity and (b) any Person that is a
     shareholder in GPF."

     10. Section 6.2 of the Original Conveyance is amended in its entirety to
read as follows:

          "Section 6.2. Assignments by Grantee. Grantee's interest in the
     Production Payment may not be transferred except in compliance with this
     section. Any Grantee and any Permitted Assign (as hereinafter defined)
     shall have the right to sell, assign, transfer or convey its interest in
     the Production Payment, in whole or in part (and either absolutely or by
     mortgage or other security instrument), at any time; provided that:

               (a) no change of ownership or right to receive payment of the
          Production Payment or of any part thereof, however accomplished, shall
          be effective or

                                       3
<PAGE>   4

          binding until notice thereof, including the Percentage Share and
          address of the transferee, shall have been registered with Grantor by
          the transferor and by the transferee (which transferee must make to
          Grantor the representations and warranties in Section 4.2 of the
          Purchase Agreement), and then only with respect to production
          occurring after receipt of such notice, and

               (b) neither Grantee nor any Permitted Assign shall assign or
          convey any rights under or any interest in the Production Payment
          Documents or the Production Payment to any Restricted Assignee.

     Any Person to whom all or any interest in the Production Payment is
     assigned or conveyed in accordance with the foregoing requirements is
     herein called a "Permitted Assign". Grantor shall keep records of all
     Permitted Assigns, their Percentage Shares, and their addresses, and shall
     give notice thereof to the other Persons, if any, from time to time holding
     the interests of Grantee hereunder."

     11. Section 8.7 of the Original Conveyance is amended in its entirety to
read as follows:

          "Section 8.7. Funds Agent; Consents, Waivers, Supplements and
     Amendments.

          (a) Fund VI has, pursuant to the Purchase Agreement, appointed Tamco
     as its agent to administer the Production Payment and to act on its behalf
     (as a Person included in Grantee) in giving any consents or waivers under
     this Conveyance, in making any amendments or supplements hereto, and in
     arranging for the marketing of the PP Hydrocarbons. GPF has, pursuant to
     other agreements, appointed TCW London as its agent to administer the
     Production Payment and to act on its behalf (as a Person included in
     Grantee) in giving any consents or waivers under this Conveyance, in making
     any amendments or supplements hereto, and in arranging for the marketing of
     the PP Hydrocarbons. Accordingly, as used herein, "Funds Agent" refers
     collectively to Tamco, acting on behalf of Fund VI, and to TCW London,
     acting on behalf of GPF. Unless and until this Conveyance is supplemented
     to reflect the termination of such agency or the appointment of a
     replacement or successor Funds Agent (which supplement may be executed by
     Fund VI and GPF without the joinder of Grantor or SPS), all Persons dealing
     with Grantor or Grantee in connection with the Production Payment shall be
     entitled to rely upon the authority of (i) Tamco to act as Funds Agent for
     Fund VI in connection herewith and (ii) TCW London to act as Funds Agent
     for GPF in connection herewith.

          (b) No consent, waiver, supplement or amendment given by Grantee in
     connection with this Conveyance or the Production Payment shall be valid or
     effective unless given be in writing and signed by each Person included in
     Grantee (provided that Tamco may sign the same on behalf of Fund VI and TCW
     London may sign the same on behalf of GPF but neither may sign on behalf of
     SPS)."

     12. Grantor and Grantee acknowledge to each other that concurrently
herewith SPS, GPF Fund V, and Fund VI are entering into an agreement which
supplements the Production Sales

                                       4
<PAGE>   5

Agreements to provide that SPS, as Marketer, will be the purchaser of the PP
Hydrocarbons belonging to GPF.

     13. All of the terms and provisions of the Original Conveyance, as the same
is amended and supplemented hereby, are ratified, adopted, affirmed and renewed,
and remain in full force and effect for the benefit of Grantee, the
Beneficiaries, Funds Agent, and their respective successors and assigns.

     14. This Supplement may be executed in multiple counterparts, all of which
are identical.

     15. This Supplement shall be binding upon and shall inure to the benefit of
the parties hereto, and their respective successors and assigns, and all of the
covenants and agreements contained in the Original Conveyance, as amended
hereby, shall be deemed to be covenants and agreements running with the lands
affected thereby.

     16. This Supplement shall be governed by and construed in accordance with
the laws of the State of Texas, without regard to principles of conflicts of
laws.

                                       5
<PAGE>   6

     IN WITNESS WHEREOF, this instrument is executed by the parties hereto on
the dates of their respective acknowledgments, effective as to deliveries of
oil, gas and other hydrocarbons as of 9:01 a.m., Texas time, on September 10,
2001 (the "Effective Time").

                                       TRANSTEXAS GAS CORPORATION

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

Grantor's address:                     1300 North Sam Houston Parkway East
                                       Suite 310
                                       Houston, Texas 77032-2949
                                       Attention: Ed Donahue, Vice President
                                       Telephone: 281/987-8600
                                       Telecopy: 281/986-8865

                                       MIRANT AMERICAS ENERGY CAPITAL, LP

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

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

SPS's address:                         1200 Smith Street
                                       Suite 2890
                                       Houston, Texas 77002
                                       Attention: David W. Stewart
                                       Telephone: 713/276-1902
                                       Telecopy: 713/276-1990

                                       6
<PAGE>   7

                                       TCW DR VI INVESTMENT PARTNERSHIP, L.P.

                                       By: TCW ASSET MANAGEMENT COMPANY, as
                                           Agent

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

Fund VI's address:                     c/o Trust Company of the West
                                       865 South Figueroa
                                       Los Angeles, California  90017
                                       Attention: Thomas F. Mehlberg
                                       Telephone: 213/244-0702
                                       Telecopy: 213/244-0604

                                       TCW GLOBAL PROJECT FUND LTD.

                                       By: TCW LONDON INTERNATIONAL, LIMITED, as
                                           Investment Advisor

                                           By:
                                               ---------------------------------
                                               Randall S. Wade
                                               Vice President

                                               By:
                                                   -----------------------------
                                                   George R. Hutchinson
                                                   Managing Director

GPF's address:                         TCW Global Project Fund Ltd.
                                       c/o Conyers, Dill & Pearman
                                       Clarendon House
                                       Church Street
                                       Hamilton, Bermuda
                                       Attention: Kevin Butler
                                       Telephone: 441/299-4993
                                       Telecopy: 441/292-4720

                                       7
<PAGE>   8

                  with a copy to:      TCW London International, Limited
                                       865 South Figueroa
                                       Los Angeles, California  90017
                                       Attention: Arthur Carlson
                                       Telephone: 213/244-0053
                                       Telecopy: 213/244-0604

This document prepared by:

John W. Rain
Thompson & Knight L.L.P.
1700 Pacific Avenue, Suite 3300
Dallas, Texas 75201

                                       8
<PAGE>   9

STATE OF TEXAS                      )
                                    )
COUNTY OF HARRIS                    )

     The foregoing instrument was acknowledged before me on this 10th day of
September, 2001, by Simon Ward, the Vice President and Treasurer of TransTexas
Gas Corporation, a Delaware corporation, on behalf of such corporation.

                                       -----------------------------------------
        [SEAL]                         Notary Public, State of Texas

STATE OF TEXAS                      )
                                    )
COUNTY OF HARRIS                    )

     The foregoing instrument was acknowledged before me on this 10th day of
September, 2001, by John A. Rigas, the Attorney-in-Fact of Mirant Americas
Development, Inc., a Georgia corporation, on behalf of such corporation acting
as general partner of Mirant Americas Energy Capital, LP, a Delaware limited
partnership, and on behalf of such limited partnership.

                                       -----------------------------------------
        [SEAL]                         Notary Public, State of Texas

STATE OF TEXAS                      )
                                    )
COUNTY OF HARRIS                    )

         The foregoing instrument was acknowledged before me on this 10th day of
September, 2001, by Kurt A. Talbot, the Senior Vice President of TCW Asset
Management Company, a California corporation, on behalf of such corporation
acting as agent as aforesaid.

                                       -----------------------------------------
        [SEAL]                         Notary Public, State of Texas

                                       9
<PAGE>   10

STATE OF TEXAS                    )
                                  )
COUNTY OF HARRIS                  )

         The foregoing instrument was acknowledged before me on this 10th day of
September, 2001, by George R. Hutchinson, a Managing Director of TCW London
International, Limited, a California corporation, on behalf of such corporation
acting as investment advisor as aforesaid.

                                       -----------------------------------------
        [SEAL]                         Notary Public, State of Texas

STATE OF TEXAS                    )
                                  )
COUNTY OF HARRIS                  )

         The foregoing instrument was acknowledged before me on this 10th day of
September, 2001, by Randall S. Wade, a Vice President of TCW London
International, Limited, a California corporation, on behalf of such corporation
acting as investment advisor as aforesaid.

                                       -----------------------------------------
        [SEAL]                         Notary Public, State of Texas

                                       10
<PAGE>   11

                                    EXHIBIT A

                         SIXTH SUPPLEMENT TO PRODUCTION
                               PAYMENT CONVEYANCE

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