Document:

<PAGE>   1
                                                                    EXHIBIT 10.4

                     SIXTH SUPPLEMENT TO PURCHASE AGREEMENT

     THIS SIXTH 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 (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", "Grantee" and "Funds Agent" have the meanings given them above.
Reference is also made to<PAGE>   1
                                                                    EXHIBIT 10.5

                         SIXTH SUPPLEMENT TO PRODUCTION
                               PAYMENT CONVEYANCE

     THIS SIXTH 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"), and TCW DR VI
Investment Partnership, L.P. ("Fund VI"). SPS and Fund VI are herein
collectively called "Grantee".

                                    RECITALS:

A.   Effective as of March 1, 2000, Grantor executed in favor of Grantee that
     certain Production Payment Conveyance dated as of March 14, 2000 (as
     heretofore amended or supplemented, the "Original Conveyance"). The
     Original Conveyance has been recorded as set forth in Schedule 1 hereto;
     all capitalized terms used but not defined herein shall have the meanings
     assigned to them in the Original Conveyance.

B.   Pursuant to the Original Conveyance, TCW Portfolio No. 1555 DR V
     Sub-Custody Partnership, L.P. ("Fund V") was originally included within
     "Grantee". 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.

C.   Effective as of July 9, 2001, SPS conveyed to Fund VI a 12.1065% undivided
     interest in and to the Production Payment, resulting in Fund VI now owing
     an aggregate 43.83362% interest in the Production Payment and SPS now
     owning a 56.16638% interest in the Production Payment (in each case prior
     to giving effect to this supplement)

D.   Grantee and Grantor desire to supplement and amend the Original Conveyance,
     as set forth herein, to account for the payment by SPS of additional funds
     to Grantor.

E.   As described in Section 8.7 of the Original Conveyance, and pursuant to the
     Purchase Agreement referred to in the Original Conveyance, Fund VI has
     appointed TCW Asset Management Company to act as its agent in connection
     with supplements and amendments to the Original Conveyance.

                           SUPPLEMENTS AND AGREEMENTS:

     FOR A GOOD AND VALUABLE CONSIDERATION, the receipt and sufficiency of which
are hereby acknowledged, Grantor and Grantee do hereby agree, act and covenant
as follows:

     1. Effective as of 9:00 a.m. Houston, Texas time, on September 10, 2001
(the "Effective Time"), the Original Conveyance is amended in order to add the
amount of Fifteen Million

                                       1
<PAGE>   2

Dollars ($15,000,000) to the unliquidated balance of the Primary Sum, as such
unliquidated balance stood as of the Effective Time after giving effect to all
applications of PP Proceeds made before the Effective Time. After giving effect
to such amendment (and to such application of PP Proceeds), the unliquidated
balance of the Primary Sum as of the Effective Time is $53,923,096. (Any PP
Proceeds received after the Effective Time on September 10, 2001 shall be deemed
to have been received on the next following Business Day.)

     2. 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.

     3. The definition of "Dedication Percentage" in Section 1.1 of the Original
Conveyance is hereby amended in its entirety to read as follows:

     " 'Dedication Percentage' means:

          (a) fifty percent (50%) from the Initial Time until 9:00 a.m., Texas
     time, on September 1, 2000;

          (b) sixty-two percent (62%) from 9:00 a.m., Texas time, on September
     1, 2000 until 9:00 a.m., Texas time, on December 1, 2000;

          (c) thirty percent (30%) from and after 9:00 a.m., Texas time, on
     December 1, 2000 until 9:00 a.m., Texas time, on August 1, 2001;

          (d) fifty percent (50%) from and after 9:00 a.m., Texas time, on
     August 1, 2001 until 9:00 a.m., Texas time, on September 1, 2001;

          (e) ninety percent (90%) from and after 9:00 a.m., Texas time, on
     September 1, 2001 until 9:00 a.m., Texas time, on February 1, 2002;

          (f) eighty-five percent (85%) from and after 9:00 a.m., Texas time, on
     February 1, 2002 until 9:00 a.m., Texas time, on January 1, 2003;

          (g) eighty percent (80%) from and after 9:00 a.m., Texas time, on
     January 1, 2003 until 9:00 a.m., Texas time, on January 1, 2004; and

          (h) seventy percent (70%) from and after 9:00 a.m., Texas time, on
     January 1, 2004."

     4. The definition of "Percentage Share" in Section 1.1 of the Original
Conveyance is hereby amended in its entirety to read as follows:

                                       2
<PAGE>   3

          " 'Percentage Share' means, with respect to each Person included in
     Grantee, the fractional undivided interest which it owns in the Production
     Payment at the time in question. From the initial grant of the Production
     Payment until June 7, 2000, the Percentage Share of each Person included in
     Grantee was as follows:

          Fund V  42.859594%
          Fund VI 21.429797%
          SPS     35.710609%

     From and after June 7, 2000, until September 8, 2000, the Percentage Share
     of each Person included in Grantee was as follows:

          Fund V  33.048697%
          Fund VI 16.524349%
          SPS     50.426954%

     From and after September 8, 2000, until November 7, 2000, the Percentage
     Share of each Person included in Grantee was as follows:

          Fund V  22.863142%
          Fund VI 11.431571%
          SPS     65.705287%

     From and after November 7, 2000, until December 1, 2000, the Percentage
     Share of each Person included in Grantee was as follows:

          Fund V   19.08294%
          Fund VI   9.54147%
          SPS      71.37559%

     From and after December 1, 2000, until February 7, 2001, the Percentage
     Share of each Person included in Grantee was as follows:

          Fund VI   9.54147%
          SPS      90.45853%

     From and after February 7, 2001, until July 9, 2001, the Percentage Share
     of each Person included in Grantee was as follows:

          Fund VI  49.82233%
          SPS      51.17767%

     From and after July 9, 2001, until September 10, 2001, the Percentage Share
     of each Person included in Grantee was as follows:

          Fund VI  43.83362%

                                       3
<PAGE>   4

          SPS      56.16638%

     As of 9:00 a.m. Houston, Texas time, on September 10, 2001, and thereafter,
     the Percentage Share of each Person included in Grantee is as follows:

          Fund VI  31.64025%
          SPS      68.35975%"

The foregoing Percentage Shares of Fund VI and SPS, respectively, taking effect
as of September 10, 2001, are in this Amendment called their "amended Percentage
Shares".

     5. In consideration of the additional purchase price payment made by SPS to
Grantor and the foregoing increase in the unliquidated balance of the Primary
Sum:

          (a) Fund VI does hereby GRANT, BARGAIN, SELL, CONVEY, ASSIGN,
     TRANSFER, SET OVER and DELIVER unto SPS such additional undivided interests
     in the Production Payment as are necessary in order to cause SPS and Fund
     VI to own the Production Payment, at and after the Effective Time, in
     undivided interests in proportion to their respective amended Percentage
     Shares, and

          (b) Fund VI does further assign unto SPS such undivided interests in
     Fund VI's accounts receivable from the sale of PP Hydrocarbons - 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 Fund VI to share in all collections
     of such accounts receivable after the Effective Time in proportion to their
     respective amended Percentage Shares.

     TO HAVE AND TO HOLD the same, upon and subject to the terms of the Original
Conveyance, as amended hereby, unto SPS and its successors and Permitted
Assigns, until the Termination Time.

     6. 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.

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

     8. 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.

     9. 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.

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<PAGE>   5

     IN WITNESS WHEREOF, this Supplement is executed by the parties hereto on
the dates set out below in their respective acknowledgments, but effective as of
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

                                       5
<PAGE>   6

                                      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

This document prepared by:

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

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<PAGE>   7

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

                                       7
<PAGE>   8

                                                                      SCHEDULE 1

                               RECORDING SCHEDULE

1.   Production Payment Conveyance among TCW Portfolio No. 1555 DR V Sub-Custody
     Partnership, L.P., TCW DR VI Investment Partnership, L.P., Southern
     Producer Services, L.P. and TransTexas Gas Corporation.

<Table>
<Caption>
     Recording Jurisdiction                   Recording Data
     ----------------------                   --------------

<S>                                           <C>
     Calhoun County, TX                       Volume 262, Page 33,
                                              Filed 9/19/2000

     Chambers County, TX                      Document No. 00-448-604,
                                              Filed 3/17/2000

     Galveston County, TX                     Film Code #014-39-1028,
                                              Filed 3/17/2000

     Jim Hogg County, TX                      Volume 17, Page 362,
                                              Filed 3/20/2000

     Live Oak County, TX                      Volume 449, Page 135,
                                              Filed 3/20/2000

     Wharton County, TX                       Volume 360, Page 596,
                                              Filed 3/17/2000

     Zapata County, TX                        Volume 629, Page 471,
                                              Filed 3/17/2000

     General Land Office of Texas             Sent for filing
</Table>

2.   First Supplement to Production Payment Conveyance among TransTexas Gas
     Corporation, Southern Producer Services, L.P., TCW Portfolio No. 1555 DR V
     Sub-Custody Partnership, L.P. and TCW DR VI Investment Partnership, L.P.

                                       1
<PAGE>   9

<Table>
<Caption>
     Recording Jurisdiction                   Recording Data
     ----------------------                   --------------

<S>                                           <C>
     Calhoun County, TX                       Volume 262, Page 114,
                                              Filed 9/19/2000

     Chambers County, TX                      Document No. 00-459-630
                                              Filed 6/9/2000

     Galveston County, TX                     Film Code 014-62-1617
                                              Filed 6/9/2000

     Jim Hogg County, TX                      Volume 19, Page 526,
                                              Filed 6/9/2000

     Live Oak County, TX                      Volume 450, Page 428,
                                              Filed 6/12/2000

     Wharton County, TX                       Volume 371, Page 458,
                                              Filed 6/9/2000

     Zapata County, TX                        Volume 634, Page 110,
                                              Filed 6/9/2000
</Table>

3.   Second Supplement to Production Payment Conveyance among TransTexas Gas
     Corporation, Southern Producer Services, L.P., TCW Portfolio No. 1555 DR V
     Sub-Custody Partnership, L.P. and TCW DR VI Investment Partnership, L.P.

<Table>
<Caption>
     Recording Jurisdiction                   Recording Data
     ----------------------                   --------------

<S>                                           <C>
     Calhoun County, TX                       Volume 262, Page 132
                                              Filed 9/19/2000

     Chambers County, TX                      Document No. 00-470-562
                                              Filed 9/18/2000

     Galveston County, TX                     File Number 2000046945
                                              Filed 9/15/2000

     Jim Hogg County, TX                      File Number 65210
                                              Filed 9/22/2000

     Live Oak County, TX                      File Number 00149214
                                              Filed 9/19/2000

     Wharton County, TX                       Volume 383, Page 740,
                                              Filed 9/19/2000
</Table>

                                       2
<PAGE>   10

<Table>
<S>                                           <C>
     Zapata County, TX                        Volume 638, Page 642,
                                              Filed 9/22/2000
</Table>

4.   Third Supplement to Production Payment Conveyance among TransTexas Gas
     Corporation, Southern Producer Services, L.P., TCW Portfolio No. 1555 DR V
     Sub-Custody Partnership, L.P. and TCW DR VI Investment Partnership, L.P.

<Table>
<Caption>
     Recording Jurisdiction                   Recording Data
     ----------------------                   --------------

<S>                                           <C>
     Calhoun County, TX                       Volume 266, Page 205,
                                              Filed 11/13/00

     Chambers County, TX                      Document No. 00-477-116,
                                              Filed 11/14/00

     Galveston County, TX                     Film Code 015-08-1029
                                              Filed 11/13/00

     Jim Hogg County, TX                      Volume 22, Page 256,
                                              Filed 11/13/00

     Live Oak County, TX                      Volume 454, Page 239,
                                              Filed 11/14/00

     Wharton County, TX                       Volume 390, Page 480,
                                              Filed 11/13/00

     Zapata County, TX                        Volume 641, Page 169,
                                              Filed 11/13/00

     General Land Office of Texas             Filed 12/22/00
</Table>

5.   Fourth Supplement to Production Payment Conveyance among TransTexas Gas
     Corporation, Mirant Americas Energy Capital, LP, formerly named Southern
     Producer Services, L.P., and TCW DR VI Investment Partnership, L.P.

<Table>
<Caption>
     Recording Jurisdiction                   Recording Data
     ----------------------                   --------------

<S>                                           <C>
     Calhoun County, TX                       Volume 272, Page 114,
                                              Filed 2/12/01

     Chambers County, TX                      Document No. 01-486-620
                                              Filed 2/12/01
</Table>

                                       3
<PAGE>   11

<Table>
<S>                                           <C>
     Galveston County, TX                     Film Code 015-31-0563
                                              Filed 2/12/01

     Jim Hogg County, TX                      Volume 23, Page 769,
                                              Filed 2/12/01

     Live Oak County, TX                      Volume 456, Page 400,
                                              Filed 2/12/01

     Wharton County, TX                       Volume 400, Page 418,
                                              Filed 2/12/01

     Zapata County, TX                        Volume 645, Page 328,
                                              Filed 2/12/01

     General Land Office of Texas             Filed
</Table>

6.   Fifth Supplement to Production Payment Conveyance among TransTexas Gas
     Corporation, Southern Producer Services, L.P., TCW Portfolio No. 1555 DR V
     Sub-Custody Partnership, L.P. and TCW DR VI Investment Partnership, L.P.

<Table>
<Caption>
     Recording Jurisdiction                   Recording Data
     ----------------------                   --------------

<S>                                           <C>
     Calhoun County, TX                       Volume 282, Page 226,
                                              Filed 7/18/01

     Chambers County, TX                      Document No. 01-507-508
                                              Filed 7/18/01

     Galveston County, TX                     Film Code 015-83-1832
                                              Filed 7/17/01

     Jim Hogg County, TX                      Volume 26, Page 417,
                                              Filed 7/18/01

     Live Oak County, TX                      Volume 461, Page 1,
                                              Filed 7/23/01

     Wharton County, TX                       Volume 419, Page 909,
                                              Filed 7/17/01

     Zapata County, TX                        Volume 652, Page 504,
                                              Filed 7/20/01

     General Land Office of Texas             Filed
</Table>

                                       4

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