<PAGE>   1
                                                                   EXHIBIT 10.72

                         FOURTH SUPPLEMENT TO PRODUCTION
                               PAYMENT CONVEYANCE

         THIS FOURTH 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
         of even date herewith (the "Fund V Assignment").

C.       Grantee and Grantor desire to supplement and amend the Original
         Conveyance, as set forth herein, to (i) take into account the Fund V
         Assignment, (ii) make additional properties subject to the Original
         Conveyance, and (iii) account for the payment by SPS of additional
         funds to Grantor.

D.       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 February 7, 2001
(the "Effective Time"), the Original Conveyance is amended in order to add the
amount of Nineteen Million Eight Hundred Thousand Dollars ($19,800,000) 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


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<PAGE>   2


such application of PP Proceeds), the unliquidated balance of the Primary Sum as
of the Effective Time is $32,532,142. (Any PP Proceeds received after the
Effective Time on February 7, 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 June 1, 2001;

                  (d) sixty-five percent (65%) from and after 9:00 a.m., Texas
         time, on June 1, 2001 until 9:00 a.m., Texas time, on June 1, 2002; and

                  (e) seventy percent (70%) from and after 9:00 a.m., Texas
         time, on June 1, 2002."

         4. Exhibit A to the Original Conveyance is hereby amended and
supplemented, effective as of the Effective Time, to include at the end thereof
Exhibit A hereto (herein called "Additional Exhibit A").

         5. The terms "NRI Percentage," "PP Hydrocarbons," "Subject
Hydrocarbons," "Subject Interests," "Subject Lands," and "Subject Wells," as
such terms are defined and used in the Original Conveyance, are hereby amended,
effective as of the Effective Time, in order to take into account and recognize
the addition of Additional Exhibit A hereto to the end of Exhibit A as attached
to the Original Conveyance, and all other direct or indirect references in the
Original Conveyance shall likewise be considered amended in order to take into
account and recognize such amendment and the addition of Additional Exhibit A
hereto to the end of Exhibit A as attached to the Original Conveyance.

         6. For the above-recited consideration, Grantor does hereby, in order
to more fully effectuate the amendments and other provisions herein contained,
GRANT, BARGAIN, SELL, CONVEY, ASSIGN, TRANSFER, SET OVER and DELIVER unto
Grantee, as a production


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payment, to be held in undivided interests in proportion to their Percentage
Shares, a term overriding royalty interest carved out of and burdening the
Subject Interests (including the "Additional Subject Interests," as defined
below) equal to and measured by all "Additional PP Hydrocarbons," as defined
below, in, under and that may be produced from (or, to the extent pooled or
unitized, allocated to) the Subject Lands (including the "Additional Subject
Lands," as defined below), with such production payment to be effective as to
deliveries of Additional PP Hydrocarbons as of the Effective Time, and to
terminate as of the Termination Time.

         TO HAVE AND TO HOLD the above-described production payment, upon and
subject to the terms of the Original Conveyance, as amended hereby, unto
Grantee, and its successors and Permitted Assigns, until the Termination Time.

         As used herein, the following terms have the following meanings:

         (a)      "Additional PP Hydrocarbons" means the Dedication Percentage
                  of the NRI Percentage (as the definitions of such terms are
                  amended herein) of:

                  (i)      all Hydrocarbons in, under or that may be produced
                           from (or, to the extent pooled or unitized, allocated
                           to) any Additional Subject Lands prior to the
                           Termination Time (as determined after giving effect
                           to this Supplement), and

                  (ii)     all other Hydrocarbons in, under or that may be
                           produced from (or, to the extent pooled or unitized,
                           allocated to) the Subject Lands prior to the
                           Termination Time, as determined after giving effect
                           to this Supplement and the instrument listed as item
                           2 on Schedule 1 hereto (this Supplement and such
                           other instruments being herein collectively called
                           the "Supplements"), to the extent such Hydrocarbons
                           are included within the PP Hydrocarbons as a result
                           of the Termination Time being changed because of the
                           Supplements.

         (b)      "Additional Subject Interests" means:

                  (i)      All of the leasehold interests and other property
                           interests described in Additional Exhibit A; and

                  (ii)     Without limitation of the foregoing, all other right,
                           title and interest (of whatever kind or character,
                           whether legal or equitable and whether vested or
                           contingent) of Grantor in and to the oil, gas and
                           other minerals in and under or that may be produced
                           from the Additional Subject Lands (including
                           interests in oil, gas or mineral leases to the extent
                           the same cover such lands, overriding royalties,
                           production payments and net profits interests in such
                           lands or such leases, and fee mineral interests, fee
                           royalty interests and other interests in such oil,
                           gas and other minerals) even


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                           though Grantor's interest in such oil, gas and other
                           minerals may be incorrectly described in, or omitted
                           from, Additional Exhibit A; and

                  (iii)    All rights, titles and interests of Grantor in and
                           to, or otherwise derived from, all presently existing
                           and valid oil, gas or mineral unitization, pooling,
                           or communitization agreements, declarations or orders
                           and in and to the properties covered and the units
                           created thereby (including all units formed under
                           orders, rules, regulations, or other official acts of
                           any federal, state, or other authority having
                           jurisdiction, voluntary unitization agreements,
                           designations or declarations, and so-called "working
                           interest units" created under operating agreements or
                           otherwise) relating to the properties described in
                           subsections (i) or (ii) above in this definition.

         (c)      "Additional Subject Lands" means the lands and depths
                  described in Additional Exhibit A hereto (where no depth limit
                  is specified, Additional Subject Lands shall include all
                  depths).

         Without limitation of the generality of the provisions of Section 5,
above, or of the foregoing provisions of this Section 6, effective as to
deliveries of Additional PP Hydrocarbons at and after the Effective Time, the
definition of PP Hydrocarbons as found in the Original Conveyance is hereby
amended to include all Additional PP Hydrocarbons, as defined herein, the
definition of Subject Lands as found in the Original Conveyance is hereby
amended to include all Additional Subject Lands, as defined herein, and the
definition of Subject Interests as found in the Original Conveyance is hereby
amended to include all Additional Subject Interests, as defined herein; the term
"Initial Time", as found in the definition of "Subject Hydrocarbons" in the
Original Conveyance, shall be deemed to refer to the Effective Time, as such
term is defined herein, but only as to Additional PP Hydrocarbons, and the
Original Conveyance is amended to so provide.

         7. 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. To the
extent, if any, required to give effect to the Supplements or to the
ratification, adoption, affirmation and renewal provided for in the preceding
sentence, Grantor does hereby GRANT, BARGAIN, SELL, CONVEY, ASSIGN, TRANSFER,
SET OVER AND DELIVER unto Grantee, as a production payment, to be held in
undivided interests in proportion to their Percentage Shares, a term overriding
royalty interest carved out of and burdening the Subject Interests equal to and
measured by all PP Hydrocarbons in and under and that may be produced from (or,
to the extent pooled or unitized, allocated to) the Subject Lands, with such
production payment to terminate as of the Termination Time and to be held upon
and subject to the terms of the Original Conveyance as amended hereby.

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


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<PAGE>   5


                  " '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 9:00 a.m. Houston, Texas time, on 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 9:00 a.m. Houston, Texas time, on June 7, 2000, until
         9:00 a.m. Houston, Texas time, on 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 9:00 a.m. Houston, Texas time, on September 8, 2000,
         until 9:00 a.m. Houston, Texas time, on 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 9:00 a.m. Houston, Texas time, on November 7, 2000,
         until 9:00 a.m. Houston, Texas time, on 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 9:00 a.m. Houston, Texas time, on December 1, 2000,
         until 9:00 a.m. Houston, Texas time, on 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 9:00 a.m. Houston, Texas time, on February 7, 2001, the
         Percentage Share of each Person included in Grantee is as follows:


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<PAGE>   6


               Fund VI         3.71408%
               SPS            96.28592%"

The foregoing Percentage Shares of Fund VI and SPS, respectively, taking effect
as of February 7, 2001, are in this Amendment called their "amended Percentage
Shares".

         9. 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.

         10. In order to take into account SPS's name change, all references to
"Southern Producer Services, L.P." in the Original Conveyance, as supplemented
hereby, shall refer to "Mirant Americas Energy Capital, LP" with respect to
matters occurring after the date hereof. In order to take into account the Fund
V Assignment, all references to "Grantee" in the Original Conveyance, as
supplemented hereby, shall refer only to SPS and Fund VI with respect to matters
occurring after the date hereof.

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

         12. 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.

         13. 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>   7


         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:
                                           -------------------------------------
                                           Ed Donahue
                                           Vice President

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:
                                                --------------------------------
                                                David W. Stewart
                                                Vice President

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


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<PAGE>   8


                                      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>   9


STATE OF TEXAS              )
                            )
COUNTY OF HARRIS            )

         The foregoing instrument was acknowledged before me on this 7th day of
February, 2001, by Ed Donahue, the Vice President 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 7th day of
February, 2001, by David W. Stewart, the Vice President 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 7th day of
February, 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


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<PAGE>   10


                                                                      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>   11

<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

         Zapata County, TX                  Volume 638, Page 642,
                                            Filed 9/22/2000
</TABLE>



                                        2
<PAGE>   12
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>


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<PAGE>   13
                                    EXHIBIT A
                                       TO
                              FOURTH SUPPLEMENT TO
                          PRODUCTION PAYMENT CONVEYANCE

                          ADDITIONAL SUBJECT INTERESTS

                              Wharton County, Texas


1.       Oil, Gas and Mineral Lease dated March 16, 2000, between Jack
         Northington Shwab and Clara Jane Lovell, as Lessor, and Prolithic
         Resources Corp., as Lessee, recorded under Memorandum of Oil, Gas and
         Mineral Lease at Volume 364, Page 306 of the Official Records of
         Wharton County, Texas, covering 567.93 acres, more or less, in the
         Alexander Edgar Survey, A-93, Wharton County, Texas. (TX 106-175/001A)

                Working Interest:                    100.00000%
                Net Revenue Interest                  75.00000%

2.       Oil, Gas and Mineral Lease dated August 15, 1997, between Robert S.
         Mortensen and Martha F. Middlebrook, Co-Trustees for Rocking M Land and
         Cattle Company Trust, as Lessor and SKH Energy Partners, L. P., as
         Lessee, recorded in Volume 248, Page 771 of the Official Records of
         Wharton County, Texas, covering 180 acres, more or less, situated in
         the J. P. Borden Survey, A-9, and the Alexander Edgar League, A-93,
         Wharton County, Texas, INSOFAR AND ONLY INSOFAR as all depths below the
         top of the Wilcox formation. (TX 106-693)

                Working Interest:                    100.00000%
                Net Revenue Interest                  76.00000%

3.       Oil, Gas and Mineral Lease dated January 2, 2001, between Robert S.
         Mortensen and Martha F. Middlebrook, Co-Trustees for Rocking M Land and
         Cattle Company Trust, as Lessor, and TransTexas Gas Corporation, as
         Lessee, recorded in Volume 396, Page 679 of the Official Records of
         Wharton County, Texas, covering 180 acres, more or less, situated in
         the J. P. Borden Survey, A-9, and the Alexander Edgar League, A-93,
         Wharton County, Texas, INSOFAR AND ONLY INSOFAR as all depths between
         the surface of the earth and the top of the Wilcox formation. (TX
         106-693A)

                Working Interest:                    100.00000%
                Net Revenue Interest                  76.00000%


                                        4
<PAGE>   14


Lease (1) above is subject to:

(a)      the terms, conditions and provisions of that certain unrecorded Egypt
         Prospect Agreement dated April 4, 2000, between Prolithic Resources
         Corporation and TransTexas Gas Corporation, as amended by First
         Amendment to Egypt Prospect Agreement dated June 20, 2000.

Lease (2) above is subject to:

(a)      the terms, conditions and provisions of that certain Assignment of Oil,
         Gas & Mineral Lease dated July 11, 2000, between Chesapeake Operating,
         Inc., as Assignor, and TransTexas Gas Corporation, as Assignee,
         recorded in Volume 376, Page 881 of the Official Records of Wharton
         County, Texas.


                                        5