Document:

<PAGE>   1
                                                                   EXHIBIT 10.70

                             SUBORDINATION AGREEMENT

         This Subordination Agreement (this "Agreement") is made as of February
7, 2001, by GMAC Commercial Credit LLC, a New York limited liability company
("Mortgage Holder") and TransTexas Gas Corporation, a Delaware corporation
(herein called "TransTexas") for the benefit of Mirant Americas Energy Capital,
LP, formerly named Southern Producer Services, L.P., and TCW DR VI Investment
Partnership, L.P. (herein collectively called the "Production Payment Owners").

                                    RECITALS:

         1. The Mortgage Holder and TransTexas have entered into that certain
Third Amended and Restated Accounts Receivable Management and Security Agreement
dated as of March 15, 2000, as amended by Amendment No. 1 to Third Amended and
Restated Accounts Receivable Management and Security Agreement dated as of
October 1, 2000 (herein, as from time to time amended or supplemented, called
the "ARM Agreement"). Pursuant to the ARM Agreement, TransTexas has executed and
delivered certain mortgages, deeds of trust, security agreements and financing
statements (collectively, the "Security Documents") to secure the Obligations
(as defined in the ARM Agreement and used herein with the same meaning). A true
and complete list of such Security Documents which burden properties in Texas is
attached as Schedule 1 hereto and made a part hereof. Each deed of trust and
mortgage listed on Schedule 1 is herein called a "Mortgage".

         2. As described in Section 9.2 of each Mortgage, TransTexas has
heretofore executed and delivered to the Production Payment Owners that certain
Production Payment Conveyance made effective as of 9:00 a.m., Houston, Texas
time on March 1, 2000 (defined in each such Section 9.2 as the "Production
Payment Conveyance" and herein called the "Original Conveyance"), pursuant to
which TransTexas conveyed and assigned to the Production Payment Owners a
certain production payment (defined in each such Section 9.2 as the "Production
Payment" and herein called the "Original Production Payment"). As described in
Section 9.2 of each Mortgage, the Original Conveyance was given pursuant to an
Order of the United States Bankruptcy Court. As contemplated in such Court
Order, the Original Conveyance has heretofore been supplemented by a First
Supplement to Production Payment Conveyance dated as of June 7, 2000 (the "First
Supplement"), by a Second Supplement to Production Payment Conveyance dated as
of September 8, 2000 (the "Second Supplement"), and by a Third Supplement to
Production Payment Conveyance dated as of November 7, 2000 (the "Third
Supplement"), to increase the unliquidated balance of the "Primary Sum"
thereunder. The Original Conveyance, the First Supplement, the Second
Supplement, and the Third Supplement have been recorded as set forth in Schedule
1 to Exhibit A hereto.

         3. Concurrently herewith, in consideration of funds to be paid to
TransTexas, TransTexas and the Production Payment Owners are executing and
delivering that certain Fourth Supplement to Production Payment Conveyance of
even date herewith (the "Fourth Supplement") which further supplements and
amends the Original Conveyance. Prior to the

<PAGE>   2

effective time of the Fourth Supplement, one of the three original owners of the
Production Payment (TCW Portfolio No. 1555 DR V Sub-Custody Partnership, L.P.)
is conveying all of its interests in the Production Payment to Mirant Americas
Energy Capital, LP, formerly named Southern Producer Services, L.P., one of the
two remaining Production Payment Owners, and after the effective time of the
Fourth Supplement, Mirant Americas Energy Capital, LP, formerly named Southern
Producer Services, L.P., is conveying part of its interests in the Production
Payment to TCW DR VI Investment Partnership, L.P., the other remaining
Production Payment Owner. True and correct copies of both such conveyances, and
a true and correct copy of the Fourth Supplement, are attached hereto as Exhibit
A-1, Exhibit A-2 and Exhibit A-3, respectively, and each is hereby made a part
hereof. As used herein, "Conveyance" means the Original Conveyance as amended
and supplemented by the First Supplement, the Second Supplement, the Third
Supplement, the Fourth Supplement, and such conveyances, and "Production
Payment" has the meaning given to such term in the Conveyance.

         4. Under the ARM Agreement, the Production Payment constitutes "First
Lien Debt" secured by a "Permitted Lien" (as such terms are defined in the ARM
Agreement). Pursuant to Sections 6(e) and 6(f) of the ARM Agreement, TransTexas
has delivered a "Subordination Request" to the Mortgage Holder, asking the
Mortgage Holder to enter into this Agreement in order to subordinate the
Mortgage Holder Liens to the Production Payment and the Conveyance (after taking
the Fourth Supplement into account), and TransTexas has satisfied the conditions
in such Sections 6(e) and 6(f). (As used herein, "Mortgage Holder Liens" means
all liens, security interests, assignments (of production or otherwise) and
other rights or interests of the Mortgage Holder at any time existing on or in
any properties or assets of TransTexas which are in any way subject to the
Production Payment or the Conveyance, whether such liens, security interests,
assignments and other rights or interests arise under the ARM Agreement or the
Security Documents or by operation of law or otherwise.)

         NOW, THEREFORE, in consideration of the foregoing, and in order to
induce the Production Payment Owners to enter into the Fourth Supplement and pay
funds to TransTexas in connection therewith, the Mortgage Holder and TransTexas
hereby agree as follows:

         Section 1. Subordination. The Production Payment and the proceeds
thereof are and hereafter shall be senior in right and priority to all Mortgage
Holder Liens, regardless of the relative priority of the Mortgage Holder Liens
and the Production Payment as determined without regard to this Agreement, and
the same shall have the same validity and effect as if the Conveyance (including
all supplements thereto) had been executed, delivered and properly recorded
prior to the date and time of execution of the ARM Agreement and the date and
time of execution and recordation of the Security Documents. All references in
Section 9.2 of each Mortgage to the "Production Payment Conveyance" shall
hereafter refer to the Conveyance, as defined herein, and all references in
Section 9.2 of each Mortgage to the "Production Payment Conveyance" shall
henceforth refer to the Production Payment Conveyance, as defined herein. As so
modified, the terms of Section 9.2 of each Mortgage are hereby ratified and
confirmed.

         Section 2. Acknowledgment of Reliance. The parties acknowledge and
agree that Production Payment Owners have entered into the Fourth Supplement in
reliance on the priority

                                       2
<PAGE>   3

of the Conveyance, and thus are relying on the waiver, relinquishment and
subordination set forth herein.

         Section 3. Governing Law. This Agreement 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 such state and the laws
of the United States of America, without regard to principles of conflicts of
law.

         Section 4. Invalidity of Particular Provisions. If any term or
provision of this Agreement shall be determined to be illegal or unenforceable,
all other terms and provisions hereof shall nevertheless remain effective and
shall be enforced to the fullest extent permitted by applicable law.

         Section 5. Successors and Assigns. This Agreement shall pass to and be
fully binding upon and inure to the benefit of the successors and assigns of
each party hereto (including the Production Payment Owners).

         Section 6. Counterparts. This Agreement may be separately executed in
any number of counterparts and by different parties hereto in separate
counterparts, each of which when so executed shall be deemed to constitute one
and the same instrument.

               IN WITNESS WHEREOF, this Agreement is executed by the parties
hereto on the dates set out below in their respective acknowledgments, to take
effect as of the date first above written.

MORTGAGE HOLDER:                       GMAC COMMERCIAL CREDIT LLC

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

TRANSTEXAS:                            TRANSTEXAS GAS CORPORATION

                                       By:
                                          --------------------------------------
                                          Ed Donahue, Vice President

                                       3
<PAGE>   4

STATE OF NEW YORK           )
                            )
COUNTY OF NEW YORK          )

         The foregoing instrument was acknowledged before me on this ____ day of
February, 2001, by ___________________________________, as
_______________________________ of GMAC Commercial Credit LLC, a New York
limited liability company, on behalf of such limited liability company.

        [SEAL]
                                       -----------------------------------------
                                       Notary Public, State of New York

My commission expires:

----------------------

STATE OF TEXAS              )
                            )
COUNTY OF HARRIS            )

         The foregoing instrument was acknowledged before me on this ____ day of
February, 2001, by Ed Donahue as Vice President of TransTexas Gas Corporation, a
Delaware corporation, on behalf of such corporation.

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

My commission expires:

----------------------

                                       4
<PAGE>   5

                                   SCHEDULE 1
                                       TO
                             SUBORDINATION AGREEMENT

                        SCHEDULE OF TEXAS RECORDING DATA

                                  COUNTERPARTS
                                     OF THE
               MORTGAGE, DEED OF TRUST, ASSIGNMENT OF PRODUCTION,
                   SECURITY AGREEMENT AND FINANCING STATEMENT,
                          DATED AS OF OCTOBER 26, 2000,
            FROM TRANSTEXAS GAS CORPORATION, AS MORTGAGOR AND DEBTOR,
                           TO JACK MACGOWAN, TRUSTEE,
                 FOR THE BENEFIT OF GMAC COMMERCIAL CREDIT LLC,
                         AS MORTGAGEE AND SECURED PARTY,
                  ARE RECORDED IN THE REAL PROPERTY RECORDS IN
                    THE COUNTIES IN THE STATE OF TEXAS AS SET
                          FORTH IN THE FOLLOWING TABLE:

<TABLE>
<CAPTION>
        COUNTY IN WHICH                                                                RECORDING DATA
        COUNTERPART IS                                       (REFERENCE IS ALSO MADE TO THE "NOTE" IMMEDIATELY FOLLOWING THIS TABLE)
           RECORDED                    DATE          -------------------------------------------------------------------------------
    (EACH IN THE STATE OF               OF                                                                   REFERENCE / FILE /
            TEXAS)                  RECORDING        VOL/BK        PAGE(S)         FILM CODE NUMBER     INSTRUMENT / DOCUMENT NUMBER
    ---------------------           ---------        ------      ----------        ----------------     ----------------------------

<S>                                 <C>              <C>           <C>             <C>                  <C>
            AUSTIN                  11/08/00                                                                       007039

           BRAZORIA                 11/08/00                                                                      00048081

            CALHOUN                 11/08/00         266             82                                            65990

           CHAMBERS                 11/08/00        00-476          535                                            7249-B

           COLORADO                 11/08/00         353            326                                            11480

           GALVESTON                11/08/00                                         015-07-08175              GAC2000057358

            HARRIS                  11/08/00                                         ###-##-####                  U723726

            HIDALGO                 11/08/00                                                                       919748

             KENT                   11/08/00          6              85                                             792

           LIVE OAK                 11/08/00         389            214                                           00149763

             STARR                  11/08/00         0877           372                                            212227

           VAL VERDE                11/08/00         758         514 - 563                                        0184052

             WEBB                   11/08/00         0981           180                                            711667
</TABLE>

<PAGE>   6

<TABLE>
<CAPTION>
        COUNTY IN WHICH                                                                RECORDING DATA
        COUNTERPART IS                                       (REFERENCE IS ALSO MADE TO THE "NOTE" IMMEDIATELY FOLLOWING THIS TABLE)
           RECORDED                    DATE          -------------------------------------------------------------------------------
    (EACH IN THE STATE OF               OF                                                                   REFERENCE / FILE /
            TEXAS)                  RECORDING        VOL/BK        PAGE(S)         FILM CODE NUMBER     INSTRUMENT / DOCUMENT NUMBER
    ---------------------           ---------        ------      ----------        ----------------     ----------------------------

<S>                                 <C>              <C>           <C>             <C>                  <C>
            WHARTON                 11/08/00         390             35                                            209698

            ZAPATA                  11/08/00         641         001 - 066                                         121125
</TABLE>

NOTE: A COUNTERPART OF THE DEED OF TRUST REFERENCED ABOVE, WITH A COMPLETE
EXHIBIT "A," IS RECORDED IN THE REAL PROPERTY RECORDS OF GALVESTON COUNTY,
TEXAS, AS SET FORTH IN THE FOREGOING TABLE. AS TO THE COUNTERPARTS OF THE DEED
OF TRUST REFERENCED ABOVE THAT ARE RECORDED IN THE REAL PROPERTY RECORDS OF
OTHER COUNTIES IN THE STATE OF TEXAS (AS INDICATED IN THE TABLE ABOVE), THE
EXHIBIT "A" ATTACHED TO EACH SUCH RECORDED COUNTERPART IDENTIFIES ONLY THOSE
REAL PROPERTIES LOCATED IN THE COUNTY IN THE STATE OF TEXAS IN WHICH SUCH
COUNTERPART IS RECORDED. FOR A COMPLETE LIST OF ALL REAL PROPERTIES IN ALL
COUNTIES IN THE STATE OF TEXAS THAT ARE SUBJECT TO THE DEED OF TRUST REFERENCED
ABOVE, REFER TO, AND REFERENCE IS HEREBY MADE FOR ALL PURPOSES TO, THE
COUNTERPART OF THE DEED OF TRUST REFERENCED ABOVE THAT IS RECORDED IN THE REAL
PROPERTY RECORDS OF GALVESTON COUNTY, TEXAS.

                          ADDITIONAL SECURITY DOCUMENTS

SECURITY AND PLEDGE AGREEMENT DATED AS OF MARCH 15, 2000, BY AND BETWEEN
TRANSTEXAS GAS CORPORATION, A DELAWARE CORPORATION, AS DEBTOR, AND GMAC
COMMERCIAL CREDIT LLC, AS AGENT FOR THE "LENDERS" DEFINED AND DESCRIBED THEREIN.

FINANCING STATEMENT OF FORM UCC-1 FROM TRANSTEXAS GAS CORPORATION, AS DEBTOR, TO
GMAC COMMERCIAL CREDIT LLC, AS AGENT, AS SECURED PARTY, FILED MARCH 22, 2000, IN
THE OFFICE OF THE SECRETARY OF STATE OF THE STATE OF TEXAS UNDER FILE NUMBER
0000457016.

FINANCING STATEMENT OF FORM UCC-1 FROM TRANSTEXAS GAS CORPORATION, AS DEBTOR, TO
GMAC COMMERCIAL CREDIT LLC, AS AGENT, AS SECURED PARTY, FILED NOVEMBER 8, 2000,
IN THE OFFICE OF THE SECRETARY OF STATE OF THE STATE OF TEXAS UNDER FILE NUMBER
0000620473.

FINANCING STATEMENT OF FORM UCC-1 FROM TRANSTEXAS GAS CORPORATION, AS DEBTOR, TO
GMAC COMMERCIAL CREDIT LLC, AS AGENT, AS SECURED PARTY, FILED NOVEMBER 8, 2000,
IN THE OFFICE OF THE SECRETARY OF STATE OF THE STATE OF DELAWARE UNDER FILE
NUMBER 0079169.

<PAGE>   7

                                   EXHIBIT A-1

                                       TO

                             SUBORDINATION AGREEMENT

                  Conveyance of Interest in Production Payment

<PAGE>   8

                                   EXHIBIT A-2

                                       TO

                             SUBORDINATION AGREEMENT

                    Partial Conveyance of Production Payment

<PAGE>   9

                                   EXHIBIT A-3

                                       TO

                             SUBORDINATION AGREEMENT

               Fourth Supplement to Production Payment Conveyance<PAGE>   1
                                                                   EXHIBIT 10.71

                     FOURTH SUPPLEMENT TO PURCHASE AGREEMENT

     THIS FOURTH SUPPLEMENT TO PURCHASE AGREEMENT dated as of February 7, 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 of even date herewith (the
"Fund V Assignment").

     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 the Original Purchase Agreement and to the Original
Conveyance for the meaning of various

<PAGE>   2

terms defined therein, all of which shall when used herein (unless otherwise
expressly defined herein) have the same meanings. For purposes of this
Supplement, unless the context otherwise requires, the following additional
terms shall have the following meanings:

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

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

     "Current Purchase Price Payment" means a Purchase Price Payment in the
amount of $19,800,000.

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

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

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

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

                                       2
<PAGE>   3

                             ARTICLE II - Amendment

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

     Section 2.2. Grantee. 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
Purchase Agreement, as supplemented hereby, shall refer only to SPS and Fund VI
with respect to matters occurring after the date hereof.

     Section 2.3. Closing. On the Current Closing Date, Grantor shall deliver
the Current Conveyance Supplement to Grantee, and Grantor shall satisfy all of
the conditions set out in Sections 3.4 and 3.5 of the Purchase Agreement (other
than the conditions in Section 3.4(e) and (f) of the Purchase Agreement, which
are hereby waived without prejudice to the rights of SPS and Fund VI to request
and receive the same in connection with any subsequent supplement to the
Purchase Agreement). Thereupon, SPS shall pay all of the Current Purchase Price
Payment to Grantor in accordance with the terms and procedures of the Purchase
Agreement.

     Section 2.4. Time and Place of Closing. The closing for the consummation of
the transactions contemplated herein shall take place on February 7, 2001 or on
such other date as may be agreed to by Grantor and Grantee (herein called the
"Current Closing Date").

                   ARTICLE III - Representations and Covenants

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

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

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

                                       3
<PAGE>   4

     (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 and to pay the Current Purchase Price Payment, Grantor
hereby covenants with Grantee that Grantor will perform all of its covenants and
duties under the Production Payment Documents, all as fully as if they were set
out in full herein.

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

                           ARTICLE IV - Miscellaneous

     Section 4.1. Ratification of Production Payment Documents. The Original
Purchase Agreement as hereby supplemented and amended is hereby ratified and
confirmed in all respects. The other Production Payments Documents, as they may
be supplemented, amended or affected by the Current Supplement Documents, are
hereby ratified and confirmed in all respects. Any reference to the Purchase
Agreement in any Production Payment Document shall be deemed to refer to this
Supplement also, and any reference in any Production Payment Document to any
other document or instrument amended, renewed, extended or otherwise affected by
any Current Supplement Document shall also refer to such Current Supplement
Document. The execution, delivery and effectiveness of this Supplement and the
other Current Supplement Documents shall

                                       4
<PAGE>   5

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.

     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]

                                       5
<PAGE>   6

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

                                       TRANSTEXAS GAS CORPORATION

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

                                       MIRANT AMERICAS ENERGY CAPITAL, LP

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

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

                                       TCW DR VI INVESTMENT PARTNERSHIP, L.P.

                                       By: TCW ASSET MANAGEMENT COMPANY, as
                                           their Agent

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

                                       TCW ASSET MANAGEMENT COMPANY, as Agent

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

                                       6
<PAGE>   7

                                 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 February 7, 2001.

GALVESTON BAY PIPELINE COMPANY

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

GALVESTON BAY PROCESSING COMPANY

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

                                       7
<PAGE>   8

                                    EXHIBIT A

                         FOURTH SUPPLEMENT TO PRODUCTION
                               PAYMENT CONVEYANCE

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