Document:

<PAGE>

                                                                     EXHIBIT 4.2

                       CENTERPOINT ENERGY RESOURCES CORP.

                     (formerly known as NorAm Energy Corp.)

                                       To

                               JPMORGAN CHASE BANK

            (successor to Chase Bank of Texas, National Association),

                                     Trustee

                                   ----------

                          SUPPLEMENTAL INDENTURE NO. 6

                           Dated as of April 14, 2003

                                   ----------

                          7.875% Senior Notes due 2013

                              Series A and Series B

<PAGE>

                       CENTERPOINT ENERGY RESOURCES CORP.

                     (formerly known as NorAm Energy Corp.)

                          SUPPLEMENTAL INDENTURE NO. 6

                          7.875% Senior Notes due 2013

                              Series A and Series B

         SUPPLEMENTAL INDENTURE No. 6, dated as of April 14, 2003, between
CENTERPOINT ENERGY RESOURCES CORP., a Delaware corporation formerly known as
NorAm Energy Corp. (the "Company"), and JPMORGAN CHASE BANK (successor to Chase
Bank of Texas, National Association), as Trustee (the "Trustee").

                                    RECITALS

         The Company has heretofore executed and delivered to the Trustee an
Indenture, dated as of February 1, 1998 (the "Original Indenture" and, as
previously and hereby supplemented and amended, the "Indenture"), providing for
the issuance from time to time of one or more series of the Company's
Securities.

         The Company has changed its name from "NorAm Energy Corp." to
"CenterPoint Energy Resources Corp." and all references in the Indenture to the
"Company" or "NorAm Energy Corp." shall be deemed to refer to CenterPoint Energy
Resources Corp.

         Pursuant to the terms of the Indenture, the Company provided for the
establishment of two series of Securities designated as the "7.875% Senior Notes
due 2013, Series A" (the "Series A Notes") and the "7.875% Senior Notes due
2013, Series B" (the "Series B Notes" and, together with the Series A Notes, the
"Notes"), the form and substance of such Notes and the terms, provisions and
conditions thereof in Supplemental Indenture No. 5 dated March 25, 2003, between
the Company and the Trustee ("Supplemental Indenture No. 5").

         Pursuant to the terms of the Indenture, the aggregate principal amount
of the Notes can be increased pursuant to a resolution of the Board of Directors
of the Company.

         The Company has furnished the Trustee with a resolution of Board of
Directors of the Company authorizing the increase of the aggregate principal
amount of the Notes by $112,000,000 and the execution of this Supplemental
Indenture No. 6 to reflect such increase.

         Section 301 of the Original Indenture provides that various matters
with respect to any series of Securities issued under the Indenture may be
established in an indenture supplemental to the Indenture.

         Subparagraph (7) of Section 901 of the Original Indenture provides that
the Company and the Trustee may enter into an indenture supplemental to the
Indenture to establish the form or terms of Securities of any series as
permitted by Sections 201 and 301 of the Original Indenture.

                                       1
<PAGE>

         For and in consideration of the premises and the issuance of the series
of Securities provided for herein, it is mutually covenanted and agreed, for the
equal and proportionate benefit of the Holders of the Securities of such series,
as follows:

                                   ARTICLE ONE

                  Relation to Indenture; Additional Definitions

         Section 101. Relation to Indenture. This Supplemental Indenture No. 6
amends Supplemental Indenture No. 5 as provided in ARTICLE TWO below and
constitutes an integral part of the Original Indenture.

                                   ARTICLE TWO

                    Amendment of Supplemental Indenture No. 5

         Section 201. Amendment of Cover Page and Title. The "$650,000,000" on
the cover page and in the fourth line of the title of Supplemental Indenture No.
5 are each hereby replaced with "$762,000,000".

         Section 202. Amendment of Section 102. The following definitions in
Section 102 of Supplemental Indenture No. 5 are hereby amended and restated to
read as follows:

         "'Initial Purchasers' means (i) Salomon Smith Barney Inc., Banc One
Capital Markets, Inc. and Wachovia Securities, Inc., as initial purchasers in
the offering of the Series A Notes pursuant to the Offering Memorandum dated
March 18, 2003 relating thereto and (ii) Citigroup Global Markets Inc. and
Credit Suisse First Boston LLC, as initial purchasers in the offering of the
Series A Notes pursuant to the Offering Memorandum dated April 7, 2003 relating
thereto;"

         "'Registration Rights Agreement' means (i) that certain Registration
Rights Agreement, dated as of March 25, 2003, by and among the Company and the
Initial Purchasers signatories thereto and (ii) that certain Registration Rights
Agreement, dated as of April 14, 2003, by and among the Company and the Initial
Purchasers signatories thereto;"

         Section 203. Amendment of Section 202. Section 202 of Supplemental
Indenture No. 5 is hereby amended and restated to read as follows:

         "The Trustee shall authenticate and deliver (i) Series A Notes for
original issue on the Issue Date in the aggregate principal amount of
$650,000,000, (ii) Series A Notes for original issue on April 14, 2003 in the
aggregate principal amount of $112,000,000 and (iii) Series B Notes from time to
time thereafter for issue only in exchange for a like principal amount of Series
A Notes, in each case upon a Company Order for the authentication and delivery
thereof and satisfaction of Sections 301 and 303 of the Original Indenture. Such
order shall specify the amount of the Notes to be authenticated, the date on
which the original issue of Notes is to be authenticated and the name or names
of the initial Holder or Holders. The aggregate principal amount of Notes that
may be outstanding at any time shall not exceed $762,000,000; provided, however,
that the authorized aggregate principal amount of the Notes may be increased
above such amount by a Board Resolution to such effect."

                                       2
<PAGE>

         Section 204. Amendment of Section 207. Section 207 of Supplemental
Indenture No. 5 is hereby amended and restated to read as follows:

         "The Notes issued on March 25, 2003 shall be initially issued at 100%
of their principal amount, and the Notes issued on April 14, 2003 shall be
initially issued at 105.826% of their principal amount (before discount to the
Initial Purchasers); plus, in each case, accrued interest, if any, from March
25, 2003."

         Section 205. Amendment of Section 211(b). Section 211(b) of
Supplemental Indenture No. 5 is hereby amended and restated to read as follows:

         "Solely with respect to the Notes issued hereby, the first sentence of
Section 1403 of the Original Indenture is hereby deleted in its entirety, and
the following is substituted in lieu thereof:

                  'Upon the Company's exercise of its option (if any) to have
                  this Section applied to any Securities or any series of
                  Securities, as the case may be, (1) the Company shall be
                  released from its obligations under Article Eight and under
                  any covenants provided pursuant to Section 301(20), 901(2) or
                  901(7) for the benefit of the Holders of such Securities and
                  (2) the occurrence of any event specified in Sections 501(4)
                  (with respect to Article Eight and to any such covenants
                  provided pursuant to Section 301(20), 901(2) or 901(7)) and
                  501(7) shall be deemed not to be or result in an Event of
                  Default, in each case with respect to such Securities as
                  provided in this Section on and after the date the conditions
                  set forth in Section 1404 are satisfied (hereinafter called
                  "Covenant Defeasance").'"

         Section 206. Amendment of Exhibit A. The form of Note attached to
Supplemental Indenture No. 5 as Exhibit A is hereby amended as follows:

         (a) The final sentence of the first paragraph on the back of the Note
is hereby amended and restated to read, with respect to any Note or replacement
Note authenticated on or after April 14, 2003, as follows:

         "This Security is one of the series designated on the face hereof,
limited in aggregate principal amount to $762,000,000; provided, however, that
the authorized aggregate principal amount of the Securities may be increased
above such amount by a Board Resolution to such effect."

         (b) The final sentence of the penultimate paragraph on the back of the
Note is hereby amended and restated to read as follows:

         "In addition to the rights provided to Holders of this Security under
the Indenture, Holders shall have all the rights set forth in that certain
Registration Rights Agreement, dated as of __________, 20__, among the Company
and the Initial Purchasers signatories thereto, including without limitation the
right to receive Additional Interest as described in Section 2.5 thereof."

                                       3
<PAGE>

                                  ARTICLE THREE

                            Miscellaneous Provisions

         Section 301. The Indenture, as supplemented and amended by this
Supplemental Indenture No. 6, is in all respects hereby adopted, ratified and
confirmed.

         Section 302. This Supplemental Indenture No. 6 may be executed in any
number of counterparts, each of which shall be an original, but such
counterparts shall together constitute but one and the same instrument.

         Section 303. THIS SUPPLEMENTAL INDENTURE NO. 6 AND EACH NOTE SHALL BE
DEEMED TO BE A CONTRACT MADE UNDER THE LAWS OF THE STATE OF NEW YORK AND SHALL
BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW
YORK WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES THEREOF.

         Section 304. If any provision in this Supplemental Indenture No. 6
limits, qualifies or conflicts with another provision hereof which is required
to be included herein by any provisions of the Trust Indenture Act, such
required provision shall control.

         Section 305. In case any provision in this Supplemental Indenture No. 6
or the Notes shall be invalid, illegal or unenforceable, the validity, legality
and enforceability of the remaining provisions shall not in any way be affected
or impaired thereby.

                                       4
<PAGE>

         IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture No. 6 to be duly executed, as of the day and year first written above.

                                         CENTERPOINT ENERGY RESOURCES CORP.

                                         By: /s/ Marc Kilbride
                                            ------------------------------------
                                         Name: Marc Kilbride
                                         Title: Vice President and Treasurer

Attest:

    /s/ Richard B. Dauphin
------------------------------------
Name: Richard B. Dauphin
Title: Assistant Corporate Secretary

(SEAL)

                                         JPMORGAN CHASE BANK, as Trustee

                                         By: /s/ Carol Logan
                                            ------------------------------------
                                         Name: Carol Logan
                                         Title: Vice President and Trust Officer

(SEAL)

                                       5<PAGE>

                                                                   EXHIBIT 10.71

                     TENTH SUPPLEMENT TO PURCHASE AGREEMENT

         THIS TENTH SUPPLEMENT TO PURCHASE AGREEMENT dated as of March 27, 2003
(this "Supplement"), is made by:

         -        TransTexas Gas Corporation, a Delaware corporation and
                  debtor-in-possession (herein called "Grantor"),

         -        TCW DR VI Investment Partnership, L.P. ("Fund VI"), acting
                  through its agent, TCW Asset Management Company,

         -        TCW Global Project Fund Ltd. ("GPF") (Fund VI and GPF are
                  herein collectively called, "Grantee"), and

         -        TCW Asset Management Company ("Tamco"), as Agent on behalf of
                  Fund VI.

                                    RECITALS

         1. Pursuant to an order of the United States Bankruptcy Court for the
Southern District of Texas, Corpus Christi Division (the "Bankruptcy Court"),
entered on February 2, 2000, Grantor, Mirant Americas Energy Capital, LP,
formerly named Southern Producer Services, L.P. ("SPS"), TCW Portfolio No. 1555
DR V Sub-Custody Partnership, L.P. ("Fund V"), Fund VI and Tamco 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 in such order and in such Purchase Agreement,
Grantor has, by means of a Production Payment Conveyance dated as of March 14,
2000 (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
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.

         4. On March 4, 2002, SPS assigned all of its right, title, and interest
under the Original Conveyance to Mirant Americas Energy Capital Assets, LLC
("Mirant").

         5. On March 13, 2003, SPS and Mirant collectively assigned their right,
title, and interest under the Original Conveyance to Hydrocarbon Capital LLC
("HCL"). Prior to the delivery of this Supplement, and by means of an Assignment
of Partial Interest in Production Payment of even date herewith, HCL assigned
its right, title and interest under the Original Conveyance to GPF, effective as
of 8:59 a.m. Houston, Texas time on March 7, 2003. As the result of such
assignment, GPF and Fund VI now constitute the "Grantee" under the Original
Conveyance.

         6. In consideration of the payment by GPF of the Current Purchase Price
Payment to Grantor, as more fully described below, Grantor and Grantee now
desire to supplement and amend the Original Conveyance to (a) increase the
amount of the unliquidated balance of the Primary Sum, and (b) adjust the
relative Percentage Shares of each Person included in Grantee to account for
such additional Purchase Price Payment.

         7. Pursuant to (a) the Order Authorizing TransTexas Gas Corporation to
Sell Certain (Eagle Bay) Assets Free and Clear of Liens and Encumbrances,
Authorizing TransTexas Gas Corporation, Galveston Bay Pipeline Company and
Galveston Bay Processing Corporation to Assume Certain Contracts and to Enter
into Certain Related Transactions entered by the United States Bankruptcy Court
for the Southern District of Texas, Corpus Christi Division (the "Bankruptcy
Court") on or about January 7, 2003 and (b) the Amended Order Authorizing
TransTexas

<PAGE>

Gas Corporation to Sell Certain (Eagle Bay) Assets Free and Clear of Liens and
Encumbrances of U.S. Bank, Indenture Trustee, entered by the Bankruptcy Court on
or about March 27, 2003 (collectively, the "Production Payment Order"), Grantor
has been authorized to make such supplement and amendment to the Original
Conveyance.

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

                     ARTICLE I - Definitions and References

         Section 1.1. Defined Terms and References. As used herein, the terms
"Bankruptcy Court", "Fund V", "Fund VI", "GPF", "Grantee", "Grantor", "Mirant",
"HCL", "Original Conveyance", "Original Purchase Agreement", "SPS", and "Tamco"
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 2.5.

         "Current Conveyance Supplement" means the Tenth 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 $5,000,000.

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

         "Production Payment" has the meaning given such term in the Production
Payment Conveyance.

         "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.

         "Subordination Agreements" means Subordination Agreements substantially
in the forms of Exhibit B and Exhibit C hereto, executed and delivered by
Grantor and the mortgagees named therein for the benefit of Grantee.

         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.

                                       25

<PAGE>

                             ARTICLE II - Amendments

         Section 2.1. Agreement to Amend Conveyance. 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. Amendments to Purchase Agreement. The definition of
Program Period in Section 1.1 of the Original Purchase Agreement is hereby
amended in its entirety to read as follows:

         " 'Program Period' means the period beginning on the Initial Closing
Date through and including March 7, 2003."

In addition, although the provisions of the Original Purchase Agreement
contemplated that Purchase Price Payments would be paid only by SPS, Grantor and
Grantee acknowledge and agree that Purchase Price Payments may be paid by any
Person included within Grantee at the time in question, and that the Purchase
Agreement shall be construed accordingly.

         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 condition in Section 3.4(f) of the Purchase Agreement, which is
hereby waived without prejudice to the rights of GPF and Fund VI to request and
receive the same in connection with any subsequent supplement to the Purchase
Agreement) and in Section 2.4 below. On the Current Closing Date, GPF 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. Additional Conditions Precedent. In addition to the
conditions mentioned above in Section 2.3, each of the following conditions must
be satisfied to Grantee's complete satisfaction, or waived by Grantee, on the
Current Closing Date:

         (a) The Production Payment Order must have been entered in form and
substance acceptable to Grantee and must be in full force and effect and not
stayed pending appeal, and Grantee must be entitled to all of the protections of
Section 363(m) of the United States Bankruptcy Code.

         (b) Each of the Subordination Agreements must have been executed and
delivered in form and substance acceptable to Grantee and must be in full force
and effect.

         (c) Galveston Bay Pipeline Company and Galveston Bay Processing
Corporation must have assumed and ratified the "Mirant Contracts" referred to in
the Production Payment Order by executing and delivering the Acknowledgment
following the signature pages of this Supplement.

         (d) In its wiring instructions to GPF, Grantor must have directed GPF
to pay directly, out of the Current Purchase Price Payment, $75,000 to Thompson
& Knight L.L.P., counsel to GPF and Fund VI, $20,000 to Mayer, Brown, Rowe and
Maw, counsel to Mirant and HCL, to be applied to the reasonable legal fees and
expenses of such counsel that have been incurred in connection with the
Production Payment and the Current Supplement Documents and that will be
incurred from time to time hereafter in connection with post-closing activities
and monitoring the Production Payment during Grantor's bankruptcy case.

         (e) Grantor's arrangements for coordinating its payments for the
Galveston Bay Development Well with the payments of the other joint interest
owner in that well must be acceptable to Grantee.

         Section 2.5. Time and Place of Closing. The closing for the
consummation of the transactions contemplated herein shall take place on March
31, 2003 or on such other date as may be agreed to by Grantor and Grantee
(herein called the "Current Closing Date").

                                       26

<PAGE>

                   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) Except as noted in the Compliance Certificate delivered
concurrently herewith, 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.

         (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 authorized by the
Bankruptcy Court, (ii) 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 (iii) 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 and Authorization. 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. Grantor further agrees that GPF, Fund VI
and their counsel are authorized to apply the $31,395.14 that TransTexas
deposited prior to commencement of the case with Thompson & Knight L.L.P. to pay
the Reimbursable Expenses that TransTexas owes as reimbursements to GPF and Fund
VI (including up to $7,500 in reimbursement for fees and expenses incurred prior
to November 14, 2002).

         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; 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.

                                       27

<PAGE>

                           ARTICLE IV - Miscellaneous

         Section 4.1. Ratification of Production Payment Documents. The Original
Purchase Agreement as hereby supplemented and amended is hereby assumed,
ratified and confirmed in all respects. The other Production Payment Documents,
as they may be supplemented, amended or affected by the Current Supplement
Documents, are hereby assumed, 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.

         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]

                                       28
<PAGE>

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

                                 TRANSTEXAS GAS CORPORATION

                                 By: ________________________________________
                                     Ed Donahue
                                     Vice President

                                 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: ________________________________________
                                      Name:
                                      Director

                                 TCW ASSET MANAGEMENT COMPANY, as Agent

                                 By: ___________________________________________
                                      Kurt A. Talbot
                                      Senior Vice President

                                       29

<PAGE>

                                 ACKNOWLEDGMENT

         To induce Grantee to enter into the above Supplement, the undersigned
Galveston Bay Pipeline Company and Galveston Bay Processing Company each hereby
assume, ratify and confirm their various agreements made in connection with the
Production Payment Documents referred to above, including without limitation the
"Mirant Contracts" referred to in the Production Payment Order, 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 and DELIVERED as of the date of the above Supplement.

GALVESTON BAY PIPELINE COMPANY

By: ________________________________
    Ed Donahue
    Vice President

GALVESTON BAY PROCESSING CORPORATION

By: ________________________________
    Ed Donahue
    Vice President

                                       30

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