<PAGE>

                                                                   EXHIBIT 10.51

                                                                [EXECUTION COPY]

                       FIRST AMENDMENT TO CREDIT AGREEMENT

      THIS FIRST AMENDMENT TO CREDIT AGREEMENT, dated as of April 5,2004 (this
"First Amendment"), is by and among NEG OPERATING LLC, a Delaware limited
liability company (the "Borrower"), MIZUHO CORPORATE BANK, LTD., as
administrative agent for the Lenders (in such capacity together with any
successors thereto, the "Administrative Agent"), THE BANK OF NOVA SCOTIA and
BANK OF TEXAS, N.A., as co-agents for the Lenders (each in such capacity, a
"Co-Agent" and together the "Co-Agents," together with any successors thereto)
and BANK OF TEXAS, N.A., as collateral agent for the Lenders (in such capacity
together with any successors thereto, the "Collateral Agent") and the various
financial institutions as are or may become parties hereto (collectively, the
"Lenders").

                                   WITNESSETH:

      1.    The Borrower, the Administrative Agent, Bank of Texas, N. A. and The
Bank of Nova Scotia, as Co-Agents, Bank of Texas, N.A. as Collateral Agent, the
Issuer and the Lenders from time to time parties thereto, have entered into that
certain Credit Agreement dated as of December 29, 2003 (as amended, modified,
supplemented, or restated from time to time, including by this First Amendment,
the "Credit Agreement"), pursuant to which the Lenders have made Loans to the
Borrower and have agreed to participate in Letters of Credit issued under the
Credit Agreement for the account of the Borrower and for the benefit of the
Borrower and its Subsidiaries.

      2.    The Borrower has entered and/or may enter into Hedging Agreements
with certain Lender Parties.

      3.    The Borrower has requested that the Administrative Agent and the
Lenders consent to (a) the formation of certain Subsidiaries and (b) the
transfer to certain Subsidiaries of certain of (i) its assets or (ii) equity
interests in other Subsidiaries.

      4.    The Administrative Agent and the Lenders are willing to consent to
the aforementioned formations and restructuring subject to (a) the
contemporaneous delivery of specified Security Documents by or with respect to
such newly-formed Subsidiaries and (b) the amendment of certain provisions of
the Credit Agreement pursuant to this First Amendment, whereby, among other
things, the Administrative Agent and the Lenders will (i) consent to the
formation of such additional Subsidiaries and such transfers and (ii)
restructure, rearrange, renew, extend and continue the Obligations of the
Borrower under the Credit Agreement and the other Loan Documents, pursuant to
which Loans to the Borrower will be made or continued by the Lenders and Letters
of Credit will be issued under the several responsibilities of the Lenders for
the account of the Borrower and for the benefit of the Borrower and its
Subsidiaries from time to time.

      5.    The Borrower has duly authorized the execution, delivery and
performance of this First Amendment.

      NOW, THEREFORE, in consideration of the premises and the mutual covenants
herein contained, the parties hereto agree as follows:

      Section 1. Defined Terms. Except as amended hereby, terms used herein when
defined in the Credit Agreement shall have the same meanings herein unless the
context otherwise requires.

<PAGE>

      Section 2. Amendments to Credit Agreement.

      a.    Amendments to Section 1.1

            (i)   Section 1.1 of the Credit Agreement is hereby amended by
inserting the following definitions of "NGX Energy," "NGX GP," "NGX LP" and
"Merger Documents," each in appropriate alphabetical order:

      "     "NGX Energy" means NGX Energy Limited Partnership, a Delaware
      limited partnership.

            "NGX GP" means NGX GP of Delaware, a Delaware limited liability
      company.

            "NGX LP" means NGX LP of Delaware, a Delaware limited liability
      company.

            "Merger Documents" means, collectively, (a) the Agreement and Plan
      of Merger by and between the Borrower and NGX GP; (b) the Agreement and
      Plan of Merger by and between the Borrower and NGX LP; (c) the Agreement
      and Plan of Merger by and between the Borrower and NGX Energy; (d) the
      Certificate of Merger with respect to the merger of NGX GP with and into
      the Borrower; (e) the Certificate of Merger with respect to the merger of
      NGX LP with and into the Borrower; and (f) the Certificate of Merger with
      respect to the merger of NGX Energy with and into the Borrower, each in
      form and substance satisfactory to the Administrative Agent or the
      Collateral Agent."

            (ii)  Section 1.1 of the Credit Agreement is hereby amended by
deleting the following definitions of "NEG Management Agreement," "Security
Document" and "Subsidiary Guaranty," and replacing those definitions with the
applicable definition below, in appropriate alphabetical order:

      "     "NEG Management Agreement" means that certain Management Agreement
      dated as of May 1, 2001, by and between NEG and the Borrower, as amended
      by that certain First Amendment to Management Agreement dated as of
      December 31,2002, by and among NEG, the Borrower and Edgemont Limited
      Partnership, and as further amended by that certain Second Amendment to
      Management Agreement dated as of April 5, 2004, by and among NEG, the
      Borrower and NGX Energy.

            "Security Document" means any Pledge Agreement, Guaranty, Security
      Agreement or Mortgage, and each other security agreement or other
      instrument or document executed and delivered pursuant to Section 5.1 or
      Section 7.1.7, including without limitation (a) that certain Pledge
      Agreement executed and delivered by Gascon; (b) that certain Pledge
      Agreement executed and delivered by NEG; (c) that certain Pledge Agreement
      executed and delivered by NEG Holding; (d) that certain Pledge Agreement
      executed and delivered by the Borrower; (e) that certain Pledge Agreement
      executed and delivered by NGX GP; (f) that certain Pledge Agreement
      executed and delivered by NGX LP; (g) that certain Guaranty executed and
      delivered

                                        2
<PAGE>

      by NEG Holding; (h) that certain Guaranty executed and delivered by Shana
      National; (i) that certain Guaranty executed and delivered by NGX GP; (j)
      that certain Guaranty executed and delivered by NGX LP; (k) that certain
      Guaranty executed and delivered by NGX Energy; (1) that certain Security
      Agreement executed and delivered by the Borrower; (m) that certain
      Security Agreement executed and delivered by Shana National; (n) that
      certain Security Agreement executed and delivered by NGX GP; (o) that
      certain Security Agreement executed and delivered by NGX LP; (p) that
      certain Security Agreement executed and delivered by NGX Energy; (q) that
      certain Mortgage executed and delivered by the Borrower; (r) that certain
      Mortgage executed and delivered by the Borrower, NGX GP, NGX LP and NGX
      Energy; and (s) any other Loan Document to secure any of the Obligations,
      and all amendments, supplements, restatements or other modifications made
      from time to time thereto, delivered pursuant to this Agreement. "Security
      Documents" means, collectively, all of the foregoing.

            "Subsidiary Guaranty" means, collectively, each Guaranty, dated as
      of the Effective Date or otherwise delivered pursuant to the Loan
      Documents, executed by Shana National, NGX GP, NGX LP and NGX Energy, each
      in favor of the Administrative Agent and substantially in the form of
      Exhibit F-2 attached hereto, together with any amendments, renewals,
      restatements or other modifications thereof from time to time. The term
      "Subsidiary Guaranty" shall include each and every Subsidiary Guaranty
      executed and delivered by a Subsidiary hereunder."

            (iii) Section 1.1 of the Credit Agreement is hereby amended by
deleting clause (d) in the definition of "Change in Control" and replacing it
with the following:

      "     (d)   a plan is adopted relating to the liquidation or dissolution
      of any of the Borrower, Shana National, NGX GP, NGX LP, NGX Energy, NEG
      Holding, NEG or Gascon;"

            (iv)  Section 1.1 of the Credit Agreement hereby is amended by
inserting the following words at the end of clause (i) of the definition of
"Change in Control":

      ", including, without limitation, NGX Energy."

      b.    The second line of Section 7.1.1(c) of the Credit Agreement hereby
is amended by deleting the words "twenty-five (25) days after the end of each of
calendar month" and replacing them with the words "one calendar month after the
end of each calendar month" in substitution therefor.

      c.    Section 7.2.2 of the Credit Agreement hereby is amended by deleting
the first three lines thereof and replacing them with the following in
substitution therefor:

      "The Borrower will not, and will not permit any of its Subsidiaries to,
      create, incur, assume or suffer to exist or otherwise become or be liable
      with respect to any Indebtedness, other than, with respect to the Borrower
      and Shana National only, the following, without duplication;"

                                        3
<PAGE>

      d.    Section 7.2.3 of the Credit Agreement hereby is amended by deleting
the first three lines thereof and replacing them with the following in
substitution therefor:

      "The Borrower will not, and will not permit any of its Subsidiaries to,
      create, incur, assume or suffer to exist any Lien upon any of its Property
      or revenues, except, with respect to the Borrower and Shana National only,
      the following:"

      e.    Amendments to Section 7.2.5. Section 7.2.5 of the Credit Agreement
hereby is amended by:

            (i)   inserting the words "by the Borrower" in the first line of
subsection (c) immediately after the word Investments".

            (ii)  deleting the word "and" in the fourth line of subsection (c).

            (iii) inserting the words "by the Borrower and its Subsidiaries
except NGX GP, NGX LP and NGX Energy" in the first line of subsection (d)
immediately after the word 'Investments".

            (iv)  inserting, after subsection (d), the following:

      "     (e)   (i) Investments by the Borrower in Equity Interests of
      NGX GP and NGX LP and (ii) Investments by NGX GP and NGX LP in Equity
      Interests of NGX Energy, in each case solely in connection with the
      formation of NGX GP, NGX LP and NGX Energy; and

            (f)   capital contributions by the Borrower in NGX GP and NGX LP,
      and the immediately subsequent capital contributions by NGX GP and NGX LP
      in NGX Energy, in each case solely in connection with the transfer of a
      "beneficial" interest in the Borrower's assets; provided, however, that,
      contemporaneously with any such capital contribution, the Borrower shall
      cause NGX Energy to execute and deliver any Security Documents required
      pursuant to the terms of this Agreement."

      f.    Amendments to Section 7.2.8. Section 7.2.8 of the Credit Agreement
hereby is amended by:

            (i)   amending subsection (a) its entirety to read as follows:

      "     (a)   (i) (A) any Subsidiary of the Borrower may be merged or
      consolidated with or into the Borrower and (B) any Subsidiary of the
      Borrower except NGX Energy may be merged or consolidated with or into any
      Wholly-Owned Subsidiary; (ii) (A) the Property or Equity Interests of any
      Subsidiary may be purchased or otherwise acquired by the Borrower and (B)
      the Property or Equity Interests of any Subsidiary except NGX Energy may
      be purchased or otherwise acquired by any Wholly-Owned Subsidiary;
      provided that in any such transaction involving the Borrower or any
      Wholly-Owned Subsidiary, the Borrower or such Wholly-Owned Subsidiary
      shall be the surviving or continuing entity; and (iii) (A) any Subsidiary
      may sell, lease, transfer or otherwise dispose of any or all of its assets
      (upon voluntary liquidation or otherwise) to the Borrower and (B) any
      Subsidiary except

                                        4
<PAGE>

      NGX Energy may sell, lease, transfer or otherwise dispose of any or all of
      its assets (upon voluntary liquidation or otherwise) to any Wholly-Owned
      Subsidiary;

            (ii)  amending subsection (b) its entirety to read as follows:

      "     (b)   so long as (i) permitted by Section 7.2.5 and (ii) no Default
      has occurred and is continuing or would occur after giving effect thereto,
      the Borrower or Shana National may purchase all or substantially all of
      the assets of any Person (except, as to Shana National, assets of NGX GP,
      NGX LP or NGX Energy), or acquire such Person by merger (except, as to
      Shana National, NGX GP, NGX LP or NGX Energy) that, in the discretion of
      the Administrative Agent, is engaged in the Oil and Gas Business; provided
      that in any merger involving the Borrower or Shana National, the Borrower
      or Shana National shall be the surviving or continuing entity; and"

            (iii) inserting, after subsection (b), the following:

      "     (c)   the Borrower may sell, transfer or otherwise dispose of a
      "beneficial" interest in any or all of its assets to NGX Energy; provided,
      however, that, contemporaneously with such sale, transfer or other
      disposition, the Borrower shall cause NGX Energy to execute and deliver
      any Security Documents required pursuant to the terms of this Agreement."

      g.    Amendments to Section 7.2.10. Section 7.2.10 of the Credit Agreement
hereby is amended by:

            (i)   inserting, in the second line of subsection (c), the words
"except NGX GP and NGX LP" after the words "and its Subsidiaries".

            (ii)  deleting, in the fourth line of subsection (c), the words "any
of its Subsidiaries" and replacing them with the words "such Subsidiaries" in
substitution therefor.

      h.    Section 7.2.11 of the Credit Agreement hereby is amended by deleting
the last sentence thereof and replacing it with the following sentence in
substitution therefor:

      "Furthermore, the Borrower will not (a) consent to any amendment,
      supplement or other modification of the NEG Management Agreement or (b)
      amend, supplement or otherwise modify, or permit any Subsidiary to amend,
      supplement or otherwise modify, any Organic Documents of a Subsidiary,
      unless previously consented to in writing by the Administrative Agent."

      i.    Amendment to Disclosure Schedule. Item 6.8 ("Subsidiaries") of the
Disclosure Schedule is hereby amended by deleting Item 6.8 thereof and inserting
in its place Item 6.8 to the Disclosure Schedule attached hereto.

      Section 3. Consent. Pursuant to Section 7.2.15 of the Credit Agreement,
the Administrative Agent and each Lender signing below hereby consent to the
formation of NGX GP, NGX LP and NGX Energy, which formations have been effected
in the following manner and involved the following transfers:

                                        5
<PAGE>

      The Borrower has formed two wholly-owned Delaware limited liability
companies, namely NGX GP and NGX LP. NGX GP and NGX LP have formed NGX Energy, a
Delaware limited partnership. NGX GP is the sole general partner of NGX Energy
and owns a .1% general partnership interest in NGX Energy, while NGX LP is the
sole limited partner and owns a 99.9% limited partnership interest in NGX
Energy. The Borrower has transferred all right, benefit and interest (other than
legal title) (the "Beneficial Interest") in its Texas assets to NGX GP and NGX
LP in proportion to their respective ownership interests in NGX Energy. NGX GP
and NGX LP have contributed all of their respective .1% and 99.9% Beneficial
Interest to NGX Energy as their respective initial capital contributions to NGX
Energy. As a result of the forgoing, all non-Texas assets remain completely
owned by the Borrower, while the Beneficial Interest in all Texas assets
previously owned by the Borrower are now owned by NGX Energy. All Beneficial
Interest in the Texas assets will at all times be held by NGX Energy. All legal
title in the same Texas assets will at all times be held by the Borrower.

      Section 4. Effectiveness. This First Amendment shall become effective upon
the satisfaction of each of the following conditions precedent set forth in this
Section 4 (the first date upon which each such condition has been satisfied,
herein the "First Amendment Effective Date"):

      a.    Certain Loan Documents. The Administrative Agent (or its counsel)
shall have received from each party thereto either a counterpart of each of the
following documents, duly executed on behalf of such party, or written evidence
satisfactory to the Administrative Agent (which may include facsimile
transmission of a signed signature page of such document) that each such party
has duly executed for delivery to the Administrative Agent a counterpart of each
of the following documents, which documents must be acceptable to the
Administrative Agent in its sole and absolute discretion: this Amendment, the
Guaranties required by Section 4(b), the Pledge Agreements required by Section
4(c), the Amendment to Pledge Agreement required by Section 4(d), the Security
Agreements required by Sections 4(e), the Mortgage required by Section 4(f), all
related financing statements and other filings, and the other Loan Documents.

      b.    Guaranties. The Administrative Agent shall have received a Guaranty
duly executed and delivered by each of NGX GP, NGX LP and NGX Energy.

      c.    Pledge Agreements; Certificates and Blank Powers. The Administrative
Agent shall have received counterparts of Pledge Agreements duly executed and
delivered by each of NGX GP and NGX LP, together with the following:

      (i)   certificates evidencing, as to each of NGX GP and NGX LP, ownership
of .1% and 99.9%, respectively, of the issued and outstanding Equity Interests
of NGX Energy, which certificates shall in each case be accompanied by undated
stock powers or other similar powers duly executed in blank; or, if such Equity
Interests are uncertificated securities, confirmation and evidence satisfactory
to the Administrative Agent or the Collateral Agent that the security interest
in such uncertificated securities has been transferred to and perfected by the
Administrative Agent for the benefit of the Lenders by control in accordance
with the Uniform Commercial Code, as in effect in the State of New York; and

      (ii)  all documents and instruments, including Uniform Commercial Code
Financing Statements (Form UCC-1), required by Governmental Rule or reasonably
requested by the

                                        6
<PAGE>

Administrative Agent or the Collateral Agent, to be filed, registered or
recorded to create or perfect the Liens intended to be created under the Pledge
Agreement.

      d.    Amendment to Borrower Pledge Agreement. The Administrative Agent
shall have received a First Amendment to Pledge Agreement duly executed and
delivered by the Borrower and in form and substance satisfactory to the
Administrative Agent or the Collateral Agent whereby the Borrower pledges 100%
of its Equity Interests in NGX GP and NGX LP.

      e.    Security Agreements and UCC Filings. The Administrative Agent shall
have received duly executed counterparts of a Security Agreement, duly executed
by NGX GP, NGX LP and NGX Energy, together with the following:

      (i)   executed or authorized Uniform Commercial Code Financing Statements
(Form UCC-1) and such evidence of filing or arrangements for filing as may be
acceptable to the Administrative Agent or the Collateral Agent, naming NGX GP,
NGX LP and NGX Energy, as applicable, as the debtor and the Collateral Agent as
the secured party, or other similar instruments or documents, filed or to be
under the Uniform Commercial Code of all jurisdictions as may be necessary or,
in the opinion of the Administrative Agent or the Collateral Agent, desirable to
perfect the security interest of the Collateral Agent pursuant to such Security
Agreement; and

      (ii)  executed or authorized copies of proper Uniform Commercial Code Form
UCC-3 termination statements, if any, necessary to release all Liens and other
rights of any Person in any Collateral described in the Security Agreement
previously granted by any Person, and together with such other Uniform
Commercial Code Form UCC-3 termination statements as the Administrative Agent or
the Collateral Agent may reasonably request.

      f.    Mortgage. The Administrative Agent shall have received counterparts
of Mortgages encumbering 100% of the interest in the Oil and Gas Properties in
Texas of the Borrower, NGX GP, NGX LP and NGX Energy, duly executed by the
Borrower, NGX GP, NGX LP and NGX Energy.

      g.    Agreement Regarding Merger Documents. The Administrative Agent shall
have received a counterpart of an agreement, in form and substance satisfactory
to the Administrative Agent or the Collateral Agent, duly executed by the
Borrower, NGX GP, NGX LP and NGX Energy, providing for, among other things, (i)
the establishment of an escrow with the Collateral Agent of all fully-executed
Merger Documents and (ii) the filing by the Administrative Agent or the
Collateral Agent of such Merger Documents with the requisite Governmental
Authorities in the event of a Default under the Credit Agreement.

      h.    Merger Documents. The Administrative Agent and the Lenders shall
have received fully-executed copies of all Merger Documents.

      i.    Organic Documents. The Administrative Agent shall have received from
the Borrower, NGX GP, NGX LP and NGX Energy a certificate from an Authorized
Officer of such Obligor dated as of the First Amendment Effective Date, and
certifying:

      (i)   that attached to each such certificate are (A) a true and complete
copy of all Organic Documents of such Obligor, as in effect on the date of such
certificate and (B) a true and complete

                                        7
<PAGE>

copy of a certificate from the Governmental Authority of the state of such
Obligor's organization certifying that such Obligor is duly organized and
validly existing in such jurisdiction;

      (ii)  that attached to such certificate is a true and complete copy of
resolutions then in full force and effect, adopted by the board of directors or
other governing body of such Obligor, authorizing the execution, delivery and
performance of this First Amendment and each other Loan Document to be executed
by it, including each Merger Document;

      (iii) that attached thereto is a true and complete copy of a certificate
from the appropriate Governmental Authority of the state of organization or
formation, as the case may be, of such Obligor as to the existence and good
standing of such Obligor, each dated within thirty (30) days prior to the date
of delivery pursuant hereto, and that such certificate of incorporation or
certificate of formation, as the case may be, has not been amended since the
date of such certified copy; and a true and complete copy of a certificate from
the appropriate Governmental Authority of each state (without duplication) as to
the good standing of and payment of franchise taxes by the Borrower or each such
Obligor, if applicable, dated within thirty (30) days prior to the date of
delivery pursuant hereto; and

      (iv)  as to the incumbency and signatures of those of its officers
authorized to act with respect to each Loan Document executed by it;

      upon which certificate each Lender may conclusively rely until it shall
have received a further certificate of the Secretary of the Borrower or such
other Obligor canceling or amending such prior certificate.

      j.    Opinions of Counsel. The Administrative Agent shall have received
opinions, dated as of the First Amendment Effective Date, addressed to the
Administrative Agent and all Lenders, from (i) Philip D. Devlin, general counsel
to the Borrower and the other Obligors hereunder, in substantially the form
delivered pursuant to the Credit Agreement and (ii) Greenberg Traurig LLP,
special New York counsel and local Texas counsel to the Borrower and the other
Obligors hereunder, in substantially the form delivered pursuant to the Credit
Agreement, which shall include an opinion that the Mortgages delivered hereunder
are effective to create a valid, perfected Lien in favor of the Collateral Agent
on the Mortgaged Properties located in the State of Texas and are in proper form
for recordation therein.

      k.    Insurance. The Administrative Agent shall have received a
certificate of the Secretary of the Borrower in form and substance satisfactory
to the Administrative Agent, certified by an Authorized Officer of the Borrower
as of the First Amendment Effective Date, as true and correct in all material
respects and describing in detail all liability insurance maintained by each of
the Borrower, NGX GP, NGX LP and NGX Energy, and certifying that (i) such
insurance satisfies the requirements of Section 7.1.4 and the Security Documents
and (ii) such insurance is in full force and effect.

      l.    UCC and Lien Searches. The Administrative Agent shall have received
(i) the UCC Searches with respect to NGX GP, NGX LP and NGX Energy, all dated
reasonably close to the First Amendment Effective Date, in the discretion of the
Administrative Agent and in form and substance satisfactory to the
Administrative Agent.

                                        8
<PAGE>

      m.    Approvals and Consents. The Administrative Agent shall have received
copies of:

      (i)   all Governmental Approvals from any applicable Governmental
Authority or regulatory body or other Person in connection with the execution
and performance of this First Amendment and the other Loan Documents executed
and delivered in connection therewith, and all applicable waiting periods and
appeal periods shall have expired, without the imposition of any burdensome
conditions, and

      (ii)  any third party consents and approvals necessary or advisable, in
connection with the execution and performance of the Loan Documents, and all
applicable waiting periods and appeal periods shall have expired, without the
imposition of any burdensome conditions. There shall be no actual government or
judicial action restraining, preventing or imposing burdensome conditions on the
execution and performance of the Loan Documents.

      n.    Priority; Security Interest. The Collateral and Borrowing Base
Properties in connection with this First Amendment shall be free and clear of
all Liens except for Liens securing payment of the Obligations and any Hedging
Obligations owed to a Lender (including the Administrative Agent or the Issuer),
granted pursuant to any Loan Document. All filings, notices, recordings and
other action necessary to perfect the Liens in such Collateral shall have been
made, given or accomplished or arrangements for the completion thereof
satisfactory to the Administrative Agent and its counsel shall have been made
and all filing fees and other expenses related to such actions either have been
paid in full or arrangements have been made for their payment in full which are
satisfactory to the Administrative Agent.

      o.    NEG Certificate. The Administrative Agent shall have received a
certificate from an Authorized Officer of NEG dated as of the First Amendment
Effective Date, and certifying that attached to such certificate is a true and
complete copy of the Second Amendment to Management Agreement, as in effect on
the date of such certificate.

      p.    Effectiveness Notice. The Administrative Agent shall have received
an "Effectiveness Notice" with respect to the conditions precedent set forth in
this Section 4.

      q.    Satisfactory Review and Legal Form. All legal matters in connection
with this First Amendment and the consummation of the transaction contemplated
hereby and by the other Loan Documents executed and delivered hereunder shall be
approved by the Administrative Agent and its legal counsel, and there shall have
been furnished to the Administrative Agent by the Borrower, at the Borrower's
expense, such agreements, opinions of counsel, title opinions, and other records
and information, in form, substance, scope and methodology satisfactory to the
Administrative Agent in its sole discretion, as it may reasonably have requested
for that purpose.

      r.    Other Documents. The Administrative Agent shall have received such
other legal opinions, instruments and documents as any of the Agents or their
counsel may have reasonably requested.

      Section 5. Representations and Warranties. To induce the Administrative
Agent and the Lenders to enter into this First Amendment, the Borrower hereby
represents and warrants or reaffirms, in the case of clauses (a) and (b), as of
the First Amendment Effective Date, the following:

                                        9
<PAGE>

      a.    After giving effect to the Subsidiary formations and the
restructuring consented to by the Administrative Agent and the Lenders pursuant
to Section 3 hereof, all of the representations and warranties set forth in the
Credit Agreement and in this First Amendment are true and correct on and as of
the date hereof (or, if stated to have been made expressly as of an earlier
date, were true and correct in all material respects as of such date).

      b.    No event or events have occurred that, individually or in the
aggregate, could reasonably be expected to have a Material Adverse Effect.

      c.    Each Obligor has all necessary power and authority to execute,
deliver and perform its obligations under the Merger Documents to which it is a
party; the execution, delivery and performance by each Obligor of the Merger
Documents to which it is a party have been duly authorized by all necessary
action on its part; and the Merger Documents to which each Obligor is a party
constitute the legal, valid and binding obligations of such Obligor, enforceable
in accordance with their terms, except to the extent that enforcement may be
subject to any applicable bankruptcy, insolvency or similar laws generally
affecting the enforcement of creditors' rights.

      d.    Neither the execution and delivery of the Merger Documents, nor
compliance with the terms and provisions thereof, will conflict with or result
in a breach of, or require any consent that has not been obtained as of the
First Amendment Effective Date, the Organic Documents of any Obligor, or any
Governmental Rule or any material agreement or instrument to which any Obligor
is a party or by which it is bound or to which it or its Properties are subject,
or constitute a default under any such agreement or instrument, or result in the
creation or imposition of any Lien upon any of the revenues or assets of any
Obligor pursuant to the terms of any such agreement or instrument other than the
Liens created by the Loan Documents to which the Obligor is a party. No
Governmental Approvals or third party consents are necessary for the execution,
delivery or performance by any Obligor of the Merger Documents to which it is a
party or for the validity or enforceability thereof.

      Section 6. Limitations; Renewal and Continuation of Existing Loans.

      a.    Except as expressly waived or agreed herein, all covenants,
obligations and agreements of the Borrower contained in the Credit Agreement
shall remain in full force and effect in accordance with their terms. Without
limitation of the foregoing, the consents, waivers and agreements set forth
herein are limited precisely to the extent set forth herein and shall not be
deemed to (i) be a consent or agreement to, or waiver or modification of, any
other term or condition of the Credit Agreement or any of the documents referred
to therein, or (ii) except as expressly set forth herein, prejudice any right
or rights that the Administrative Agent or any Lender may now have or may have
in the future under or in connection with the Credit Agreement or any of the
documents referred to therein. Except as expressly modified hereby, the terms
and provisions of the Credit Agreement and any other documents or instruments
executed in connection with any of the foregoing, are and shall remain in full
force and effect, and the same are hereby ratified and confirmed by the Borrower
in all respects. The parties hereto expressly acknowledge and agree that nothing
herein or in any instrument or document issued pursuant to this First Amendment,
and no act or thing done in connection with or in furtherance of this First
Amendment, constitutes or will constitute any extinguishment of any Obligations
of the Borrower or any other Obligor under the Loan Documents, or the creation
of any new debt obligations relating to the Loans.

                                       10
<PAGE>

      b.    Upon the effectiveness of this First Amendment, all of the
Obligations outstanding under the Credit Agreement as of the date of such
effectiveness shall hereby be restructured, rearranged, renewed, extended and
continued under the Credit Agreement (as amended hereby) and all Loans
outstanding under the Credit Agreement as of the date of such effectiveness
shall hereby become Loans outstanding under the Credit Agreement (as amended
hereby).

      Section 7. Governing Law; Entire Agreement. THIS FIRST AMENDMENT, THE
CREDIT AGREEMENT, AS AMENDED HEREBY, AND EACH OTHER LOAN DOCUMENT SHALL EACH BE
DEEMED TO BE A CONTRACT MADE UNDER AND GOVERNED BY, AND CONSTRUED IN ACCORDANCE
WITH, THE LAWS OF THE STATE OF NEW YORK (WITHOUT GIVING EFFECT TO THE PRINCIPLES
THEREOF RELATING TO CONFLICT OF LAW, EXCEPT SECTION 5-1401 OF THE NEW YORK
GENERAL OBLIGATIONS LAW). This First Amendment, the Credit Agreement, as amended
hereby, and the other Loan Documents constitute the entire understanding among
the parties hereto with respect to the subject matter hereof and supersede any
prior agreements, written or oral, with respect thereto.

      Section 8. Severability. Any provision of this First Amendment that is
prohibited or unenforceable in any jurisdiction shall, as to such provision and
such jurisdiction, be ineffective to the extent of such prohibition or
unenforceability without invalidating the remaining provisions of this First
Amendment or affecting the validity or enforceability of such provision in any
other jurisdiction.

      Section 9. Execution in Counterparts. This First Amendment may be executed
by the parties hereto in several counterparts, each of which shall be deemed to
be an original and all of which shall constitute together but one and the same
agreement.

      Section 10. Headings. The various headings of this First Amendment are
inserted for convenience only and shall not affect the meaning or interpretation
of this First Amendment or any other Loan Document, or any provisions hereof or
thereof.

      Section 11. Successors and Assigns. The provisions of this First Amendment
shall be binding upon and inure to the benefit of the parties hereto and their
respective successors and assigns permitted under the terms of the Credit
Agreement.

            THERE ARE NO UNWRITTEN ORAL AGREEMENTS AMONG THE PARTIES.

                      [SIGNATURES BEGIN ON FOLLOWING PAGE]

                                       11
<PAGE>

      IN WITNESS WHEREOF, the Borrower, the Administrative Agent and the Lenders
have executed this First Amendment as of the date first above written.

                                  BORROWER

                                  NEC OPERATING LLC

                                  By: NEG Holding LLC,
                                      sole Member

                                      By: Gascon Partners,
                                          Managing Member

                                          By: Cigas Corp.,
                                              Managing General Partner

                                              By: /s/ Edward E. Mattner
                                                  ------------------------------
                                              Name: Edward E. Mattner
                                              Title: President

                                              Address:

                                              1400 One Energy Square
                                              4925 Greenville Avenue
                                              Dallas, TX 75206
                                              Attention: Mr. Randall D. Cooley
                                              Telephone No.: (214) 692-9211
                                              Facsimile No.: (214) 692-9310
                                              E-Mail: rcooley@negx.com

                        Signature Page to First Amendment

                                       S-1
<PAGE>

                                  MIZUHO CORPORATE BANK, LTD.,
                                  as Administrative Agent and as a Lender

                                  By: /s/ Thomas W. Boylan
                                      --------------------------------
                                      Name: THOMAS W. BOYLAN
                                      Title: SR. VICE PRESIDENT

                        Signature Page to First Amendment

                                       S-2
<PAGE>

                                  BANK OF TEXAS, N.A.,
                                  as Co-Agent, Collateral Agent and as a Lender

                                  By: /s/ Timothy E. Merrell
                                      --------------------------------
                                      Name: Timothy E. Merrell
                                      Title: Senior Vice President

                        Signature Page to First Amendment

                                       S-3
<PAGE>

                                  THE BANK OF NOVA SCOTIA,
                                  as Co-Agent and as a Lender

                                  BY: /s/ N. Bell
                                      --------------------------------
                                      Name: N. Bell
                                      Title: Senior Manager

                        Signature Page to First Amendment

                                       S-4
<PAGE>

                                  FIRST AMERICAN BANK, SSB,
                                  as Issuer and as a Lender

                                  By: /s/ Angela McCracken
                                      ------------------------------
                                      Name: Angela McCracken
                                      Title: Vice President
                                             Energy Banking Group

                        Signature Page to First Amendment

                                       S-5
<PAGE>

                                             Item 6.8 of the Disclosure Schedule
                                to the Credit Agreement, as amended by the First

Item 6.8        Subsidiaries.

                NAME: Shana National LLC
                STATE OF ORGANIZATION: Delaware
                ORGANIZATIONAL NUMBER: 3432958
                PRINCIPAL PLACE OF BUSINESS: c/o National Energy Group, Inc.
                1400 One Energy Square 4925 Greenville Avenue Dallas Texas 75206
                CHIEF EXECUTIVE OFFICE: c/o National Energy Group, Inc. 1400 One
                Energy Square 4925 Greenville Avenue Dallas Texas 75206
                FEDERAL TAX IDENTIFICATION No: 75-2958833

                NAME: NGX GP of Delaware LLC
                STATE OF ORGANIZATION: Delaware
                ORGANIZATIONAL NUMBER: 3777770
                PRINCIPAL PLACE OF BUSINESS: c/o Starfire Holding Corporation,
                100 South Bedford Road, Suite 210, Mt. Kisco, New York 10549
                CHIEF EXECUTIVE OFFICE: c/o Starfire Holding Corporation, 100
                South Bedford Road, Suite 210, Mt. Kisco, New York 10549
                FEDERAL TAX IDENTIFICATION No: N/A

                NAME: NGX LP of Delaware LLC
                STATE OF ORGANIZATION: Delaware
                ORGANIZATIONAL NUMBER: 3773017
                PRINCIPAL PLACE OF BUSINESS: 10650 West Charleston Blvd.
                Suite 130, Las Vegas, Nevada 89135
                CHIEF EXECUTIVE OFFICE: 10650 West Charleston Blvd. Suite 130,
                Las Vegas, Nevada 89135
                FEDERAL TAX IDENTIFICATION No: N/A

                NAME: NGX Energy Limited Partnership
                STATE OF ORGANIZATION: Delaware
                ORGANIZATIONAL NUMBER: 3777773
                PRINCIPAL PLACE OF BUSINESS: c/o National Energy Group, Inc.
                400 One Energy Square 4925 Greenville Avenue Dallas Texas 75206
                CHIEF EXECUTIVE OFFICE: c/o National Energy Group, Inc. 1400 One
                Energy Square 4925 Greenville Avenue Dallas Texas 75206
                FEDERAL TAX IDENTIFICATION No: N/A