<PAGE>
                                                                 EXHIBIT 10.15.6

            FIRST AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT

         THIS FIRST AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT
(hereinafter referred to as the "First Amendment") executed as of the 27th day
of December, 2002, by and among RANGE RESOURCES CORPORATION, a Delaware
corporation ("Borrower") and BANK ONE, NA, a national banking association ("Bank
One"), and each of the financial institutions which is a party hereto (as
evidenced by the signature pages to this Amendment) or which may from time to
time become a party hereto pursuant to the provisions of Section 29 of the
Credit Agreement or any successor or assignee thereof (hereinafter collectively
referred to as "Lenders", and individually, "Lender") and Bank One, as
Administrative Agent ("Agent"), Fleet National Bank, as Co-Documentation Agent,
Fortis Capital Corp., as Co-Documentation Agent, JPMorgan Chase Bank, as
Co-Syndication Agent, Credit Lyonnais, New York Branch, as Co-Syndication Agent,
Banc One Capital Markets, Inc., as Joint Lead Arranger and Joint Bookrunner and
JPMorgan Chase Bank, as Joint Lead Arranger and Joint Bookrunner.

                                   WITNESSETH:

         WHEREAS, as of May 2, 2002, Borrower, Agent and the Lenders entered
into an Amended and Restated Credit Agreement pursuant to which the Lenders made
a credit facility available to Borrower (the "Credit Agreement"); and

         WHEREAS, the Borrower has requested that the Lenders agree to make
certain amendments to the Credit Agreement and the Lenders have agreed to do so
on the terms and conditions hereinafter set forth.

         NOW, THEREFORE, the parties agree to amend the Credit Agreement as
follows:

         1. Unless otherwise defined herein all defined terms used herein shall
have the same meaning as ascribed to such terms in the Credit Agreement.

         2. Section 1 of the Credit Agreement is hereby amended by deleting the
reference therein to "Guarantor" and substituting the following in lieu thereof:

                  "Guarantor" means GulfStar Energy, Inc., Range Energy I, Inc.,
         Range HoldCo, Inc., Range Production Company, Range Energy Ventures
         Corporation and Range Energy Finance Corporation."

         3. Section 13 of the Credit Agreement is hereby amended in the
following respects:

         (a)   By deletion of the references to "REFC" in Section 13(a)(ii).

         (b)   By deletion of the references to "REFC" in Section 13(h)(vi).

         (c)   By deletion of the references to "REFC" in Section 13(i)(ii).

         (d)   By the addition of new Subsections 13(j)(iv) and 13(j)(v) as
               follows:



<PAGE>
                           "(iv) loans or advances to REFC on or before December
                  31, 2002 not exceeding $12,900,000; or

                           (v) loans or advances to REFC after December 31, 2002
                  not exceeding at any time the lesser of (i) the total amount
                  of cash received by Borrower from REFC or (ii) $10,000,000."

                  (c) By the addition of the following sentence to the end of
         Section 13(b), (c), (d), (e) and (f) as follows:

                           "GLEP and the results of its financial performance
                  shall be excluded from the aforesaid calculation."

         4. Except to the extent its provisions are specifically amended,
modified or superseded by this First Amendment, the representations, warranties
and affirmative and negative covenants of the Borrower contained in the Credit
Agreement are incorporated herein by reference for all purposes as if copied
herein in full. The Borrower hereby restates and reaffirms each and every term
and provision of the Credit Agreement, as amended, including, without
limitation, all representations, warranties and affirmative and negative
covenants. Except to the extent its provisions are specifically amended,
modified or superseded by this First Amendment, the Credit Agreement, as
amended, and all terms and provisions thereof shall remain in full force and
effect, and the same in all respects are confirmed and approved by the Borrower
and the Lenders.

         5. This First Amendment shall be effective as of the date first above
written, but only upon the satisfaction of the conditions precedent set forth in
Paragraph 6 hereof (the "First Amendment Effective Date").

         6. The obligations of Lenders under this First Amendment shall be
subject to the following conditions precedent:

                  (a) Execution and Delivery. The Borrower and each Guarantor
         shall have executed and delivered this First Amendment, and other
         required documents, and, REFC shall, in addition, have executed and
         delivered its Guaranty in the form of Exhibit "A" hereto, all in form
         and substance satisfactory to the Agent;

                  (b) Legal Opinion. The Agent shall have received from REFC's
         legal counsel a favorable legal opinion in form and substance
         satisfactory to Agent;

                  (c) Resolutions. The Agent shall have received appropriate
         certified resolutions of Borrower and each Guarantor;

                  (d) Good Standing. The Agent shall have received evidence of
         existence and good standing for Borrower and each Guarantor;




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<PAGE>
                  (e) Certificates of Incorporation and Bylaws. The Agent shall
         have received copies of Certificates of Incorporation for Borrower and
         each Guarantor (including REFC) together with all amendments thereto,
         appropriately certified by governmental authority in the jurisdiction
         of incorporation of Borrower and each Guarantor, and a copy of the
         Bylaws of Borrower and each Guarantor, and all amendments thereto,
         certified by one or more officers of Borrower and each Guarantor, as
         the case may be, as being true, correct and complete;

                  (f) Incumbency. To the extent not already received, the Agent
         shall have received a signed Certificate of Borrower and each
         Guarantor, certifying the names of the officers of Borrower and each
         Guarantor authorized to sign loan documents on behalf of Borrower and
         each Guarantor, together with the true signatures of each such officer.
         The Agent may conclusively rely on each such Certificate until the
         Agent receives a further Certificate of Borrower and/or any Guarantor
         canceling or amending the prior Certificate and submitting signatures
         of the officers named in such further Certificate;

                  (g) Payoff of Indebtedness of REFC. The Agent shall have
         received satisfactory evidence that all indebtedness owed pursuant to
         that certain Credit Agreement dated as of December 14, 1999 among Range
         Energy Finance Corporation, Credit Lyonnais New York Branch, and
         Compass Bank shall have been paid in full and all Liens given to secure
         the same shall have been, or shall be, within a reasonable time,
         released:

                  (h) Representations and Warranties. The representations and
         warranties of the Borrowers under this First Amendment are true and
         correct in all material respects as of such date, as if then made
         (except to the extent that such representations and warranties related
         solely to an earlier date);

                  (i) No Event of Default. No Event of Default shall have
         occurred and be continuing nor shall any event have occurred or failed
         to occur which, with the passage of time or service of notice, or both,
         would constitute an Event of Default;

                  (j) Other Documents. The Agent shall have received such other
         instruments and documents incidental and appropriate to the transaction
         provided for herein as the Agent or its counsel may reasonably request,
         and all such documents shall be in form and substance satisfactory to
         the Agent;

                  (k) Legal Matters Satisfactory. All legal matters incident to
         the consummation of the transactions contemplated hereby shall be
         reasonably satisfactory to special counsel for the Agent retained at
         the expense of Borrower.

         7. Borrower hereby represents and warrants that all factual information
heretofore and contemporaneously furnished by or on behalf of Borrower to Agent
for purposes of or in connection with this First Amendment does not contain any
untrue statement of a material fact or omit to state any material fact necessary
to keep the statements contained herein or therein from being misleading. Each
of the foregoing representations and warranties shall constitute a


                                       -3-


<PAGE>
representation and warranty of Borrower made under the Credit Agreement, and it
shall be an Event of Default if any such representation and warranty shall prove
to have been incorrect or false in any material respect at the time given. Each
of the representations and warranties made under the Credit Agreement (including
those made herein) shall survive and not be waived by the execution and delivery
of this First Amendment or any investigation by Lenders.

         8. The Borrower agrees to indemnify and hold harmless the Lenders and
their respective officers, employees, agents, attorneys and representatives
(singularly, an "Indemnified Party", and collectively, the "Indemnified
Parties") from and against any loss, cost, liability, damage or expense
(including the reasonable fees and out-of-pocket expenses of counsel to the
Lender, including all local counsel hired by such counsel) ("Claim") incurred by
the Lenders in investigating or preparing for, defending against, or providing
evidence, producing documents or taking any other action in respect of any
commenced or threatened litigation, administrative proceeding or investigation
under any federal securities law, federal or state environmental law, or any
other statute of any jurisdiction, or any regulation, or at common law or
otherwise, which is alleged to arise out of or is based upon any acts, practices
or omissions or alleged acts, practices or omissions of the Borrower or its
agents or arises in connection with the duties, obligations or performance of
the Indemnified Parties in negotiating, preparing, executing, accepting,
keeping, completing, countersigning, issuing, selling, delivering, releasing,
assigning, handling, certifying, processing or receiving or taking any other
action with respect to the Loan Documents and all documents, items and materials
contemplated thereby even if any of the foregoing arises out of an Indemnified
Party's ordinary negligence. The indemnity set forth herein shall be in addition
to any other obligations or liabilities of the Borrower to the Lenders hereunder
or at common law or otherwise, and shall survive any termination of this First
Amendment, the expiration of the Loan and the payment of all indebtedness of the
Borrower to the Lenders hereunder and under the Notes, provided that the
Borrower shall have no obligation under this section to the Lenders with respect
to any of the foregoing arising out of the gross negligence or willful
misconduct of the Lenders. If any Claim is asserted against any Indemnified
Party, the Indemnified Party shall endeavor to notify the Borrower of such Claim
(but failure to do so shall not affect the indemnification herein made except to
the extent of the actual harm caused by such failure). The Indemnified Party
shall have the right to employ, at the Borrower's expense, counsel of the
Indemnified Parties' choosing and to control the defense of the Claim. The
Borrower may at its own expense also participate in the defense of any Claim.
Each Indemnified Party may employ separate counsel in connection with any Claim
to the extent such Indemnified Party believes it reasonably prudent to protect
such Indemnified Party. THE PARTIES INTEND FOR THE PROVISIONS OF THIS SECTION TO
APPLY TO AND PROTECT EACH INDEMNIFIED PARTY FROM THE CONSEQUENCES OF STRICT
LIABILITY IMPOSED OR THREATENED TO BE IMPOSED ON ANY INDEMNIFIED PARTY AS WELL
AS FROM THE CONSEQUENCES OF ITS OWN NEGLIGENCE, WHETHER OR NOT THAT NEGLIGENCE
IS THE SOLE, CONTRIBUTING, OR CONCURRING CAUSE OF ANY CLAIM.

         9. This First Amendment may be executed in any number of counterparts
and all of such counterparts taken together shall be deemed to constitute one
and the same instrument.

         10. WRITTEN CREDIT AGREEMENT. THE CREDIT AGREEMENT, AS AMENDED BY THIS
FIRST AMENDMENT REPRESENTS THE FINAL AGREEMENT BETWEEN AND AMONG THE PARTIES AND
MAY NOT BE CONTRADICTED BY

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EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES.
THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN AND AMONG THE PARTIES.

         11. The Guarantors hereby consent to the execution of this First
Amendment by the Borrower and reaffirms their guaranties of all of the
obligations of the Borrower to the Lenders. Borrower and Guarantors acknowledge
and agree that the renewal, extension and amendment of the Credit Agreement
shall not be considered a novation of account or new contract but that all
existing rights, titles, powers, and estates in favor of the Lenders constitute
valid and existing obligations in favor of the Lenders. Borrower and Guarantors
each confirm and agree that (a) neither the execution of this First Amendment or
any other Loan Document nor the consummation of the transactions described
herein and therein shall in any way effect, impair or limit the covenants,
liabilities, obligations and duties of the Borrower and the Guarantors under the
Loan Documents and (b) the obligations evidenced and secured by the Loan
Documents continue in full force and effect. Each Guarantor hereby further
confirms that it unconditionally guarantees to the extent set forth in their
respective Guaranties the due and punctual payment and performance of any and
all amounts and obligations owed to the Lenders under the Credit Agreement or
the other Loan Documents.

         IN WITNESS WHEREOF, the parties have caused this First Amendment to
Credit Agreement to be duly executed as of the date first above written.

                                    BORROWER:

                                    RANGE RESOURCES CORPORATION
                                    a Delaware corporation



                                    By: /s/ EDDIE LEBLANC
                                        --------------------------------------
                                        Eddie LeBlanc, Chief Financial Officer


                                    GUARANTORS:

                                    RANGE ENERGY I, INC.
                                    a Delaware corporation



                                    By: /s/ EDDIE LEBLANC
                                        --------------------------------------
                                    Name:   Eddie LeBlanc
                                         -------------------------------------
                                            Senior Vice President and
                                    Title:  Chief Financial Officer
                                          ------------------------------------






                                      -5-
<PAGE>
                                    RANGE HOLDCO, INC.
                                    a Delaware corporation



                                    By: /s/ EDDIE LEBLANC
                                        --------------------------------------
                                    Name:   Eddie LeBlanc
                                         -------------------------------------
                                            Senior Vice President and
                                    Title:  Chief Financial Officer
                                          ------------------------------------


                                    RANGE PRODUCTION COMPANY
                                    a Delaware corporation



                                    By: /s/ EDDIE LEBLANC
                                        --------------------------------------
                                    Name:   Eddie LeBlanc
                                         -------------------------------------
                                            Senior Vice President and
                                    Title:  Chief Financial Officer
                                          ------------------------------------


                                    RANGE ENERGY VENTURES
                                    CORPORATION, a Delaware corporation



                                    By: /s/ EDDIE LEBLANC
                                        --------------------------------------
                                    Name:   Eddie LeBlanc
                                         -------------------------------------
                                            Senior Vice President and
                                    Title:  Chief Financial Officer
                                          ------------------------------------


                                    GULFSTAR ENERGY, INC.
                                    a Delaware corporation



                                    By: /s/ EDDIE LEBLANC
                                        --------------------------------------
                                    Name:   Eddie LeBlanc
                                         -------------------------------------
                                            Senior Vice President and
                                    Title:  Chief Financial Officer
                                          ------------------------------------


                                    RANGE ENERGY FINANCE CORPORATION
                                    a Delaware corporation



                                    By: /s/ EDDIE LEBLANC
                                        --------------------------------------
                                    Name:   Eddie LeBlanc
                                         -------------------------------------
                                            Senior Vice President and
                                    Title:  Chief Financial Officer
                                          ------------------------------------






                                      -6-
<PAGE>
                                    LENDERS:

                                    BANK ONE, NA, a national
                                    banking association (Main Office Chicago)
                                    as a Lender and Administrative Agent



                                    By: /s/ WM. MARK CRANMER
                                        --------------------------------------
                                            Wm. Mark Cranmer
                                            Director, Capital Markets









                                      -7-
<PAGE>
                                    BANK OF SCOTLAND



                                    By: /s/ ELIZABETH WILSON
                                        --------------------------------------
                                    Name:   Elizabeth Wilson
                                         -------------------------------------
                                    Title:  Senior Director
                                          ------------------------------------












                                      -8-
<PAGE>
                                    JPMORGAN CHASE BANK



                                    By: /s/ ROBERT C. MERTENSOTTO
                                        --------------------------------------
                                    Name:   Robert C. Mertensotto
                                         -------------------------------------
                                    Title:   Managing Director
                                          ------------------------------------












                                      -9-
<PAGE>
                                    COMPASS BANK



                                    By: /s/ JOHN M. FALBO
                                        ----------------------------------------
                                            John M. Falbo, Senior Vice President










                                      -10-
<PAGE>
                                    CREDIT LYONNAIS, NEW YORK BRANCH



                                    By: /s/ OLIVIER AUDEMARD
                                        --------------------------------------
                                    Name:   Olivier Audemard
                                         -------------------------------------
                                    Title:  Senior Vice President
                                          ------------------------------------












                                      -11-
<PAGE>
                                    FLEET NATIONAL BANK



                                    By: /s/ JEFFREY H. RATHKAMP
                                        --------------------------------------
                                    Name:   Jeffrey H. Rathkamp
                                         -------------------------------------
                                    Title:  Vice President
                                          ------------------------------------








                                      -12-
<PAGE>
                                    FORTIS CAPITAL CORP.



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




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









                                      -13-
<PAGE>
                                    NATEXIS BANQUES POPULAIRES



                                    By: /s/ DONOVAN C. BROUSSARD
                                        --------------------------------------
                                    Name:   Donovan C. Broussard
                                         -------------------------------------
                                    Title:  Vice President
                                          ------------------------------------



                                    By: /s/ LOUIS P. LAVILLE, III
                                        --------------------------------------
                                    Name:   Louis P. Laville, III
                                         -------------------------------------
                                    Title:  Vice President and Group Manager
                                          ------------------------------------








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