FIRST AMENDMENT TO THIRD AMENDED AND RESTATED CREDIT AGREEMENT

Exhibit 10.51

FIRST AMENDMENT TO THIRD AMENDED AND RESTATED CREDIT AGREEMENT

This FIRST AMENDMENT TO THIRD AMENDED AND RESTATED CREDIT AGREEMENT (this “Amendment”) is entered into as of March 11, 2013, by and among MRC ENERGY COMPANY, a Texas corporation (the “Borrower”), the LENDERS party hereto and ROYAL BANK OF CANADA, as Administrative Agent for the Lenders (in such capacity, the “Administrative Agent”). Unless otherwise expressly defined herein, capitalized terms used but not defined in this Amendment have the meanings assigned to such terms in the Credit Agreement (as defined below).

WITNESSETH:

WHEREAS, the Borrower, the Administrative Agent and the Lenders have entered into that certain Third Amended and Restated Credit Agreement, dated as of September 28, 2012 (as the same has been and may hereafter be amended, restated, supplemented or otherwise modified from time to time, the “Credit Agreement”); and

WHEREAS, the Borrower has requested that the Administrative Agent and the Lenders amend the Credit Agreement in certain respects and the Administrative Agent and the Lenders have agreed to do so on the terms and conditions hereinafter set forth.

NOW, THEREFORE, for and in consideration of the mutual covenants and agreements herein contained and other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged and confessed, the Borrower, the Administrative Agent and the Lenders hereby agree as follows:

SECTION 1. Amendments to Credit Agreement. Subject to the satisfaction or waiver in writing of each condition precedent set forth in Section 4 of this Amendment, and in reliance on the representations, warranties, covenants and agreements contained in this Amendment, the Credit Agreement shall be amended in the manner provided in this Section 1.

1.1 Amended Definitions. The following definitions in Section 1.1 of the Credit Agreement shall be and they hereby are amended and restated in their respective entireties to read as follows:

“Indebtedness” means (a) all indebtedness, obligations and liabilities of every nature, contingent or otherwise, of Borrower or any Guarantor to any of the Lenders, any of the Lenders’ Affiliates, the Administrative Agent, or the Issuing Lender, individually or collectively, under any Loan Document, whether for principal, interest, reimbursement of amounts drawn under any Letter of Credit, funding indemnification amounts, fees, expenses, indemnification or otherwise, (b) Lender Hedging Obligations, and (c) Lender Product Obligations, in each case whether existing on the date of this Agreement or arising thereafter, direct or indirect, joint or several, absolute or contingent, matured or unmatured, liquidated or unliquidated, secured or unsecured, including interest accruing subsequent to the filing of a petition or other action concerning bankruptcy or other similar proceedings, and all renewals, extensions, refinancings and replacements for the foregoing; provided; however, that Indebtedness shall not include any Excluded Swap Obligations.

“Lender Hedging Obligations” means all obligations arising from time to time under Commodity Hedging Agreements and Interest Rate Agreements permitted hereunder and entered into from time to time between any Credit Party, on the one hand and a Lender Counterparty on the other hand (including any such obligations under any Existing Commodity Hedging Agreements); provided, however, that if a Lender Counterparty ceases to be a Lender hereunder or an Affiliate of a Lender hereunder, Lender Hedging Obligations shall only include such obligations to the extent arising from transactions and confirmations entered into under Commodity Hedging Agreements and Interest Rate Agreements at any time such Person was a Lender or an Affiliate of a Lender hereunder, without giving effect to any extension, increases or modifications thereof which are made after such Person ceases to be a Lender or an Affiliate of a Lender hereunder.

“Secured Party” means each of the Administrative Agent, any Lender, any Lender Counterparty, or any Affiliate of any Lender to which any Indebtedness is owed, including any Lender Hedging Obligations and Lender Product Obligations, provided, however, that a Lender Counterparty and Lenders and Affiliates of Lenders to whom Lender Product Obligations or Lender Hedging Obligations are owed shall be a Secured Party (a) in the case of Lender Product Obligations, only while such Person (or, in the case of an Affiliate of a Lender, such Lender) is a Lender under this Agreement and (b) in the case of Lender Hedging Obligations, only with respect to obligations arising from transactions and confirmations entered into under Commodity Hedging Agreements and Interest Rate Agreements at any time such Person (or, in the case of an Affiliate of a Lender, such Lender) is a Lender under this Agreement.

1.2 Additional Definitions. The following definitions shall be and they hereby are added to Section 1.1 of the Credit Agreement in appropriate alphabetical order:

“Commodity Exchange Act” means the Commodity Exchange Act (7 U.S.C. § 1 et seq.), as amended from time to time, and any successor statute.

“Excluded Swap Obligation” means, with respect to any Guarantor, any Swap Obligation if, and to the extent that, all or a portion of the guarantee of such Guarantor of, or the grant by such Guarantor of a security interest to secure, such Swap Obligation (or any guarantee thereof) is or becomes illegal under the Commodity Exchange Act or any rule, regulation or order of the Commodity Futures Trading Commission (or the application or official interpretation of any thereof) by virtue of such Guarantor’s failure for any reason to constitute an “eligible contract participant” as defined in the Commodity Exchange Act and the regulations thereunder at the time the guarantee of such Guarantor or the grant of such security interest becomes effective with respect to such Swap Obligation. If a Swap Obligation arises under a master agreement governing more than one swap, such exclusion shall apply only to the portion of such Swap Obligation that is attributable to swaps for which such guarantee or security interest is or becomes illegal.

“Swap Obligation” means, with respect to any Guarantor, any obligation to pay or perform under any agreement, contract or transaction that constitutes a “swap” within the meaning of section 1a(47) of the Commodity Exchange Act.

1.3 Current Ratio. Clause (b) of Section 7.9 of the Credit Agreement shall be and it hereby is amended and restated in its entirety to read as follows:

(b) Current Ratio. Maintain as of the last day of each Fiscal Quarter, commencing with the Fiscal Quarter ending March 31, 2014, a Current Ratio of not less than 1.00 to 1.00.

1.4 Application of Proceeds of Collateral. Section 10.2 of the Credit Agreement shall be and it hereby is amended by deleting in its entirety the phrase “owing to a Lender or an Affiliate of a Lender” located in the second clause (d) thereof.

1.5 Concerning Lender Hedging Obligations and Lender Product Obligations. Section 13.19 of the Credit Agreement shall be and it hereby is amended and restated in its entirety to read as follows:

13.19 Concerning Lender Hedging Obligations and Lender Product Obligations. The benefit of the Collateral Documents and of the provisions of this Agreement relating to any collateral securing the Indebtedness shall also extend to the Lender Hedging Obligations and the Lender Product Obligations; provided that if any Lender Counterparty ceases to be a Lender or an Affiliate of a Lender hereunder, (a) such Lender Hedging Obligations shall only include obligations under Commodity Hedging Agreements and Interest Rate Agreements entered into with such Person to the extent arising from transactions and confirmations entered into at any time such Person was a Lender or an Affiliate of a Lender hereunder, without giving effect to any extension, increases or modifications thereof which are made after such Person ceases to be a Lender or an Affiliate of a Lender hereunder and (b) any obligations arising under Lender Products extended by such Person shall no longer constitute Lender Product Obligations. No Lender or any Affiliate of a Lender shall have any voting rights under any Loan Document or with respect to any Collateral, as a result of the existence of obligations owed to it under any Commodity Hedging Agreements, Interest Rate Agreements or agreements relating to Lender Products. All Commodity Hedging Agreements, Interest Rate Agreements and agreements relating to Lender Products, if any, are independent agreements governed by the written provisions of such agreements, which will remain in full force and effect, unaffected by any repayment, prepayment, acceleration, reduction, increase or change in the terms of the Advances or this Agreement, except as otherwise expressly provided in such agreements, and any payoff statement from any Lender relating to this Agreement shall not apply to such agreements except as otherwise expressly provided in such payoff statement.

1.6 Schedules. Schedule 1.2 of the Credit Agreement shall be and it hereby is amended and restated in its entirety and replaced with Schedule 1.2 attached hereto.

SECTION 2. Redetermined Borrowing Base. This Amendment shall constitute notice of a redetermination of the Borrowing Base and the Conforming Borrowing Base pursuant to Section 4.2 of the Credit Agreement, and the Administrative Agent, the Lenders and the Borrower hereby acknowledge that effective as of the date hereof, the Borrowing Base shall be $255,000,000 and the Conforming Borrowing Base shall be $220,000,000 and such redetermined Borrowing Base and Conforming Borrowing Base shall remain in effect until the date the Borrowing Base and the Conforming Borrowing Base are otherwise adjusted pursuant to the terms of the Credit Agreement. The redetermination of the Borrowing Base and the Conforming Borrowing Base contained herein shall constitute the Determination Date to occur on or about May 1, 2013.

SECTION 3. New Lenders and Reallocation and Increase of Revolving Credit Commitment Amounts. The Lenders have agreed among themselves to reallocate their respective Revolving Credit Commitment Amounts, and to, among other things, allow certain financial institutions identified by RBC Capital Markets¹ (“RBC Capital”), in its capacity as a Joint Lead Arranger, in consultation with the Borrower, to become a party to the Credit Agreement as a Lender (each, a “New Lender”). Each of the Administrative Agent and the Borrower hereby consent to (i) the reallocation of the Revolving Credit Commitment Amounts and (ii) each New Lender’s agreement to provide a Revolving Credit Commitment Amount. On the date this Amendment becomes effective and after giving effect to such reallocation and increase of the Revolving Credit Aggregate Commitment, the Revolving Credit Commitment Amount of each Lender shall be as set forth on Schedule 1.2 of this Amendment. Each Lender hereby consents to the Revolving Credit Commitment Amount set forth on Schedule 1.2 of this Amendment. The reallocation of the Revolving Credit Commitment Amounts among the Lenders, including the acquisition by each New Lender of an interest in the Revolving Credit Aggregate Commitment, shall be deemed to have been consummated pursuant to the terms of the Assignment and Assumption attached as Exhibit D to the Credit Agreement as if the Lenders, including each New Lender, had executed an Assignment and Assumption with respect to such reallocation. The Administrative Agent hereby waives the $3,500 processing and recordation fee set forth in Section 13.7(b)(iv) of the Credit Agreement with respect to the assignments and reallocations contemplated by this Section 3. To the extent requested by any Lender, and in accordance with Section 11.1 of the Credit Agreement, the Borrower shall pay to such Lender, within the time period prescribed by Section 11.1 of the Credit Agreement, any amounts required to be paid by the Borrower under Section 11.1 of the Credit Agreement in the event the payment of any principal of any Eurodollar-based Advance or the conversion of any Eurodollar-based Advance other than on the last day of an Interest Period applicable thereto is required in connection with the reallocation contemplated by this Section 3.

SECTION 4. Conditions. The amendments to the Credit Agreement contained in Section 1 of this Amendment, the redetermination of the Borrowing Base contained in Section 2 of this Amendment and the reallocation of the Revolving Credit Commitment Amounts contained in Section 3 of this Amendment shall be effective upon the satisfaction of each of the conditions set forth in this Section 4.

4.1 Execution and Delivery. The Administrative Agent shall have received a duly executed counterpart of (a) this Amendment signed by the Borrower and the Lenders and (b) the Consent and Reaffirmation attached hereto signed by each Guarantor.

4.2 No Default. No Default or Event of Default shall have occurred and be continuing or shall result from the effectiveness of this Amendment.

4.3 Fees. The Borrower and the Administrative Agent shall have executed and delivered a fee letter in connection with this Amendment, and the Administrative Agent shall have received the fees separately agreed upon in such fee letter.

4.4 Notes. The Administrative Agent shall have received Notes duly executed by the Borrower for each New Lender that requests a Note in accordance with Section 2.2(e) of the Credit Agreement.

4.5 Mortgages. The Credit Parties shall have executed and delivered to the Administrative Agent Mortgages and title information, in each case, reasonably satisfactory to the Administrative Agent with respect to the Oil and Gas Properties of the Credit Parties, or the portion thereof, as required by Sections 7.16 and 7.17 of the Credit Agreement.

4.6 Other Documents. The Administrative Agent shall have received such other instruments and documents incidental and appropriate to the transactions provided for herein as the Administrative Agent or its special counsel may reasonably request, and all such documents shall be in form and substance satisfactory to the Administrative Agent.

SECTION 5. Representations and Warranties. To induce the Lenders to enter into this Amendment, the Borrower hereby represents and warrants to the Lenders as follows:

5.1 Reaffirmation of Representations and Warranties. After giving effect to the amendments herein, each representation and warranty of the Borrower, the Parent and each other Credit Party contained in the Credit Agreement and in each of the other Loan Documents is true and correct in all material respects as of the date hereof (without duplication of any materiality qualifier contained therein), except to the extent any such representations and warranties are expressly limited to an earlier date, in which case, such representations and warranties shall continue to be true and correct in all material respects (without duplication of any materiality qualifier contained therein) as of such specified earlier date.

5.2 Corporate Authority; No Conflicts. The execution, delivery and performance by the Borrower of this Amendment and all documents, instruments and agreements contemplated herein are within the Borrower’s corporate powers, have been duly authorized by necessary action, require no action by or in respect of, or filing with, any court or agency of government and do not violate or constitute a default under any provision of any applicable law or other agreements binding upon the Borrower or result in the creation or imposition of any Lien upon any of the assets of the Borrower except for Liens permitted by Section 8.2 of the Credit Agreement and otherwise as permitted in the Credit Agreement.

5.3 Enforceability. This Amendment constitutes the valid and binding obligation of the Borrower enforceable in accordance with its terms, except as (i) the enforceability thereof may be limited by bankruptcy, insolvency or similar laws affecting creditor’s rights generally, and (ii) the availability of equitable remedies may be limited by equitable principles of general application.

5.4 No Default. As of the date hereof, both before and immediately after giving effect to this Amendment, no Default or Event of Default has occurred and is continuing.

SECTION 6. Miscellaneous.

6.1 Reaffirmation of Loan Documents and Liens. Any and all of the terms and provisions of the Credit Agreement and the Loan Documents shall, except as amended and modified hereby, remain in full force and effect and are hereby in all respects ratified and confirmed by the Borrower. The Borrower hereby agrees that the amendments and modifications herein contained shall in no manner affect or impair the liabilities, duties and obligations of the Borrower, the Parent or any other Credit Party under the Credit Agreement and the other Loan Documents or the Liens securing the payment and performance thereof, except as amended and modified hereby.

6.2 Parties in Interest. All of the terms and provisions of this Amendment shall bind and inure to the benefit of the parties hereto and their respective successors and assigns.

6.3 Further Assurances. The Borrower covenants and agrees from time to time, as and when requested by the Administrative Agent or the Lenders, to execute and deliver or cause to be executed or delivered, all such documents, instruments and agreements and to take or cause to be taken such further or other action as the Administrative Agent or the Lenders may reasonably deem necessary or desirable in order to carry out the intent and purposes of this Amendment.

6.4 Legal Expenses. The Borrower hereby agrees to pay all reasonable and documented out-of-pocket fees and expenses of special counsel to the Administrative Agent incurred by the Administrative Agent in connection with the preparation, negotiation and execution of this Amendment and all related documents.

6.5 Counterparts. This Amendment may be executed in one or more counterparts and by different parties hereto in separate counterparts each of which when so executed and delivered shall be deemed an original, but all such counterparts together shall constitute but one and the same instrument; signature pages may be detached from multiple separate counterparts and attached to a single counterpart so that all signature pages are physically attached to the same document. Delivery of photocopies of the signature pages to this Amendment by facsimile or electronic mail shall be effective as delivery of manually executed counterparts of this Amendment.

6.6 Complete Agreement. THIS AMENDMENT, THE CREDIT AGREEMENT, AND THE OTHER LOAN DOCUMENTS REPRESENT THE FINAL AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR ORAL AGREEMENTS OF THE PARTIES. THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

6.7 Headings. The headings, captions and arrangements used in this Amendment are, unless specified otherwise, for convenience only and shall not be deemed to limit, amplify or modify the terms of this Amendment, nor affect the meaning thereof.

6.8 Governing Law. This Amendment shall be construed in accordance with and governed by the laws of the State of Texas.

6.9 Severability. Any provision of this Amendment held to be invalid, illegal or unenforceable in any jurisdiction shall, as to such jurisdiction, be ineffective to the extent of such invalidity, illegality or unenforceability without affecting the validity, legality and enforceability of the remaining provisions hereof; and the invalidity of a particular provision in a particular jurisdiction shall not invalidate such provision in any other jurisdiction.

6.10 Reference to and Effect on the Loan Documents.

(a) This Amendment shall be deemed to constitute a Loan Document for all purposes and in all respects. Each reference in the Credit Agreement to “this Agreement,” “hereunder,” “hereof,” “herein” or words of like import, and each reference in the Credit Agreement or in any other Loan Document, or other agreements, documents or other instruments executed and delivered pursuant to the Credit Agreement to the “Credit Agreement”, shall mean and be a reference to the Credit Agreement as amended by this Amendment.

(b) The execution, delivery and effectiveness of this Amendment shall not operate as a waiver of any right, power or remedy of any Lender or the Administrative Agent under any of the Loan Documents, nor constitute a waiver of any provision of any of the Loan Documents.

[Signature pages follow.]

IN WITNESS WHEREOF, the parties have caused this Amendment to be duly executed by their respective authorized officers to be effective as of the date first above written.

Schedule 1.2

Percentages and Allocations

Revolving Credit

CONSENT AND REAFFIRMATION

Each of the undersigned (each a “Guarantor”) hereby (i) acknowledges receipt of a copy of the foregoing First Amendment to Third Amended and Restated Credit Agreement (the “First Amendment”); (ii) consents to the Borrower’s execution and delivery thereof; (iii) agrees to be bound by the terms of the First Amendment; (iv) affirms that nothing contained therein shall modify in any respect whatsoever its guaranty of the Indebtedness pursuant to the terms of the Guaranty or the Liens granted by it pursuant to the terms of the other Loan Documents to which it is a party securing payment and performance of the Indebtedness, (v) reaffirms that the Guaranty and the other Loan Documents to which it is a party and such Liens are and shall continue to remain in full force and effect and are hereby ratified and confirmed in all respects and (vi) represents and warrants to the Administrative Agent and the Lenders that, as of the date hereof, (x) all of the representations and warranties made by it in each of the Loan Documents to which it is a party are true and correct in all material respects (without duplication of any materiality qualifier contained therein), except to the extent any such representations and warranties are expressly limited to an earlier date, in which case, such representations and warranties shall continue to be true and correct in all material respects (without duplication of any materiality qualifier contained therein) as of such specified earlier date, and (y) no Default or Event of Default has occurred and is continuing. Although each Guarantor has been informed of the matters set forth herein and has acknowledged and agreed to same, each Guarantor understands that neither the Administrative Agent nor any of the Lenders have any obligation to inform any Guarantor of such matters in the future or to seek any Guarantor’s acknowledgment or agreement to future amendments or waivers for the Guaranty and other Loan Documents to which it is a party to remain in full force and effect, and nothing herein shall create such duty or obligation.

[SIGNATURE PAGES FOLLOW]

IN WITNESS WHEREOF, the undersigned has executed this Consent and Reaffirmation on and as of the date of the First Amendment.