FIFTH AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT

Exhibit 10.22.5.6

FIFTH AMENDMENT TO

AMENDED AND RESTATED CREDIT AGREEMENT

AMONG

PRIMEENERGY CORPORATION

PRIMEENERGY MANAGEMENT CORPORATION

PRIME OPERATING COMPANY

EASTERN OIL WELL SERVICE COMPANY

SOUTHWEST OILFIELD CONSTRUCTION COMPANY

EOWS MIDLAND COMPANY

GUARANTY BANK, FSB

AS AGENT AND LETTER OF CREDIT ISSUER

AND

THE LENDERS SIGNATORY HERETO

October 30, 2008

TABLE OF CONTENTS

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FIFTH AMENDMENT TO

AMENDED AND RESTATED CREDIT AGREEMENT

This FIFTH AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT (this “ Amendment ”) is made and entered into effective as of October 30, 2008 (the “ Effective Date ”), by and among PRIMEENERGY CORPORATION, a Delaware corporation, PRIMEENERGY MANAGEMENT CORPORATION, a New York corporation, PRIME OPERATING COMPANY, a Texas corporation, EASTERN OIL WELL SERVICE COMPANY, a West Virginia corporation, SOUTHWEST OILFIELD CONSTRUCTION COMPANY, an Oklahoma corporation, and EOWS MIDLAND COMPANY, a Texas corporation (collectively, the “ Borrower ”), each lender that is a signatory hereto (individually, together with its successors and assigns, a “ Lender ” and collectively together, with their respective successors and assigns, the “ Lenders ”) and GUARANTY BANK, FSB, a federal savings bank, as agent for the Lenders (in such capacity, together with its successors in such capacity pursuant to the terms of the Amended and Restated Credit Agreement referred to hereinafter, the “ Agent ”) and letter of credit issuer.

WITNESSETH :

WHEREAS, the above named parties did execute and exchange counterparts of that certain Amended and Restated Credit Agreement dated December 28, 2006, as amended to the Effective Date (as so amended, the “ Agreement ”), to which reference is here made for all purposes;

WHEREAS, the Borrower, the Lenders and the Agent are desirous of amending the Agreement in the particulars hereinafter set forth;

NOW, THEREFORE, in consideration of the mutual covenants and agreements of the parties to the Agreement, as set forth therein, and the mutual covenants and agreements of the parties hereto, as set forth herein, the Borrower, the Lenders and the Agent agree as follows:

ARTICLE I

DEFINITIONS

1.1 Terms Defined Above . As used herein, each of the terms “ Agent ,” “ Agreement ,” “ Amendment ,” “ Borrower ,” “ Effective Date ,” “ Lender ” and “ Lenders ” shall have the meaning assigned to such term hereinabove.

1.2 Terms Defined in Agreement . As used herein, each term defined in the Agreement shall have the meaning assigned thereto in the Agreement, unless expressly provided herein to the contrary.

1.3 References . References in this Amendment to Schedule, Exhibit, Article, or Section numbers shall be to Schedules, Exhibits, Articles, or Sections of this Amendment, unless expressly stated to the contrary. References in this Amendment to “hereby,” “herein,” “hereinafter,” “hereinabove,” “hereinbelow,” “hereof,” “hereunder” and words of similar import shall be to this Amendment in its entirety and not only to the particular Schedule, Exhibit,

Article, or Section in which such reference appears. Specific enumeration herein shall not exclude the general and, in such regard, the terms “includes” and “including” used herein shall mean “includes, without limitation,” or “including, without limitation,” as the case may be, where appropriate. Except as otherwise indicated, references in this Amendment to statutes, sections, or regulations are to be construed as including all statutory or regulatory provisions consolidating, amending, replacing, succeeding, or supplementing the statute, section, or regulation referred to. References in this Amendment to “writing” include printing, typing, lithography, facsimile reproduction, and other means of reproducing words in a tangible visible form. References in this Amendment to amendments and other contractual instruments shall be deemed to include all exhibits and appendices attached thereto and all subsequent amendments and other modifications to such instruments, but only to the extent such amendments and other modifications are not prohibited by the terms of this Amendment. References in this Amendment to Persons include their respective successors and permitted assigns.

1.4 Articles and Sections . This Amendment, for convenience only, has been divided into Articles and Sections; and it is understood that the rights and other legal relations of the parties hereto shall be determined from this instrument as an entirety and without regard to the aforesaid division into Articles and Sections and without regard to headings prefixed to such Articles or Sections.

1.5 Number and Gender . Whenever the context requires, reference herein made to the single number shall be understood to include the plural; and likewise, the plural shall be understood to include the singular. Definitions of terms defined in the singular or plural shall be equally applicable to the plural or singular, as the case may be, unless otherwise indicated. Words denoting sex shall be construed to include the masculine, feminine and neute