COMMITMENT INCREASE AGREEMENT

Exhibit 4.5.1

COMMITMENT INCREASE AGREEMENT

This COMMITMENT INCREASE AGREEMENT (this “Agreement”) is made as of the 1^st day of September, 2006 by and among Chesapeake Exploration Limited Partnership, an Oklahoma limited partnership and Chesapeake Appalachia, L.L.C., an Oklahoma limited liability company (“Co-Borrowers”), Chesapeake Energy Corporation, an Oklahoma corporation (the “Company”), Union Bank of California, N.A., as administrative agent (“Administrative Agent”), Union Bank of California, N.A. and BNP Paribas, as Issuing Lenders (“Issuing Lenders”), and the Lenders listed on the attached Exhibit A (each a “Supplemental Lender”, and together the “Supplemental Lenders”).

1. Co-Borrowers, the Company, Administrative Agent and certain Lenders, as defined therein, are parties to that certain Sixth Amended and Restated Credit Agreement dated as of February 3, 2006 (as amended, supplemented, or restated, the “Credit Agreement”). All terms used herein and not otherwise defined shall have the same meaning given to them in the Credit Agreement.

2. Pursuant to Section 2.15 of the Credit Agreement, Co-Borrowers have the right to increase the Total Revolving Commitments by obtaining increases in Revolving Commitments of existing Lenders or additional Revolving Commitments from new Lenders, upon satisfaction of certain conditions. This Agreement requires only the signature of Co-Borrowers, Administrative Agent and the Supplemental Lenders so long as the Total Revolving Commitments are not increased above the amount permitted by Section 2.15 of the Credit Agreement.

3. The Supplemental Lenders are each either (a) an existing Lender which is increasing its Revolving Commitment or (b) a new Lender which is a lending institution whose identity Co-Borrowers, Administrative Agent and Issuing Lenders hereby approve by their signatures below.

4. In consideration of the foregoing, each Supplemental Lender, from and after the date hereof, shall have the Revolving Commitment listed next to its name on the attached Exhibit A, which, in the case of any existing Lender, shall replace the prior Revolving Commitment of such Lender in effect immediately prior to this Agreement, as listed next to its name on the attached Exhibit A, and any Supplemental Lender that is a new Lender hereby assumes all of the rights and obligations of a Lender under the Credit Agreement. This Agreement shall not affect the Revolving Commitments of Lenders who are not Supplemental Lenders.

5. Co-Borrowers have executed and delivered to each Supplemental Lender as of the date hereof, if requested by such Supplemental Lender, a new or replacement Note in the form attached to the Credit Agreement as Exhibit H to evidence the new or increased Revolving Commitment of such Supplemental Lender.

6. Each Supplemental Lender hereby represents and warrants that it has received a copy of the Credit Agreement and such financial statements, documents and information as it has

deemed appropriate to make its own credit analysis and decision to enter into this Agreement, on the basis of which it has made such analysis and decision independently and without reliance on Administrative Agent, Issuing Lenders or any other Lender. Each Supplemental Lender agrees that (1) it will, independently and without reliance on Administrative Agent, Issuing Lenders or any other Lender, and based on such documents and information as it shall deem appropriate at the time, continue to make its own credit decisions in taking or not taking action under the Loan Documents, and (2) it will perform in accordance with their terms all of the obligations which by the terms of the Loan Documents are required to be performed by it as a Lender.

7. On the effective date hereof, each Supplemental Lender shall make a Revolving Loan to Co-Borrowers to implement the provisions of Section 2.15 of the Credit Agreement.

8. This Agreement may not be amended, changed, waived or modified, except by a writing executed by the parties hereto. This Agreement, together with the Loan Documents, embodies the entire agreement among each Supplemental Lender, Co-Borrowers, the Company, Administrative Agent and Issuing Lenders with respect to the subject matter hereof and supersedes all other prior arrangements and understandings relating to the subject matter hereof.

9. This Agreement may be executed in any number of counterparts, each of which shall be deemed to be an original. Each such counterpart shall become effective when counterparts have been executed by all parties hereto. Delivery of an executed counterpart of this Agreement by facsimile transmission shall be effective as delivery of a manually executed counterpart of this Agreement.

This Agreement shall be governed by, and construed in accordance with, the laws governing the Credit Agreement.

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IN WITNESS WHEREOF, Administrative Agent, Co-Borrowers, the Company and the Supplemental Lenders have executed this Agreement as of the date shown above.

Exhibit A