CHESAPEAKE ENERGY CORPORATION

Exhibit 4.2.1

CHESAPEAKE ENERGY CORPORATION

and

the Subsidiary Guarantors named herein

7.00% SENIOR NOTES DUE 2014

FOURTH SUPPLEMENTAL INDENTURE

DATED AS OF JULY 15, 2005

THE BANK OF NEW YORK

as Trustee

THIS FOURTH SUPPLEMENTAL INDENTURE, dated as of July 15, 2005, is among Chesapeake Energy Corporation, an Oklahoma corporation (the “Company”), each of the parties identified under the caption “Subsidiary Guarantors” on the signature page hereto (the “Subsidiary Guarantors”) and The Bank of New York, as Trustee.

RECITALS

WHEREAS, the Company, the Subsidiary Guarantors a party thereto and the Trustee entered into an Indenture, dated as of August 2, 2004, as supplemented prior to the date hereof (the “Indenture”), pursuant to which the Company has originally issued $300,000,000 in principal amount of 7.00% Senior Notes due 2014 (the “Notes”); and

WHEREAS, Section 9.01(3) of the Indenture provides that the Company, the Subsidiary Guarantors and the Trustee may amend or supplement the Indenture without notice to or consent of any Holder to reflect the addition of any Subsidiary Guarantor, as provided for in the Indenture; and

WHEREAS, the Board of Directors of the Company has designated Chesapeake Eagle Canada Corp. as a Restricted Subsidiary of the Company and desires to add the entity as a Subsidiary Guarantor under the Indenture; and

WHEREAS, all acts and things prescribed by the Indenture, by law and by the charter and the bylaws (or comparable constituent documents) of the Company, of the Subsidiary Guarantors and of the Trustee necessary to make this Fourth Supplemental Indenture a valid instrument legally binding on the Company, the Subsidiary Guarantors and the Trustee, in accordance with its terms, have been duly done and performed;

NOW, THEREFORE, to comply with the provisions of the Indenture and in consideration of the above premises, the Company, the Subsidiary Guarantors and the Trustee covenant and agree for the equal and proportionate benefit of the respective Holders of the Notes as follows:

ARTICLE 1

Section 1.01. This Fourth Supplemental Indenture is supplemental to the Indenture and does and shall be deemed to form a part of, and shall be construed in connection with and as part of, the Indenture for any and all purposes.

Section 1.02. This Fourth Supplemental Indenture shall become effective immediately upon its execution and delivery by each of the Company, the Subsidiary Guarantors and the Trustee.

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ARTICLE 2

Section 2.01. From this date, in accordance with Section 10.03 of the Indenture and by executing this Fourth Supplemental Indenture, Chesapeake Eagle Canada Corp., a New Brunswick corporation, is subject to the provisions of the Indenture as Subsidiary Guarantor to the extent provided for in Article Ten thereunder.

ARTICLE 3

Section 3.01. Except as specifically modified herein, the Indenture and the Notes are in all respects ratified and confirmed (mutatis mutandis) and shall remain in full force and effect in accordance with their terms with all capitalized terms used herein without definition having the same respective meanings ascribed to them as in the Indenture.

Section 3.02. Except as otherwise expressly provided herein, no duties, responsibilities or liabilities are assumed, or shall be construed to be assumed, by the Trustee by reason of this Fourth Supplemental Indenture. This Fourth Supplemental Indenture is executed and accepted by the Trustee subject to all the terms and conditions set forth in the Indenture with the same force and effect as if those terms and conditions were repeated at length herein and made applicable to the Trustee with respect hereto.

Section 3.03. The Company hereby notifies the Trustee that Chesapeake Eagle Canada Corp. has been designated by the Board of Directors of the Company as a Restricted Subsidiary (as that term is defined in the Indenture).

Section 3.04. THE LAW OF THE STATE OF NEW YORK SHALL GOVERN AND BE USED TO CONSTRUE AND ENFORCE THIS FOURTH SUPPLEMENTAL INDENTURE.

Section 3.05. The parties may sign any number of copies of this Fourth Supplemental Indenture. Each signed copy shall be an original, but all of such executed copies together shall represent the same agreement.

[NEXT PAGE IS SIGNATURE PAGE]

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IN WITNESS WHEREOF, the parties hereto have caused this Fourth Supplemental Indenture to be duly executed, all as of the date first written above.

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