the Subsidiary Guarantors named herein 8 1/2% SENIOR NOTES DUE 2012

EXHIBIT 4.2.1

CHESAPEAKE ENERGY CORPORATION

and

the Subsidiary Guarantors named herein

8 1/2% SENIOR NOTES DUE 2012

THIRTEENTH SUPPLEMENTAL INDENTURE

DATED AS OF May 1, 2003

THE BANK OF NEW YORK

as successor Trustee to

United States Trust Company of New York

THIS THIRTEENTH SUPPLEMENTAL INDENTURE, dated as of May 1, 2003, 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 successor to United States Trust Company 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 March 15, 1997, as supplemented prior to the date hereof (the “Indenture”), pursuant to which the Company has originally issued $150,000,000 in principal amount of 8 1/2% Senior Notes due 2012 (the “Notes”); and

WHEREAS, Section 9.1(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 MC Mineral Company, L.L.C. as a Restricted Subsidiary of the Company and desires to add such 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 Thirteenth 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 Thirteenth 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 Thirteenth Supplemental Indenture shall become effective immediately upon its execution and delivery by each of the Company, the Subsidiary Guarantors and the Trustee.

ARTICLE 2

From this date, in accordance with Section 10.3 and by executing this Thirteenth Supplemental Indenture, MC Mineral Company, L.L.C., an Oklahoma limited liability company, is subject to the provisions of the Indenture as a Subsidiary Guarantor to the extent provided for in Article X 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 Thirteenth Supplemental Indenture. This Thirteenth 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 MC Mineral Company, L.L.C. 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 THIRTEENTH SUPPLEMENTAL INDENTURE.

Section 3.05. The parties may sign any number of copies of this Thirteenth 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]

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

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