SUPPLEMENT AND AMENDMENT TO DEED OF TRUST, MORTGAGE, LINE OF CREDIT MORTGAGE, ASSIGNMENT, SECURITY AGREEMENT, FIXTURE FILING AND FINANCING STATEMENT

EXHIBIT 10.3

A CARBON, PHOTOGRAPHIC, FACSIMILE, OR OTHER REPRODUCTION OF THIS INSTRUMENT IS SUFFICIENT AS A FINANCING STATEMENT.

THIS INSTRUMENT CONTAINS AFTER-ACQUIRED PROPERTY PROVISIONS, SECURES PAYMENT OF FUTURE ADVANCES, AND COVERS PROCEEDS OF COLLATERAL.

THIS INSTRUMENT COVERS, AMONG OTHER THINGS, (A) GOODS WHICH ARE OR ARE TO BECOME FIXTURES RELATED TO THE REAL PROPERTY DESCRIBED HEREIN, AND (B) AS-EXTRACTED COLLATERAL RELATED TO THE REAL PROPERTY DESCRIBED HEREIN (INCLUDING, WITHOUT LIMITATION, OIL, GAS, OTHER MINERALS AND OTHER SUBSTANCES OF VALUE WHICH MAY BE EXTRACTED FROM THE EARTH AND ACCOUNTS ARISING OUT OF THE SALE AT THE WELLHEAD OR MINEHEAD THEREOF).  THIS INSTRUMENT IS TO BE FILED FOR RECORD, AMONG OTHER PLACES, IN THE REAL ESTATE OR COMPARABLE RECORDS OF THE COUNTIES AND/OR PARISHES REFERENCED IN EXHIBIT A-1 HERETO AND SUCH FILING SHALL SERVE, AMONG OTHER PURPOSES, AS A FIXTURE FILING AND AS A FINANCING STATEMENT COVERING AS-EXTRACTED

COLLATERAL. THE MORTGAGOR HAS AN INTEREST OF RECORD IN THE REAL ESTATE AND/OR IMMOVABLE PROPERTY CONCERNED, WHICH INTEREST IS DESCRIBED IN SECTION 1.1 OF THIS INSTRUMENT.

A POWER OF SALE HAS BEEN GRANTED IN THIS MORTGAGE. A POWER OF SALE MAY ALLOW AGENT (AS HEREINAFTER DEFINED) OR TRUSTEE (AS HEREINAFTER DEFINED) TO TAKE THE MORTGAGED PROPERTIES AND SELL THEM WITHOUT GOING TO COURT IN A FORECLOSURE ACTION UPON DEFAULT BY THE MORTGAGOR (AS HEREINAFTER DEFINED) UNDER THIS MORTGAGE .

Note to North Dakota Recording Officer: Attached to counterparts hereto to be filed in the State of North Dakota (as Annex II-A) is an adequate statement of the interest.

EMPLOYER IDENTIFICATION NUMBER OF MORTGAGOR: 41-05 18430

ORGANIZATIONAL IDENTIFICATION NUMBER OF MORTGAGOR: 0044728

THIS SUPPLEMENT AND AMENDMENT TO DEED OF TRUST, MORTGAGE, LINE OF CREDIT MORTGAGE, ASSIGNMENT, SECURITY AGREEMENT, FIXTURE FILING AND FINANCING STATEMENT (this “ Supplement ”) is entered into as of the effective time and date hereinafter stated on the signature page hereto (the “ Effective Date ”) by ST. MARY LAND & EXPLORATION COMPANY), a Delaware corporation (Taxpayer I.D. No. 41-0518430) (“ Mortgagor ”); to WACHOVIA BANK, NATIONAL ASSOCIATION, as Administrative Agent (in such capacity, together with its successors in such capacity, by operation of law or as otherwise provided in the hereinafter defined Credit Agreement, the “ Agent ”) for the lenders (collectively, the “ Lenders ”) now or hereafter party to the Credit Agreement, or, alternatively, to JAY CHERNOSKY, as Trustee with respect to Property located in the State of Texas, whose address for notice is 1001 Fannin Street, Suite 2255, Houston, Texas  77002 (the “ Trustee ”), for the benefit of the Agent.

A.    Mortgagor, the Agent and the Lenders entered into that certain Amended and Restated Credit Agreement dated as of April 7, 2005, by and among Mortgagor, the Agent, and each of the Lenders (the “ Amended and Restated Credit Agreement ”).

B.    The indebtedness of the Mortgagor under or in connection with the Amended and Restated Credit Agreement is secured by, among other things, that certain Deed of Trust, Mortgage, Line of Credit Mortgage, Assignment, Security Agreement, Fixture Filing and Financing Statement, from Mortgagor to Trustee for the benefit of the Agent, duly recorded as set forth on Annex I attached hereto and made a part hereof for all purposes (the “ Mortgage ”).

C.    Mortgagor, the Agent and the Lenders entered into that certain Second Amended and Restated Credit Agreement dated as of April 10, 2008, by and among Mortgagor, the Agent, and each of the Lenders (the “ Existing Credit Agreement ”), which amended and restated the Amended and Restated Credit Agreement in its entirety.

D.    Mortgagor, the Agent and the Lenders desire to enter into that certain Third Amended and Restated Credit Agreement dated as of even date herewith to amend and restate the Existing Credit Agreement in its entirety to, among other things, increase the Aggregate Commitment (as defined therein) to $700,000,000 and extend the Maturity Date (as defined therein) to July 31, 2012 (the “ Credit Agreement ”).

E.    Mortgagor hereby desires to further supplement and amend the Mortgage by adding to the Mortgaged Properties described therein and covered thereby all rights, titles, interests and estates now owned or hereafter acquired by Mortgagor in and to the properties described on Exhibit A-1 attached hereto and made a part hereof for all purposes.

NOW, THEREFORE, for good and valuable consideration in hand paid by Mortgagor to Agent and in consideration of the debts and trusts hereinafter mentioned, the receipt and

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sufficiency of all of which is hereby acknowledged, Mortgagor and Agent do hereby agree as follows:

Section 1.1    Mortgagor, for and in consideration of the sum of Ten Dollars ($10.00) to Mortgagor in hand paid, and in order to secure the payment of the secured indebtedness referred to in the Mortgage, as supplemented and amended hereby, and the performance of the obligations, covenants, agreements, warranties and undertakings of Mortgagor described in the Mortgage, as supplemented and amended hereby, does hereby (a) GRANT, BARGAIN, SELL, CONVEY, TRANSFER, ASSIGN AND SET OVER to Trustee, and grant to Trustee a POWER OF SALE (pursuant to the Mortgage, as supplemented and amended hereby, and applicable law) with respect to, those of the following described properties, rights and interests which are located in (or cover properties located in) the State of Texas or which are located within (or cover properties located in) the offshore area over which the United States of America asserts jurisdiction and to which the laws of any such state are applicable with respect to the Mortgage, as supplemented and amended hereby, and/or the liens or security interests created hereby (the “ Additional Deed of Trust Mortgaged Properties ”); and (b) MORTGAGE, ASSIGN, WARRANT, PLEDGE AND HYPOTHECATE to Agent, and grant to Agent a POWER OF SALE (pursuant to the Mortgage, as supplemented and amended hereby, and applicable law) with respect to, all of the following described rights, interests and properties which were not granted to Trustee in clause (a) above (including, without limitation, those of the following described properties, rights and interests which are located in (or cover properties located in) the States of Colorado, Louisiana, Montana, New Mexico, North Dakota, Oklahoma or Wyoming or which are located within (or cover properties located within) the offshore area over which the United States of America asserts jurisdiction and to which the laws of any such state are applicable with respect to the Mortgage, as supplemented and amended hereby, and/or the liens or security interests created hereby) (the “ Additional Other Mortgaged Properties ”):

A.    The oil, gas and/or other mineral properties, mineral servitudes, and/or mineral rights which are described in Exhibit A-1 attached hereto and made a part hereof;

B.    Without limitation of the foregoing, all other right, title and interest of Mortgagor of whatever kind or character (whether now owned or hereafter acquired by operation of law or otherwise) in and to (i) the oil, gas and/or mineral leases or other agreements described in Exhibit A-1 hereto, (ii) the lands described or referred to in Exhibit A-1 (or described in any of the instruments described or referred to in Exhibit A-1 ), without regard to any limitations as to specific lands or depths that may be set forth in Exhibit A-1 hereto or in any of the leases or other agreements described in Exhibit A-1 hereto and (iii) any other lands (including submerged lands) located anywhere in the United States of America or located offshore the United States of America but within the offshore area over which the United States of America or any State thereof asserts jurisdiction;

C.    All of Mortgagor’s interest (whether now owned or hereafter acquired by operation of law or otherwise) in and to all presently existing and hereafter created oil, gas and/or mineral unitization, pooling and/or communitization agreements, declarations and/or

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orders, and in and to the properties, rights and interests covered and the units created thereby (including, without limitation, units formed under orders, rules, regulations or other official acts of any federal, state or other authority having jurisdiction), which cover, affect or otherwise relate to the properties, rights and interests described in clause A or B above;

D.    All of Mortgagor’s interest in and rights under (whether now owned or hereafter acquired by operation of law or otherwise) all presently existing and hereafter created operating agreements, equipment leases, production sales contracts, processing agreements, transportation agreements, gas balancing agreements, farmout and/or farm-in agreements, salt water disposal agreements, area of mutual interest agreements, and other contracts and/or agreements which cover, affect, or otherwise relate to the properties, rights and interests described in clause A, B or C above or to the operation of such properties, rights and interests or to the treating, handling, storing, processing, transporting or marketing of oil, gas, other hydrocarbons, or other minerals produced from (or allocated to) such properties, rights and interests (including, but not limited to, those contracts listed in Exhibit A-1 hereto), as same may be amended or supplemented from time to time;

E.    All of Mortgagor’s interest (whether now owned or hereafter acquired by operation of law or otherwise) in and to all improvements, fixtures, movable or immovable property and other real and/or personal property (including, without limitation, all wells, pumping units, wellhead equipment, tanks, pipelines, flow lines, gathering lines, compressors, dehydration units, separators, meters, buildings, injection facilities, salt water disposal facilities, and power, telephone and telegraph lines), and all easements, servitudes, rights-of-way, surface leases, licenses, permits and other surface rights, which are now or hereafter used, or held for use, in connection with the properties, rights and interests described in clause A, B or C above, or in connection with the operation of such properties, rights and interests, or in connection with the treating, handling, storing, processing, transporting or marketing of oil, gas, other hydrocarbons, or other minerals produced from (or allocated to) such properties, rights and interests; and

F.    All rights, estates, powers and privileges appurtenant to the foregoing rights, interests and properties.

TO HAVE AND TO HOLD (a) the Additional Deed of Trust Mortgaged Properties unto the Trustee, and its successors or substitutes in this trust, and to its or their successors and assigns, in trust, however, upon the terms, provisions and conditions set forth in the Mortgage, as amended and supplemented hereby, and (b) the Additional Other Mortgaged Properties unto Agent, and Agent’s successors and assigns, upon the terms, provisions and conditions herein set forth (the Additional Deed of Trust Mortgaged Properties and the Additional Other Mortgaged Properties are herein sometimes collectively called the “ Additional Mortgaged Properties ”).

Section 1.2    Mortgagor hereby confirms that it has heretofore granted, bargained, sold, conveyed, transferred, assigned, set over, mortgaged, warranted, pledged and hypothecated to the Agent, and granted a security interest to the Agent in, the “Property” (as such term is amended hereby), and Mortgagor hereby further grants, bargains, sells, conveys, transfers, assigns, sets over, mortgages, warrants, pledges and hypothecates to Agent, and grants a security

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interest to Agent in, the Property, to secure the payment and performance of the “secured indebtedness” (as defined in the Mortgage, as supplemented and amended hereby).

Section 1.3    Mortgagor hereby confirms that it has heretofore absolutely and unconditionally assigned, transferred and set over and does hereby absolutely and unconditionally assign, transfer and set over to Agent, its successors and assigns, all of the “Production” (as defined in the Mortgage, as supplemented and amended hereby) which accrues to Mortgagor’s interest in the “Mortgaged Properties” (as such term is amended hereby), and all “Production Proceeds” (as defined in the Mortgage, as supplemented and amended hereby), together with the immediate and continuing right to collect and receive all such Production Proceeds.

Section 2.1    Amendment to Section 1.3 (Secured Indebtedness) .  Section 1.3 of the Mortgage is hereby amended and restated in its entirety to read as follows:

“Section 1.3   Secured Indebtedness .  This Mortgage is executed and delivered by the Mortgagor to secure and enforce the payment and performance of the following:

(a)           Payment of and performance of any and all indebtedness, obligations and liabilities, whether now in existence or hereafter arising, whether by acceleration or otherwise, including the principal of, interest on (including, without limitation, interest accruing after the maturity of the “Loans” (as defined in the hereinafter defined Credit Agreement) made by each Lender and interest accruing after the filing of any petition in bankruptcy, or the commencement of any insolvency, reorganization or like proceeding, relating to the Mortgagor, whether or not a claim for post-filing or post-petition interest is allowed in such proceeding) of the Mortgagor and all other amounts, payments and premiums due under or in connection with that certain Third Amended and Restated Credit Agreement dated as of the Effective Date, by and among Mortgagor, Agent and the Lenders as the same may from time to time be amended or supplemented, the “ Credit Agreement ”) or any other “Loan Document” (as defined in the Credit Agreement), including, without limitation, the “Notes” (as defined in the Credit Agreement) in the aggregate original principal amount of $678,000,000 with final maturity on or before July 31, 2012, and further including any additional Loans or any increases to the Loans which may be made pursuant to the Credit Agreement; provided, the Lenders have no obligation to make any such additional Loans or increase.

(b)           Payment and performance of any and all indebtedness, obligations and liabilities of any Guarantor (as defined in the Credit Agreement) whether now existing or hereafter arising under or in connection with the “Guaranty Agreement” (as defined in the Credit Agreement).

(c)           Any sums which may be advanced or paid by the Agent or any Lender under the terms hereof or of the Credit Agreement or any Loan Document on account of the failure of the Mortgagor to comply with the covenants of the Mortgagor contained herein or in the Credit Agreement or any other Loan Document; and all other indebtedness of the Mortgagor arising pursuant to the provisions of this Mortgage.

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(d)           Payment of and performance of any and all present or future obligations of the Mortgagor according to the terms of any present or future interest rate or currency swap, rate cap, rate floor, rate collar, forward rate agreement or other exchange or rate protection agreements or any o