UNDERWRITING AGREEMENT

EXHIBIT 1.2

Execution Version

$ 1,250,000,000

XTO ENERGY INC.

$300,000,000 5.90% Senior Notes due 2012

$450,000,000 6.25% Senior Notes due 2017

$500,000,000 6.75% Senior Notes due 2037

UNDERWRITING AGREEMENT

July 12, 2007

L EHMAN B ROTHERS I NC .

J.P. M ORGAN S ECURITIES I NC .

UBS Securities LLC

as Representatives of the Underwriters

set forth on Schedule I hereto

c/o Lehman Brothers Inc.

745 Seventh Avenue

New York, New York 10019

Ladies and Gentlemen:

XTO Energy Inc., a Delaware corporation (the “ Company ”), proposes to issue and sell $300,000,000 aggregate principal amount of its 5.90% Senior Notes due 2012 (the “ 2012 Notes ”), $450,000,000 aggregate principal amount of its 6.25% Senior Notes due 2017 (the “ 2017 Notes ”) and $500,000,000 aggregate principal amount of its 6.75% Senior Notes due 2037 (the “ 2037 Notes ”) (collectively, the 2012 Notes, 2017 Notes and the 2037 Notes are herein referred to as the “ Notes ”) to you (the “ Underwriters ”). The Notes will be issued pursuant to an Indenture dated as of July 19, 2007, as amended and supplemented to date, and as further amended and supplemented by a First Supplemental Indenture to be dated on or before the Delivery Date (as hereinafter defined), (collectively, the “ Indenture ”) between the Company and The Bank of New York Trust Company, N.A., as Trustee (the “ Trustee ”). This agreement (this “ Agreement ”) is to confirm the agreement concerning the purchase of the Notes from the Company by the Underwriters.

Section 1. Representations, Warranties and Agreements of the Company . The Company represents, warrants and agrees that:

(a) A registration statement on Form S-3 relating to the Notes (File No. 333-135136) (i) has been prepared by the Company in conformity with the requirements of the Securities Act of 1933, as amended (the “ Securities Act ”), and the rules and regulations

(the “ Rules and Regulations ”) of the Securities and Exchange Commission (the “ Commission ”) thereunder; (ii) has been filed with the Commission under the Securities Act; and (iii) upon its filing with the Commission automatically became and is effective under the Securities Act. Copies of such registration statement, any amendment thereto and any supplement to the prospectus included therein have been delivered by the Company to you as the representatives (the “ Representatives ”) of the Underwriters; and no other document relating to the Notes with respect to such registration statement or any such document incorporated by reference therein has heretofore been filed or transmitted for filing with the Commission. As used in this Agreement:

(i) “ Applicable Time ” means 5:30 p.m. (New York City time) on the date of this Agreement;

(ii) “ Base Prospectus ” means the base prospectus filed as part of the Registration Statement, in the form in which it has most recently been amended on or prior to the date hereof, relating to the Notes;

(iii) “ Effective Date ” means any date as of which any part of such registration statement relating to the Notes became, or is deemed to have become, effective under the Securities Act in accordance with the Rules and Regulations;

(iv) “ Final Term Sheet ” means the term sheet prepared pursuant to Section 5(a) of the Agreement and substantially in the form attached in Schedule III hereto;

(v) “ Issuer Free Writing Prospectus ” means each “ free writing prospectus ” (as defined in Rule 405 of the Rules and Regulations) prepared by or on behalf of the Company or used or referred to by the Company in connection with the offering of the Notes, including the Final Term Sheet;

(vi) “ Preliminary Prospectus ” means any preliminary prospectus relating to the Notes included in such registration statement, including the Base Prospectus, or filed with the Commission pursuant to Rule 424(b) of the Rules and Regulations, including any preliminary prospectus supplement thereto relating to the Notes, and provided to the Representatives for use by the Underwriters;

(vii) “ Disclosure Package ” means, as of the Applicable Time, the most recent Preliminary Prospectus, together with each Issuer Free Writing Prospectus filed or used by the Company on or before the Applicable Time and identified on Schedule II, other than a road show that is an Issuer Free Writing Prospectus under Rule 433 of the Rules and Regulations;

(viii) “ Prospectus ” means the final prospectus relating to the Notes, including the Base Prospectus and any final prospectus supplement thereto relating to the Notes, as filed with the Commission pursuant to Rule 424(b) of the Rules and Regulations and provided to the Representatives for use by the Underwriters; and

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(ix) “ Registration Statement ” means, collectively, the various parts of such registration statement, each as amended as of the Effective Date for such part, including any Preliminary Prospectus or the Prospectus and all exhibits to such registration statement.

Any reference to any Preliminary Prospectus or the Prospectus shall be deemed to refer to and include any documents incorporated by reference therein pursuant to Form S-3 under the Securities Act as of the date of such Preliminary Prospectus or the Prospectus, as the case may be. Any reference to the “ most recent Preliminary Prospectus ” shall be deemed to refer to the latest Preliminary Prospectus included in the Registration Statement or filed pursuant to Rule 424(b) prior to or on the date hereof (including, for purposes hereof, any documents incorporated by reference therein prior to or on the date hereof). Any reference to any amendment or supplement to any Preliminary Prospectus or the Prospectus shall be deemed to refer to and include any document filed under the Securities Exchange Act of 1934, as amended (the “ Exchange Act ”), after the date of such Preliminary Prospectus or the Prospectus, as the case may be, and incorporated by reference in such Preliminary Prospectus or the Prospectus, as the case may be; and any reference to any amendment to the Registration Statement shall be deemed to include any annual report of the Company on Form 10-K filed with the Commission pursuant to Section 13(a) or 15(d) of the Exchange Act after the Effective Date that is incorporated by reference in the Registration Statement. The Commission has not issued any order preventing or suspending the use of any Preliminary Prospectus, any Issuer Free Writing Prospectus or the Prospectus, or of any document incorporated by reference in the most recent Preliminary Prospectus or the Prospectus, or suspending the effectiveness of the Registration Statement, and no proceeding or examination for such purpose has been instituted or threatened by the Commission. The Commission has not notified the Company of any objection to the use of the form of the Registration Statement.

(b) The Company has been since the time of initial filing of the Registration Statement and continues to be a “well-known seasoned issuer” (as defined in Rule 405) eligible to use Form S-3 for the offering of the Notes, including not having been an “ineligible issuer” (as defined in Rule 405) at any such time or date. The Registration Statement is an “automatic shelf registration statement” (as defined in Rule 405) and was filed not earlier than the date that is three years prior to the Delivery Date (as defined in Section 4).

(c) The Registration Statement conformed and will conform in all material respects on the Effective Date and on the Delivery Date, and any amendment to the Registration Statement filed after the date hereof will conform in all material respects when filed, to the requirements of the Securities Act and the Rules and Regulations. The most recent Preliminary Prospectus conforms on the date hereof, and the Prospectus and any amendment or supplement thereto will conform, in all material respects when filed with the Commission pursuant to Rule 424(b) and on the Delivery Date to the requirements of the Securities Act and the Rules and Regulations.

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(d) The Disclosure Package did not, as of the Applicable Time, contain an untrue statement of a material fact or omit to state a material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading; provided , that no representation or warranty is made as to information contained in or omitted from the Registration Statement in reliance upon and in conformity with written information furnished to the Company through the Representatives by or on behalf of any Underwriter specifically for inclusion therein, which information is specified in Section 9(e).

(e) Each Issuer Free Writing Prospectus (including, without limitation, any road show that is a free writing prospectus under Rule 433), when considered together with the Disclosure Package as of the Applicable Time, did not contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading. Each Issuer Free Writing Prospectus (including, without limitation, any road show that is a free writing prospectus under Rule 433) does not include any information that conflicts with the information contained in the Registration Statement, including any document incorporated by reference therein that has not been superseded or modified, or the Disclosure Package.

(f) Each Issuer Free Writing Prospectus (including, without limitation, any road show that is a free writing prospectus under Rule 433) conformed or will conform in all material respects to the requirements of the Securities Act and the Rules and Regulations on the date of first use, and the Company has complied with any filing requirements applicable to such Issuer Free Writing Prospectus pursuant to the Rules and Regulations. The Company has not made any offer relating to the Notes that would constitute an Issuer Free Writing Prospectus without the prior written consent of the Representatives. The Company has retained, and will retain, in accordance with the Rules and Regulations all Issuer Free Writing Prospectuses that were not required to be filed pursuant to the Rules and Regulations.

(g) The Registration Statement did not, as of the Effective Date, and the Prospectus will not, as of its date and on the Delivery Date, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading; provided that no representation or warranty is made as to information contained in or omitted from the Registration Statement or the Prospectus in reliance upon and in conformity with written information furnished to the Company through the Representatives by or on behalf of any Underwriter specifically for inclusion therein, which information is specified in Section 9(e).

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(h) The documents incorporated by reference in any Preliminary Prospectus or the Prospectus conformed, and any further documents so incorporated will conform, when filed with the Commission, in all material respects to the requirements of the Exchange Act or the Securities Act, as applicable, and the rules and regulations of the Commission thereunder. None of such documents, when filed with the Commission, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein, in light of the circumstances under which they were made, not misleading.

(i) Neither the Company nor any of its subsidiaries has sustained, since the date of the latest audited financial statements included in the Preliminary Prospectus, any material loss or interference with its business from fire, explosion, flood or other calamity, whether or not covered by insurance, or from any labor dispute or court or governmental action, order or decree, otherwise than as set forth or contemplated in the Preliminary Prospectus or Prospectus; and, since the respective dates as of which information is given in the Registration Statement and the Preliminary Prospectus, there has not been any change in the long-term debt of the Company or any of its subsidiaries, other than changes to indebtedness outstanding under the Company’s Amended and Restated 5-Year Revolving Credit Agreement dated April 1, 2005 between the Company and certain commercial banks named therein, as amended to date, and the Company’s commercial paper program, or any material adverse change, or any development involving a prospective material adverse change, in or affecting the general affairs, management, financial position, stockholders’ equity or results of operations of the Company and its subsidiaries, otherwise than as set forth or contemplated in the Preliminary Prospectus.

(j) (1) The Company has good and defensible title to all real property, free and clear of all liens, encumbrances and defects, except (A) royalties, overriding royalties and other burdens under oil and gas leases, (B) easements, restrictions, rights-of-way and other matters that commonly affect property, (C) liens securing taxes and other governmental charges, or claims of materialmen, mechanics and similar persons, not yet due and payable, (D) liens and encumbrances under operating agreements, farmout agreements, unitization, pooling and commutation agreements, declarations and orders, and gas sales contracts, securing payment of amounts not yet due and payable and of a scope and nature customary in the oil and gas industry and (E) liens, encumbrances and defects that do not in the aggregate materially affect the value of the real property or materially interfere with the use made or proposed to be made of such real property by the Company; and (2) the working interests in oil, gas and mineral leases or mineral interests which constitute a portion of the real property held by the Company reflect in all material respects the right of the Company to explore or receive production from such real property, and the care taken by the Company and its subsidiaries with respect to acquiring or otherwise procuring such leases or mineral interests was generally consistent with standard industry practices for acquiring or procuring leases and interests therein to explore for hydrocarbons.

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(k) The Company has been duly incorporated and is validly existing as a corporation in good standing under the laws of Delaware, is duly qualified to do business and is in good standing as a foreign corporation in each jurisdiction in which its ownership or lease of property or the conduct of its business requires such qualification, and has all power and authority necessary to own or hold its properties and to conduct the businesses in which it is engaged; and none of the subsidiaries of the Company is a significant subsidiary, as such term is defined in Rule 405 of the Rules and Regulations.

(l) The Company has an authorized capitalization as set forth in the most recent Preliminary Prospectus and the Prospectus. All of the issued shares of capital stock of the Company have been duly and validly authorized and issued, were issued in compliance with federal and state securities laws, are fully paid and non-assessable and conform, or will conform, to the description thereof contained in the Registration Statement, the most recent Preliminary Prospectus and the Prospectus. All of the Company’s options, warrants and other rights to purchase or exchange any securities for shares of the Company’s capital stock have been duly and validly authorized and issued, were issued in compliance with federal and state securities laws, and conform, or will conform, to the description thereof contained in the Registration Statement, the most recent Preliminary Prospectus and the Prospectus. All of the issued shares of capital stock of each subsidiary of the Company have been duly authorized and validly issued, are fully paid and non-assessable and are owned directly or indirectly by the Company, free and clear of all liens, encumbrances, equities or claims, except for such liens, encumbrances, equities or claims as could not, in the aggregate, reasonably be expected to have a material adverse effect.

(m) Neither (i) the execution or delivery hereof by the Company, nor (ii) the consummation of the transactions contemplated hereby, nor (iii) the execution and delivery of the Indenture and the Notes by the Company, nor (iv) compliance by the Company with all of the provisions of this Agreement, the Indenture and the Notes, will (A) conflict with or result in a breach or violation of, or constitute a default under, the certificate of incorporation, by-laws, partnership agreement or other governing documents of the Company or any of its subsidiaries, or any material agreement, indenture or other instrument to which the Company or any of its subsidiaries is a party or by which any of them is bound, or to which any of their properties is subject or (B) violate any law, rule, administrative regulation or decree of any court, or any governmental agency or body having jurisdiction over the Company, its subsidiaries or any of their respective properties or assets, or result in the creation or imposition of any lien, charge, claim or encumbrance upon any property or asset of the Company or any of its subsidiaries, which in any such event described in this subclause (B) would have a material adverse effect on the Company. Except for (i) the registration of the Notes under the Securities Act, (ii) such consents, approvals, authorizations, registrations or qualifications as may be required under the Exchange Act and applicable state securities laws in connection with the purchase and sale of the Notes by the Underwriters and such permits, consents, approvals and similar authorizations required under the securities or “Blue Sky” laws of certain

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jurisdictions, and (iii) such permits, consents, approvals and authorizations which have been obtained, no permit, consent, approval, authorization or order of, or filing or registration with, any court, governmental agency or body or financial institution is required in connection with the consummation of the transactions contemplated by this Agreement.

(n) Neither the Company nor any of its subsidiaries is in violation of its certificate of incorporation or bylaws or in default in the performance or observance of any material obligation, agreement, covenant or condition contained in any indenture, mortgage, deed of trust, loan agreement, lease or any other agreement or instrument to which it is a party or by which it or any of its properties is bound.

(o) The Company has all requisite corporate power and authority to execute, deliver and perform its obligations under this Agreement. This Agreement has been duly authorized, executed and delivered by the Company.

(p) The Notes will rank pari passu with all existing and future unsecured and unsubordinated indebtedness of the Company.

(q) The Indenture has been duly and validly authorized, and, on or prior to the Delivery Date, will be executed and delivered by the Company and will be a valid and binding agreement of the Company, enforceable against the Company in accordance with its terms, except as the enforceability thereof may be limited by bankruptcy, insolvency, reorganization or similar laws relating to or affecting creditors’ rights generally and by general equitable principles (regardless of whether such enforceability is considered in a proceeding in equity or at law). The Indenture (i) is duly qualified under the Trust Indenture Act of 1939, as amended (the “Trust Indenture Act”), (ii) complies as to form with the requirements of the Trust Indenture Act and (iii) conforms, or will conform, to the description thereof in the Registration Statement, the Preliminary Prospectus and the Prospectus.

(r) The Notes have been duly and validly authorized by the Company for issuance and sale to the Underwriters pursuant to this Agreement and, when executed by the Company and authenticated by the Trustee in accordance with the Indenture and delivered to the Underwriters against payment therefor in accordance with the terms hereof, will have been validly issued and delivered, free of any preemptive or similar rights to subscribe to or purchase the same arising by operation of law or under the charter or by-laws of the Company or otherwise, and will constitute valid and binding obligations of the Company entitled to the benefits of the Indenture and enforceable in accordance with their terms, except as enforcement thereof may be limited by bankruptcy, insolvency, reorganization or other similar laws relating to or affecting the enforcement of creditors’ rights generally and by general equitable principles, and the Notes conform, or will conform, to the description thereof in the Registration Statement, the Preliminary Prospectus and the Prospectus. The Company has all requisite corporate power and

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authority to issue, sell and deliver the Notes in accordance with and upon the terms and conditions set forth in this Agreement and in the Registration Statement, the Preliminary Prospectus and Prospectus. All corporate action required to be taken by the Company for the authorization, issuance, sale and delivery of the Notes to be sold by the Company hereunder has been validly and sufficiently taken.

(s) The financial statements (including the related notes and supporting schedules) filed as part of the Registration Statement or included or incorporated by reference in the most recent Preliminary Prospectus and the Prospectus comply as to form in all material respects with the requirements of Regulation S-X under the Securities Act and present fairly the financial condition, results of operations and the cash flows of the entities purported to be shown thereby, at the dates and for the periods indicated, and have been prepared in conformity with generally accepted accounting principles applied on a consistent basis throughout the periods involved.

(t) KPMG LLP (“ KPMG ”), who have certified certain financial statements of the Company, whose reports appear in the Preliminary Prospectus and the Prospectus or are incorporated by reference therein and who have delivered the initial letter referred to in Section 8(f) hereof, are independent registered public accountants within the meaning of Rule 101 of the Code of Professional Conduct of the American Institute of Certified Public Accountants and its interpretations and rulings thereunder.

(u) The Company and each of its subsidiaries carry, or are covered by, insurance in such amounts and covering such risks as is adequate for the conduct of their respective businesses and the value of their respective properties as is customary for companies engaged in similar businesses in similar industries.

(v) The Company and each of its subsidiaries own or possess adequate rights to use all material patents, patent applications, trademarks, service marks, trade names, trademark registrations, service mark registrations, copyrights and licenses necessary for the conduct of their respective businesses and have no reason to believe that the conduct of their respective businesses will conflict with, and have not received any notice of any claim of conflict with, any such rights of others.

(w) Except as described in the most recent Preliminary Prospectus and the Prospectus, there are no legal or governmental proceedings pending to which the Company or any of its subsidiaries is a party or of which any property or assets of the Company or any of its subsidiaries is the subject which, if determined adversely to the Company or any of its subsidiaries, would have a material adverse effect on the consolidated financial position, stockholders’ equity, results of operations, business or prospects of the Company and its subsidiaries; and to the best of the Company’s knowledge, no such proceedings are threatened or contemplated by governmental authorities or threatened by others.

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(x) No labor disturbance by the employees of the Company or its subsidiaries exists or, to the knowledge of the Company, is imminent, which could be reasonably expected to have a material adverse effect on the general affairs, management, consolidated financial position, stockholders’ equity, results of operations, business or prospects of the Company and its subsidiaries.

(y) Since the date as of which information is given in the most recent Preliminary Prospectus through the date hereof, and except as may otherwise be disclosed in the most recent Preliminary Prospectus and the Prospectus, the Company has not (i) issued or granted any securities other than pursuant to any of its employee benefit plans, (ii) incurred any liability or obligation, direct or contingent, other than liabilities and obligations which were incurred in the ordinary course of business or (iii) entered into any transaction not in the ordinary course of business.

(z) The Company (i) makes and keeps accurate books and records and (ii) maintains internal accounting controls which provide reasonable assurance that (A) transactions are executed in accordance with management’s authorization, (B) transactions are recorded as necessary to permit preparation of its financial statements and to maintain accountability for its assets, (C) access to its assets is permitted only in accordance with management’s authorization and (D) the reported accountability for its assets is compared with existing assets at reasonable intervals.

(aa) Neither the Company nor any of its subsidiaries, nor any director, officer, agent, employee or other person acting on behalf of the Company or any of its subsidiaries, has (i) used any corporate funds for any unlawful contribution, gift, entertainment or other unlawful expense relating to political activity, (ii) made any direct or indirect unlawful payment to any foreign or domestic government official or employee from corporate funds, (iii) violated or is in violation of any provision of the Foreign Corrupt Practices Act of 1977, or (iv) made any bribe, rebate, payoff, influence payment, kickback or other unlawful payment. Neither the Company nor any of its subsidiaries conducts business or has any operations in any foreign jurisdiction.

(bb) There has been no storage, disposal, generation, manufacture, refinement, transportation, handling or treatment of toxic wastes, medical wastes, hazardous wastes or hazardous substances by the Company or any of its subsidiaries (or, to the knowledge of the Company, any of their predecessors in interest) at, upon or from any of the property now or previously owned or leased by the Company or its subsidiaries in violation of any laws, regulations, ordinances, rules, orders, judgments, decrees, permits or other legal requirements of any governmental authority, including without limitation any international, national, state, provincial, regional, or local authority, relating to the protection of human health or safety, the environment, or natural resources, or to hazardous or toxic substances or wastes, pollutants or contaminants (“ Environmental Laws ”) or which would require remedial action under any applicable Environmental Laws, except for any violation or remedial action which would not be reasonably likely to

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have, singularly or in the aggregate with all such violations and remedial actions, a material adverse effect on the general affairs, management, consolidated financial position, stockholders’ equity or results of operations of the Company and its subsidiaries; there has been no material spill, discharge, leak, emission, injection, escape, dumping or release of any kind onto such property or into the environment surrounding such property of any toxic wastes, medical wastes, solid wastes, hazardous wastes or hazardous substances due to or caused by the Company or any of its subsidiaries or with respect to which the Company or any of its subsidiaries have knowledge, except for any such spill, discharge, leak, emission, injection, escape, dumping or release which would not be reasonably likely to have, singularly or in the aggregate with all such spills, discharges, leaks, emissions, injections, escapes, dumpings and releases, a material adverse effect on the general affairs, management, consolidated financial position, stockholders’ equity or results of operations of the Company and its subsidiaries; and the terms “hazardous wastes,” “toxic wastes,” “hazardous substances” and “medical wastes” shall have the meanings specified in any applicable Environmental Laws; none of the Company or any of its subsidiaries has received notice of any actual or alleged violation of such Environmental Laws, or of any potential liability for or other obligation concerning the presence, disposal or release of hazardous or toxic substances or wastes, pollutants or contaminants, which violations or liabilities could reasonably be expected to have a material effect on the capital expenditures, earnings or competitive position of the Company and its subsidiaries. Except as described in the most recent Preliminary Prospectus, (A) there are no proceedings that are pending, or known to be contemplated, against the Company or any of its subsidiaries under Environmental Laws in which a governmental authority is also a party, other than such proceedings regarding which it is reasonably believed no monetary sanctions of $100,000 or more will be imposed, (B) the Company and its subsidiaries are not aware of any issues regarding compliance with Environmental Laws, or liabilities or other obligations under Environmental Laws or concerning hazardous or toxic substances or wastes, pollutants or contaminants, which could reasonably be expected to have a material effect on the capital expenditures, earnings or competitive position of the Company and its subsidiaries, and (C) none of the Company and its subsidiaries anticipates material capital expenditures relating to Environmental Laws.

(cc) The Company is not, and after giving effect to the offer and sale of the Notes and the application of the proceeds therefrom as described under “Use of Proceeds” in the most recent Preliminary Prospectus and the Prospectus will not be, an investment company as defined in the Investment Company Act of 1940, as amended (the “ Investment Company Act ”).

(dd) The information supplied by the Company to its independent petroleum engineering consultants for purposes of preparing the reserve reports used to calculate estimates of reserves of the Company included in the Registration Statement, Preliminary Prospectus and Prospectus, including, without limitation, production, costs of operation and development, current prices for production, agreements relating to current and future

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operations and sales of production, was true and correct in all material respects on the date supplied and was prepared in accordance with customary industry practices; Miller and Lents, Ltd., independent consulting petroleum engineers, who prepared estimates of the extent and value of proved oil and natural gas reserves, are independent with respect to the Company.

(ee) The Company has not taken and will not take, directly or indirectly, any action which is designed to or which has constituted or which would reasonably be expected to cause or result in stabilization or manipulation of the price of any security of the Company to facilitate the sale or resale of the Notes.

(ff) There are no contracts, agreements or understandings between the Company and any person granting such person the right (other than rights which have been waived or satisfied) to require the Company to file a registration statement under the Securities Act with respect to any securities of the Company owned or to be owned by such person or to require the Company to include such securities in the securities registered pursuant to the Registration Statement or in any securities being registered pursuant to any other registration statement filed by the Company under the Securities Act (other than (i) the Registration Rights Agreement dated May 11, 1993 by and between the Company and certain stockholders (the “ 1993 Registration Rights Agreement ”) under which all registrable securities are eligible for trading pursuant to Rule 144 under the Securities Act, (ii) the Registration Rights Agreement dated April 1, 2005 by and between the Company and former stockholders of Antero Resources Corporation) and (iii) the Registration Rights Agreement dated June 30, 2006 by and between the Company and former securityholders of Peak Energy Resources, Inc.).

(gg) There are no contracts or other documents which are required by the Rules and Regulations to be described in the most recent Preliminary Prospectus and Prospectus or filed as exhibits to the Registration Statement by the Securities Act or by the Rules and Regulations which have not been described in such Preliminary Prospectus and Prospectus or filed as exhibits to the Registration Statement.

(hh) No relationship, direct or indirect, exists between or among the Company on the one hand, and the directors, officers, stockholders, customers or suppliers of the Company on the other hand, which is required by the Rules and Regulations to be described in the most recent Preliminary Prospectus and Prospectus which is not so described.

(ii) The Company has filed all federal, state and local income and franchise tax returns required to be filed through the date hereof and has paid all taxes due thereon, and no tax deficiency has been determined adversely to the Company or any of its subsidiaries which has had (nor does the Company have any knowledge of any asserted tax deficiency which, if determined adversely to the Company or any of its subsidiaries, would have) a material adverse effect on the consolidated financial position, stockholders’ equity, results of operations, business or prospects of the Company and its subsidiaries.

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(jj) There is and has been no failure on the part of the Company and any of the Company’s directors or officers, in their capacities as such, to comply with the provisions of the Sarbanes-Oxley Act of 2002 and the rules and regulations promulgated in connection therewith.

(kk) The Company has not received any written comments from the Commission staff in connection with the Company’s reports under the Exchange Act that remain unresolved.

(ll) The Company has not distributed and, prior to the later to occur of any Delivery Date and completion of the distribution of the Notes, will not distribute any offering material in connection with the offering and sale of the Notes other than any Preliminary Prospectus, the Prospectus, any Issuer Free Writing Prospectus to which the Representatives have consented in accordance with Section 1(f) or 5(g).

Any certificate signed by any officer of the Company and delivered to the Representatives or counsel for the Underwriters in connection with the offering of the Notes shall be deemed a representation and warranty by the Company, as to matters covered thereby, to each Underwriter.

Section 2. Purchase of the Notes by the Underwriters . On the basis of the representations and warranties contained in, and subject to the terms and conditions of, this Agreement, the Company agrees to issue and sell to the several Underwriters, and each of the Underwriters, severally and not jointly, agrees to purchase from the Company, at a price equal to 99.264% of the principal amount of the 2012 Notes, at a price equal to 99.060% of the principal amount of the 2017 Notes, and at a price equal to 99.006% of the principal amount of the 2037 Notes, in each case plus accrued interest, if any, from the Delivery Date (as defined below), the principal amount of each of the 2012 Notes, 2017 Notes and 2037 Notes set forth opposite that Underwriter’s name in Schedule I hereto.

Section 3. Offering of Notes by the Underwriters . Upon authorization by the Company of the release of the Notes, the several Underwriters propose to offer the Notes for sale upon the terms and conditions set forth in the Prospectus.

Section 4. Delivery of and Payment for the Notes . Delivery of and payment for the Notes shall be made at the offices of Kelly, Hart & Hallman LLP, Fort Worth, Texas at 10:00 A.M., New York City time, on July 19, 2007, or at such other date or place as shall be determined by agreement between the Representatives and the Company. This date and time are sometimes referred to as the “ Delivery Date .” On the Delivery Date, the Company shall deliver or cause to be delivered the Notes to the Representatives for the account of each Underwriter against payment by the several Underwriters through the Representatives of the respective aggregate purchase prices of the Notes being sold by the Company to or upon the order of the

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Company of the purchase price by wire transfer in immediately available funds to the accounts specified by the Company. Time shall be of the essence, and delivery at the time and place specified pursuant to this Agreement is a further condition of the obligation of each Underwriter hereunder.

Section 5. Further Agreements of the Company . The Company agrees:

(a) To prepare the Prospectus in a form approved by the Representatives and to file such Prospectus pursuant to Rule 424(b) under the Securities Act not later than the Commission’s close of business on the second business day following the execution and delivery of this Agreement; to make no further amendment or any supplement to the Registration Statement or the Prospectus prior to the Delivery Date except as provided herein; to prepare the Final Term Sheet, substantially in the form of Schedule III hereto and approved by the Representatives and file the Final Term Sheet pursuant to Rule 433(d) of the Rules and Regulations within the time period prescribed by such Rule; to advise the Representatives, promptly after it receives notice thereof, of the time when any amendment or supplement to the Registration Statement or the Prospectus has been filed and to furnish the Representatives with copies thereof; to file promptly all reports and any definitive proxy or information statements required to be filed by the Company with the Commission pursuant to Section 13(a), 13(c), 14 or 15(d) of the Exchange Act subsequent to the date of the Prospectus and for so long as the delivery of a prospectus is required in connection with the offering or sale of the Notes; to advise the Representatives, promptly after it receives notice thereof, of the issuance by the Commission of any stop order or of any order preventing or suspending the use of the Preliminary Prospectus, Prospectus or any Issuer Free Writing Prospectus, of the suspension of the qualification of the Notes for offering or sale in any jurisdiction, of the initiation or threatening of any proceeding or examination for any such purpose, of any notice from the Commission objecting to the use of the form of the Registration Statement or any post-effective amendment thereto or of any request by the Commission for the amending or supplementing of the Registration Statement, Preliminary Prospectus or the Prospectus or any Issuer Free Writing Prospectus or for additional information; and, in the event of the issuance of any stop order or of any order preventing or suspending the use of the Preliminary Prospectus, Prospectus or any Issuer Free Writing Prospectus or suspending any such qualification, to use promptly its best efforts to obtain its withdrawal;

(b) To pay the applicable Commission filing fees relating to the Notes within the time required by Rule 456(b)(1) without regard to the proviso therein;

(c) To furnish promptly to each of the Representatives and to counsel for the Underwriters a copies of the executed Registration Statement as originally filed with the Commission, and each amendment thereto filed with the Commission, including all consents and exhibits filed therewith;

(d) To deliver promptly to the Representatives such number of the following documents as the Representatives shall reasonably request: (A) conformed copies of the Registration Statement as originally filed with the Commission and each amendment thereto (in

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each case excluding exhibits other than this Agreement and the computation of per share earnings), (B) each Preliminary Prospectus, the Prospectus and any amended or supplemented Prospectus, (C) each Issuer Free Writing Prospectus, (D) the Final Term Sheet, and (E) any document incorporated by reference in any Preliminary Prospectus or the Prospectus;

(e) During the period in which the Prospectus relating to the Notes (or in lieu thereof, the notice referred to in Rule 173(a) of the Rules and Regulations) is required to be delivered under the Securities Act, to comply with all requirements imposed upon it by the Securities Act and by the Rules and Regulations, as from time to time in force, so far as is necessary to permit the continuance of sales of or dealings in the Notes as contemplated by the provisions of this Agreement and by the Prospectus. If during such period any event occurs as a result of which the Disclosure Package or the Prospectus as then amended or supplemented would include an untrue statement of a material fact or omit to state a material fact necessary to make the statements therein, in the light of the circumstances then existing, not misleading, or if during such period it is necessary to amend the Registration Statement or amend or supplement the Disclosure Package or the Prospectus or file any document to comply with the Securities Act, the Company will promptly notify the Representatives and will, subject to Section 5(a) hereof, amend the Registration Statement, amend or supplement the Disclosure Package or the Prospectus, as the case may be, or file any document (in each case, at the expense of the Company) so as to correct such statement or omission or to effect such compliance, and will furnish without charge to each Underwriter as many written and electronic copies of any such amendment or supplement as the Representatives may from time to time reasonably request

(f) To file promptly with the Commission any amendment or supplement to the Registration Statement or the Prospectus that may, in the judgment of the Company or the Representatives, be required by the Securities Act or requested by the Commission;

(g) For so long as the delivery of a prospectus is required in connection with the offering or sale of the Notes, prior to filing with the Commission any amendment or supplement to the Registration Statement or the Prospectus, any document incorporated by reference in the Prospectus or any amendment to any document incorporated by reference in the Prospectus, to furnish a copy thereof to the Representatives and counsel for the Underwriters and obtain the consent of the Representatives to the filing, which consent shall not be unreasonably withheld;

(h) Not to make any offer relating to the Notes that would constitute an Issuer Free Writing Prospectus without the prior written consent of the Representatives (which consent being deemed to have been given with respect to (A)&