Exhibit 1.1

1,500,000 Shares

ATP Oil & Gas Corporation

8.00% Convertible Perpetual Preferred Stock, Series B

UNDERWRITING AGREEMENT

June 15, 2011

CREDIT SUISSE SECURITIES (USA) LLC

As Representative of the several Underwriters

c/o Credit Suisse Securities (USA) LLC

    Eleven Madison Avenue,

New York, N.Y. 10010-3629

1. Introductory. ATP Oil & Gas Corporation, a Texas corporation (“Company”), agrees with the several underwriters named in Schedule A hereto (the “Underwriters”), for whom you are acting as the representative (the “Representative”), subject to the terms and conditions stated herein, to issue and sell to the several Underwriters 1,500,000 shares of its 8.00% Convertible Perpetual Preferred Stock, Series B, par value $0.001 per share (the “Firm Securities”), and also proposes to grant to the Underwriters an option, exercisable from time to time by the Representative, to purchase an aggregate of up to 225,000 additional shares (the “Optional Securities”) of its 8.00% Convertible Perpetual Preferred Stock, Series B. The Firm Securities and the Optional Securities which the Representative may elect to purchase pursuant to Section 3 hereof are herein collectively called the “Offered Securities”. The Offered Securities will be convertible, subject to certain conditions set forth in the Statement of Resolutions establishing the Offered Securities, at the option of the holder thereof for shares of common stock, par value $0.001 per share, of the Company (“Common Stock”), in accordance with the terms of the Offered Securities.

2. Representations and Warranties of the Company. The Company represents and warrants to, and agrees with, the several Underwriters that:

(a) Filing and Effectiveness of Registration Statement; Certain Defined Terms. The Company has filed with the Commission a registration statement on Form S-3 (No. 333-171268), including a related prospectus or prospectuses, covering the registration of the Offered Securities under the Act, which has become effective. “Registration Statement” at any particular time means such registration statement in the form then filed with the Commission, including any amendment thereto, any document incorporated by reference therein and all 430B Information and all 430C Information with respect to such registration statement, that in any case has not been superseded or modified. “Registration Statement” without reference to a time means the Registration Statement as of the Effective Time. For purposes of this definition, 430B Information shall be considered to be included in the Registration Statement as of the time specified in Rule 430B.

For purposes of this Agreement:

“430B Information” means information included in a prospectus then deemed to be a part of the Registration Statement pursuant to Rule 430B(e) or retroactively deemed to be a part of the Registration Statement pursuant to Rule 430B(f).

“430C Information” means information included in a prospectus then deemed to be a part of the Registration Statement pursuant to Rule 430C.

“Act” means the Securities Act of 1933, as amended.

“Applicable Time” means 8:00 a.m. New York City time on the date of this Agreement.

“Closing Date” has the meaning set forth in Section 3 hereof.

“Commission” means the Securities and Exchange Commission.

“Effective Time” of the Registration Statement relating to the Offered Securities means the time of the first contract of sale for the Offered Securities.

“Exchange Act” means the Securities Exchange Act of 1934.

“Final Prospectus” means the Statutory Prospectus that discloses the public offering price, other 430B Information and other final terms of the Offered Securities and otherwise satisfies Section 10(a) of the Act.

“General Use Issuer Free Writing Prospectus” means any Issuer Free Writing Prospectus that is intended for general distribution to prospective investors, as evidenced by its being so specified in Schedule B to this Agreement.

“Issuer Free Writing Prospectus” means any “issuer free writing prospectus,” as defined in Rule 433, relating to the Offered Securities in the form filed or required to be filed with the Commission or, if not required to be filed, in the form retained in the Company’s records pursuant to Rule 433(g).

“Limited Use Issuer Free Writing Prospectus” means any Issuer Free Writing Prospectus that is not a General Use Issuer Free Writing Prospectus.

“Rules and Regulations” means the rules and regulations of the Commission.

“Securities Laws” means, collectively, the Sarbanes-Oxley Act of 2002 (“Sarbanes-Oxley”), the Act, the Exchange Act, the Rules and Regulations, the auditing principles, rules, standards and practices applicable to auditors of “issuers” (as defined in Sarbanes-Oxley) promulgated or approved by the Public Company Accounting Oversight Board and, as applicable, the rules of the New York Stock Exchange and the NASDAQ Stock Market (“Exchange Rules”).

“Statutory Prospectus” with reference to any particular time means the prospectus relating to the Offered Securities that is included in the Registration Statement immediately prior to that time, including all 430B Information and all 430C Information with respect to the Registration Statement. For purposes of the foregoing definition, 430B Information shall be considered to be included in the Statutory Prospectus only as of the actual time that form of prospectus (including a prospectus supplement) is filed with the Commission pursuant to Rule 424(b) and not retroactively.

“Underlying Shares” means the shares of Common Stock into which the Offered Securities are convertible or the shares of Common Stock that may be issued as dividends on the Offered Securities or may otherwise be issued in respect of the Offered Securities.

Unless otherwise specified, a reference to a “rule” is to the indicated rule under the Act.

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(b) Compliance with Act Requirements. (i) (A) At the time the Registration Statement initially became effective, (B) at the time of each amendment thereto for the purposes of complying with Section 10(a)(3) of the Act (whether by post-effective amendment, incorporated report or form of prospectus), (C) at the Effective Time relating to the Offered Securities and (D) on the Closing Date, the Registration Statement conformed and will conform in all respects to the requirements of the Act and the Rules and Regulations and did not and will not include any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein not misleading and (ii) (A) on its date, (B) at the time of filing the Final Prospectus pursuant to Rule 424(b) and (C) on the Closing Date, the Final Prospectus will conform in all respects to the requirements of the Act and the Rules and Regulations, and will not include any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein not misleading. The preceding sentence does not apply to statements in or omissions from any such document based upon written information furnished to the Company by any Underwriter through the Representative, if any, specifically for use therein, it being understood and agreed that the only such information furnished by any Underwriter consists of the information described as such in Section 7(b) hereof.

(c) Automatic Shelf Registration Statement. (i) Well-Known Seasoned Issuer Status. (A) At the time of initial filing of the Registration Statement, (B) at the time of the most recent amendment thereto for the purposes of complying with Section 10(a)(3) of the Act (whether such amendment was by post-effective amendment, incorporated report filed pursuant to Section 13 or 15(d) of the Exchange Act or form of prospectus), and (C) at the time the Company or any person acting on its behalf (within the meaning, for this clause only, of Rule 163(c)) made any offer relating to the Offered Securities in reliance on the exemption of Rule 163, the Company was a “well known seasoned issuer” as defined in Rule 405, including not having been an “ineligible issuer” as defined in Rule 405.

(ii) Effectiveness of Automatic Shelf Registration Statement. The Registration Statement is an “automatic shelf registration statement,” as defined in Rule 405, that initially became effective within three years of the date hereof.

(iii) Eligibility to Use Automatic Shelf Registration Form. The Company has not received from the Commission any notice pursuant to Rule 401(g)(2) objecting to use of the automatic shelf registration statement form. If, at any time when Offered Securities remain unsold by the Underwriters, the Company receives from the Commission a notice pursuant to Rule 401(g)(2) or otherwise ceases to be eligible to use the automatic shelf registration statement form, the Company will (i) promptly notify the Representative, (ii) promptly file a new registration statement or post-effective amendment on the proper form relating to the Offered Securities, in a form satisfactory to the Representative, (iii) use its reasonable best efforts to cause such registration statement or post-effective amendment to be declared effective as soon as practicable, and (iv) promptly notify the Representative of such effectiveness. The Company will take all other action necessary or appropriate to permit the public offering and sale of the Offered Securities to continue as contemplated in the registration statement that was the subject of the Rule 401(g)(2) notice or for which the Company has otherwise become ineligible. References herein to the Registration Statement shall include such new registration statement or post-effective amendment, as the case may be.

(iv) Filing Fees. The Company has paid or shall pay the required Commission filing fees relating to the Offered Securities within the time required by Rule 456(b)(1) without regard to the proviso therein and otherwise in accordance with Rules 456(b) and 457(r).

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(d) Ineligible Issuer Status. (i) At the earliest time after the filing of the Registration Statement that the Company or another offering participant made a bona fide offer (within the meaning of Rule 164(h)(2)) of the Offered Securities and (ii) at the date of this Agreement, the Company was not and is not an “ineligible issuer,” as defined in Rule 405, including (x) the Company or any other subsidiary in the preceding three years not having been convicted of a felony or misdemeanor or having been made the subject of a judicial or administrative decree or order as described in Rule 405 and (y) the Company in the preceding three years not having been the subject of a bankruptcy petition or insolvency or similar proceeding, not having had a registration statement be the subject of a proceeding under Section 8 of the Act and not being the subject of a proceeding under Section 8A of the Act in connection with the offering of the Offered Securities, all as described in Rule 405.

(e) General Disclosure Package. As of the Applicable Time, neither (i) the General Use Issuer Free Writing Prospectus issued at or prior to the Applicable Time, the preliminary prospectus supplement, dated June 14, 2011, together with the base prospectus, dated December 17, 2010 (which is the most recent Statutory Prospectus distributed to investors generally), and the other information, if any, stated in Schedule B to this Agreement to be included in the General Disclosure Package, all considered together (collectively, the “General Disclosure Package”), nor (ii) any individual Limited Use Issuer Free Writing Prospectus, when considered together with the General Disclosure Package, included any untrue statement of a material fact or omitted to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading. The preceding sentence does not apply to statements in or omissions from any Statutory Prospectus or any Issuer Free Writing Prospectus in reliance upon and in conformity with written information furnished to the Company by any Underwriter through the Representative specifically for use therein, it being understood and agreed that the only such information furnished by any Underwriter consists of the information described as such in Section 7(b) hereof.

(f) Issuer Free Writing Prospectuses. Each Issuer Free Writing Prospectus, as of its issue date and at all subsequent times through the completion of the public offer and sale of the Offered Securities or until any earlier date that the Company notified or notifies the Representative as described in the next sentence, did not, does not and will not include any information that conflicted, conflicts or will conflict with the information then contained in the Registration Statement. If at any time following issuance of an Issuer Free Writing Prospectus there occurred or occurs an event or development as a result of which such Issuer Free Writing Prospectus conflicted or would conflict with the information then contained in the Registration Statement or as a result of which such Issuer Free Writing Prospectus, if republished immediately following such event or development, would include an untrue statement of a material fact or omitted or would omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading, (i) the Company has promptly notified or will promptly notify the Representative and (ii) the Company has promptly amended or will promptly amend or supplement such Issuer Free Writing Prospectus to eliminate or correct such conflict, untrue statement or omission.

(g) Good Standing of the Company. The Company has been duly incorporated and is existing and in good standing under the laws of the State of Texas, with power and authority (corporate and other) to own its properties and conduct its business as described in the General Disclosure Package and the Final Prospectus; and the Company is duly qualified to do business as a foreign corporation in good standing in all other jurisdictions in which its ownership or lease of property or the conduct of its business requires such qualification.

(h) Subsidiaries. Each subsidiary of the Company has been duly incorporated and is existing and in good standing under the laws of the jurisdiction of its incorporation, with power and authority

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(corporate and other) to own its properties and conduct its business as described in the General Disclosure Package and the Final Prospectus; and each subsidiary of the Company is duly qualified to do business as a foreign corporation in good standing in all other jurisdictions in which its ownership or lease of property or the conduct of its business requires such qualification except where the failure to be so qualified would not reasonably be expected to have, individually or in the aggregate, a material adverse effect on the business, financial position or results of operations of the Company and its subsidiaries taken as a whole or in the performance by the Company of its obligations under this Agreement (a “Material Adverse Effect”); all of the issued and outstanding capital stock of each subsidiary of the Company has been duly authorized and validly issued and is fully paid and nonassessable; and, except as disclosed in the General Disclosure Package and the Final Prospectus, the capital stock of each subsidiary owned by the Company, directly or through subsidiaries, is owned free from liens, encumbrances and defects.

(i) Offered Securities. The Offered Securities and all other outstanding shares of capital stock of the Company have been duly authorized; all outstanding shares of capital stock of the Company are, and, when the Offered Securities have been delivered and paid for in accordance with this Agreement on the Closing Date, such Offered Securities will have been, validly issued, fully paid and nonassessable; all outstanding shares of capital stock of the Company, and, when the Offered Securities have been delivered and paid for in accordance with this Agreement on the Closing Date, such Offered Securities will, conform to the information in the General Disclosure Package and to the description of such capital stock and Offered Securities contained in the Final Prospectus; the shareholders of the Company have no preemptive rights with respect to the Offered Securities; none of the outstanding shares of capital stock of the Company are or will have been issued in violation of any preemptive or similar rights of any security holder; and the authorized equity capitalization of the Company is as set forth in the General Disclosure Package.

(j) Conversion. When the Offered Securities are delivered and paid for pursuant to this Agreement on each Closing Date, such Offered Securities will be convertible into the Underlying Shares of the Company in accordance with their terms; the Underlying Shares initially issuable upon conversion of such Offered Securities have been duly authorized and reserved for issuance upon such conversion and will conform to the information in the General Disclosure Package and will conform to the description of such Underlying Shares contained in the Final Prospectus; all outstanding shares of capital stock of the Company are, and when issued upon conversion of the Offered Securities the Underlying Shares will be, validly issued, fully paid and nonassessable; and the shareholders of the Company have no preemptive rights with respect to the issuance of the Underlying Shares upon the conversion of the Offered Securities.

(k) No Finder’s Fee. Except as disclosed in the General Disclosure Package and the Final Prospectus, there are no contracts, agreements or understandings between the Company (or any of its subsidiaries) and any person that would give rise to a valid claim against the Company, any of its subsidiaries or any Underwriter for a brokerage commission, finder’s fee or other like payment in connection with the offering and sale of the Offered Securities.

(l) Registration Rights. Except as disclosed in the General Disclosure Package and the Final Prospectus, there are no contracts, agreements or understandings between the Company and any person granting such person the right to require the Company to file a registration statement under the 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 Act (collectively, “registration rights”), and any person to whom the Company has granted registration rights has agreed not to exercise such rights until after the expiration of the Blackout Period referred to in Section 4 hereof.

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(m) Absence of Further Requirements. No consent, approval, authorization, order, license, registration or qualification of or with any court or arbitrator or governmental or regulatory authority is required for the execution, delivery and performance by the Company of this Agreement, the issuance and sale of the Offered Securities and the Underlying Shares and the consummation of the transactions contemplated by this Agreement and described in the General Disclosure Package and Final Prospectus.

(n) Stock Options. With respect to the stock options (the “Stock Options”) granted pursuant to the stock-based compensation plans of the Company and its subsidiaries (the “Company Stock Plans”), (A) each Stock Option intended to qualify as an “incentive stock option” under Section 422 of the Code so qualifies, (B) each grant of a Stock Option was duly authorized no later than the date on which the grant of such Stock Option was by its terms to be effective (the “Grant Date”) by all necessary corporate action, including, as applicable, approval by the board of directors of the Company (or a duly constituted and authorized committee thereof) and any required shareholder approval by the necessary number of votes or written consents, and the award agreement governing such grant (if any) was duly executed and delivered by each party thereto, (C) each such grant was made in accordance with the terms of the Company Stock Plans, the Exchange Act and all other applicable laws and regulatory rules or requirements, including the rules of the Nasdaq Global Select Market and any other exchange on which Company securities are traded, and (D) each such grant was properly accounted for in accordance with generally accepted accounting principles in the financial statements (including the related notes) of the Company and disclosed in the Company’s filings with the Commission in accordance with the Exchange Act and all other applicable laws. The Company has not knowingly granted, and there is no and has been no policy or practice of the Company of granting, Stock Options prior to, or otherwise coordinating the grant of Stock Options with, the release or other public announcement of material information regarding the Company or its subsidiaries or their results of operations or prospects.

(o) Title to Property. Except as disclosed in the General Disclosure Package and the Final Prospectus, or except as would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect, the Company and its subsidiaries have title to their properties as follows: (A) with respect to their wells (including leasehold interests and appurtenant personal property) and their non-producing oil and gas properties (including undeveloped locations on leases held by production and those leases not held by production), such title is valid and defensible and free and clear of all liens, security interests, pledges, charges, encumbrances, mortgages and restrictions, (B) with respect to their non-producing properties in exploration prospects, such title was investigated in accordance with customary industry procedures prior to the Company or its subsidiaries’ acquisition thereof; (C) with respect to their real property other than oil and gas interests, such title is good and marketable free and clear of all liens, security interests, pledges, charges, encumbrances, mortgages and restrictions; and (D) with respect to their personal property other than that appurtenant to its oil and gas interests, such title is free and clear of all liens, security interests, pledges, charges, encumbrances, mortgages and restrictions. No real property owned, leased, licensed, or used by the Company or its subsidiaries lies in an area that is, or to the knowledge of the Company will be, subject to restrictions that would prohibit, and no statements of facts relating to the actions or inaction of another person or entity or his or its ownership, leasing, licensing, or use of any real or personal property exists or will exist which would prevent, the continued effective ownership, leasing, licensing, exploration, development or production or use of such real property in the business of the Company as presently conducted or as the General Disclosure Package or the Final Prospectus indicates they contemplate conducting, except as disclosed in the General Disclosure Package and the Final Prospectus or except as would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect.

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(p) Possession of Intellectual Property. The Company and its subsidiaries own or possess, or can acquire on reasonable terms, adequate patents, patent rights, licenses, inventions, copyrights, know-how, confidential information, trademarks, service marks, trade names or other intellectual property (collectively, “Intellectual Property”) necessary to carry on the business now operated by them, except where the failure to own or possess, or have the ability to acquire on reasonable terms such Intellectual Property, would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect. Neither the Company nor any subsidiary has received any notice of any infringement of or conflict with asserted rights of others with respect to any Intellectual Property that if determined adversely to the Company would reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect.

(q) Absence of Defaults and Conflicts Resulting from Transaction. The execution, delivery and performance by the Company of each of this Agreement, the issuance and sale of the Offered Securities and the Underlying Shares and the consummation of the transactions contemplated by this Agreement and described in the General Disclosure Package and Final Prospectus will not (i) conflict with or result in a breach or violation of any of the terms or provisions of, or constitute a default under, or result in the creation or imposition of any lien, charge or encumbrance upon any property or assets of the Company or any of its subsidiaries pursuant to, any indenture, mortgage, deed of trust, loan agreement or other agreement or instrument to which the Company or any of its subsidiaries is a party or by which the Company or any of its subsidiaries is bound or to which any of the property or assets of the Company or any of its subsidiaries is subject, (ii) result in any violation of the provisions of the charter or by-laws or similar organizational documents of the Company or any of its subsidiaries or (iii) result in the violation of any law or statute or any judgment, order, rule or regulation of any court or arbitrator or governmental or regulatory authority, except, in the case of clauses (i) and (iii) above, for any such conflict, breach, violation or default that would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect.

(r) Absence of Existing Defaults and Conflicts. Neither the Company nor any of its subsidiaries is (i) in violation of its charter or by-laws or similar organizational documents; (ii) in default, and no event has occurred that, with notice or lapse of time or both, would constitute such a default, in the due performance or observance of any term, covenant or condition contained in any indenture, mortgage, deed of trust, loan agreement or other agreement or instrument to which the Company or any of its subsidiaries is a party or by which the Company or any of its subsidiaries is bound or to which any of the property or assets of the Company or any of its subsidiaries is subject; or (iii) in violation of any law or statute or any judgment, order, rule or regulation of any court or arbitrator or governmental or regulatory authority, except, in the case of clauses (ii) and (iii) above, for any such default or violation that would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect.

(s) Authorization. The Company has full right, power and authority to execute and deliver this Agreement and to perform its obligations hereunder; and all action required to be taken for the due and proper authorization, execution and delivery by it of this Agreement and the consummation by it of the transactions contemplated hereby has been duly and validly taken. This Agreement has been duly authorized, executed and delivered by the Company.

(t) Possession of Licenses and Permits. The Company and its subsidiaries possess all licenses, certificates, permits and other authorizations issued by the appropriate Federal, state, local or foreign governmental or regulatory authorities that are necessary for the ownership or lease of their respective properties or the conduct of their respective businesses as described in the General Disclosure Package and the Final Prospectus, except where the failure to so possess would not

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reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect; except as described in the General Disclosure Package and the Final Prospectus, the Company and its subsidiaries are in compliance with the terms and conditions of all such licenses, certificates, permits and authorizations, except where the failure to so comply would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect; and except as described in the General Disclosure Package and the Final Prospectus, neither the Company nor any of its subsidiaries has received notice of any revocation or modification of any such license, certificate, permit or authorization which, individually or in the aggregate, if subject to an unfavorable decision, ruling or finding, would reasonably be expected to have a Material Adverse Effect.

(u) Absence of Labor Dispute. No labor disturbance by or dispute with the employees of the Company or any of its subsidiaries exists or, to the knowledge of the Company, is imminent, and the Company is not aware of any existing or imminent labor disturbance by, or dispute with, the employees of any of its or its subsidiaries’ principal suppliers, contractors or customers, except as would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect.

(v) Environmental Laws. (A) The Company and its subsidiaries (1) are in compliance with any and all applicable Federal, state, local and foreign laws, rules, regulations, requirements, decisions, judgments, decrees, and orders relating to pollution or the protection of the environment, natural resources or human health or safety, including those relating to the generation, storage, treatment, use, handling, transportation, Release or threat of Release of Hazardous Materials (collectively, “Environmental Laws”), (2) have received and are in compliance with all permits, licenses, certificates or other authorizations or approvals required of them under applicable Environmental Laws to conduct their respective businesses, (3) have not received notice of any actual or potential liability under or relating to, or actual or potential violation of, any Environmental Laws, including for the investigation or remediation of any Release or threat of Release of Hazardous Materials, and have no knowledge of any event or condition that would reasonably be expected to result in any such notice, (4) are not conducting or paying for, in whole or in part, any investigation, remediation or other corrective action pursuant to any Environmental Law at any location, and (5) are not a party to any order, decree or agreement that imposes any obligation or liability under any Environmental Law, except for any such matter as would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect; (B) there are no costs or liabilities associated with Environmental Laws of or relating to the Company or its subsidiaries, except for any such matter as would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect; and (C) except as described in the General Disclosure Package and the Final Prospectus, (1) there are no proceedings that are pending, or that are known to be contemplated, against the Company or any of its subsidiaries under any Environmental Laws in which a governmental entity 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, and (2) the Company and its subsidiaries are not aware of any facts or issues regarding compliance with Environmental Laws, or liabilities or other obligations under Environmental Laws, including the Release or threat of Release of Hazardous Materials, that could reasonably be expected to have a material effect on the capital expenditures, earnings or competitive position of the Company and its subsidiaries.

(w) Hazardous Materials. There has been no storage, generation, transportation, use, handling, treatment, Release or threat of Release of Hazardous Materials by, relating to or caused by the Company or any of its subsidiaries (or, to the knowledge of the Company and its subsidiaries, any other entity (including any predecessor) for whose acts or omissions the Company or any of its subsidiaries is or could reasonably be expected to be liable) at, on, under or from any property or facility now or previously owned, operated or leased by the Company or any of its subsidiaries, or at, on, under or from any other property or facility, in violation of any

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Environmental Laws or in a manner or amount or to a location that could reasonably be expected to result in any liability of the Company or any of its subsidiaries under any Environmental Law, except for any violation or liability that would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect. “Hazardous Materials” means any material, chemical, substance, waste, pollutant, contaminant, compound, mixture, or constituent thereof, in any form or amount, including petroleum (including crude oil or any fraction thereof) and petroleum products, natural gas, natural gas liquids, asbestos and asbestos containing materials, naturally occurring radioactive materials, brine, and drilling mud, regulated or which can give rise to liability under any Environmental Law. “Release” means any spilling, leaking, seepage, pumping, pouring, emitting, emptying, discharging, injecting, escaping, leaching, dumping, disposing, depositing, dispersing, or migrating in, into or through the environment, or in, into, from or through any building or structure

(x) Accurate Disclosure. The statements in the General Disclosure Package and the Final Prospectus under the headings “Description of Series B Convertible Preferred Stock” and “Certain United States Federal Income Tax And Estate Tax Considerations”, insofar as such statements summarize legal matters, agreements, documents or proceedings discussed therein, are accurate and fair summaries of such legal matters, agreements, documents or proceedings and present the information required to be shown.

(y) Absence of Manipulation. Neither the Company nor any of its affiliates has, either alone or with one or more other persons, bid for or purchased for any account in which it or any of its affiliates had a beneficial interest any Offered Securities or attempted to induce any person to purchase any Offered Securities.

(z) Statistical and Market-Related Data. Any third-party statistical and market-related data included in the Registration Statement, a Statutory Prospectus or the General Disclosure Package are based on or derived from sources that the Company believes to be reliable and accurate.

(aa) Disclosure Controls. The Company and its subsidiaries maintain an effective system of “disclosure controls and procedures” (as defined in Rule 13a-15(e) of the Exchange Act) that complies with the requirements of the Exchange Act and that has been designed to ensure that information required to be disclosed by the Company in reports that it files or submits under the Exchange Act is recorded, processed, summarized and reported within the time periods specified in the Commission’s rules and forms, including controls and procedures designed to ensure that such information is accumulated and communicated to the Company’s management as appropriate to allow timely decisions regarding required disclosure. The Company and its subsidiaries have carried out evaluations of the effectiveness of their disclosure controls and procedures as required by Rule 13a-15 of the Exchange Act.

(bb) Accounting Controls. The Company and its subsidiaries maintain systems of “internal control over financial reporting” (as defined in Rule 13a-15(f) of the Exchange Act) that comply with the requirements of the Exchange Act and have been designed by, or under the supervision of, their respective principal executive and principal financial officers, or persons performing similar functions, to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles, including, but not limited to, internal accounting controls sufficient to provide reasonable assurance that (A) transactions are executed in accordance with management’s general or specific authorizations; (B) transactions are recorded as necessary to permit preparation of financial statements in conformity with generally accepted accounting principles and to maintain asset accountability; (C) access to assets is permitted only in accordance with management’s general or specific authorization; and (D) the recorded accountability for assets is compared with

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the existing assets at reasonable intervals and appropriate action is taken with respect to any differences. Except as disclosed in the General Disclosure Package and the Final Prospectus, the Company is not aware of (i) any significant deficiencies or material weaknesses in the design or operation of internal controls or (ii) any fraud, whether or not material, that involves management or other employees who have a significant role in the Company’s internal controls over financial reporting which are likely to adversely affect the Company’s ability to record, process, summarize and report financial information. The Company’s auditors and the Audit Committee of the Board of Directors (the “Audit Committee”) of the Company have been advised of: (1) all significant deficiencies and material weaknesses in the design or operation of internal controls over financial reporting which have adversely affected or are reasonably likely to adversely affect the Company’s ability to record, process, summarize and report financial information; and (2) any fraud, whether or not material, that involves management or other employees who have a significant role in the Company’s internal controls over financial reporting.

(cc) Absence of Accounting Issues. A member of the Audit Committee has confirmed to the Chief Executive Officer, Chief Financial Officer or General Counsel that, except as set forth in the General Disclosure Package and the Final Prospectus, the Audit Committee is not reviewing or investigating, and neither the Company’s independent auditors nor its internal auditors have recommended that the Audit Committee review or investigate, (i) adding to, deleting, changing the application of, or changing the Company’s disclosure with respect to, any of the Company’s material accounting policies; (ii) any matter which could result in a restatement of the Company’s financial statements for any annual or interim period during the current or prior three fiscal years; or (iii) any Internal Control Event.

(dd) Litigation. Except as disclosed in the General Disclosure Package and the Final Prospectus, there are no legal, governmental or regulatory investigations, actions, suits or proceedings pending to which the Company or any of its subsidiaries is or may be a party or to which any property of the Company or any of its subsidiaries is or may be the subject that, individually or in the aggregate, if determined adversely to the Company or any of its subsidiaries, would reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect; no such investigations, actions, suits or proceedings are threatened by any governmental or regulatory authority.

(ee) Taxes. The Company and its subsidiaries have on a timely basis paid all Federal, state, local and foreign taxes and filed all tax returns required to be paid or filed through the date hereof (except in any case in which the failure to so file has not had and would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect); and except as otherwise disclosed in the General Disclosure Package and the Final Prospectus, there is no tax deficiency that has been, or could reasonably be expected to be, asserted against the Company or any of its subsidiaries or any of their respective properties or assets, except for any tax deficiency that is currently being contested in good faith or would not reasonably be expected to have a Material Adverse Effect.

(ff) Financial Statements. The financial statements (including the related notes thereto) of the Company and its consolidated subsidiaries included or incorporated by reference in the General Disclosure Package and the Final Prospectus comply in all material respects with the applicable requirements of the Securities Act and the Exchange Act, as applicable, and present fairly the financial position of the Company and its consolidated subsidiaries as of the dates indicated and the results of their operations and the changes in their cash flows for the periods specified; such financial statements have been prepared in conformity with U.S. generally accepted accounting principles applied on a consistent basis throughout the periods covered thereby, and any supporting schedules included or incorporated by reference in the General Disclosure Package and the Final

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Prospectus present fairly the information required to be stated therein; and the other financial information included or incorporated by reference in the General Disclosure Package and the Final Prospectus has been derived from the accounting records of the Company and its consolidated subsidiaries and presents fairly the information shown thereby.

(gg) No Material Adverse Change in Business. Since the end of the period covered by the latest audited financial statements included in the General Disclosure Package (i) there has been no change, nor any development or event involving a prospective change, in the condition (financial or otherwise), results of operations, business, properties or prospects of the Company and its subsidiaries, taken as a whole, that is material and adverse, (ii) except as disclosed in or contemplated by the General Disclosure Package and the Final Prospectus, there has been no dividend or distribution of any kind declared, paid or made by the Company on any class of its capital stock, (iii) except as disclosed in or contemplated by the General Disclosure Package and the Final Prospectus, there has been no material adverse change in the capital stock, short-term indebtedness, long-term indebtedness, net current assets or net assets of the Company and its subsidiaries, (iv) neither the Company nor any of its subsidiaries has entered into any transaction or agreement (whether or not in the ordinary course of business) that is material to the Company and its subsidiaries taken as a whole or incurred any liability or obligation, direct or contingent, that is material to the Company and its subsidiaries taken as a whole and (v) neither the Company nor any of its subsidiaries has sustained any loss or interference with its business that is material to the Company and its subsidiaries taken as a whole and that is either from fire, explosion, flood or other calamity, whether or not covered by insurance, or from any labor disturbance or dispute or any action, order or decree of any court or arbitrator or governmental or regulatory authority, except in each case as otherwise disclosed in the General Disclosure Package and the Final Prospectus.

(hh) Investment Company Act. The Company is not and, after giving effect to the offering and sale of the Offered Securities and the application of the proceeds thereof as described in the General Disclosure Package and the Final Prospectus, will not be required to register as an “investment company” or an entity “controlled” by an “investment company” within the meaning of the Investment Company Act of 1940, as amended, and the rules and regulations of the Commission thereunder (collectively, the “Investment Company Act”).

(ii) Regulations T, U and X. The application of the proceeds received by the Company from the issuance, sale and delivery of the Offered Securities as described in the General Disclosure Package and the Final Prospectus will not violate Regulation T, U or X of the Board of Governors of the Federal Reserve System or any other regulation of such Board of Governors.

(jj) Ratings. No “nationally recognized statistical rating organization” as such term is defined for purposes of Rule 436(g)(2) (i) has imposed (or has informed the Company that it is considering imposing) any condition (financial or otherwise) on the Company’s retaining any rating assigned to the Company or any securities of the Company or (ii) has indicated to the Company that it is considering any of the actions described in Section 6(d)(ii) hereof.

(kk) Insurance. The Company and its subsidiaries have insurance covering their respective properties, operations, personnel and businesses, including business interruption insurance, which insurance is in amounts and insures against such losses and risks as are customary for the businesses in which they are engaged; all such policies insuring the Company and its subsidiaries are in full force and effect; and neither the Company nor any of its subsidiaries has received notice from any insurer or agent of such insurer that it will not be able to renew insurance coverage as and when such coverage expires or to obtain coverage acceptable to the Company at reasonable cost from similar insurers as may be necessary to continue its business.

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(ll) Compliance with ERISA. The minimum funding standard under Section 302 of the Employee Retirement Income Security Act of 1974, as amended, and the regulations and published interpretations thereunder (“ERISA”), has been satisfied by each “pension plan” (as defined in Section 3(2) of ERISA) which has been established or maintained by the Company, and the trust forming part of each such plan which is intended to be qualified under Section 401 of the Internal Revenue Code of 1986, as amended (the “Code”), is so qualified; each of the Company and each of its subsidiaries has fulfilled its obligations, if any, under Section 515 of ERISA; neither the Company nor any of its subsidiaries maintains and is required to contribute to a “welfare plan” (as defined in Section 3(1) of ERISA) which provides retiree or other post-employment welfare benefits or insurance coverage (other than “continuation coverage” (as defined in Section 602 of ERISA)); each pension plan and welfare plan established or maintained by the Company and/or one of the subsidiaries is in compliance with the currently applicable provisions of ERISA, except where the failure to comply would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect; no prohibited transaction, within the meaning of Section 406 of ERISA or Section 4975 of the Code, has occurred with respect to any pension plan or welfare plan (excluding transactions effected pursuant to statutory or administrative exemption) that could reasonably be expected to result in a material liability to the Company or its subsidiaries; and neither the Company nor any of its subsidiaries has incurred or could reasonably be expected to incur any withdrawal liability under Section 4201 of ERISA, any liability under Section 4062, 4063, or 4064 of ERISA, or any other liability under Title IV of ERISA.

(mm) Reserve Report Data. The oil and gas reserve estimates of the Company contained or incorporated by reference into the General Disclosure Package and the Final Prospectus have been prepared by (A) Ryder Scott Company, L.P. and (B) Collarini Associates (collectively, the “Engineers”), and the Company has no reason to believe that such estimates do not fairly reflect the oil and gas reserves of the Company at the dates indicated. The information underlying the estimates of the reserves of the Company and its subsidiaries supplied by the Company to the Engineers, for the purposes of preparing the reserve reports of Company and its subsidiaries referenced in the General Disclosure Package and the Final Prospectus (the “Reserve Reports”), was true and correct in all material respects on the date of each such Reserve Report; the estimates of future capital expenditures and other future exploration and development costs supplied to the Engineers were prepared in good faith and with a reasonable basis; the information provided to the Engineers was prepared in good faith and with a reasonable basis. Other than production of the reserves in the ordinary course of business and intervening product price fluctuations described in the General Disclosure Package and the Final Prospectus, the Company is not aware of any facts or circumstances that would not reasonably be expected to have, individually or in the aggregate, a Material Adverse Effect in the reserves or the present value of future net cash flows therefrom as described in the General Disclosure Package and the Final Prospectus. Each of the Engineers is an independent reserve engineer with respect to the Company and its subsidiaries.

(nn) Related Party Transactions. All business relationships or related-party transactions involving the Company or any of its subsidiaries or any of their respective directors, managers, executive officers, nominees for election as director, manager or executive officer, any security holder, member interest holder or partnership interest holder, each as applicable, who is known to the Company or any of its subsidiaries to own of record or beneficially more than five percent of any class of the Company’s or any of its subsidiaries’ voting securities or interests, as applicable, any member of the immediate family of the foregoing persons, or any other person, required to be disclosed pursuant to the requirements of Item 404 of Regulation S-K under the Act, are accurately described in the General Disclosure Package and the Final Prospectus, and all other related-party transactions required to be disclosed pursuant to the requirements of Item 404 of Regulation S-K under the Act have been accurately described in the General Disclosure Package.

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(oo) Independent Accountants. PricewaterhouseCoopers LLP, who have certified certain financial statements of the Company and its subsidiaries, and Deloitte & Touche LLP, who have certified certain financial statements of the Company and its subsidiaries, are each an independent registered public accounting firm with respect to the Company and its subsidiaries within the applicable rules and regulations adopted by the Commission and the Public Company Accounting Oversight Board (United States) and as required by the Act.

(pp) Incorporated Documents. The documents incorporated by reference in the General Disclosure Package and the Final Prospectus, when they were filed with the Commission, conformed in all material respects to the requirements of the Exchange Act, and none of such documents contained any untrue statement of a material fact or omitted to state a material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading; and any further documents so filed and incorporated by reference in the General Disclosure Package and the Final Prospectus, when such documents are filed with the Commission, will conform in all material respects to the requirements of the Exchange Act and will not contain any 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.

(qq) No Unlawful Payments. Neither the Company nor any of its subsidiaries nor, to the knowledge of the Company, any director, officer, agent, employee or other person associated with or acting on behalf of the Company or any of its subsidiaries has (A) used any corporate funds for any unlawful contribution, gift, entertainment or other unlawful expense relating to political activity; (B) made any direct or indirect unlawful payment to any foreign or domestic government official or employee from corporate funds; (C) violated or is in violation of any provision of the Foreign Corrupt Practices Act of 1977; or (D) made any bribe, rebate, payoff, influence payment, kickback or other unlawful payment.

(rr) Compliance with Money Laundering Laws. The operations of the Company and its subsidiaries are and have been conducted at all times in compliance with applicable financial recordkeeping and reporting requirements of the Currency and Foreign Transactions Reporting Act of 1970, as amended, the money laundering statutes of all jurisdictions, the rules and regulations thereunder and any related or similar rules, regulations or guidelines, issued, administered or enforced by any governmental agency (collectively, the “Money Laundering Laws”) and no action, suit or proceeding by or before any court or governmental agency, authority or body or any arbitrator involving the Company or any of its subsidiaries with respect to the Money Laundering Laws is pending or, to the knowledge of the Company, threatened.

(ss) Compliance with OFAC. None of the Company, any of its subsidiaries or, to the knowledge of the Company, any director, officer, agent, employee or affiliate of the Company or any of its subsidiaries is currently subject to any U.S. sanctions administered by the Office of Foreign Assets Control of the U.S. Department of the Treasury (“OFAC”); and the Company will not, directly or indirectly, use the proceeds of the offering of the Offered Securities hereunder, or lend, contribute or otherwise make available such proceeds to any subsidiary, joint venture partner or other person or entity, for the purpose of financing the activities of any person currently subject to any U.S. sanctions administered by OFAC.

3. Purchase, Sale and Delivery of Offered Securities. On the basis of the representations, warranties and agreements and subject to the terms and conditions set forth herein, the Company agrees to sell to the several Underwriters, and each of the Underwriters agrees, severally and not jointly, to purchase from the Company, at a purchase price of U.S. $87.00 per share plus accumulated dividends from June 20, 2011 to the

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First Closing Date, the respective number of shares of Firm Securities set forth opposite the names of the several Underwriters in Schedule A hereto.

The Company will deliver the Firm Securities to or as instructed by the Representative in a form reasonably acceptable to the Representative against payment of the purchase price by the Underwriters in Federal (same day) funds by wire transfer to an account at a bank designated by the Company and acceptable to the Representative drawn to the order of the Company at the office of Cravath, Swaine & Moore LLP, at 10:00 a.m., New York City time, on June 20, 2011 or at such other time not later than seven full business days thereafter as the Representative and the Company determine, such time being herein referred to as the “First Closing Date”. The Firm Securities so to be delivered or evidence of their issuance will be made available for checking at the above office of Cravath, Swaine & Moore LLP at least 24 hours prior to the First Closing Date. For purposes of Rule 15c6-1 under the Exchange Act, the First Closing Date (if later than the otherwise applicable settlement date) shall be the date for payment of funds and delivery of securities for all the Firm Securities sold pursuant to the offering. It is understood that the Underwriters propose to offer the Firm Securities for sale as set forth in the Final Prospectus.

In addition, upon written notice from the Representative given to the Company from time to time not more than 30 days subsequent to the date of this Agreement, the Underwriters may purchase all or less than all of the Optional Securities at the purchase price per Offered Security (including any accumulated dividends thereon to the related Optional Closing Date) to be paid for the Firm Securities. The Company agrees to sell to the Underwriters the number of Optional Securities specified in such notice and the Underwriters agree, severally and not jointly, to purchase such Optional Securities. Such Optional Securities shall be purchased from the Company for the account of each Underwriter in the same proportion as the number of Firm Securities set forth opposite such Underwriter’s name in Schedule A hereto bears to the total number of Firm Securities (subject to adjustments by the Representative to eliminate fractions). No Optional Securities shall be sold or delivered unless the Firm Securities previously have been, or simultaneously are, sold and delivered. The right to purchase the Optional Securities or any portion thereof may be exercised from time to time and to the extent not previously exercised may be surrendered and terminated at any time upon notice by the Representative to the Company.

Each time for the delivery of and payment for the Optional Securities, being herein referred to as the “Optional Closing Date”, which may be the First Closing Date (the First Closing Date and each Optional Closing Date, if any, being sometimes referred to as a “Closing Date”), shall be determined by the Representative but shall not be later than seven full business days after written notice of election to purchase Optional Securities is given. Payment for the Optional Securities being purchased on each Optional Closing Date shall be made by the Underwriters in United States dollars in Federal (same day) funds by wire transfer to an account at a bank acceptable to the Representative drawn to the order of the Company at 10:00 a.m. (New York City time) on the Optional Closing Date against delivery to or as instructed by the Representative for the account of the several Underwriters of the Optional Securities in a form reasonably acceptable to the Representative. The Optional Securities being purchased on each Optional Closing Date or evidence of their issuance will be made available for checking at the above offices of Cravath, Swaine & Moore LLP at a reasonable time in advance of such Optional Closing Date.

4. Certain Agreements of the Company. The Company agrees with the Underwriters that it will furnish to counsel for the Underwriters a copy of the registration statement relating to the Registered Securities, including all exhibits, in the form it became effective and of all amendments thereto and that, in connection with each offering of Offered Securities:

(a) Filing of Prospectuses. The Company has filed or will file each Statutory Prospectus (including the Final Prospectus) pursuant to and in accordance with Rule 424(b)(2) (or, if applicable and consented to by the Representative, subparagraph (5)) not later than the second business day

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following the earlier of the date it is first used and the date of this Agreement. The Company has complied and will comply with Rule 433.

(b) Filing of Amendments; Response to Commission Requests. The Company will promptly advise the Representative of any proposal to amend or supplement the Registration Statement or any Statutory Prospectus at any time and will afford the Representative a reasonable opportunity to comment on any such proposed amendment or supplement; and the Company will also advise the Representative promptly of (i) the filing of any such amendment or supplement, (ii) any request by the Commission or its staff for any amendment to any Registration Statement, for any supplement to any Statutory Prospectus or for any additional information, (iii) the institution by the Commission of any stop order proceedings in respect of a Registration Statement or the threatening of any proceeding for that purpose, and (iv) the receipt by the Company of any notification with respect to the suspension of the qualification of the Offered Securities in any jurisdiction or the institution or threatening of any proceedings for such purpose. The Company will use its commercially reasonable efforts to prevent the issuance of any such stop order or the suspension of any such qualification and, if issued, to obtain as soon as possible the withdrawal thereof.

(c) Continued Compliance with Securities Laws. If, at any time when a prospectus relating to the Offered Securities is (or but for the exemption in Rule 172 under the Act would be) required to be delivered under the Act in connection with sales by any Underwriter or any dealer, any event occurs as a result of which the Final Prospectus as then amended or supplemented would include an untrue statement of a material fact or omit to state any material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading, or if it is necessary at any time to amend the Registration Statement or supplement the Final Prospectus to comply with the Act, the Company will promptly notify the Representative of such event and will promptly prepare and file with the Commission and furnish, at its own expense, to the Underwriters and the dealers and any other dealers upon request of the Representative, an amendment or supplement which will correct such statement or omission or an amendment which will effect such compliance. Neither the Representative’s consent to, nor the Underwriters’ delivery of, any such amendment or supplement shall constitute a waiver of any of the conditions set forth in Section 6 hereof.

(d) Rule 158. As soon as practicable, but not later than 16 months, after the date of this Agreement, the Company will make generally available to its securityholders an earnings statement covering a period of at least 12 months beginning after the date of this Agreement and satisfying the provisions of Section 11(a) of the Act and Rule 158.

(e) Furnishing of Prospectuses. The Company will furnish to the Representative copies of the Registration Statement, including all exhibits, any Statutory Prospectus relating to the Offered Securities, the Final Prospectus and all amendments and supplements to such documents, in each case as soon as available and in such quantities as the Representative reasonably requests. The Company will pay the expenses of printing and distributing to the Underwriters all such documents.

(f) Blue Sky Qualifications. The Company will arrange for the qualification of the Offered Securities and the Underlying Shares for sale and the determination of their eligibility for investment under the laws of such jurisdictions as the Representative designates and will continue such qualifications in effect so long as required for the distribution.

(g) Reporting Requirements. For so long as the Offered Securities remain outstanding, the Company will furnish to the Representative and, upon request, to the other Underwriter, as soon as practicable after the end of each fiscal year, a copy of its annual report to shareholders for such year; and the Company will furnish to the Representative (i) as soon as available, a copy of each report and any definitive proxy statement of the Company filed with the Commission under the Exchange Act or

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mailed to shareholders, and (ii) from time to time, such other information concerning the Company as the Representative may reasonably request. However, so long as the Company is subject to the reporting requirements of either Section 13 or Section 15(d) of the Exchange Act and is timely filing reports with the Commission on its Electronic Data Gathering, Analysis and Retrieval system (“EDGAR”), it is not required to furnish such reports or statements to the Underwriters.

(h) Payment of Expenses. The Company will pay all expenses incident to the performance of its obligations under this Agreement, including but not limited to any filing fees and other expenses (including fees and disbursements of counsel to the Underwriters) incurred in connection with qualification of the Offered Securities for sale and any determination of their eligibility for investment under the laws of such jurisdictions as the Representative may designate and the preparation and printing of memoranda relating thereto, for any fees charged by investment rating agencies for the rating of the Offered Securities, for any costs and expenses related to the review by the the Financial Industry Regulatory Authority, Inc. (including filing fees and the fees and expenses of counsel for the Underwriters relating to such review), costs and expenses relating to investor presentations or any “road show” in connection with the offering and sale of the Offered Securities including, without limitation, any travel expenses of the Company’s officers and employees and any other expenses of the Company including the chartering of airplanes, fees and expenses in connection with the registration of the Offered Securities under the Exchange Act, and expenses incurred in distributing the preliminary prospectus, any Statutory Prospectuses and the Final Prospectus (including any amendments and supplements thereto) to the Underwriters and for expenses incurred for preparing, printing and distributing any Issuer Free Writing Prospectuses to investors or prospective investors.

(i) Use of Proceeds. The Company will use the net proceeds received in connection with the offering of the Offered Securities in the manner described in the “Use of Proceeds” section of the General Disclosure Package and, except as disclosed in the General Disclosure Package and the Final Prospectus, the Company does not intend to use any of the proceeds from the sale of the Offered Securities hereunder to repay any outstanding debt owed to any affiliate of any Underwriter.

(j) Absence of Manipulation. The Company will not take, directly or indirectly, any action designed to or that would constitute or that might reasonably be expected to cause or result in, stabilization or manipulation of the price of any securities of the Company to facilitate the sale or resale of the Offered Securities.

(k) Restriction on Sale of Securities. For a period of 90 days after the date of the Final Prospectus (the “Blackout Period”), the Company will not, directly or indirectly, take any of the following actions with respect to the Offered Securities, its Common Stock or any securities convertible into or exchangeable or exercisable for its Common Stock (collectively, “Lock-Up Securities”): (i) offer, sell, issue, contract to sell, pledge or otherwise dispose of, Lock-Up Securities, (ii) offer, sell, issue, contract to sell, contract to purchase or grant any option, right or warrant to purchase Lock-Up Securities, (iii) enter into any swap, hedge or any other agreement that transfers, in whole or in part, the economic consequences of ownership of Lock-Up Securities, (iv) establish or increase a put equivalent position or liquidate or decrease a call equivalent position in Lock-Up Securities within the meaning of Section 16 of the Exchange Act or (v) except as required by the statement of resolutions relating to the Offered Securities or the Company’s currently outstanding 8.00% convertible perpetual preferred stock, par value $0.001 per share, file with the Commission a registration statement under the Act relating to Lock-Up Securities, or publicly disclose the intention to take any such action, without the prior written consent of the Representative, other than the Offered Securities to be sold hereunder and any shares of Common Stock issued upon the exercise of options granted under stock option plans described in the General Disclosure Package and the Final

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Prospectus. Notwithstanding the foregoing, if (1) during the last 17 days of the Blackout Period, the Company issues an earnings release or material news or a material event relating to the Company occurs; or (2) prior to the expiration of the Blackout Period, the Company announces that it will release earnings results during the 16-day period beginning on the last day of the 90-day period, the restrictions imposed by this Agreement shall continue to apply until the expiration of the 18-day period beginning on the issuance of the earnings release or the occurrence of the material news or material event.

5. Free Writing Prospectuses. (a) Issuer Free Writing Prospectuses. The Company represents and agrees that, unless it obtains the prior consent of the Representative, and each Underwriter represents and agrees that, unless it obtains the prior consent of the Company, it has not made and will not make any offer relating to the Offered Securities that would constitute an Issuer Free Writing Prospectus, or that would otherwise constitute a “free writing prospectus,” as defined in Rule 405, required to be filed with the Commission. Any such free writing prospectus consented to by the Company and the Representative is hereinafter referred to as a “Permitted Free Writing Prospectus.” The Company represents that it has treated and agrees that it will treat each Permitted Free Writing Prospectus as an “issuer free writing prospectus,” as defined in Rule 433, and has complied and will comply with the requirements of Rules 164 and 433 applicable to any Permitted Free Writing Prospectus, including timely Commission filing where required, legending and record keeping.

(b) Term Sheets. The Company will prepare a final term sheet relating to the Offered Securities, containing only information that describes the final terms of the Offered Securities and otherwise in a form consented to by the Representative, and will file such final term sheet within the period required by Rule 433(d)(5)(ii) following the date such final terms have been established for all classes of the offering of the Offered Securities. Any such final term sheet is an Issuer Free Writing Prospectus and a Permitted Free Writing Prospectus for purposes of this Agreement. The Company also consents to the use by any Underwriter of a free writing prospectus that contains only (i)(x) information describing the preliminary terms of the Offered Securities or their offering or (y) information that describes the final terms of the Offered Securities or their offering and that is included in the final term sheet of the Company contemplated in the first sentence of this subsection or (ii) other information that is not “issuer information,” as defined in Rule 433, it being understood that any such free writing prospectus referred to in clauses (i) or (ii) above shall not be an Issuer Free Writing Prospectus for purposes of this Agreement.

6. Conditions of the Obligations of the Underwriters. The obligations of the several Underwriters to purchase and pay for the Offered Securities will be subject to the accuracy of the representations and warranties of the Company herein (as though made on each Closing Date), to the accuracy of the statements of Company officers made pursuant to the provisions hereof, to the performance by the Company of its obligations hereunder and to the following additional conditions precedent:

(a) Accountants’ Comfort Letter. The Underwriters shall have received letters dated, respectively, the date hereof and each Closing Date of PricewaterhouseCoopers LLP, confirming that they are a registered public accounting firm and independent public accountants within the meaning of the Securities Laws and substantially in the form of Schedule C hereto (except that, in any letter dated the Closing Date, the specified date referred to in Schedule C shall be a date no more than three days prior to the Closing Date).

(b) Filing of Prospectus. The Final Prospectus shall have been filed with the Commission in accordance with the Rules and Regulations and Section 4(a) hereof. No stop order suspending the effectiveness of the Registration Statement or of any part thereof shall have been issued and no proceedings for that purpose shall have been instituted or, to the knowledge of the Company or any Underwriter, shall be contemplated by the Commission.

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(c) Reserve Engineers’ Letters. The Underwriters shall have received a letter dated the date hereof and each Closing Date of each of Ryder Scott Company, L.P. and Collarini Associates (i) confirming that as of the date of its Reserve Reports, it was an independent reserve engineer for the Company and/or its subsidiaries and that as of the date of such letter no information had come to its attention that could reasonably have been expected to cause it to withdraw its Reserve Report and (ii) otherwise in form and substance acceptable to the Representative.

(d) No Material Adverse Change. Subsequent to the execution and delivery of this Agreement, there shall not have occurred (i) any change, or any development or event involving a prospective change, in the condition (financial or otherwise), results of operations, business, properties or prospects of the Company and its subsidiaries taken as a whole which, in the judgment of the Representative, is material and adverse and makes it impractical or inadvisable to market the Offered Securities; (ii) any downgrading in the rating of any debt securities or preferred stock of the Company by any “nationally recognized statistical rating organization” (as defined for purposes of Rule 436(g)), or any public announcement that any such organization has under surveillance or review its rating of any debt securities or preferred stock of the Company (other than an announcement with positive implications of a possible upgrading, and no implication of a possible downgrading, of such rating) or any announcement that the Company has been placed on negative outlook; (iii) any change in U.S. or international financial, political or economic conditions or currency exchange rates or exchange controls, the effect of which is such as to make it, in the judgment of the Representative, impractical to market or to enforce contracts for the sale of the Offered Securities, whether in the primary market or in respect of dealings in the secondary market; (iv) any suspension or material limitation of trading in securities generally on the New York Stock Exchange, or any setting of minimum or maximum prices for trading on such exchange; (v) or any suspension of trading of any securities of the Company on any exchange or in the over-the-counter market; (vi) any banking moratorium declared by any U.S. federal or New York authorities; (vii) any major disruption of settlements of securities, payment, or clearance services in the United States or any other country where such securities are listed or (viii) any attack on, outbreak or escalation of hostilities or act of terrorism involving the United States, any declaration of war by Congress or any other national or international calamity or emergency if, in the judgment of the Representative, the effect of any such attack, outbreak, escalation, act, declaration, calamity or emergency is such as to make it impractical or inadvisable to market the Offered Securities or to enforce contracts for the sale of the Offered Securities.

(e) Opinion of Counsel for the Company. The Underwriters shall have received an opinion, dated the Closing Date, of Jackson Walker L.L.P., counsel for the Company, substantially in the form of Schedule D hereto.

(f) Opinion of Counsel for the Underwriters. The Underwriters shall have received from Cravath, Swaine & Moore LLP, counsel for the Underwriters, such opinion or opinions, dated the Closing Date, with respect to such matters as the Representative may require, and the Company shall have furnished to such counsel such documents as they request for the purpose of enabling them to pass upon such matters.

(g) Officers’ Certificate. The Underwriters shall have received a certificate, dated the Closing Date, of an executive officer of the Company and a principal financial or accounting officer of the Company in which such officers shall state that: the representations and warranties of the Company in this Agreement are true and correct; the Company has complied with all agreements and satisfied all conditions on its part to be performed or satisfied hereunder at or prior to the Closing Date; no stop order suspending the effectiveness of the Registration Statement or of any part thereof has been issued and no proceedings for that purpose have been instituted or, to the best of their

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knowledge and after reasonable investigation, are contemplated by the Commission; and subsequent to the date of the most recent financial statements in the General Disclosure Package, there has been no material adverse change, nor any development or event involving a prospective material adverse change, in the condition (financial or otherwise), results of operations, business, properties or prospects of the Company and its subsidiaries taken as a whole except as set forth in the General Disclosure Package or as described in such certificate.

(h) Lockup Letters. On or prior to the date hereof, the Representative shall have received a lockup letter from each of the directors of the Company and each of the executive officers of the Company that is a reporting person under Section 16 of the Exchange Act.

(i) No Downgrade. Subsequent to the earlier of (A) the Applicable Time and (B) the execution and delivery of this Agreement, if there are any debt securities or preferred stock of, or guaranteed by, the Company or any of its subsidiaries that are rated by a “nationally recognized statistical rating organization,” as such term is defined by the Commission for purposes of Rule 436(g)(2) under the Act, (i) no downgrading shall have occurred in the rating accorded any such debt securities or preferred stock and (ii) no such organization shall have publicly announced that it has under surveillance or review, or has changed its outlook with respect to, its rating of any such debt securities or preferred stock (other than an announcement with positive implications of a possible upgrading).

(j) Chief Financial Officer’s Certificate. The Underwriters shall have received on and as of such Closing Date a certificate of the Chief Financial Officer of the Company to the effect that as of such Closing Date in the form and substance reasonably satisfactory to the Representative, to the effect set forth in Schedule E hereto.

(k) No Legal Impediment to Issuance. No action shall have been taken and no statute, rule, regulation or order shall have been enacted, adopted or issued by any federal, state or foreign governmental or regulatory authority that would as of such Closing Date prevent the issuance or sale of the Offered Securities; and no injunction or order of any federal, state or foreign court shall have been issued that would as of such Closing Date prevent the issuance or sale of the Offered Securities.

(l) Good Standing. The Underwriters shall have received on and as of such Closing Date satisfactory evidence of the good standing of the Company in writing or any standard form of telecommunication from the appropriate governmental authorities of such jurisdictions.

The Company will furnish the Underwriters with such conformed copies of such opinions, certificates, letters and documents as the Representative reasonably requests. The Representative may in its sole discretion waive compliance with any conditions to the obligations of the Underwriters under this Agreement.

7. Indemnification and Contribution. (a) Indemnification of Underwriters. The Company will indemnify and hold harmless each Underwriter, its partners, members, directors, officers, employees, agents, affiliates and each person, if any, who controls such Underwriter within the meaning of Section 15 of the Act or Section 20 of the Exchange Act (each, an “Indemnified Party”), against any and all losses, claims, damages or liabilities, joint or several, to which such Indemnified Party may become subject, under the Act, the Exchange Act, other Federal or state statutory law or regulation or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based upon any untrue statement or alleged untrue statement of any material fact (i) contained in any part of the Registration Statement, when such part became effective or was deemed effective, or arise out of or are based upon the omission or alleged

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omission of a material fact required to be stated therein or necessary in order to make the statements therein not misleading, or (ii) contained in any part of any Statutory Prospectus as of any time, the Final Prospectus or any Issuer Free Writing Prospectus, or arise out of or are based upon the omission or alleged omission of 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, and will reimburse each Indemnified Party for any legal or other expenses reasonably incurred by such Indemnified Party in connection with investigating or defending against any such loss, claim, damage, liability, action, litigation, investigation or proceeding whatsoever (whether or not such Indemnified Party is a party thereto), whether threatened or commenced, and in connection with the enforcement of this provision with respect to any of the above as such expenses are incurred; provided, however, that the Company will not be liable in any such case to the extent that any such loss, claim, damage or liability arises out of or is based upon an untrue statement or alleged untrue statement in or omission or alleged omission from any of such documents in reliance upon and in conformity with written information furnished to the Company by any Underwriter, through the Representative, if any, specifically for use therein, it being understood and agreed that the only such information furnished by any Underwriter consists of the information described in subsection (b) below.

(b) Indemnification of Company. Each Underwriter will severally and not jointly indemnify and hold harmless the Company, each of its directors and each of its officers who signs the Registration Statement and each person, if any, who controls the Company within the meaning of Section 15 of the Act or Section 20 of the Exchange Act (each, an “Underwriter Indemnified Party”), against any losses, claims, damages or liabilities to which such Underwriter Indemnified Party may become subject, under the Act, the Exchange Act, other Federal or state statutory law or regulation or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based upon any untrue statement or alleged untrue statement of any material fact (i) contained in any part of the Registration Statement, when such part became effective or was deemed effective, or arise out of or are based upon the omission or alleged omission of a material fact required to be stated therein or necessary in order to make the statements therein not misleading, or (ii) contained in any part of any Statutory Prospectus as of any time, the Final Prospectus or any Issuer Free Writing Prospectus, or arise out of or are based upon the omission or alleged omission of 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, in each case to the extent, but only to the extent, that such untrue statement or alleged untrue statement or omission or alleged omission was made in reliance upon and in conformity with written information furnished to the Company by such Underwriter through the Representative, if any, specifically for use therein, and will reimburse any legal or other expenses reasonably incurred by such Underwriter Indemnified Party in connection with investigating or defending against any such loss, claim, damage, liability, action, litigation, investigation or proceeding whatsoever (whether or not such Underwriter Indemnified Party is a party thereto), whether threatened or commenced, based upon any such untrue statement or omission, or any such alleged untrue statement or omission as such expenses are incurred, it being understood and agreed that the only such information furnished by any Underwriter consists of the following information in Final Prospectus furnished on behalf of such Underwriter: (i) information in the third paragraph of the section titled “Underwriting” with respect to concessions or reallowances and (ii) information in the ninth and tenth paragraphs of the section titled “Underwriting” with respect to stabilizing transactions, over-allotment transactions, syndicate covering transactions and penalty bids.

(c) Actions against Parties; Notification. Promptly after receipt by an indemnified party under this Section of notice of the commencement of any action, such indemnified party will, if a claim in respect thereof is to be made against the indemnifying party under subsection (a) or (b) above, notify the indemnifying party of the commencement thereof; provided that the failure to notify the indemnifying party shall not relieve it from any liability that it may have under subsection (a) or (b) above except to the extent that it has been materially prejudiced (through the forfeiture of substantive rights or defenses) by such failure; and provided further that the failure to notify the indemnifying party shall not relieve it from any liability that it may have to an indemnified party otherwise than under subsection (a) or (b) above. In case any such action is brought against any indemnified party and it notifies the indemnifying party of the commencement thereof, the

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indemnifying party will be entitled to participate therein and, to the extent that it may wish, jointly with any other indemnifying party similarly notified, to assume the defense thereof, with counsel satisfactory to such indemnified party (who shall not, except with the consent of the indemnified party, be counsel to the indemnifying party), and after notice from the indemnifying party to such indemnified party of its election so to assume the defense thereof, the indemnifying party will not be liable to such indemnified party under this Section for any legal or other expenses subsequently incurred by such indemnified party in connection with the defense thereof other than reasonable costs of investigation. No indemnifying party shall, without the prior written consent of the indemnified party, effect any settlement of any pending or threatened action in respect of which any indemnified party is or could have been a party and indemnity could have been sought hereunder by such indemnified party unless such settlement (i) includes an unconditional release of such indemnified party from all liability on any claims that are the subject matter of such action and (ii) does not include a statement as to, or an admission of, fault, culpability or a failure to act by or on behalf of an indemnified party.

(d) Contribution. If the indemnification provided for in this Section is unavailable or insufficient to hold harmless an indemnified party under subsection (a) or (b) above, then each indemnifying party shall contribute to the amount paid or payable by such indemnified party as a result of the losses, claims, damages or liabilities referred to in subsection (a) or (b) above (i) in such proportion as is appropriate to reflect the relative benefits received by the Company on the one hand and the Underwriters on the other from the offering of the Offered Securities or (ii) if the allocation provided by clause (i) above is not permitted by applicable law, in such proportion as is appropriate to reflect not only the relative benefits referred to in clause (i) above but also the relative fault of the Company on the one hand and the Underwriters on the other in connection with the statements or omissions which resulted in such losses, claims, damages or liabilities as well as any other relevant equitable considerations. The relative benefits received by the Company on the one hand and the Underwriters on the other shall be deemed to be in the same proportion as the total net proceeds from the offering (before deducting expenses) received by the Company bear to the total underwriting discounts and commissions received by the Underwriters. The relative fault shall be determined by reference to, among other things, whether the untrue or alleged untrue statement of a material fact or the omission or alleged omission to state a material fact relates to information supplied by the Company or the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such untrue statement or omission. The amount paid by an indemnified party as a result of the losses, claims, damages or liabilities referred to in the first sentence of this subsection (d) shall be deemed to include any legal or other expenses reasonably incurred by such indemnified party in connection with investigating or defending any action or claim which is the subject of this subsection (d). Notwithstanding the provisions of this subsection (d), no Underwriter shall be required to contribute any amount in excess of the amount by which the total price at which the Offered Securities underwritten by it and distributed to the public were offered to the public exceeds the amount of any damages which such Underwriter has otherwise been required to pay by reason of such untrue or alleged untrue statement or omission or alleged omission. No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation. The Underwriters’ obligations in this subsection (d) to contribute are several in proportion to their respective underwriting obligations and not joint. The Company and the Underwriters agree that it would not be just and equitable if contribution pursuant to this Section 7(d) were determined by pro rata allocation (even if the Underwriters were treated as one entity for such purpose) or by any other method of allocation which does not take account of the equitable considerations referred to in this Section 7(d).

8. Default of Underwriters. If one of the Underwriters defaults in its obligations to purchase Offered Securities hereunder on either the First Closing Date or any Optional Closing Date and the aggregate number of Offered Securities that such defaulting Underwriter agreed but failed to purchase does not exceed 10% of the total number of Offered Securities that the Underwriters are obligated to purchase on such Closing Date, the non-defaulting Underwriter may make arrangements satisfactory to the Company for the purchase of such Offered Securities by other persons, including the non-defaulting Underwriter, but if no such arrangements are made by such Closing Date, the non-defaulting Underwriter shall be obligated to

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purchase the Offered Securities that such defaulting Underwriter agreed but failed to purchase on such Closing Date. If one of the Underwriters so defaults and the aggregate number of Offered Securities with respect to which such default occurs exceeds 10% of the total number of shares of Offered Securities that the Underwriters are obligated to purchase on such Closing Date and arrangements satisfactory to the non-defaulting Underwriter and the Company for the purchase of such Offered Securities by other persons are not made within 36 hours after such default, this Agreement will terminate without liability on the part of the non-defaulting Underwriter or the Company, except as provided in Section 9 (provided that if such default occurs with respect to Optional Securities after the First Closing Date, this Agreement shall not terminate as to the Firm Securities or any Optional Securities purchased prior to such termination). As used in this Agreement, the term “Underwriter” includes any person substituted for an Underwriter under this Section. Nothing herein will relieve a defaulting Underwriter from liability for its default.

9. Survival of Certain Representations and Obligations. The respective indemnities, agreements, representations, warranties and other statements of the Company or its officers and of the several Underwriters set forth in or made pursuant to this Agreement will remain in full force and effect, regardless of any investigation, or statement as to the results thereof, made by or on behalf of any Underwriter, the Company or any of their respective representatives, officers or directors or any controlling person, and will survive delivery of and payment for the Offered Securities. If the purchase of the Offered Securities by the Underwriters is not consummated for any reason other than solely because of the termination of this Agreement pursuant to Section 8 hereof or the occurrence of any event specified in clause (iii), (iv), (vi), (vii) or (viii) of Section 6(d) hereof, the Company will reimburse the Underwriters for all out-of-pocket expenses (including fees and disbursements of counsel) reasonably incurred by them in connection with the offering of the Offered Securities, and the respective obligations of the Company and the Underwriters pursuant to Section 7 hereof shall remain in effect. In addition, if any Offered Securities have been purchased under this Agreement, the representations and warranties in Section 2 hereof and all obligations under Section 4 hereof shall also remain in effect.

10. Notices. All communications hereunder will be in writing and, if sent to the Underwriters, will be mailed, delivered or telegraphed and confirmed to them at their address furnished to the Company in writing for the purpose of communications hereunder or, if sent to the Company, will be mailed, delivered or telegraphed and confirmed to it at 4600 Post Oak Place, Suite 100, Houston, Texas 77027, Attention: John Tschirhart, with a copy to Jackson Walker L.L.P., 1401 McKinney St., Suite 1900, Houston, Texas 77010, Attention: Richard Roth.

11. Successors. This Agreement will inure to the benefit of and be binding upon the Company and the Underwriters and their respective successors and the officers and directors and controlling persons referred to in Section 7, and no other person will have any right or obligation hereunder.

12. Representation of Underwriters. The Representative will act for the several Underwriters in connection with the offering described herein, and any action under this Agreement taken by the Representative will be binding upon all the Underwriters.

13. Counterparts. This Agreement may be executed in any number of counterparts, each of which shall be deemed to be an original, but all such counterparts shall together constitute one and the same Agreement.

14. Absence of Fiduciary Relationship. The Company acknowledges and agrees that:

(a) No Other Relationship. The Representative has been retained solely to act as underwriter in connection with the sale of Offered Securities and that no fiduciary, advisory or agency relationship between the Company and the Representative has been created in respect of any of the transactions contemplated by this Agreement (including the provisions of this Agreement incorporated by reference

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therein) or the Final Prospectus, irrespective of whether the Representative has advised or is advising the Company on other matters;

(b) Arm’s-Length Negotiations. The price of the Offered Securities set forth in this Agreement was established by the Company following discussions and arms-length negotiations with the Representative and the Company is capable of evaluating and understanding and understands and accepts the terms, risks and conditions of the transactions contemplated by this Agreement;

(c) Absence of Obligation to Disclose. The Company has been advised that the Representative and its affiliates are engaged in a broad range of transactions which may involve interests that differ from those of the Company and that the Representative has no obligation to disclose such interests and transactions to the Company by virtue of any fiduciary, advisory or agency relationship; and

(d) Waiver. The Company waives, to the fullest extent permitted by law, any claims it may have against the Representative for breach of fiduciary duty or alleged breach of fiduciary duty and agrees that the Representative shall have no liability (whether direct or indirect) to the Company in respect of such a fiduciary duty claim or to any person asserting a fiduciary duty claim on behalf of or in right of the Company, including shareholders, employees or creditors of the Company.

15. Applicable Law. This Agreement shall be governed by, and construed in accordance with, the laws of the State of New York.

The Company hereby submits to the non-exclusive jurisdiction of the Federal and state courts in the Borough of Manhattan in The City of New York in any suit or proceeding arising out of or relating to this Agreement or the transactions contemplated hereby. The Company irrevocably and unconditionally waives any objection to the laying of venue of any suit or proceeding arising out of or relating to this Agreement or the transactions contemplated hereby in Federal and state courts in the Borough of Manhattan in The City of New York and irrevocably and unconditionally waives and agrees not to plead or claim in any such court that any such suit or proceeding in any such court has been brought in an inconvenient forum.

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If the foregoing is in accordance with the Representative’s understanding of our agreement, kindly sign and return to the Company one of the counterparts hereof, whereupon it will become a binding agreement between the Company and the several Underwriters in accordance with its terms.

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SCHEDULE A

SCHEDULE B

“General Use Issuer Free Writing Prospectus” includes each of the following documents:

1. Final term sheet, dated June 15, 2011.

The following information is also included in the General Disclosure Package:

None.

SCHEDULE C

[Form of PwC Comfort Letter]

[OMITTED]

SCHEDULE D

[Form of Opinion of Jackson Walker L.L.P.]

1. The Company is validly existing as a corporation and in good standing under the laws of the State of Texas.

2. The Company has the corporate power and authority to own its properties and to otherwise carry on its business under the laws of the State of Texas and the Company Certificate of Formation and Company Bylaws as described in the Registration Statement, Prospectus and Disclosure Package, and to execute, deliver and perform its obligations under the Underwriting Agreement.

3. The execution by the Company of the Underwriting Agreement has been duly authorized by all requisite corporate action on the part of the Company under the laws of the State of Texas, and the Company has duly executed and delivered the Underwriting Agreement.

4. The authorized capital stock of the Issuer consists of (i) 100,000,000 shares of Common Stock and (ii) 10,000,000 shares of preferred stock, par value $0.001 per share.

5. The Securities have been duly authorized, and when issued by the Company and delivered to and paid for by the Underwriters in accordance with the Underwriting Agreement, will be validly issued, fully paid and non-assessable. The Securities conform to the description thereof contained in the Disclosure Package and the Prospectus and the shareholders of the Company have no statutory preemptive rights with respect to the Securities.

6. The Securities delivered on the Closing Date are convertible into Common Stock of the Company in accordance with their terms; the shares of Common Stock of the Company initially issuable upon conversion of the Securities have been duly authorized and reserved for issuance upon such conversion and conform to the description thereof contained in the Disclosure Package and the Prospectus; the shareholders of the Company have no statutory preemptive rights with respect to the issuance of the Underlying Shares upon the conversion of the Securities in any case pursuant to Company Certificate of Formation, the Company Bylaws, the Texas Business Organizations Code; and all Underlying Shares, when issued upon conversion of the Securities, will be validly issued, fully paid and non-assessable.

7. The Registration Statement became effective upon filing under Rule 462(e) of the Rules and Regulations; the Preliminary Prospectus was filed with the Commission pursuant to Rule 424(b)(5) on June     , 2011 and the Prospectus was filed with the Commission pursuant to Rule 424(b)(5) on June     , 2011 and, based solely upon telephonic conversations with the Commission, no order suspending the effectiveness of the Registration Statement has been issued and no proceeding for that purpose or pursuant to Section 8A of the Securities Act against the Company or in connection with the offering is pending before the Commission.

8. The Registration Statement, the Preliminary Prospectus, the Issuer Free Writing Prospectus and the Prospectus, and each amendment or supplement to the Registration Statement and the Prospectus, including the documents incorporated by reference therein, in each case as of their respective effective (and deemed effective) or issue dates (other than (i) the financial statements and notes thereto and schedules included or incorporated by reference therein or omitted therefrom, (ii) the summary reserve report of the independent petroleum engineers included or incorporated by reference therein, or (iii) the other financial, statistical or reserve engineering data contained therein or omitted therefrom, as to which we express no opinion) appeared on their faces to have complied as to form in all material respects with the requirements of the Securities Act or the Exchange Act, as applicable (except that we express no opinion as to Regulation S-T).

9. The execution, delivery and performance by the Company of the Underwriting Agreement and the issuance and sale of the Securities and the Underlying Shares and the consummation of the transactions contemplated by the Underwriting Agreement (except as contemplated by the Registration Statement, the Prospectus or the Disclosure Package) do not (a) breach, conflict with or violate, or necessitate any filing or registration under (i) any Included Law, other than the filing of the Statement of Resolutions with the Secretary of State of the State of Texas, (ii) any Applicable Order, or (iii) any provision of the Company Certificate of Formation or the Company Bylaws, or (b) breach or conflict with any indenture, instrument or other agreement that is a Filed Exhibit except, in the case of clause (b), as could not reasonably be expected to have a Material Adverse Effect.

10. No consent, approval, authorization, order, license, registration or qualification of or with, any court or arbitrator or governmental or regulatory authority is required for the consummation of the transactions contemplated by the Underwriting Agreement in connection with the offering, issuance or sale of the Securities and Underlying Securities by the Company except for (i) those that have been obtained or made, (ii) those required under Federal and state securities laws or blue sky laws and (iii) those that may be required under the rules and regulations of The NASDAQ Global Select Market.

11. The statements under the captions “Certain United States Federal Income and Estate Tax considerations” and “Description of Series B Convertible Preferred Stock” in the Disclosure Package and Prospectus, and “Description of Capital Stock” in the Base Prospectus insofar as such statements purport to summarize certain provisions of documents and legal matters referred to therein and reviewed by us as described above, fairly summarize such provisions and legal matters in all material respects, subject to the qualifications and assumptions stated therein.

12. The Company is not and, after giving effect to the offering and sale of the Securities and the application of the proceeds thereof as described in the Disclosure Package, will not be an “investment company” within the meaning of the Investment Company Act of 1940, as amended.

SCHEDULE E

[Form of CFO’s Certificate]

I, Albert L. Reese Jr., do hereby certify that I am the Chief Financial Officer of ATP OIL & GAS CORPORATION, a Texas corporation (the “Company”), and, in my capacity as Chief Financial Officer, and based upon an examination of the Company’s financial records and schedules undertaken by myself or members of my staff who are responsible for the Company’s financial and accounting matters, do hereby certify that:

1. I am providing this certificate in connection with the marketing of 1,500,000 shares (or a total of 1,725,000 shares if the Underwriters exercise in full their option to purchase up to 225,000 additional shares) of the Company’s 8.00% Convertible Preferred Stock, Series B, as described in that certain prospectus supplement, dated June 14, 2011 (the “Prospectus Supplement”).

2. I am familiar with the accounting, operations and records systems of the Company.

3. I have read (1) the Prospectus Supplement, (2) the prospectus dated December 17, 2010 forming part of the Company’s shelf registration statement on Form S-3 (No. 333-171286) (the “Prospectus”), (3) the Company’s Annual Reports on Form 10-K for the years ended December 31, 2010, 2009, 2008, 2007 and 2006 and (4) the Company’s Quarterly Report on Form 10-Q for the quarter ended March 31, 2011.

4. No consolidated financial statements of the Company as of any date for any period subsequent to March 31, 2011 are currently available.

5. I have no reason to believe that for the period from March 31, 2011 to May 31, 2011, there were any decreases, as compared with the corresponding period in the preceding year, in consolidated net sales or in the total or per-share amounts of income before extraordinary items or of net income, except in all instances for changes, increases or decreases that the Prospectus Supplement and the Prospectus disclose have occurred or may occur.

6. I have supervised the compilation of and reviewed the circled information contained on the attached Exhibit A, which is included or incorporated by reference into the Prospectus Supplement. I have performed the following procedures with respect to the circled information identified on Exhibit A, which were applied as indicated with respect to the capital letters as explained below, and such information is correct, complete and accurate in all material respects:

A. Recomputed from, or compared to, the corresponding amounts in the Company’s audited consolidated balance sheets as of December 31, 2007 and 2006 and the related consolidated statements of operations, shareholders’ equity, comprehensive income (loss) and cash flows for the years ended December 31, 2007 and 2006, included in the Company’s Annual Reports on Form 10-K for the years ended December 31, 2007 and 2006.

B. Recomputed from, or compared to, corresponding amounts appearing in the accounting records of the Company and found the amounts (as adjusted for rounding, where appropriate) to be in agreement.

C. Compared and agreed (as adjusted for rounding, where appropriate), or recomputed and agreed (as adjusted for rounding, where appropriate), to a schedule derived from the Company’s accounting records, and compared the amounts on the schedule to corresponding amounts appearing in the accounting records and found the amounts to be in agreement (as adjusted for rounding, where appropriate) and determined that the amounts on the schedule were arithmetically correct.

D. Proved the arithmetic accuracy of the percentages and ratios (as adjusted for rounding, where applicable) based on data in the above-mentioned accounting records and schedules.

7. The Company’s consolidated balance sheets as of December 31, 2007 and 2006 and the related consolidated statements of operations, shareholders’ equity, comprehensive income (loss) and cash flows for the years ended December 31, 2007 and 2006 were audited by Deloitte & Touche LLP, the Company’s independent registered public accounting firm during such periods. Nothing has come to my attention that would indicate that such financial statements do not fairly present in all material respects the financial position of the Company and its subsidiaries at December 31, 2007 and 2006, and the results of their operations for the years ended December 31, 2007 and 2006, in each case in conformity with accounting principles generally accepted in the United States.