PARTNERSHIP INTEREST PURCHASE AGREEMENT

Exhibit 2.1

PARTNERSHIP INTEREST PURCHASE AGREEMENT

BY AND AMONG

TURBO LNG LLC,

CONTANGO SUNDANCE, INC.

AND

OSAKA GAS CO., LTD., AS GUARANTOR

January 7, 2008

TABLE OF CONTENTS

i

ii

PARTNERSHIP INTEREST PURCHASE AGREEMENT

Partnership Interest Purchase Agreement (this “Agreement”) entered into as of January 7, 2008 by and among TURBO LNG LLC, a Delaware limited liability company (“Purchaser”), CONTANGO SUNDANCE, INC., a Delaware corporation (the “Company”) and OSAKA GAS CO., LTD., a joint stock corporation incorporated under the laws of Japan, as Guarantor.

W I T N E S S E T H

WHEREAS, the Company owns a ten percent (10%) limited partnership interest (“Freeport LNG Interest”) in Freeport LNG Development, LP (“Freeport”), a limited partnership engaged in developing a liquefied natural gas receiving and gasification terminal in Freeport, Texas (the “Project”);

WHEREAS, the Company desires to sell the Freeport LNG Interest to Purchaser, and Purchaser desires to purchase from the Company, the Freeport LNG Interest, subject to the terms and conditions set forth in this Agreement;

WHEREAS, the Guarantor wishes to guarantee certain obligations of the Purchaser as set forth in this Agreement;

WHEREAS, the Board of Directors of the Company deems it advisable and in the best interests of the stockholders to consummate the sale provided for herein;

WHEREAS, the Board of Directors of the Guarantor and Osaka Gas Freedom Energy Corp., the sole member of Purchaser, are expected to determine on or before January 30, 2008, whether they deem it advisable and in the best interests of the stockholders or the holders of the capital interest, as the case may be, to consummate the sale provided for herein;

WHEREAS, in furtherance thereof, the Board of Directors of the Company has approved this Agreement; and

WHEREAS, the Board of Directors of the Company has recommended to the stockholders of the Company the approval of this Agreement.

NOW THEREFORE, in consideration of the representations, warranties, covenants, promises and the mutual agreements contained herein, the Parties agree as follows:

ARTICLE I

PURCHASE AND SALE OF FREEPORT LNG INTEREST

1.1 Purchase and Sale of Freeport LNG Interest . Upon and subject to the terms and conditions of this Agreement, on the Closing Date the Company shall sell to Purchaser, and Purchaser shall purchase from the Company all right, title and interest of the Company in and to the Freeport LNG Interest.

1.2 Payment of Purchase Price . In consideration of the sale to Purchaser of the Freeport LNG Interest pursuant to Section 1.1 hereof, on the Closing Date Purchaser shall pay or cause to be paid to the Company an aggregate amount of Sixty-Eight Million United States Dollars (U.S.$68,000,000) (the “Purchase Price”) by wire transfer of immediately available funds denominated in U.S. Dollars or by such other means as agreed upon by the Company and Purchaser.

1.3 The Closing . The Closing shall take place at the offices of Morgan, Lewis & Bockius LLP, 1000 Louisiana Avenue, Suite 1300, Houston, Texas 77002, commencing at 9:00 a.m. local time on the Closing Date. The “Closing Date” shall be two (2) business days after the satisfaction or waiver of all of the conditions to the obligations of the Parties to consummate the transactions contemplated hereby (excluding the delivery at the Closing of any of the documents set forth in Article V ), or such other date as may be mutually acceptable to the Parties.

1.4 Actions at the Closing . At the Closing:

(a) The Company shall deliver to Purchaser the various certificates, instruments and documents referred to in Section 5.2 ;

(b) Purchaser shall deliver to the Company the various certificates, instruments and documents referred to in Section 5.3 ;

(c) The Company shall deliver to Purchaser the Freeport LNG Interest; and

(d) Purchaser shall issue and deliver to the Company, the Purchase Price.

ARTICLE II

REPRESENTATIONS AND WARRANTIES OF THE COMPANY

As an inducement to Purchaser to enter into this Agreement and to consummate the transactions contemplated herein, except as set forth in Section 2 of the Company Disclosure Letter, the Company represents and warrants the following to Purchaser, each of which representations and warranties is material to and is relied upon by Purchaser:

2.1 Organization and Qualification . The Company is a corporation, duly organized, validly existing and in good standing under the laws of the State of Delaware, with full corporate power and authority to execute and deliver this Agreement and the Company Related Documents (as hereinafter defined) and to perform its obligations hereunder and thereunder, to carry on its business as currently being conducted and to own or lease and operate the properties it owns or leases as and in the places now owned, leased or operated, respectively. The Company is not in default under or in violation of any provision of its Certificate of Incorporation or Bylaws. The Company is duly qualified or licensed to do business and is in good standing as a foreign corporation in each jurisdiction in which the character or location of its assets or properties (whether owned, leased or licensed) or the nature of its business make such qualification necessary, except where the failure to be so qualified or in good standing, individually or in the aggregate, has not had and would not reasonably be expected to have a Company Material Adverse Effect.

2

2.2 Authority; Binding Effect .

(a) The execution and delivery by the Company of this Agreement and the consummation by the Company of the transactions contemplated hereby have been duly and validly authorized by all necessary corporate action on the part of the Company. Without limiting the generality of the foregoing, the Board of Directors of the Company, by unanimous written consent has (i) determined that the transactions contemplated by this Agreement are fair and in the best interests of the Company and its stockholders, (ii) adopted this Agreement in accordance with the provisions of the Certificate of Incorporation, Bylaws and DGCL, and (iii) directed that this Agreement be submitted to stockholders of the Company for their adoption and approval and resolved to recommend that the stockholders of the Company vote in favor of the adoption of this Agreement. Further, the sole stockholder of the Company has by action by written consent in accordance with the provisions of the DGCL, the Certificate of Incorporation, and Bylaws of the Company, adopted and approved this Agreement and the transactions contemplated hereby.

(b) This Agreement constitutes and each agreement, instrument or document being or to be executed and delivered by the Company in connection with the transactions contemplated thereby (“Company Related Documents”), upon due execution and delivery by the Company, will constitute, assuming the due execution and delivery by the other parties thereto, the legal, valid, and binding obligation of the Company, enforceable in accordance with its respective terms (except as enforcement may be limited by bankruptcy, insolvency, reorganization, moratorium or similar laws relating to or limiting creditors’ rights generally or by application of equitable principles).

2.3 Governmental Entities . Except as otherwise expressly set forth herein, the Company is not required to submit any material notice, report or other filing with any Governmental Entity in connection with its execution or delivery of this Agreement or any of the Company Related Documents or the consummation of the transactions contemplated hereby and no consent, approval or authorization of any Governmental Entity is required to be obtained by the Company in connection with the execution, delivery and performance of this Agreement, except (a) for such filings as may be required under the Hart-Scott-Rodino Act, and (b) for the filing with the SEC of any reports under the Exchange Act as may be required in connection with this Agreement and the transactions contemplated hereby.

2.4 No Conflicts . Except as set forth in Section 2.4 of the Company Disclosure Letter, the execution, delivery and performance of this Agreement and any of the Company Related Documents by the Company does not and will not:

(a) Conflict with or result in any breach of the provisions of, or constitute a default under the organizational documents of the Company;

(b) (i) Violate any restriction to which the Company is subject or, with or without the giving of notice, the passage of time, or both, (ii) violate (or give rise to any right of termination, cancellation or acceleration under) any mortgage, deed of trust, license, lease, indenture, contract or other material agreement or instrument, whether oral or written, to which the Company is a party, or by which it or any of the assets of the Company is bound (which will

3

not be satisfied, assigned or terminated on or prior to the Closing as a result of the transactions contemplated by this Agreement), (iii) result in the termination of any such instrument or termination of any provisions in such instruments or (iv) result in the creation or imposition of any Security Interest upon the properties or assets of the Company in any such case or cases, that would reasonably be expected to have a Company Material Adverse Effect; or

(c) Constitute a violation of any applicable rule, regulation, law, statute, ordinance, or any judgment, decree, writ, injunction or order of any Governmental Entity.

2.5 Ownership of Freeport LNG Interest .

(a) The Company has good and valid title to, and sole record and beneficial ownership of the Freeport LNG Interest. Except as set forth in Section 2.5(a) of the Company Disclosure Letter, the Company owns beneficially and of record the Freeport LNG Interest free and clear of any claims, liens, pledges, options, security interests, trusts, encumbrances or other rights or interests of any Person (other than those in favor of Purchaser under this Agreement) including, without limitation, any conditional sale or other title retention agreement, any lease in the nature thereof, and the filing of or the agreement to give any financing statement under the Uniform Commercial Code of any jurisdiction, other than mechanics’, materialmen’s and similar statutory liens incurred in the ordinary course of business, with respect to which payment is not delinquent, and liens for taxes not yet due and payable (collectively, “Liens”).

(b) There are no outstanding options, rights of first refusal, warrants, subscription rights, securities that are convertible into or exchangeable for, or any other commitments of any character relating to, the Freeport LNG Interest (other than those in favor of Purchaser under this Agreement) and the Company has not entered into any agreement with respect to the foregoing or voting the Freeport LNG Interest other than the Partnership Agreement.

(c) Except as set forth in Section 2.5(c) of the Company Disclosure Letter, the Company has not sold, transferred, conveyed, assigned, pledged or granted any Lien with respect to all or any portion of the Freeport LNG Interest to any person or entity other than Purchaser, or entered into any agreement to do any of the foregoing.

(d) Following the Closing, Purchaser shall own the Freeport LNG Interest free and clear of any Liens of any Person claiming by or through the Company.

(e) Attached hereto as Exhibit A is a true, correct and complete copy of the Partnership Agreement. There are no capital calls that have been made by Freeport to the Company that have not been funded by the Company.

2.6 No Litigation . There are no actions, suits, dispute resolution proceedings, claims, governmental investigations or other legal or administrative proceedings, or any orders decrees or judgments in progress, pending or in effect, or, to the Knowledge of the Company, threatened against or relating to the Company, or relating to the transactions contemplated by this Agreement, and there are none pending in state courts, or in any federal courts, or, to the Knowledge of the Company, pending in other jurisdictions or threatened in writing, at law or in equity, by or before any federal, state or municipal court or other governmental agency, department, commission, board, bureau, instrumentality or other Governmental Entity.

4

2.7 Brokers’ Fees . Except as set forth in Section 2.7 of the Company Disclosure Letter, the Company has no liability or obligation to pay any fees or commissions to any broker, finder or agent with respect to the transactions contemplated by this Agreement.

2.8 Disclaimer . Except as expressly set forth in this Agreement or the Company Related Documents, the Company makes no representation or warranty, express or implied, at law or in equity, in respect of the Company or any of its assets, liabilities or operations and any such other representations or warranties are hereby expressly disclaimed.

ARTICLE III

REPRESENTATIONS AND WARRANTIES OF PURCHASER

As an inducement to the Company to enter into this Agreement and to consummate the transactions contemplated herein, except as set forth in Section 3 of the Purchaser Disclosure Letter, Purchaser represents and warrants the following to the Company, each of which representations and warranties is material to and is relied upon by the Company.

3.1 Organization and Qualification . Purchaser is a limited liability company duly organized, validly existing and in good standing under the laws of the State of Delaware, with full power and authority to execute and deliver this Agreement and the Purchaser Related Documents (as hereinafter defined) and to perform its obligations hereunder and thereunder, to carry on its business as currently being conducted and to own or lease and operate the properties it owns or leases as and in the places now owned, leased or operated, respectively. Purchaser is not in default under or in violation of any provision of its Certificate of Formation or Operating Agreement. Purchaser is duly qualified or licensed to do business and is in good standing as a foreign organization in each jurisdiction in which the character or location of its assets or properties (whether owned, leased or licensed) or the nature of its business make such qualification necessary, except where the failure to be so qualified or in good standing, individually or in the aggregate, has not had and would not reasonably be expected to have a Purchaser Material Adverse Effect.

3.2 Authority; Binding Effect .

(a) Subject to the approval by the Board of Directors of each of the Guarantor and Osaka Gas Freedom Energy Corp., the sole member of Purchaser, which is a condition precedent to the Closing pursuant to Sections 5.2(i) and 5.3(g), the execution and delivery by Purchaser of this Agreement and the consummation by Purchaser of the transactions contemplated hereby have been duly and validly authorized by all necessary action on the part of Purchaser.

(b) This Agreement constitutes and each agreement, instrument or document being or to be executed and delivered by Purchaser in connection with the transactions contemplated thereby (“Purchaser Related Documents”), upon due execution and delivery by Purchaser, will constitute, assuming the due execution and delivery by the other parties thereto, the legal, valid, and binding obligation of Purchaser, enforceable in accordance with its

5

respective terms (except as enforcement may be limited by bankruptcy, insolvency, reorganization, moratorium or similar laws relating to or limiting creditors’ rights generally or by application of equitable principles).

3.3 Governmental Entities . Except as set forth in Section 3.3 of the Purchaser Disclosure Letter or as otherwise expressly set forth herein, Purchaser is not required to submit any material notice, report or other filing with any Governmental Entity in connection with its execution or delivery of this Agreement or any of the Purchaser Related Documents or the consummation of the transactions contemplated hereby and no consent, approval or authorization of any Governmental Entity is required to be obtained by Purchaser in connection with the execution, delivery and performance of this Agreement, except (a) for such filings as may be required under the Hart-Scott-Rodino Act and (b) for the filing with the SEC of any reports under the Exchange Act as may be required in connection with this Agreement and the transactions contemplated hereby.

3.4 No Conflicts . Except as set forth in Section 3.4 of the Purchaser Disclosure Letter, the execution, delivery and performance of this Agreement and any of the Purchaser Related Documents by Purchaser does not and will not:

(a) Conflict with or result in any breach of the provisions of, or constitute a default under the organizational documents of Purchaser;

(b) (i) Violate any restriction to which Purchaser or any of its Subsidiaries is subject or, with or without the giving of notice, the passage of time, or both, (ii) violate (or give rise to any right of termination, cancellation or acceleration under) any mortgage, deed of trust, license, lease, indenture, contract or other material agreement or instrument, whether oral or written, to which Purchaser or any of its Subsidiaries is a party, or by which it or any of the assets of Purchaser and its Subsidiaries are bound (which will not be satisfied, assigned or terminated on or prior to the Closing Date as a result of the transactions contemplated by this Agreement), (iii) result in the termination of any such instrument or termination of any provisions in such instruments or (iv) result in the creation or imposition of any Security Interest upon the properties or assets of Purchaser and its Subsidiaries, in any such case or cases, that will have a Purchaser Material Adverse Effect; or

(c) Constitute a violation of any applicable rule, regulation, law, statute, ordinance, or any judgment, decree, writ, injunction or order of any Governmental Entity.

3.5 Securities Laws . Purchaser, as an indirect wholly-owned subsidiary of Guarantor, is an experienced and knowledgeable investor in the U.S. oil and gas business. Prior to entering into this Agreement, Purchaser was advised by its counsel, accountants, financial advisors, and such other Persons it has deemed appropriate concerning this Agreement and has relied solely on the Company’s representations and warranties expressly contained herein and an independent investigation and evaluation of, and appraisal and judgment with respect to, the Freeport LNG Interest. Purchaser hereby acknowledges that the Freeport LNG Interest has not been registered under the Securities Act, or registered or qualified for sale under any state securities laws and cannot be resold without registration thereunder or exemption therefrom. Purchaser is an “accredited investor,” as such term is defined in Regulation D of the Securities Act, and will

6

acquire the Freeport LNG Interest for its own account and not with a view to a sale or distribution thereof in violation of the Securities Act, and the rules and regulations thereunder, any applicable state blue sky laws or any other applicable securities laws. Purchaser has sufficient knowledge and experience in financial and business matters to enable it to evaluate the risks of investment in the Freeport LNG Interest and has the ability to bear the economic risk of this investment for an indefinite period of time.

3.6 Brokers’ Fees . Except as set forth in Section 3.6 of the Purchaser Disclosure Letter, neither Purchaser nor any of its Subsidiaries has any liability or obligation to pay any fees or commissions to any broker, finder or agent with respect to the transactions contemplated by this Agreement.

3.7 Disclaimer . Except as expressly set forth in this Agreement or the Purchaser Related Documents, Purchaser makes no representations or warranties, express or implied, at law or in equity, in respect of Purchaser, each of its Subsidiaries, or any of their respective assets, liabilities or operations, including, without limitation, with respect to merchantability or fitness for any particular purpose, and any such other representations or warranties are hereby expressly disclaimed.

ARTICLE IV

COVENANTS

4.1 Closing Efforts . Each of the Parties shall use its Reasonable Best Efforts to take all actions and to do all things necessary, proper or advisable to consummate the transactions contemplated by this Agreement, including using its Reasonable Best Efforts to ensure that (a) its representations and warranties are true and correct in all material respects on the Closing Date and (b) the conditions to the obligations of the other Parties to consummate the transactions contemplated by this Agreement are satisfied.

4.2 Governmental and Third-Party Notices and Consents .

(a) Each Party shall use its Reasonable Best Efforts to, as promptly as practical, obtain, at its expense, all waivers, permits, consents, approvals or other authorizations from Governmental Entities and to effect all registrations, filings and notices with or to Governmental Entities, as may be required for such Party to consummate the transactions contemplated by this Agreement and to otherwise comply with all applicable laws and regulations in connection with the consummation of the transactions contemplated by this Agreement. Without limiting the generality of the foregoing, each of the Parties shall promptly file any Notification and Report Forms and related material that it may be required to file with the Federal Trade Commission and the Antitrust Division of the United States Department of Justice under the Hart-Scott-Rodino Act, shall use its Reasonable Best Efforts to obtain an early termination of the applicable waiting period, and shall make any further filings or information submissions pursuant thereto that may be necessary, proper or advisable; provided that in no event shall Purchaser or any of its Affiliates be required to agree or commit to divest, hold separate, offer for sale, abandon, limit its operation of or take similar action with respect to any assets (tangible or intangible) or any business interest of it or any of its Affiliates in connection with or as a condition to receiving the consent or approval of any Governmental Entity (including, without limitation, under the Hart-Scott-Rodino Act).

7

(b) The Company shall use its Reasonable Best Efforts (without the obligation to expend money except reasonable out-of-pocket costs including filing fees and such other fees and costs as are required pursuant to the terms of the applicable contract or agreement) to obtain, at its expense, all such waivers, consents or approvals from third parties, and to give all such notices to third parties, as are required to be listed in the Company Disclosure Letter or as may be required for the Company to consummate the transactions contemplated by this Agreement and to otherwise comply with all applicable laws and regulations in connection with the consummation of the transactions contemplated by this Agreement, including, without limitation, any waivers, consents or approvals from third parties arising or delivered after the Closing.

4.3 Operation of Business of the Company . Except as contemplated by this Agreement, during the period from the date of this Agreement to the earlier of the Closing or the termination of this Agreement, the Company shall conduct its operations in the ordinary course of business, consistent with past practice, and in compliance in all material respects with all applicable laws and regulations and shall not take any action that would cause Freeport to conduct its operations other than in the ordinary course of business, consistent with past practice, and in compliance in all material respects with all applicable laws and regulations.

4.4 Expenses . Except as otherwise provided in this Agreement, each of the Parties shall bear its own costs and expenses (including legal and accounting fees and expenses) incurred in connection with this Agreement and the transactions contemplated hereby.

4.5 Notification . Each of the Company and Purchaser shall notify the other Party in writing of the existence or happening of any fact, event or occurrence which should be included in the Company Disclosure Letter or Purchaser Disclosure Letter, as applicable, in order to make the representations and warranties set forth in Article II or Article III , as applicable, true and correct in all material respects as of the Closing Date, it being understood and agreed that the delivery of such information shall not in any manner constitute a waiver by Purchaser or the Company, as applicable, of any of the conditions precedent to the Closing Date hereunder; provided, however, that in determining whether there is a breach of any representation or warranty contained in Article II or Article III , as applicable, for purposes of or otherwise affect the indemnification to be provided by Indemnifying Party pursuant to Article VI , such representation or warranty shall be qualified by any information provided pursuant to this Section 4.5 .

4.6 Access to Information . Subject to the terms of the Confidentiality Agreement by and between Purchaser and the Company, dated October 4, 2007 (the “Confidentiality Agreement”), the Company shall, and shall cause its Subsidiaries to, afford to the other Party’s officers, directors, employees, accountants, counsel and other agents reasonable access during normal business hours to its employees, properties, assets and records, during the period prior to the Closing Date, to obtain all information concerning its business as such other Party may reasonably request. The Company shall furnish to the other Party all such documents and copies of documents and records and information with respect to itself and its Subsidiaries and copies of any working papers relating thereto as the other Party may reasonably request. Nothing in this

8

Section 4.6 shall require the Company to provide any access, or to disclose any information, if permitting such access or disclosing such information would (a) violate applicable law, (b) violate any of its obligations with respect to confidentiality (provided that each Party shall, upon the request of the other Party, use its Reasonable Best Efforts to obtain the required consent of any third party to such access or disclosure), or (c) result in the loss of attorney-client privilege (provided that each Party shall use its Reasonable Best Efforts to allow for such access or disclosure in a manner that does not result in a loss of attorney-client privilege). The Company also will consult with the other Party regarding its business on a regular basis.

4.7 Further Assurances . Upon the terms and subject to the conditions hereof, each of the parties hereto shall execute such documents and other instruments and take such further actions as may be reasonably required to carry out the provisions hereof and consummate the transactions contemplated by this Agreement.

ARTICLE V

CONDITIONS PRECEDENT

5.1 Conditions to Each Party’s Obligations . The respective obligations of each Party to consummate the transactions contemplated by this Agreement are subject to all applicable waiting periods (and any extensions thereof) under the Hart-Scott-Rodino Act having expired or otherwise been terminated.

5.2 Conditions to Obligations of Purchaser . Each and every obligation of Purchaser under this Agreement, (except for the obligations of Purchaser to be fulfilled prior to the Closing Date and obligations that survive termination of this Agreement), shall be subject to the satisfaction, on or before the Closing Date, of each of the conditions set forth in this Section 5.2 , unless waived in writing by Purchaser