SUMMARY

This summary highlights information contained elsewhere in this joint proxy statement/prospectus and may not contain all the information that is important to you with respect to the merger and the other matters being considered at the Contango and Crimson special meetings. Contango and Crimson urge you to read the remainder of this joint proxy statement/prospectus carefully, including the attached Annexes, and the other documents to which we have referred you. See also the section entitled “Where You Can Find More Information” beginning on page 160. We have included page references in this summary to direct you to a more complete description of the topics presented below.

The Companies

Contango Oil & Gas Company

Contango is a Houston, TX-based, independent natural gas and oil company. Contango’s core business is to explore, develop, produce and acquire natural gas and oil properties offshore in the Gulf of Mexico in water-depths of less than 300 feet. Contango has 12 operating wells and three production platforms in the Gulf of Mexico. Contango has additional onshore investments in (i) Alta Resources Investments, LLC, whose primary area of focus is the liquids-rich Kaybob Duvernay in Alberta, Canada; (ii) Exaro Energy III LLC (“Exaro”), which is primarily focused on the development of proved natural gas reserves in the Jonah Field in Wyoming; and (iii) the Tuscaloosa Marine Shale where Contango leases approximately 24,000 net acres. As of March 31, 2013, Contango had estimated proved reserves of 215.5 billion cubic feet equivalent (Bcfe) including 19.6 Bcfe of proved developed reserves attributable to its investment in Exaro. Contango has an active exploration program, and plans to drill two new exploratory wells in the central Gulf of Mexico in 2013. As of March 31, 2013, Contango had no debt and approximately $100 million of working capital. For the quarter ended March 31, 2013, Contango’s average production was approximately 64.6 million cubic feet equivalent per day (“Mmcfed”).

Contango’s common stock is traded on the NYSE MKT under the symbol “MCF.”

The principal executive offices of Contango are located at 3700 Buffalo Speedway, Houston, Texas 77098, and Contango’s telephone number is (713) 960-1901. Additional information about Contango and its subsidiaries is included in documents incorporated by reference into this joint proxy statement/prospectus. See “Where You Can Find More Information” on page 160.

Crimson Exploration Inc.

Crimson is a Houston, TX-based independent energy company engaged in the exploitation, exploration, development and acquisition of crude oil and natural gas, primarily in the onshore Gulf Coast regions of the United States. Crimson currently owns approximately 95,000 net acres onshore in Texas, Louisiana, Colorado and Mississippi. Crimson refers to its four corporate areas as (i) Southeast Texas, focusing on the Woodbine, Eagle Ford and Georgetown formations, (ii) South Texas, focusing on the Eagle Ford and Buda formations, (iii) East Texas, focusing on the Haynesville, Mid-Bossier and James Lime formations, and (iv) Rockies and Other, focusing on the Niobrara and D&J Sand formations. Crimson’s strategy is to continue to increase crude oil and liquids-rich reserves and production from an extensive inventory of drilling prospects, de-risk unproved prospects in core operating areas, and opportunistically grow reserves through acquisitions complementary to its existing asset base.

As of June 30, 2013, Crimson had estimated proved reserves of 117.1 Bcfe of natural gas equivalents, based on SEC reporting guidelines. For the quarter ended June 30, 2013, Crimson’s average production was approximately 44.2 Mmcfed.

Crimson’s common stock is traded on the NASDAQ under the symbol “CXPO.”

The principal executive offices of Crimson are located at 717 Texas Ave., Suite 2900, Houston, Texas 77002, and Crimson’s telephone number is (713) 236-7400. Additional information about Crimson and its subsidiaries is included in documents incorporated by reference into this joint proxy statement/prospectus. See “Where You Can Find More Information” on page 160.

Contango Acquisition, Inc.

Contango Acquisition, Inc., a wholly owned subsidiary of Contango, is a Delaware corporation that was formed on March 14, 2013 for the sole purpose of effecting the merger. In the merger, Contango Acquisition, Inc. will be merged with and into Crimson, with Crimson surviving as a wholly owned subsidiary of Contango.

The Meetings

The Contango Special Meeting (see page 40)

The special meeting of Contango stockholders will be held at 3700 Buffalo Speedway, Second Floor, Houston, Texas 77098, on October 1, 2013, at 9:30 a.m., local time. The special meeting of Contango stockholders is being held to consider and vote on:

Completion of the merger is conditioned on approval of the issuance of shares of Contango common stock to Crimson stockholders in connection with the merger.

Only record holders of shares of Contango common stock at the close of business on August 20, 2013, the record date for the Contango special meeting, are entitled to receive notice of, and to vote at, the Contango special meeting or any adjournment or postponement thereof. At the close of business on the record date, the only outstanding voting securities of Contango were common stock, and 15,194,952 shares of Contango common stock were issued and outstanding, approximately 11.2% of which were owned and entitled to be voted by Contango directors and executive officers and the Estate of Mr. Peak, Contango’s former Chairman and Chief Executive Officer. The Contango directors and executive officers and the Estate of Mr. Peak are currently expected to vote their shares in favor of each Contango proposal listed above.

The Estate of Kenneth R. Peak, Brad Juneau and certain of the executive officers of Contango have each entered into an agreement with Crimson whereby, subject to the terms and conditions of that agreement, such stockholder has agreed to vote all of the Contango shares held by such stockholder in favor of the issuance of shares of Contango common stock to Crimson stockholders in connection with the merger. As of the date of this joint proxy statement/prospectus, such parties hold in the aggregate approximately 10.6% of the outstanding shares of Contango common stock.

With respect to each Contango proposal listed above, Contango stockholders may cast one vote for each share of Contango common stock that they own as of the Contango record date. The proposal to approve the issuance of shares of Contango common stock to Crimson stockholders in connection with the merger requires the affirmative vote of a majority of the shares of Contango common stock, present in person or represented by proxy at the Contango special meeting and entitled to vote thereon. No business may be transacted at the Contango special meeting unless a quorum is present. If a quorum is not present, or if fewer shares are voted in favor of the proposal to approve the issuance of shares of Contango common stock to Crimson stockholders in connection with the merger than is required, to allow additional time for obtaining additional proxies, the special meeting may be adjourned if the requisite stockholder approval to adjourn the meeting is obtained. No notice of an adjourned meeting need be given unless the adjournment is for more than 30 days or, if after the adjournment, a new record date is fixed for the adjourned meeting, in which case a notice of the adjourned meeting shall be given to each stockholder of record entitled to vote at the meeting.

Reasons for the Merger; Recommendation of the Board of Directors of Contango (see page 61)

After careful consideration, the Contango board of directors unanimously determined that the merger and the other transactions contemplated by the merger agreement are advisable and in the best interests of Contango

and its stockholders, approved the merger and the merger agreement and recommended to the holders of Contango common stock the approval of the issuance of Contango common stock to Crimson stockholders in connection with the merger. For more information regarding the factors considered by the Contango board of directors in reaching its decisions relating to its recommendations, see the section entitled “The Merger — Contango’s Reasons for the Merger; Recommendation of the Contango Board of Directors.” The Contango board of directors unanimously recommends that Contango stockholders vote “FOR” the proposal to approve the issuance of shares of Contango common stock to Crimson stockholders in connection with the merger, “FOR” the proposal to approve on an advisory (non-binding) basis, the compensation that may be paid or become payable to Contango’s named executive officers that is based on or otherwise related to the proposed transactions and “FOR” the proposal to approve the adjournment of the Contango special meeting to a later date or dates, if necessary or appropriate to solicit additional proxies in the event there are not sufficient votes at the time of the special meeting to approve the issuance of shares of Contango common stock.

Opinion of Contango’s Financial Advisor (see page 63)

In connection with the merger, on February 6, 2013, the Contango board of directors retained Petrie Partners Securities, LLC (referred to in this joint proxy statement/prospectus as “Petrie”) to act as financial advisor to the Contango board of directors. On April 29, 2013, at a meeting of the Contango board of directors, Petrie rendered its oral opinion, subsequently confirmed by delivery of a written opinion soon after the meeting, that, as of April 29, 2013 and based upon and subject to the factors, procedures, assumptions, qualifications and limitations set forth in its opinion, the exchange ratio was fair, from a financial point of view, to Contango.

The full text of the written opinion of Petrie, dated as of April 29, 2013, which sets forth, among other things, the procedures followed, assumptions made, matters considered and qualifications and limitations on the scope of review undertaken in rendering its opinion, is attached as Annex B to this joint proxy statement/prospectus and is incorporated by reference in its entirety into this joint proxy statement/prospectus. You are urged to read the opinion carefully and in its entirety. Petrie’s opinion was addressed to, and provided for the information and benefit of, the Contango board of directors (in its capacity as such) in connection with its evaluation of whether the exchange ratio was fair, from a financial point of view, to Contango. Petrie’s opinion does not address the fairness of the proposed merger, or any consideration received in connection with the proposed merger, to the holders of any securities, creditors or other constituencies of Contango, nor does it address the fairness of the amount or nature of any compensation to be paid or payable to any of the officers, directors or employees of Contango, or any class of such persons, whether relative to the exchange ratio or otherwise. Petrie assumed that any modification to the structure of the merger would not vary in any respect material to its analysis. Petrie’s opinion does not address the relative merits of the merger as compared to any other alternative business transaction or strategic alternative that might be available to Contango, nor does it address the underlying business decision of Contango to engage in the merger. Petrie’s opinion does not constitute a recommendation to the Contango board of directors or to any other persons in respect of the merger, including as to how any holder of shares of common stock of Contango should act or vote in respect of any of the transactions contemplated by the merger agreement. Finally, Petrie did not express any opinion as to the price at which shares of Contango or Crimson common stock will trade at any time.

The Crimson Special Meeting (see page 45)

The special meeting of Crimson stockholders will be held at Crimson’s offices at 717 Texas Avenue, Suite 2900, Houston, Texas 77002, on October 1, 2013, at 9:30 a.m., local time. The special meeting of Crimson stockholders is being held in order to consider and vote on:

Only record holders of shares of Crimson common stock at the close of business on August 20, 2013, the record date for the Crimson special meeting, are entitled to notice of, and to vote at, the Crimson special meeting or any adjournment or postponement thereof. At the close of business on the record date, the only outstanding voting securities of Crimson were common stock, and 44,764,423 shares of Crimson common stock were issued and outstanding and entitled to vote at the Crimson special meeting, approximately 2.95% of which were owned and entitled to be voted by Crimson directors and executive officers. The Crimson directors and executive officers are currently expected to vote their shares in favor of each of the Crimson proposals listed above.

OCM GW Holdings, LLC, a Delaware limited liability company, and OCM Crimson Holdings, LLC, a Delaware limited liability company (collectively, “Oaktree”) and each executive officer of Crimson have entered into an agreement with Contango and Merger Sub whereby, subject to the terms and conditions of that agreement, such stockholder has agreed to vote all of the Crimson shares held by such stockholder in favor of the adoption of the merger. As of the date of this joint proxy statement/prospectus, Oaktree holds and is entitled to vote, in the aggregate, approximately 34.71% of the issued and outstanding shares of Crimson common stock entitled to vote at the Crimson special meeting and the executive officers of Crimson hold and are entitled to vote, in the aggregate, approximately 2.54% of the issued and outstanding shares of Crimson common stock entitled to vote at the Crimson special meeting.

With respect to each Crimson proposal listed above, Crimson stockholders may cast one vote for each share of Crimson common stock that they own as of the Crimson record date. The proposal to adopt the merger agreement requires the affirmative vote of a majority of the issued and outstanding shares of Crimson common stock that are entitled to vote thereon. No business may be transacted at the Crimson special meeting unless a quorum is present. If a quorum is not present, or if fewer shares are voted in favor of the proposal to adopt the merger agreement than is required, to allow additional time for obtaining additional proxies, the special meeting may be adjourned if the requisite stockholder approval to adjourn the meeting is obtained. No notice of an adjourned meeting need be given unless the adjournment is for more than 30 days or, if after the adjournment, a new record date is fixed for the adjourned meeting, in which case a notice of the adjourned meeting shall be given to each stockholder of record entitled to vote at the meeting.

Reasons for the Merger; Recommendation of the Board of Directors of Crimson (see page 76)

After careful consideration, the Crimson board of directors unanimously adopted the merger agreement, determined that the merger agreement and the transactions contemplated thereby, including the merger, are advisable and in the best interests of Crimson’s stockholders, and recommended that the merger agreement, the merger and the other transactions contemplated by the merger agreement be adopted by Crimson’s stockholders. For more information regarding the factors considered by the Crimson board of directors in reaching its decision to recommend the approval of the merger agreement, see the section entitled “The Merger — Crimson’s Reasons

for the Merger; Recommendation of the Crimson Board of Directors.” The Crimson board of directors unanimously recommends that Crimson stockholders vote “FOR” the proposal to adopt the merger agreement, the merger and the other transactions contemplated by the merger agreement at the Crimson special meeting, “FOR” the proposal to approve on an advisory (non-binding) basis, the compensation that may be paid or become payable to Crimson’s named executive officers that is based on or otherwise related to the proposed transactions and “FOR” the proposal to approve the adjournment of the Crimson special meeting to a later date or dates, if necessary or appropriate, to solicit additional proxies in the event there are not sufficient votes at the time of the special meeting to approve and adopt the merger agreement.

Opinion of Crimson’s Financial Advisor (see page 79)

In connection with the merger, Crimson’s board of directors received a written opinion, dated April 29, 2013, from Barclays Capital Inc. (referred to in this joint proxy statement/prospectus as “Barclays”) as to the fairness, as of the date of the opinion, from a financial point of view, to holders of Crimson common stock of the exchange ratio provided for in the merger agreement. The full text of Barclays’ written opinion, which is attached to this joint proxy statement/prospectus as Annex C, sets forth the assumptions made, procedures followed, matters considered and limitations on the review undertaken. Barclays’ opinion was provided for the information of Crimson’s board of directors (in its capacity as such) in connection with its evaluation of the exchange ratio from a financial point of view and did not address any other aspects or implications of the merger. Barclays expressed no view as to, and its opinion does not address, the underlying business decision of Crimson to effect the merger, the relative merits of the merger as compared to any alternative business strategies that might exist for Crimson or the effect of any other transaction in which Crimson might engage. Barclays’ opinion is not intended to be and does not constitute a recommendation to any stockholder as to how such stockholder should vote or act on any matters relating to the proposed merger or otherwise.

The Merger

A copy of the merger agreement is attached as Annex A to this joint proxy statement/prospectus. Contango and Crimson encourage you to read the entire merger agreement carefully because it is the principal document governing the merger. For more information on the merger agreement, see the section entitled “The Merger Agreement” beginning on page 111.

Form of the Merger (see page 111)

Subject to the terms and conditions of the merger agreement, at the effective time of the merger, Merger Sub, a wholly owned subsidiary of Contango formed for the sole purpose of effecting the merger, will be merged with and into Crimson. Crimson will survive the merger as a wholly owned subsidiary of Contango.

Merger Consideration (see page 111)

Crimson stockholders will have the right to receive 0.08288 shares of Contango common stock for each share of Crimson common stock they hold at the effective time of the merger (the “exchange ratio”) and, in lieu of any fractional shares, cash (without interest) in an amount equal to the product of (i) such fractional part of a share of Contango common stock multiplied by (ii) the closing price for a share of Contango common stock as reported on the NYSE MKT on the first trading day following the effective date of the merger. The exchange ratio is fixed and will not be adjusted for changes in the market value of the common stock of Crimson or Contango. As a result, the implied value of the consideration to Crimson stockholders will fluctuate between the date of this joint proxy statement/prospectus and the effective date of the merger. Based on the closing price of Contango common stock on the NYSE MKT on April 29, 2013, the last trading day before public announcement of the merger, the exchange ratio represented approximately $3.19 in value for each share of Crimson common stock. Based on the closing price of Contango common stock on the NYSE MKT on August 21, 2013, the latest practicable trading day before the date of this joint proxy statement/prospectus, the exchange ratio represented approximately $3.07 in value for each share of Crimson common stock.

Treatment of Crimson Stock Options and Other Equity-Based Awards (see page 122)

Stock Options. Upon completion of the merger, each outstanding option to acquire Crimson common stock will be converted into a fully vested and immediately exercisable option to purchase shares of Contango common stock. The number of shares of Contango common stock that will be subject to such Contango stock options will be the number of shares of Crimson common stock subject to each such Crimson stock option multiplied by 0.08288, rounded down to the nearest whole share of Contango common stock. The exercise price per share of Contango common stock for such Contango stock option will equal the exercise price per share of Crimson common stock for such Crimson stock option divided by 0.08288, rounded up to the nearest whole cent.

Restricted Stock. Effective simultaneously with the effective time of the merger, each outstanding share of Crimson restricted stock will vest and be converted into an unrestricted share of Crimson common stock with the right to receive 0.08288 fully vested shares of Contango common stock.

Expected Timing of the Merger

Contango and Crimson currently expect the closing of the merger to occur in September or October of 2013. However, the merger is subject to the satisfaction or waiver of conditions as described in the merger agreement, and it is possible that factors outside the control of Contango and Crimson could result in the merger being completed at an earlier time, a later time or not at all.

Conditions to Completion of the Merger (see page 123)

The obligations of Contango, Crimson and Merger Sub to complete the merger are subject to the satisfaction or waiver of the following conditions on or prior to the closing date:

In addition, each of Contango’s and Crimson’s obligations to effect the merger is subject to the satisfaction or waiver of the following additional conditions:

No Solicitation of Alternative Proposals (see page 117)

The merger agreement precludes Contango and Crimson from soliciting or engaging in discussions or negotiations with respect to a proposal regarding an alternative transaction. However, if Contango or Crimson receives an unsolicited acquisition proposal from a third party, and Contango’s or Crimson’s board of directors, as applicable, among other things, reasonably determines in good faith (after consultation with its outside legal advisors) that such unsolicited proposal is, or is reasonably likely to lead to, a superior proposal to the merger, Contango or Crimson, as applicable, may furnish non-public information to and enter into discussions with, and only with, that third party regarding such acquisition proposal. See the section entitled “The Merger Agreement — No Solicitation of Alternative Proposals” for a discussion of these and other rights of each of Contango and Crimson to terminate the merger agreement.

Termination of the Merger Agreement (see page 124)

Contango and Crimson may mutually agree to terminate the merger agreement at any time, notwithstanding approval of the merger by stockholders. Either company may also terminate the merger agreement if the merger is not consummated by October 31, 2013, subject to certain exceptions. See the section entitled “The Merger Agreement — Termination of the Merger Agreement” for a discussion of these and other rights of each of Contango and Crimson to terminate the merger agreement.

Effect of Termination of the Merger Agreement (see page 125)

If the merger agreement is terminated by either party in accordance with its terms, the merger agreement (except for the confidentiality agreement between Contango and Crimson and certain provisions expressly listed in the merger agreement, which will survive such termination) will become void, and neither of the parties nor their affiliates or representatives will have any liability under the merger agreement or in connection with the transactions contemplated thereby, except: (a) with respect to any applicable termination fees and the reimbursement of certain expenses; (b) for willful and material breach of the merger agreement; (c) for fraud; or (d) for breaches of the confidentiality agreement between Contango and Crimson. In the case of clauses (b) and (c), the aggrieved party will be entitled to all rights and remedies available at law or equity; provided that in the case of clause (b), if such breach is of the stockholder meetings provision or the no solicitation provision and the breaching party pays the applicable termination fee, such party and its affiliates or representatives will have no further liability in connection with the merger agreement or such termination. In the case of clause (d), the aggrieved party shall be entitled to all rights and remedies provided in the confidentiality agreement between Contango and Crimson.

Termination Fees and Expenses (see page 125)

Generally, all fees and expenses incurred in connection with the merger agreement and the transactions contemplated by the merger agreement will be paid by the party incurring those expenses, subject to the specific exceptions discussed in this joint proxy statement/prospectus where Contango may be required to pay a termination fee of $28 million or an expense reimbursement of $4.5 million and Crimson may be required to

pay a termination fee of $7 million or an expense reimbursement of $4.5 million. See the section entitled “The Merger Agreement — Termination Fees and Expenses” for a discussion of the circumstances under which such termination fee or expense reimbursement will be required to be paid.

Comparison of Stockholder Rights and Corporate Governance Matters (see page 146)

Crimson stockholders receiving merger consideration will have different rights once they become stockholders of Contango due to differences between the governing corporate documents of Crimson and the governing corporate documents of Contango. These differences are described in detail under the section entitled “Comparison of Rights of Contango Stockholders and Crimson Stockholders.”

Listing of Shares of Contango Common Stock; De-Listing and Deregistration of Shares of Crimson Common Stock (see page 110)

It is a condition to the completion of the merger that the shares of Contango common stock to be issued to Crimson stockholders and the shares of Contango common stock reserved for issuance pursuant to the merger (including those shares of Contango common stock to be issued upon conversion of the Crimson restricted stock) be authorized for listing on the NYSE MKT at the effective time of the merger, subject to official notice of issuance. Upon completion of the merger, shares of Crimson common stock currently listed on the NASDAQ will cease to be listed on the NASDAQ and will be subsequently deregistered under the Exchange Act.

Interests of Contango Directors and Executive Officers in the Merger (see page 97)

Certain of Contango’s directors and executive officers have financial interests in the merger that may be different from, or in addition to, the interests of Contango stockholders generally. As detailed below under “The Merger Agreement — Stockholder Support Agreements,” certain executive officers of Contango along with Brad Juneau and the Estate of Kenneth R. Peak, former Chairman of the Board, have entered into support agreements with Crimson in connection with the execution of the merger agreement.

As detailed below under “The Merger — Board of Directors and Management, following the Merger,” Joseph J. Romano will serve as the Chairman of the board of directors of Contango and certain of the members of Contango’s board of directors immediately prior to the merger will serve as directors of Contango upon completion of the merger.

As detailed below under “The Merger — Interests of Contango Directors and Executive Officers in the Merger,” a significant corporate transaction is one of the annual performance measures for Joseph J. Romano, so it is anticipated that Mr. Romano will be entitled to receive a $4.0 million bonus payment as a result of the closing of the merger, if Mr. Romano has not already earned the bonus payment through satisfaction of other performance measures.

As of August 20, 2013, the record date for the Contango special meeting, the directors and executive officers of Contango beneficially owned and were entitled to vote 1,705,861 shares of Contango common stock, collectively representing approximately 11.2% of the shares of Contango common stock outstanding and entitled to vote at the Contango special meeting. In order to approve the issuance of shares of Contango common stock to Crimson stockholders in connection with the merger, the affirmative vote of a majority of the shares of Contango common stock, present in person or represented by proxy at the Contango special meeting and entitled to vote on the proposal is required. See “The Contango Special Meeting — Required Vote” below on page 41.

The Contango board of directors was aware of these interests and considered them, among other matters, in evaluating the merger and in making its recommendations to Contango stockholders.

Interests of Crimson Directors and Executive Officers in the Merger (see page 99)

Certain of Crimson’s directors and executive officers have financial interests in the merger that may be different from, or in addition to, the interests of Crimson stockholders generally. As detailed below under “The Merger Agreement — Stockholder Support Agreements,” each of Allan D. Keel, E. Joseph Grady, A. Carl Isaac, Jay S. Mengle, Thomas H. Atkins and John A. Thomas have entered into support agreements with Contango in connection with the execution of the merger agreement. Moreover, the parties expect that Crimson’s existing executive officers will become executive officers of Contango at or shortly after the consummation of the merger.

As detailed below under “The Merger — Board of Directors and Management, following the Merger,” Allan D. Keel will serve as President and Chief Executive Officer of the combined company, E. Joseph Grady will serve as Senior Vice President and Chief Financial Officer of the combined company, Thomas H. Atkins will serve as Senior Vice President of Exploration of the combined company, Jay S. Mengle will serve as Senior Vice President of Engineering of the combined company, A. Carl Isaac will serve as Senior Vice President of Operations of the combined company, and certain of the members of Crimson’s board of directors immediately prior to the merger will continue to serve as directors of the combined company upon completion of the merger. Messrs. Keel, Grady, Atkins, Mengle and Isaac have entered into employment agreements with Contango as discussed below under “Interests of Crimson Directors and Executive Officers in the Merger.”

As of August 20, 2013, the record date for the Crimson special meeting, the directors and executive officers of Crimson beneficially owned and were entitled to vote 1,319,048 shares of Crimson common stock, collectively representing approximately 2.95% of the shares of Crimson common stock outstanding and entitled to vote at the Crimson special meeting. In order to approve the adoption of the merger agreement, the merger and the other transactions contemplated by the merger agreement, the affirmative vote of a majority of the issued and outstanding shares of Crimson common stock that are entitled to vote at the special meeting is required. See “The Crimson Special Meeting — Required Vote” below on page 46.

The Crimson board of directors was aware of these interests and considered them, among other matters, in evaluating the merger and in making its recommendations to Crimson stockholders.

Regulatory Clearances Required to Complete the Transactions (see page 108)

Contango and Crimson are not required to file notifications with the Federal Trade Commission and the Antitrust Division of the Department of Justice or observe a mandatory pre-merger waiting period before completing the merger under the U.S. Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (referred to in this joint proxy statement/prospectus as the “HSR Act”). Contango and Crimson cannot assure you, however, that other government agencies or private parties will not initiate actions to challenge the merger before or after it is completed.

Board of Directors and Executive Management Following the Merger (see page 108)

Effective as of, and subject to the occurrence of, the effective time of the merger, the following will occur:

No Appraisal Rights (see page 110)

Neither the holders of shares of Contango common stock nor the holders of shares of Crimson common stock are entitled to appraisal rights in connection with the merger in accordance with Delaware law, nor do the certificates of incorporation or bylaws of either company confer such appraisal rights.

Exchange of Shares in the Merger (see page 112)

Prior to the effective time of the merger, Contango will appoint an exchange agent to handle the exchange of shares of Crimson common stock for shares of Contango common stock. At the effective time of the merger, shares of Crimson common stock will be converted into the right to receive 0.08288 shares of Contango common stock without the need for any action by the holders of Crimson common stock.

Promptly after the effective time of the merger, but in no event later than three business days after the closing of the merger, Contango will cause the exchange agent to mail to each holder of a Crimson stock certificate a letter of transmittal specifying, among other things, that delivery will be effected, and risk of loss and title to any certificates representing Crimson common stock shall pass, only upon proper delivery of such certificates to the exchange agent. The letter will also include instructions explaining the procedure for surrendering Crimson stock certificates in exchange for shares of Contango common stock. Crimson stockholders should not return Crimson stock certificates with the enclosed proxy card. Holders of uncertificated shares of Crimson common stock in book-entry form will automatically receive the merger consideration and will not be required to deliver a certificate or an executed letter of transmittal to the exchange agent.

After the effective time of the merger, shares of Crimson common stock will no longer be outstanding, will be automatically canceled and will cease to exist and each certificate, if any, that previously represented shares of Crimson common stock will represent only the right to receive the merger consideration as described above. With respect to such shares of Contango common stock deliverable upon the surrender of Crimson stock certificates, until holders of such Crimson stock certificates have surrendered such stock certificates to the exchange agent for exchange, those holders will not receive dividends or distributions with respect to such shares of Contango common stock with a record date after the effective time of the merger.

Crimson stockholders will not receive any fractional shares of Contango common stock pursuant to the merger. Instead of any fractional shares, Crimson stockholders will be paid an amount in cash for such fraction of a share calculated by multiplying the fractional share interest to which such holder would otherwise be entitled by the closing price for a share of Contango common stock as reported on the NYSE MKT on the first trading day following the effective date of the merger.

Contango stockholders need not take any action with respect to their stock certificates.

Anticipated Accounting Treatment (see page 135)

Under generally accepted accounting principles, the merger will be accounted for using the acquisition method of accounting with Contango being considered the acquirer of Crimson for accounting purposes. This means that Contango will allocate the purchase price to the fair value of Crimson’s tangible and intangible assets and liabilities as of the acquisition date, with the excess purchase price being recorded as goodwill.

Material U.S. Federal Income Tax Consequences (see page 132)

The merger is intended to be treated for U.S. federal income tax purposes as a “reorganization” within the meaning of Section 368(a) of the Code. Assuming the merger qualifies as a reorganization, a U.S. holder of Crimson common stock will not recognize any gain or loss for U.S. federal income tax purposes upon the exchange of the holder’s shares of Crimson common stock for shares of Contango common stock pursuant to the merger, except with respect to cash received in lieu of fractional shares of Contango common stock.

As a condition to the completion of the merger, Crimson and Contango will each have received an opinion, dated as of the closing date of the merger, that the merger will be treated for U.S. federal income tax purposes as a “reorganization” within the meaning of Section 368(a) of the Code. An opinion of counsel represents such counsel’s best legal judgment but is not binding on the Internal Revenue Service (the “IRS”) or any court, so there can be no certainty that the IRS will not challenge the conclusions reflected in the opinion or that a court would not sustain such a challenge.

The tax opinions regarding the merger will not address any state, local or foreign tax consequences of the merger. The opinions will be based on certain assumptions and representations as to factual matters from Contango and Crimson, as well as certain covenants and undertakings by Contango and Crimson. If any of the assumptions, representations, covenants or undertakings is incorrect, incomplete, inaccurate or violated in any material respect, the validity of the conclusions reached by counsel in their opinions would be jeopardized and the tax consequences of the merger could differ from those described in this joint proxy statement/prospectus. Neither Contango nor Crimson is currently aware of any facts or circumstances that would cause the assumptions, representations, covenants and undertakings to be incorrect, incomplete, inaccurate or violated in any material respect.

You are urged to consult your own tax advisor regarding the particular consequences to you of the merger.

Risk Factors (see page 32)

In evaluating the merger, in addition to the other information contained in this joint proxy statement/prospectus, you should carefully consider the risk factors relating to the merger and each of Contango and Crimson beginning on page 32.

Summary Selected Consolidated Financial Data

Summary Selected Consolidated Historical Financial Data of Contango

The following selected statement of income data for the years ended June 30, 2012, 2011 and 2010 and selected balance sheet data as of June 30, 2012 and 2011 have been derived from the audited consolidated financial statements of Contango contained in its Annual Report on Form 10-K for the fiscal year ended June 30, 2012, which is incorporated into this joint proxy statement/prospectus by reference. The selected statement of income data for the years ended June 30, 2009 and 2008 and selected balance sheet data as of June 30, 2010, 2009 and 2008 have been derived from Contango’s audited consolidated financial statements for such years, which have not been incorporated into this joint proxy statement/prospectus by reference.

The selected statement of income data for the nine months ended March 31, 2013 and 2012, and selected balance sheet data as of March 31, 2013 have been derived from Contango’s unaudited interim consolidated financial statements contained in its Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2013, which is incorporated into this joint proxy statement/prospectus by reference. The selected balance sheet data as of March 31, 2012 has been derived from Contango’s unaudited interim consolidated financial statements contained in its Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2012, which has not been incorporated into this joint proxy statement/prospectus by reference. These financial statements are unaudited, but, in the opinion of Contango’s management, contain all adjustments necessary to fairly state Contango’s financial position and results of operations for the periods indicated.

You should read this summary financial data together with the financial statements that are incorporated by reference into this joint proxy statement/prospectus and their accompanying notes and management’s discussion and analysis of financial condition and results of operations of Contango contained in such reports. See “Where You Can Find More Information” beginning on page 160.

Summary Selected Consolidated Historical Financial Data of Crimson

The following selected income statement data for the years ended December 31, 2012, 2011 and 2010 and selected balance sheet data as of December 31, 2012 and 2011 have been derived from the audited consolidated financial statements of Crimson contained in its Annual Report on Form 10-K for the fiscal year ended December 31, 2012, which is incorporated into this joint proxy statement/prospectus by reference. The selected income statement data for the years ended December 31, 2009 and 2008 and selected balance sheet data as of December 31, 2010, 2009 and 2008 have been derived from Crimson’s audited consolidated financial statements for such years, which have not been incorporated into this joint proxy statement/prospectus by reference.

The selected income statement data for the six months ended June 30, 2013 and 2012, and selected balance sheet data as of June 30, 2013 have been derived from Crimson’s unaudited interim consolidated financial statements contained in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2013, which is incorporated into this joint proxy statement/prospectus by reference. The selected balance sheet data as of June 30, 2012 has been derived from Crimson’s unaudited interim consolidated financial statements contained in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2012, which has not been incorporated into this joint proxy statement/prospectus by reference. These financial statements are unaudited, but, in the opinion of Crimson’s management, contain all adjustments necessary to fairly state Crimson’s financial position and results of operations for the periods indicated.

You should read this summary selected financial data together with the financial statements that are incorporated by reference into this joint proxy statement/prospectus and their accompanying notes and management’s discussion and analysis of financial condition and results of operations of Crimson contained in such reports. See “Where You Can Find More Information” beginning on page 160.

Summary Selected Unaudited Pro Forma Condensed Combined Financial Information

The following table presents selected unaudited pro forma combined financial information including Contango’s consolidated balance sheet and statements of income, after giving effect to the merger with Crimson. The information under “Pro Forma Income Statement Data” in the table below gives effect to the merger as if it had been consummated on July 1, 2011. The information under “Pro Forma Balance Sheet Data” in the table below assumes the merger had been consummated on March 31, 2013. This unaudited pro forma combined financial information was prepared assuming the merger will be accounted for using the acquisition method of accounting with Contango considered the acquirer of Crimson. See “Accounting Treatment” on page 135.

The unaudited pro forma combined financial information represents a current estimate based on currently available information and includes adjustments that are preliminary and will likely be revised. The unaudited pro forma balance sheet data includes adjustments to record the assets and liabilities of Crimson at their estimated fair values and is subject to further adjustment as additional information becomes available and as additional analyses are performed. There can be no assurance that such revisions will not result in material changes. The unaudited pro forma combined financial information is presented for illustrative purposes only and does not indicate the financial results of the combined company.

The information presented below should be read in conjunction with the historical consolidated financial statements of Contango and Crimson, including the related notes, filed by each of them with the SEC, and with the pro forma condensed combined financial statements of Contango and Crimson, including the related notes, appearing elsewhere in this joint proxy statement/prospectus. See “Where You Can Find More Information” beginning on page 160 and “Unaudited Pro Forma Condensed Combined Financial Information” beginning on page 136. The unaudited pro forma financial information, while helpful in illustrating the financial characteristics of the combined company under one set of assumptions, does not reflect the impact of possible revenue enhancements, expense efficiencies and asset dispositions, among other factors that may result as a consequence of the merger and, accordingly, does not attempt to predict or suggest future results. The unaudited pro forma condensed combined financial data are not necessarily indicative of results that actually would have occurred or that may occur in the future had the merger been completed on the dates indicated.

Summary Pro Forma Combined Oil and Natural Gas Reserve and Production Data

The following table presents selected unaudited pro forma information regarding Contango’s proved reserves as of March 31, 2013, giving effect to the acquisition of Crimson’s proved reserves as if they were acquired on March 31, 2013. Contango’s reserve disclosures are based on a reserve report prepared by Contango’s independent reserve engineer as of March 31, 2013, in accordance with the guidelines established by the SEC. Crimson’s reserve information as of March 31, 2013 is based on the internal reserve report as of March 31, 2013 and was not reviewed by any independent reserve engineer.

There are numerous uncertainties inherent in estimating quantities and values of proved reserves and in projecting future rates of production and the amount and timing of development expenditures, including many factors beyond the property owner’s control. The following reserve data represents estimates only and should not be construed as being precise. The assumptions used in preparing these estimates may not be realized, causing the quantities of oil and gas that are ultimately recovered, the timing of the recovery of oil and gas reserves, the production and operating costs incurred and the amount and timing of future development expenditures to vary from the estimates presented herein. Actual production, revenues and expenditures with respect to reserves will vary from estimates and the variances may be material.

These estimates were calculated using the twelve-month average of the first-day-of-the-month reference prices as adjusted for location and quality differentials. Any significant price changes will have a material effect on the quantity and present value of the reserves shown below. These estimates depend on a number of variable factors and assumptions, including historical production from the area compared with production from other comparable producing areas, the assumed effects of regulations by governmental agencies, assumptions concerning future oil and gas prices, and assumptions concerning future operating costs, transportation costs, severance and excise taxes, development costs and workover and remedial costs.

The following table sets forth summary pro forma information with respect to Contango’s and Crimson’s combined oil and natural gas production for the year ended June 30, 2012 and nine months ended March 31, 2013. This pro forma information gives effect to the merger as if it occurred on July 1, 2011.

The Contango production data presented below was derived from its Annual Report on Form 10-K for the year ended June 30, 2012 and its quarterly report on Form 10-Q for the nine months ended March 31, 2013, which are incorporated by reference in this joint proxy/prospectus.

The Crimson production data for the twelve months ended June 30, 2012 and nine months ended March 31, 2013 was recalculated based on information derived from its Annual Report on Form 10-K for the year ended December 31, 2012, its quarterly report on Form 10-Q for the three months ended March 31, 2013, both incorporated by reference in this registration statement; and quarterly report on Form 10-Q for the three months ended March 31, 2012, not incorporated by reference in this joint proxy/prospectus.

Unaudited Comparative Per Share Data

The following table sets forth certain historical net income (loss) per share of Contango and Crimson and per share book value information on an unaudited pro forma combined basis after giving effect to the merger.

Historical per share data of Contango for the nine months ended March 31, 2013 and the year ended June 30, 2012 was derived from Contango’s historical financial statements for the respective periods. Historical per share data of Crimson for the nine months ended March 31, 2013 and the twelve months ended June 30, 2012 was calculated based on historical financial information included in its financial statements. This information should be read together with the consolidated financial statements and related notes of Contango and Crimson that are incorporated by reference into this joint proxy statement/prospectus. See “Where You Can Find More Information” beginning on page 160.

Unaudited pro forma combined per share data for the nine months ended March 31, 2013 and the twelve months ended June 30, 2012 was derived and should be read in conjunction with the unaudited pro forma combined financial data included under “Unaudited Pro Forma Condensed Combined Financial Information”

beginning on page 136. The pro forma information is presented for illustrative purposes only and is not necessarily indicative of the operating results or financial position that would have occurred if the merger had been completed as of the beginning of the period.