SUMMARY

      This summary highlights selected information from this prospectus, but does not contain all information that you should consider before investing in the shares. You should read this entire prospectus carefully, including the “Risk Factors” beginning on page 24 of this prospectus and the financial statements included elsewhere in this prospectus. References to “Mariner,” “the Company,” “we,” “us,” and “our” refer to Mariner Energy, Inc. The estimates of our proved reserves as of December 31, 2002, 2003 and 2004 included in this prospectus are based on reserve reports prepared by Ryder Scott Company, L.P., independent petroleum engineers (“Ryder Scott”). A summary of their report on our proved reserves as of December 31, 2004 is attached to this prospectus as Annex A. We have provided definitions for some of the industry terms used in this prospectus in the “Glossary of Oil and Natural Gas Terms” beginning on page 191 of this prospectus.

      In this prospectus:

About Mariner Energy, Inc.

      Mariner Energy, Inc. is an independent oil and gas exploration, development and production company with principal operations in the Gulf of Mexico, both shelf and deepwater, and the Permian Basin in West Texas. As of December 31, 2004, we had 237.5 Bcfe of estimated proved reserves, of which approximately 64% were natural gas and 36% were oil and condensate. As of December 31, 2004, the present value, discounted at 10% per annum, of estimated future net revenues from our estimated proved reserves, before income tax (“PV10”), was approximately $668 million, and our standardized measure of discounted future net cash flows attributable to its estimated proved reserves was approximately $494.4 million. Please see “Business— Estimated Proved Reserves” for a reconciliation of PV10 to the standardized measure of discounted future net cash flows. As of December 31, 2004, approximately 46% of our estimated proved reserves were classified as proved developed. For the year ended December 31, 2004, our total net production was 37.6 Bcfe. Of our estimated proved reserves, 48% are located in the Permian Basin in West Texas, 37% in the Gulf of Mexico deepwater and 15% on the Gulf of Mexico shelf as of December 31, 2004. In the three-year period ended December 31, 2004, we deployed approximately $337 million of capital on acquisitions, exploration and development while adding approximately 191 Bcfe of estimated proved reserves and producing approximately 111 Bcfe.

Significant Properties

      We own oil and gas properties, producing and non-producing, onshore in Texas and offshore in the Gulf of Mexico, primarily in federal waters. Our largest properties, based on the present value of estimated future net proved reserves as of December 31, 2004, are shown in the following table.

      The distribution of our proved reserves reflects our efforts over the last three years to diversify our asset base, which in prior years had been focused primarily in the Gulf of Mexico deepwater. We have shifted some of our focus on deepwater activities to increased exploration and development on the Gulf of Mexico shelf and exploitation of our West Texas Permian Basin properties. By allocating our resources among these three areas, we expect to balance the risks associated with the exploration and development of our asset base. We intend to continue to pursue moderate-risk exploratory and development drilling projects in the Gulf of Mexico deepwater and on the Gulf of Mexico shelf, including select deep shelf

prospects, and also target low-risk infill drilling projects in West Texas. It is our practice to generate most of our prospects internally, but from time to time we also acquire third-party generated prospects. We then drill to find oil and natural gas reserves, a process that we refer to as “growth through the drill bit.”

      We operate and own working interests in individual wells ranging from 33% to 84% (with an average working interest of approximately 66.5%), in the 18,500-acre Aldwell Unit. The field is located in the heart of the Spraberry geologic trend southeast of Midland, Texas, and has produced oil and gas since 1949. We began our recent redevelopment of the Aldwell Unit by drilling eight wells in the fourth quarter of 2002, 43 wells in 2003, and 54 wells in 2004. As of December 31, 2004, there were a total of 185 wells producing or capable of producing in the field. Our aggregate net capital expenditures for the 2004 drilling program in the field were approximately $20.3 million, and we added 27 Bcfe of proved reserves, while producing 4.0 Bcfe.

      During 2005, we have accelerated our development program in West Texas. Through September 30, 2005, we had drilled 65 new wells at our Aldwell and North Stiles Units. All of our drilling in the Aldwell and North Stiles Units has resulted in commercially successful wells that are expected to produce in quantities sufficient to exceed costs of drilling and completion. Our net production from onshore wells for the nine months ended September 30, 2005 averaged approximately 17 MMcfe per day. We have completed construction of our own oil and gas gathering system and compression facilities in the Aldwell Unit. We began flowing gas production through the new facilities on June 1, 2005. We have also entered into new contracts with third parties to provide processing of our natural gas and transportation of our oil produced in the unit. The new gas arrangement also provides us with the option to sell our gas to one of four firm or five interruptible sales pipelines versus a single outlet under the former arrangement. We expect these arrangements to improve the economics of production from the Aldwell Unit.

      In August 2005, but effective in October 2005, we entered into an agreement covering approximately 33,000 acres in West Texas, pursuant to which, upon closing, we acquired an approximate 35% working interest in approximately 200 existing producing wells effective November 1, 2005, and committed to drill an additional 150 wells within a four year period, funding $36.5 million of our partner’s share of drilling costs for such 150-well drilling program. We will obtain an assignment of an approximate 35% working interest in the entire committed acreage upon completion of the 150-well program.

      As of September 30, 2005 we held interests in 11 fields in the Gulf of Mexico deepwater, four of which we operate. The Gulf of Mexico deepwater accounts for 37%, or 86.7 Bcfe, of our December 31, 2004 proved reserves. Our net production from deepwater wells for the nine months ended September 30, 2005 averaged approximately 33 MMcfe per day (see “Recent Developments” below for a discussion of the effects of hurricanes Katrina and Rita). As of September 30, 2005, we held interests in 55 Gulf of Mexico blocks with water depths of over 1,300 feet and had approximately 132,000 net undeveloped acres under lease. In 2004, we spent approximately $63.5 million net on drilling and completion activities in the deepwater. We drilled five exploratory wells, four of which were successful, and one development well, which was also successful.

      In 2004, four subsea tiebacks were in the development phase in the deepwater: Mississippi Canyon 718 (Pluto), Viosca Knoll 917 (Swordfish), Green Canyon 178 (Baccarat) and Mississippi Canyon 296 (Rigel). These four subsea tieback projects contain approximately 49 Bcfe of proved reserves as of December 31, 2004. Swordfish, Baccarat and Rigel are the results of Mariner-generated prospects. The Swordfish and Pluto projects are operated by Mariner, and the Baccarat and Rigel projects are operated by other working interest owners. Currently approximately 7 MMcfe per day of production remains shut-in awaiting repairs due to Hurricanes Katrina and Rita, primarily associated with the Baccarat property. While we believe physical damage to our existing platforms and facilities was relatively minor from both hurricanes, the effects of the storms caused damage to onshore pipeline and processing

facilities that resulted in a portion of our production being temporarily shut-in, or in the case of our Swordfish project, postponed. In addition, Hurricane Katrina caused damage to platforms that host three of our development projects: Pluto, Rigel, and Mississippi Canyon 66 (Ochre). Repairs to these facilities may take up to six months, pushing commencement of production on these projects into 2006.

      In the past two years, we have increased our drilling activities on the Gulf of Mexico shelf. As of September 30, 2005, we held interests in 21 fields on the Gulf of Mexico shelf, eight of which we operate. Gulf of Mexico shelf properties comprise 15%, or 36 Bcfe, of our proved reserves as of December 31, 2004. Our net production from these wells for the nine months ended September 30, 2005 averaged approximately 32 MMcfe per day (see “Recent Developments” below for a discussion of the effects of hurricanes Katrina and Rita). As of September 30, 2005, we held interests in 59 Gulf of Mexico shelf blocks and had approximately 81,000 net undeveloped acres under lease. During 2004, we spent approximately $38.3 million to drill nine exploratory wells, three of which were successful, and two development wells, one of which was successful, on the Gulf of Mexico shelf.

      First production from our Ewing Bank 977 (Dice) project, a subsea tieback, and High Island 46 (Green Pepper) commenced in January 2005. First production from our two West Cameron 333 wells (Royal Flush) commenced during February 2005.

Recent Developments

      Approximately 29 Mmcfe per day of natural gas and approximately 3,000 bbls per day of oil and condensate net to our interest were initially shut-in as a result of the effects of Hurricane Katrina in August 2005. The majority of this production was returned within two weeks of the hurricane, and substantially all within three weeks of the hurricane. Additionally, we are experiencing delays in startup of three of our projects primarily as a result of Hurricane Katrina which is anticipated to defer commencement of production to as late as the second quarter of 2006. Approximately 60 MMcfe per day of production net to our interest was shut-in initially as a result of the effects of Hurricane Rita in late September 2005. Approximately 53 MMcfe per day of production, or approximately 90% of our pre-hurricane production, was restored within two weeks of the hurricane. Our operated platforms appear to have sustained minimal damage attributable to the storm. First reports from operators of other facilities handling our production indicated varying degrees of damage to their facilities, the full extent of which may not be known for some time. Although a submersible rig engaged in drilling operations on our East Cameron Block 79 property was moved off location by Hurricane Rita, a substitute rig was subsequently provided, the damage to the well was repaired and drilling recommenced in the last quarter of 2005. Other planned operations also are delayed as a result of the effects of both hurricanes. We cannot estimate a range of loss arising from the hurricanes until we are able to more completely assess the impacts on our properties and the properties of our operational partners. Until we are able to complete all the repair work and submit costs to our insurance underwriters for review, the full extent of our insurance recovery and the resulting net cost to us for Hurricanes Katrina and Rita will be unknown. For the insurance period ending September 30, 2005, we carry a $3.0 million annual deductible and a $.375 million single occurrence deductible.

      We entered into an agreement effective in October 2005 covering approximately 33,000 acres in West Texas, pursuant to which, upon closing, we acquired an approximate 35% working interest in approximately 200 existing producing wells effective November 1, 2005, and committed to drill an additional 150 wells within a four year period, funding $36.5 million of our partner’s share of drilling costs for such 150-well drilling program. We will obtain an assignment of an approximate 35% working interest in the entire committed acreage upon completion of the 150-well program.

The Offering

Risk Factors

      You should carefully consider all of the information contained in this prospectus prior to investing in the common stock. In particular, we urge you to carefully consider the information under “Risk Factors,” beginning on page 24 of this prospectus so that you understand the risks associated with an investment in our company and the common stock. These risks include the following:

Corporate Information

      We were incorporated in August 1983 as a Delaware corporation. We have three subsidiaries, Mariner LP LLC, a Delaware limited liability company, Mariner Energy Texas LP, a Delaware limited partnership, and MEI Sub, Inc., a Delaware corporation.

      On March 2, 2004, Mariner was acquired by MEI Acquisitions Holdings, LLC, an affiliate of the private equity funds, Carlyle/ Riverstone Global Energy and Power Fund II, L.P. and ACON Investments LLC, through a merger of Mariner’s former indirect parent with MEI. Prior to the merger, we were owned indirectly by Joint Energy Development Investments Limited Partnership (“JEDI”), which was an indirect wholly owned subsidiary of Enron Corp. As a result of the merger, we are no longer affiliated with Enron Corp. See “Business— Enron Related Matters.”

      In March 2005, we completed a private placement of 16,350,000 shares of our common stock to qualified institutional buyers, non-U.S. persons and accredited investors. Our former sole stockholder, MEI Acquisitions Holdings, LLC, also sold 15,102,500 shares of our common stock in the private placement. We used the net proceeds from the sale of 12,750,000 shares of our common stock to purchase and retire an equal number of shares of our common stock from our former sole stockholder. As a result, after the private placement an affiliate of our former sole stockholder beneficially owned 5.3% of our outstanding common stock. See “Security Ownership of Certain Beneficial Owners and Management.”

      Our principal executive office is located at One Briar Lake Plaza, Suite 2000, 2000 West Sam Houston Parkway South, Houston, Texas 77042. Our telephone number is (713) 954-5500.

Proposed Merger with Forest Energy Resources, Inc.

      On September 9, 2005, we entered into a merger agreement with Forest Oil Corporation (which we refer to as Forest), Forest Energy Resources, Inc. (which we refer to as Forest Energy Resources), and MEI Sub, Inc. The consummation of the transactions contemplated by the merger agreement is subject to several conditions, including the adoption of the merger agreement by our stockholders. Accordingly, we cannot assure you that the merger and related transactions will ever be consummated. Our annual stockholder meeting, at which Mariner stockholders will vote to adopt the merger agreement, is scheduled to occur on March 2, 2006.

      The following provides a summary of the material terms of the transactions contemplated by the merger agreement.

Overview of the Proposed Transactions

      Forest has transferred and contributed the assets and certain liabilities associated with its offshore Gulf of Mexico operations to Forest Energy Resources, a newly formed subsidiary of Forest. Immediately prior to the merger, Forest will distribute all of the outstanding shares of Forest Energy Resources to Forest shareholders on a pro rata basis. Forest Energy Resources will then merge with a newly formed subsidiary of Mariner, and become a new wholly owned subsidiary of Mariner. When the merger is complete, approximately 58% of the Mariner common stock will be held by shareholders of Forest and approximately 42% of Mariner common stock will be held by the pre-merger stockholders of Mariner, each on a pro forma basis.

      Following the merger, Mariner will:

About Forest and Forest Energy Resources

      Forest is an independent oil and gas company engaged in the acquisition, exploration, development and production of natural gas and liquids in North America and selected international locations. Forest was incorporated in New York in 1924, as the successor to a company formed in 1916, and has been a publicly held company since 1969. Forest operates from offices located in Denver, Colorado; Lafayette and Metairie, Louisiana; Anchorage, Alaska; and Calgary, Alberta, Canada.

      Forest Energy Resources is a wholly owned subsidiary of Forest. Forest Energy Resources was formed in Delaware on August 18, 2005 for the purpose of completing the spin-off of the Forest Gulf of Mexico operations. As of December 31, 2004, the Forest Gulf of Mexico operations that have been contributed to Forest Energy Resources had 339.7 Bcfe of estimated proved reserves, of which approximately 79% were natural gas and 21% were oil and condensate. As of December 31, 2004, the PV10 of the Forest Gulf of Mexico operations was approximately $1,222.2 million, and the standardized measure of discounted future net cash flows attributable to its estimated proved reserves was approximately $925.8 million. Please see “The Forest Gulf of Mexico Operations— Estimated Proved Reserves” for a reconciliation of PV10 to the standardized measure of discounted future net cash flows. As of December 31, 2004, approximately 76% of the Forest Gulf of Mexico operations’ estimated proved reserves were classified as proved developed. For the year ended December 31, 2004, the Forest Gulf of Mexico operations’ total net production was 81.1 Bcfe. In the three-year period ended December 31, 2004, the Forest Gulf of Mexico operations deployed approximately $560 million of capital on acquisitions, exploration and development while adding approximately 182 Bcfe of estimated proved reserves and producing approximately 215 Bcfe.

Transaction Structure

      The following diagrams and accompanying descriptions serve to describe generally the transactions that will take place in connection with the spin-off and merger. For more information, please read “The Spin-off and Merger.”

      Forest Energy Resources is a wholly owned subsidiary of Forest. MEI Sub is a wholly owned subsidiary of Mariner.

      Forest has contributed the assets and certain liabilities associated with its Gulf of Mexico operations to Forest Energy Resources. Forest will, immediately prior to the merger, distribute all of the shares of Forest Energy Resources to its shareholders on a pro rata basis.

      MEI Sub will merge with and into Forest Energy Resources, with Forest Energy Resources surviving as a wholly owned subsidiary of Mariner. Forest Energy Resources will be renamed Mariner Energy Resources, Inc. In conjunction with the merger, shares of Forest Energy Resources stock will automatically be converted into shares of Mariner stock.

      At the conclusion of the merger, Forest shareholders will own approximately 58% of Mariner and the stockholders of Mariner who owned shares prior to the merger will own the remaining approximately 42% of Mariner.

What Forest and Mariner Stockholders Will Receive

      If the merger is completed, each Forest shareholder will ultimately receive shares of Mariner common stock. As a result of the spin-off, Forest shareholders will initially receive shares of Forest Energy Resources, which will then be converted in the merger into the right to receive shares of Mariner. After the merger, Forest shareholders will be entitled to receive approximately 0.8 shares of Mariner for each Forest share that they own. Forest shareholders will not be required to pay for the shares of Forest Energy Resources distributed in the spin-off transaction or the shares of Mariner issued in the merger.

      Mariner stockholders will keep the shares of Mariner common stock they currently own, but will not receive any additional shares in the merger.

Proposal to Amend Mariner’s Certificate of Incorporation

      We are proposing to amend Mariner’s certificate of incorporation to increase the number of authorized shares of stock from 90 million to 200 million, subject to completion of the merger. Mariner’s certificate of incorporation currently does not authorize a sufficient number of shares of common stock to complete the merger. Mariner currently is authorized to issue 70 million shares of Mariner common stock and 20 million shares of Mariner preferred stock. As of February 1, 2006, approximately 35.6 million shares of Mariner common stock were issued and outstanding. Under the terms of the merger agreement, Mariner must issue approximately 50.6 million shares (representing approximately 0.8 shares of Mariner common stock for each share of Forest common stock) of common stock in the merger, which would result in approximately 86 million shares of Mariner common stock outstanding. Therefore, the number of authorized shares of Mariner common stock must be increased in order to complete the merger.

Recommendation of Mariner’s Board of Directors

      The Mariner board of directors has determined that the merger is fair to and in the best interests of Mariner and its stockholders, and that the merger agreement is advisable. The Mariner board of directors has unanimously approved the merger agreement and the other proposals and recommends that the Mariner stockholders vote “for” the adoption of the merger agreement and the other proposals. A more detailed description of the background and reasons for the merger is set forth under “The Spin-Off and Merger” beginning on page 95.

      When considering the recommendations of the Mariner board of directors, you should be aware that the directors and executive officers of Mariner have interests and arrangements that may be different from your interests as stockholders, including:

      At the close of business on February 1, 2006, directors and executive officers of Mariner and their affiliates as a group beneficially owned and were entitled to vote approximately 3.7 million shares of Mariner common stock (including restricted stock subject to vesting), representing approximately 10.4% of the shares of Mariner common stock outstanding on that date. All of the directors and executive officers of Mariner who are entitled to vote at the annual meeting of stockholders have indicated that they intend to vote their shares of Mariner common stock in favor of adoption of the merger agreement.

      In reaching its decision on the merger, the Mariner board of directors considered a number of factors, including the following among others:

      The Mariner board of directors also identified and considered some risks and potential disadvantages associated with the merger, including, among others, the following:

      In the judgment of the Mariner board of directors, the potential benefits of the merger outweigh the risks and the potential disadvantages.

Opinion of Mariner’s Financial Advisor

      Lehman Brothers Inc., Mariner’s financial advisor, has delivered to Mariner’s board of directors a written opinion that, as of September 9, 2005, based upon and subject to the factors and assumptions set forth in the opinion, the exchange ratio in the merger was fair from a financial point of view to Mariner.

Directors and Officers of Mariner Following the Merger

      If the merger is completed, Mariner’s board will consist of seven members, five of whom will be the current directors of Mariner, and two of whom will be mutually agreed between Mariner and Forest prior to the completion of the merger. The Chairman of the Mariner board will be Mr. Scott D. Josey, the current Chairman, Chief Executive Officer and President of Mariner. The two Mariner directors to be mutually agreed by Forest and Mariner pursuant to the terms of the merger agreement have not yet been designated.

      The current executive officers of Mariner will remain in their current positions following the merger.

Material United States Federal Tax Consequences of the Merger

      It is a condition to the completion of the merger that Forest, Forest Energy Resources and Mariner receive opinions from their respective tax counsels to the effect that the merger will constitute a tax-free reorganization for U.S. federal income tax purposes. As a tax-free reorganization for U.S. federal income tax purposes, the merger will be tax-free to the stockholders of Mariner and tax-free to the shareholders of Forest, except for cash received in lieu of fractional shares of Mariner for shares of Forest Energy Resources.

      We encourage you to consult your own tax advisor for a full understanding of the tax consequences of the merger to you.

Conditions to the Completion of the Merger

      The merger will be completed only if certain conditions, including the following, are satisfied (or waived in certain cases):

      On November 14, 2005, the waiting period under the Hart-Scott-Rodino Act with respect to the merger expired. On October 19, 2005, Forest received the consent required pursuant to its credit facility. On February 7, 2006, Mariner’s common stock was approved for listing on the New York Stock Exchange upon the completion of the merger. As of February 7, 2006, no other conditions to closing have been satisfied. Mariner is currently negotiating the definitive documents for its new credit facility, which documents also will grant the consent required pursuant to its existing facility. Mariner and Forest are actively working to obtain necessary consents, approvals and authorizations from governmental authorities, including the Minerals Management Service.

      Based on its current valuation of the Forest Gulf of Mexico operations and the current amount of distributions permitted by the covenants contained in the indentures governing Forest’s outstanding bonds, Forest believes that no consents of its bondholders will be required for the spin-off and the merger. If Forest’s belief that bondholder consents are not necessary remains unchanged as the merger closing approaches, it intends to waive conditions in the merger agreement and distribution agreement related to such consents.

      Neither Mariner nor Forest currently believes that any other condition to closing is likely to be waived.

      Pursuant to the terms of the merger agreement, the closing of the merger will occur as promptly as practicable, and in no event later than the second business day following the satisfaction or, if permissible, waiver of the conditions to closing set forth in the merger agreement, or at such other time as Mariner and Forest Energy Resources mutually agree. Unless Mariner consents otherwise, the closing will not occur earlier than the fifth business day following the record date for the spin-off.

Termination of the Merger Agreement

      Forest and Mariner may mutually agree to terminate the merger agreement without completing the merger. In addition, either party may terminate the merger agreement if:

      In addition, Mariner may terminate the merger agreement if it receives a proposal to acquire Mariner that Mariner’s board of directors determines in good faith to be more favorable to Mariner’s stockholders than the merger. Forest may terminate the merger agreement if Mariner’s board of directors withdraws or modifies its approval of the merger to Mariner’s stockholders.

Termination Fee and Expenses

      Mariner must pay Forest a termination fee of $25 million and out-of-pocket fees and expenses of up to $5 million if Mariner terminates the merger agreement to accept an alternative proposal that Mariner’s board of directors determines in good faith to be more favorable to Mariner’s stockholders than the merger. In addition, Mariner must pay Forest a termination fee of $25 million and reimbursement of

out-of-pocket fees and expenses of up to $5 million if the merger agreement is terminated for the other reasons set forth under “The Merger Agreement— Termination Fees and Expenses” on page 130.

Financing Arrangements Relating to the Spin-Off and the Merger

      At the closing of the merger Mariner and Mariner Energy Resources expect to enter into a new $500 million senior secured revolving credit facility, and Mariner will enter into an additional $40 million senior secured letter of credit facility. The revolving credit facility will mature on the fourth anniversary of the closing, and the letter of credit facility will mature on the third anniversary of the closing. The outstanding principal balance of loans under the revolving credit facility may not exceed the borrowing base, which will be initially set at $400 million. In addition, Forest Energy Resources expects to enter into a new senior term loan facility in connection with the spin-off, which facility is expected to be repaid with borrowings under Mariner’s and Mariner Energy Resources’ $500 million revolving credit facility.

Ancillary Agreements

      In addition to the merger agreement and the distribution agreement, Forest, Forest Energy Resources and Mariner have entered into a tax sharing agreement relating to the allocation of certain tax liabilities. See “Ancillary Agreements— Tax Sharing Agreement” beginning on page 135. In addition, Forest and Forest Energy Resources have entered into an employee benefits agreement addressing certain benefits matters for former Forest employees who become employees of Forest Energy Resources in connection with the spin-off and the m