SUMMARY

      This summary highlights selected information from this joint proxy statement/prospectus and does not contain all of the information that may be important to you. To understand the merger fully and for a more complete description of the legal terms of the merger, you should carefully read this entire joint proxy statement/prospectus and the other documents to which we refer you. See “Where You Can Find More Information” on page 135. We have included page references parenthetically to direct you to a more complete description of the topics presented in this summary.

The Companies

      Pioneer is an independent oil and gas exploration and production company with ownership interests in oil and gas properties located in the United States, Argentina, Canada, Gabon, South Africa and Tunisia. Pioneer’s proved reserves are approximately balanced between oil and gas. Approximately 65% of Pioneer’s proved reserves are in three areas in the United States: the Hugoton gas field, the West Panhandle gas field and the Spraberry oil and gas field.

      Evergreen is an independent exploration and production company engaged primarily in the operation, development, production, exploration and acquisition of North American unconventional gas properties. Evergreen is one of the leading developers of coal bed methane reserves in the United States. Evergreen’s current operations principally focus on developing and expanding its coal bed methane project located in the Raton Basin in southern Colorado.

The Combined Company

      Upon completion of the merger, Evergreen will be a wholly-owned subsidiary of Pioneer. The merger will result in a strategic alliance between the two companies that will provide an attractive opportunity for the combined company to realize the value of both Pioneer’s and Evergreen’s long-lived assets and provide stockholders with exposure to high-impact exploration opportunities.

      The combined company will continue to balance low-risk growth from its onshore foundation assets with deploying excess cash flow into high-impact, potentially high-return exploration opportunities. The addition of Evergreen’s assets will expand Pioneer’s existing long-lived North American asset foundation and add a new core area with a significant inventory of low-risk drilling opportunities, including over 2,000 low-risk onshore drilling locations in the United States. The new asset base is expected to provide Pioneer with additional free cash flow, which it can use to rebalance its portfolio and invest in future growth opportunities. The combined company plans to pursue high-potential exploration programs in Alaska, the deepwater Gulf of Mexico, North Africa and West Africa.

      Expansion into the Rocky Mountains. The combined company will have significant reserves in the Rocky Mountain region. Evergreen’s 2003 reported year-end proved reserves amounted to approximately 1,495 Bcfe of gas equivalents, or 249 MMBOE, that are concentrated primarily in the Rocky Mountains in

      Proved Reserves of the Combined Company. Based on the December 31, 2003 reserve estimates of both Pioneer and Evergreen, the combined company expects to have proved reserves of 1,059 MMBOE or 6.4 trillion cubic feet of gas equivalent. These proved reserves are 60% gas, with 86% of these proved reserves located in North America. Netherland, Sewell & Associates, Inc., which we refer to in this joint proxy statement/ prospectus as Netherland, Sewell, has audited 91% of the combined company’s December 31, 2003 proved reserves. In order to conform the reporting of Evergreen’s gas used in field compression to that of Pioneer, Evergreen’s gas reserve volumes were increased, where applicable, to include the estimate of future gas to be used in field compression. Accordingly, oil and gas revenues and production costs were increased by corresponding dollar amounts to account for the gas revenues and production costs associated with gas used in field compression.

      Production of the Combined Company. The combined company’s current production is approximately 838 MMcf per day of gas and 71 MBbls of liquids per day. The combined company also expects to have substantial exploration potential and a proved reserves to production ratio of 16 years.

      Ratings of the Combined Company. Pioneer will continue its commitment to achieve a mid-investment grade rating by the end of 2005. The combined company expects to have debt to book capitalization of approximately 45% by the end of 2004, and is targeting debt to book capitalization below 40% by the end of 2005.

      Commodity Hedges of the Combined Company. In order to mitigate the impact of commodity price changes on the merger and to stabilize product prices so that Pioneer will have sufficient cash to achieve its debt reduction targets by the end of 2005, Evergreen and Pioneer have added commodity hedges for their 2004 and 2005 forecasted oil and gas production. As of June 9, 2004, Evergreen has hedged 119 MMcf per day of its July through December 2004 gas production at an average fixed price of $4.70 per Mcf and 100 MMcf per day of its 2005 gas production at an average fixed price of $5.15 per Mcf. Also as of June 9, 2004, Pioneer has hedged 310 MMcf per day of its July through December 2004 gas production at an average fixed price of $4.40 per Mcf, 175 MMcf per day of its 2005 forecasted gas production at an average fixed price of $5.15 per Mcf, 23 MBbls per day of its July through December 2004 oil production at an average fixed price of $29.45 per Bbl, and 27 MBbls per day of its forecasted 2005 oil production at an average fixed price of $27.97 per Bbl. Pioneer has also hedged portions of its 2006 and 2007 forecasted gas production and portions of its 2006, 2007 and 2008 forecasted oil production.

      Increased Production of the Combined Company. If the merger is completed by the end of the third quarter of 2004, the combined company will realize an increase in production levels over those currently being produced by Pioneer. For 2004, Pioneer estimates that total production will range from 70 to 73 MMBOE, including a full quarter of production from the Evergreen assets. The combined company is expected to realize production growth during 2005 of approximately 10% to 15% over Pioneer’s forecasted 2004 production, reflecting the assumed combined production during the fourth quarter of 2004.

      Directors of the Combined Company. At the time of the merger, two of Evergreen’s directors, Mark Sexton and Andrew Lundquist, will join Pioneer’s board of directors.

      Environmental Focus of the Combined Company. The combined company will continue to strive to use environmentally responsible operating techniques, a hallmark of both Pioneer and Evergreen.

The Merger

      The merger agreement is attached as Annex A to this document. We encourage you to read the merger agreement carefully and in its entirety. It is the principal document governing the merger.

      Evergreen stockholders have the option to elect to receive one of three forms of merger consideration for each share of Evergreen common stock held: (i) 1.1635 shares of Pioneer common stock, subject to allocation and proration; (ii) $39.00 in cash, subject to allocation and proration; or (iii) 0.58175 shares of Pioneer common stock and $19.50 in cash. Each holder must make the same election with respect to all of its shares of Evergreen common stock. Evergreen stockholders who do not make an election will receive 0.58175 shares of Pioneer common stock and $19.50 in cash for each share of Evergreen common stock held. In addition, Evergreen stockholders are entitled to receive an additional cash payment equal to the sum of (i) $0.35 per share of Evergreen common stock as consideration from Pioneer for Evergreen’s properties located in Kansas; plus (ii) an amount per share of Evergreen common stock equal to a pro rata share of the net proceeds in excess of $15 million from Evergreen’s sale, if any, of its Kansas properties to a third party if a sale occurs prior to the closing of the merger. For further discussion of the consideration each Evergreen stockholder is entitled to receive, see “The Merger Agreement — Merger Consideration” beginning on page 84.

      At its meeting on May 3, 2004, after due consideration, the Pioneer board of directors, except for Scott Sheffield, who recused himself from voting and did not participate in the meeting in which such vote was taken, unanimously adopted resolutions (i) determining that the merger agreement, the merger in accordance with the terms of the merger agreement, the issuance of shares of Pioneer common stock pursuant to the merger, and the other transactions contemplated by the merger agreement are advisable and in the best interests of Pioneer and its stockholders, (ii) approving the merger agreement, the merger, and the other transactions contemplated by the merger agreement and approving the issuance of Pioneer common stock pursuant to the merger, and (iii) recommending that the Pioneer stockholders vote “FOR” the approval of the issuance of shares of Pioneer common stock in the merger. Mr. Sheffield recused himself from voting with respect to these matters and did not participate in the meeting in which such vote was taken because he is a member of Evergreen’s board of directors in addition to serving as Chairman of the Board, President and Chief Executive Officer of Pioneer.

      At its meeting on May 3, 2004, after due consideration, the Evergreen board of directors, except for Scott Sheffield, who recused himself from voting and did not participate in the meeting in which such vote was taken, unanimously adopted resolutions (i) determining that the merger agreement, the merger in accordance with the terms of the merger agreement, and the other transactions contemplated thereby are advisable and in the best interests of Evergreen and its stockholders, (ii) approving and adopting the merger agreement, the merger, and the other transactions contemplated by the merger agreement, and (iii) recommending that the Evergreen stockholders vote “FOR” the approval of the merger agreement. Mr. Sheffield recused himself from voting with respect to these matters and did not participate in the meeting in which such vote was taken because he is Chairman of the Board, President and Chief Executive Officer of Pioneer in addition to serving as a member of Evergreen’s board of directors.

      The approval of the issuance of shares of Pioneer common stock in the merger requires the affirmative vote of a majority of the votes cast by holders of Pioneer common stock by proxy or in person and entitled to vote as of the record date for the special meeting. The total vote cast by Pioneer stockholders at the meeting must represent more than 50% of all shares of Pioneer common stock issued and outstanding and entitled to vote as of the record date for the special meeting.

      As of June 9, 2004, shares representing less than 1% of the total outstanding shares of Pioneer common stock were held by Pioneer’s directors, executive officers and their respective affiliates.

      The approval of the merger agreement requires the affirmative vote by proxy or in person of the holders of a majority of Evergreen’s common stock issued and outstanding and entitled to vote as of the record date for the Evergreen special meeting.

      As of June 9, 2004, shares representing approximately 4% of the total outstanding shares of Evergreen common stock were held by Evergreen’s directors, executive officers and their respective affiliates.

      In deciding to approve the merger agreement, Pioneer’s board of directors considered the oral opinion of J.P. Morgan Securities Inc., which we refer to in this joint proxy statement/ prospectus as JPMorgan, delivered on May 3, 2004, which was subsequently confirmed in writing, that, as of that date and based upon and subject to the matters set forth in the opinion, the consideration to be paid by Pioneer in the merger was fair, from a financial point of view, to Pioneer. The written opinion of JPMorgan confirming their oral opinion is attached as Annex B to this document. We urge Pioneer stockholders to read the JPMorgan opinion carefully and in its entirety. JPMorgan’s advisory services and opinion were provided for the information of the Pioneer board of directors in its evaluation of the proposed merger and did not constitute a recommendation of the merger to Pioneer or a recommendation to any holder of Pioneer common stock as to how that stockholder should vote on any matters relating to the merger.

      In deciding to approve the merger agreement, Evergreen’s board of directors considered the oral opinion of Citigroup Global Markets Inc., which we refer to in this joint proxy statement/ prospectus as Citigroup, delivered on May 3, 2004, which was subsequently confirmed in writing, that, as of that date and based upon and subject to the matters set forth in the opinion, the merger consideration to be received by the holders of Evergreen common stock in the merger was fair, from a financial point of view, to the holders of Evergreen common stock. The written opinion of Citigroup is attached as Annex C to this document. We urge Evergreen stockholders to read the Citigroup opinion carefully and in its entirety. Citigroup provided its advisory services and opinion for the information of the Transactions Committee and the Evergreen board of directors in connection with their evaluation of the merger. The Citigroup opinion is not a recommendation, and Citigroup makes no recommendation, to any stockholder regarding how such stockholder should vote on any matters relating to the merger.

      Scott Sheffield, Pioneer’s Chairman of the Board, President and Chief Executive Officer, is also a director of Evergreen and owns 6,400 shares of Evergreen common stock, options to purchase 4,800 shares of Evergreen common stock that are fully exercisable, options to purchase 19,200 shares of Evergreen

      Some of the directors and officers of Evergreen have interests in the merger that are different from or in addition to the interests of other Evergreen stockholders. These interests include positions as directors of Pioneer after the merger, possible substantial payments and benefits under change of control agreements that are triggered upon the completion of the merger, payments under consulting and non-competition agreements entered into with Pioneer, and accelerated vesting of stock options and restricted stock awards as a result of the merger. Pioneer and Messrs. Carlton, Collins and Sexton disagree about the amount of cash that would be payable to those three executives under their change in control agreements if the merger is completed, and the three executives have made a written request to submit the matter to arbitration.

      The U.S. federal income tax consequences of the merger to you will depend on whether the transactions contemplated by the merger agreement qualify as a “reorganization” under U.S. federal income tax laws and, if so, whether you receive solely cash or a combination of Pioneer common stock and cash in the merger.

      If the transactions contemplated by the merger agreement qualify as a “reorganization” under U.S. federal income tax laws, you generally will recognize gain only to the extent of any cash you receive in the merger. You generally will not recognize loss in the merger unless you receive solely cash in exchange for your shares of Evergreen common stock.

      If the transactions contemplated by the merger agreement do not qualify as a “reorganization” under U.S. federal income tax laws, you will recognize gain or loss in an amount equal to the fair market value of any shares of Pioneer common stock and the amount of any cash you receive minus your basis in the Evergreen common stock surrendered.

      Whether the transactions contemplated by the merger agreement qualify as a “reorganization” under U.S. federal income tax laws will depend on the aggregate value of the Pioneer common stock delivered in the merger to holders of shares of Evergreen common stock (other than Pioneer common stock delivered in exchange for shares of Evergreen common stock issuable pursuant to restricted stock awards for which the applicable restrictions lapse upon the effective time of the merger or thereafter) relative to the aggregate amount of cash consideration delivered in the merger in exchange for those shares of Evergreen common stock. The aggregate values of the Pioneer common stock and cash delivered in the merger will depend, under the administrative practice of the IRS, upon the value of a share of Pioneer common stock when the merger occurs and upon other matters such as the amount of cash that is paid to dissenters and in respect of the Kansas properties, none of which can be determined or predicted at this time. See “The Merger — Material United States Federal Income Tax Consequences of the Merger” beginning on page 75 for a discussion of what value of Pioneer common stock should be sufficient for this purpose. As an illustration of the interaction of the relevant variables, if (i) no shares dissent from the merger and (ii) the consideration paid in respect of the Kansas properties is $0.35 for each share of Evergreen common stock, then based on Evergreen’s estimates of the number of shares of Evergreen common stock that will be outstanding when the merger occurs and the number of those shares of Evergreen common stock that are issuable pursuant to restricted stock awards for which the applicable restrictions lapse upon the effective time of the merger or thereafter, there should be sufficient value in the Pioneer common stock that is delivered in the merger to qualify the transactions as a “reorganization” if the fair market

      Qualification of the transactions contemplated by the merger agreement as a “reorganization” is not a condition to the closing of the merger, and it will not be known at the time of the stockholders’ meeting or at the time you elect which form of consideration you wish to receive whether the transactions contemplated by the merger agreement will qualify as a “reorganization” under U.S. federal income tax laws.

      We intend to make a public announcement on, or soon after, the effective date of the merger indicating whether we intend to treat the transactions contemplated by the merger agreement as qualifying as a “reorganization.”

      The rights of Pioneer stockholders are governed by Delaware law and are subject to Pioneer’s amended and restated certificate of incorporation and restated bylaws. The rights of Evergreen’s stockholders are governed by Colorado law and are subject to Evergreen’s articles of incorporation and bylaws. Upon completion of the merger, the rights of both stockholder groups will be governed by Delaware law and Pioneer’s amended and restated certificate of incorporation and restated bylaws.

      As a result of the merger, Evergreen stockholders will have dissenters’ rights of appraisal under Colorado law. In order to perfect such rights, Evergreen stockholders who demand appraisal of their shares should follow the procedures described under “The Merger — Dissenters’ Rights of Appraisal of Evergreen Stockholders” beginning on page 80 and in Annex D. Pioneer stockholders will not have any dissenters’ rights.

      Pioneer’s and Evergreen’s obligations to complete the merger are each subject to the fulfillment or waiver, if applicable, of a number of conditions, including the following:

      Pioneer and Evergreen can jointly agree to terminate the merger agreement at any time. Either company may also terminate the merger agreement:

      In addition, Pioneer may terminate the merger agreement in the event that (i) holders of more than 5% of the total number of issued and outstanding shares of Evergreen common stock exercise their dissenters’ rights, (ii) Evergreen’s board of directors changes its recommendation that Evergreen stockholders approve the merger, (iii) Evergreen fails to comply with the provisions of the merger agreement relating to acquisition proposals, or (iv) within ten days after commencement of any tender or exchange offer for shares of Evergreen common stock, Evergreen’s board of directors fails to recommend against acceptance of any such tender or exchange offer or takes no position with respect to the tender or exchange offer. Evergreen may terminate the merger agreement, subject to certain conditions, if its board of directors changes its recommendation that Evergreen stockholders approve the merger in order to accept a superior offer from a third party and Pioneer has not made a new offer that is at least as favorable to Evergreen stockholders from a financial point of view as the superior offer.

      Pioneer and Evergreen have each agreed to pay a termination fee of $35 million to the other company if the merger agreement is terminated under the circumstances described under “The Merger Agreement — Termination Fee” on page 100.

      The merger agreement generally prohibits Evergreen and its officers, directors, employees, agents and representatives from taking any action to solicit an acquisition proposal as described on page 93. The merger agreement does not, however, prohibit Evergreen or its board of directors from considering, and potentially recommending, an unsolicited, bona fide, written superior offer from a third party in the circumstances described under “The Merger Agreement — Acquisition Proposals” on page 93.

Comparative Market Price Information

      Shares of Pioneer common stock are listed on the New York Stock Exchange under the symbol “PXD,” and shares of Evergreen common stock are listed on the New York Stock Exchange under the symbol “EVG.” The following table presents the last reported sale price per share of Pioneer common stock and Evergreen common stock, as reported on the New Stock Exchange reporting system on May 3, 2004, the last full trading day prior to the public announcement of the merger, and on June 10, 2004, the last trading day for which this information could be obtained prior to the date of this joint proxy statement/ prospectus.

Selected Historical Financial Information of Pioneer

     The following selected consolidated financial data is derived from Pioneer’s consolidated financial statements. This information is only a summary and does not provide all of the information contained in Pioneer’s consolidated financial statements, including the related notes and Management’s Discussion and Analysis of Financial Condition and Results of Operations, which are part of Pioneer’s Quarterly Report on Form 10-Q for the three months ended March 31, 2004 and Annual Report on Form 10-K for the year ended December 31, 2003. Pioneer’s financial statements and other information filed with the SEC should be read in conjunction with the following information.

Selected Historical Financial Information of Evergreen

      The following selected consolidated financial data is derived from Evergreen’s consolidated financial statements. This information is only a summary and does not provide all of the information contained in Evergreen’s consolidated financial statements, including the related notes and Management’s Discussion and Analysis of Financial Condition and Results of Operations, which are part of Evergreen’s Quarterly Report on Form 10-Q for the three months ended March 31, 2004 and Annual Report on Form 10-K for the year ended December 31, 2003. Evergreen’s financial statements and other information filed with the SEC should be read in conjunction with the following information.