SUMMARY

This summary highlights some of the information in this proxy statement/prospectus. It may not contain all of the information that is important to you. To understand the merger fully and for a more complete description of the terms of the merger agreement, you should read carefully this proxy statement/prospectus, the documents incorporated by reference and the Annexes to this proxy statement/prospectus, including the full text of the merger agreement included as Annex A and the amendment included as Annex B. Please also read “Where You Can Find More Information” on page 150.

The Merger Parties’ Businesses

Pioneer Natural Resources Company

Pioneer, a Delaware corporation formed in 1997, is a large independent oil and gas exploration and production company with operations in the United States. Pioneer is a holding company whose assets consist of direct and indirect ownership interests in, and whose business is conducted substantially through, its subsidiaries. Pioneer’s common stock is listed and traded on the NYSE under the symbol “PXD.”

Pioneer’s mission is to enhance stockholder investment returns through strategies that maximize its long-term profitability and net asset value. The strategies employed to achieve this mission are predicated on maintaining financial flexibility, capital allocation discipline and enhancing net asset value through accretive drilling programs, joint ventures and acquisitions. These strategies are anchored primarily by drilling in the Spraberry oil field located in West Texas (the “Spraberry field”), the liquid-rich Eagle Ford Shale field located in South Texas and the liquid-rich Barnett Shale Combo field in North Texas. Complementing these growth areas, Pioneer has oil and gas production activities and development opportunities in the Raton gas field located in southern Colorado, the Hugoton gas and liquid field located in southwest Kansas, the West Panhandle gas and liquid field located in the Texas Panhandle, and the Edwards gas field located in South Texas.

As of December 31, 2012, Pioneer had proved oil, NGL and gas reserves of 1.1 billion BOE. For the year ended December 31, 2012, Pioneer had net income attributable to Pioneer common stockholders of $192.3 million, or $1.50 per diluted share, and revenues and other income of $3.2 billion. For the nine months ended September 30, 2013, Pioneer had net income attributable to common stockholders of $529.1 million, or $3.82 per diluted share, and revenues and other income of $2.8 billion.

Pioneer’s executive offices are located at 5205 N. O’Connor Blvd., Suite 200, Irving, Texas 75039. Pioneer’s telephone number is (972) 444-9001.

Pioneer Southwest Energy Partners L.P.

Pioneer Southwest is a Delaware limited partnership that was formed in June 2007 by Pioneer to own, acquire, explore and develop oil and gas assets in Pioneer Southwest’s area of operations.

All of Pioneer Southwest’s properties are located in the Spraberry field. Pioneer Southwest’s only operating segment is oil and gas producing activities. As of December 31, 2012, Pioneer Southwest had proved oil, NGL and gas reserves of 49.4 million BOE. For the year ended December 31, 2012, Pioneer Southwest had net income of $107.6 million, or $3.00 per common unit, and total revenues and other income of $208.3 million. For the nine months ended September 30, 2013, Pioneer Southwest had net income of $62.8 million, or $1.75 per common unit, and total revenues and other income of $156.6 million.

Pioneer Southwest’s executive offices are located at 5205 N. O’Connor Blvd., Suite 200, Irving, Texas 75039. Pioneer Southwest’s telephone number is (972) 969-3586.

Relationship of Pioneer and Pioneer Southwest

Pioneer and Pioneer Southwest are closely related. Pioneer formed Pioneer Southwest in 2007, and Pioneer Southwest completed its initial public offering in 2008. The operations and activities of Pioneer Southwest are managed by its general partner, Pioneer Southwest GP, an indirect wholly-owned subsidiary of Pioneer. Pioneer indirectly owns a 52.5% interest in Pioneer Southwest, including the 0.1% general partner interest. Pioneer Southwest, its operating subsidiary and Pioneer Southwest GP have no employees. Pioneer Southwest, Pioneer Southwest GP and Pioneer have entered into an administrative services agreement pursuant to which Pioneer manages all of Pioneer Southwest’s assets and performs administrative services for Pioneer Southwest. Please see “Summary — Organizational Chart — Before the Merger.”

Each of the executive officers of Pioneer Southwest GP is also an executive officer of Pioneer. For information about the common executive officers of Pioneer and Pioneer Southwest GP and the resulting interests of Pioneer and Pioneer Southwest GP directors and executive officers in the merger, please read “Certain Relationships; Interests of Certain Persons in the Merger.”

Structure of the Merger

Pursuant to the merger agreement, at the effective time of the merger, MergerCo, a direct wholly-owned subsidiary of Pioneer, will merge with and into Pioneer Southwest with Pioneer Southwest surviving the merger as an indirect wholly-owned subsidiary of Pioneer, and each outstanding Pioneer Southwest common unit other than those Pioneer Southwest common units owned by Pioneer USA, will be cancelled and, other than dissenting units, converted into the right to receive 0.2325 of a share of Pioneer common stock. This merger consideration represents a 19% premium to the closing price of the Pioneer Southwest common units based on the closing prices of the Pioneer Southwest common units and shares of Pioneer common stock on May 6, 2013, the last trading day before Pioneer announced its proposal to acquire all of the Pioneer Southwest common units owned by the public.

If the exchange ratio results in a Pioneer Southwest unitholder being entitled to receive a fraction of a share of Pioneer common stock, that Pioneer Southwest unitholder will not receive any fractional share of Pioneer common stock. In lieu of receiving any fractional share of Pioneer common stock to which any Pioneer Southwest unitholder would otherwise have been entitled, after aggregating all fractions of shares to which such unitholder would be entitled, any fractional share will be rounded up to a whole share of Pioneer common stock.

Once the merger is completed, former Pioneer Southwest unitholders who surrender their Pioneer Southwest common units in accordance with the merger agreement will be eligible, in their capacity as Pioneer stockholders, to receive dividends declared by the Pioneer Board on Pioneer common stock, if any, after the effective time of the merger. For a description of Pioneer’s dividend policy, please read “Summary — Market Prices and Dividend and Distribution Information — Pioneer’s Dividend Policy.”

Based on the 16,992,500 Pioneer Southwest common units outstanding on November 5, 2013, and eligible to be converted into shares of Pioneer common stock pursuant to the merger agreement (which number does not include the Pioneer Southwest common units owned by Pioneer USA), and without giving effect to rounding of fractional shares and assuming no unitholder exercises appraisal rights, Pioneer expects to issue approximately 3.95 million shares of Pioneer common stock in connection with the merger. This number will represent approximately 3% of Pioneer’s outstanding shares of common stock after the merger, based on 138,667,689 shares of Pioneer common stock outstanding as of November 5, 2013.

Pioneer plans to pay off the Pioneer Southwest credit facility shortly following the closing of the merger, and, through a separate, independent transaction, expects to merge Pioneer Southwest GP, Pioneer Southwest and their subsidiaries into Pioneer USA after the closing of the merger.

Voting Agreement

In connection with the merger agreement, Pioneer, Pioneer USA, MergerCo, Pioneer Southwest and Pioneer Southwest GP entered into the voting agreement on August 9, 2013. Pursuant to the voting agreement, Pioneer, Pioneer USA and MergerCo have agreed to vote the Pioneer Southwest common units owned by them in favor of the merger proposal, including the 18,721,200 Pioneer Southwest common units currently held by Pioneer USA, which units represent 52.4% of the outstanding Pioneer Southwest common units and therefore constitute a sufficient number of Pioneer Southwest common units to approve the merger proposal at the Pioneer Southwest special meeting. The voting agreement will terminate upon the earliest of (i) the completion of the merger, (ii) the termination of the merger agreement, and (iii) the mutual written agreement of the parties.

Directors and Executive Officers of Pioneer Following the Merger

The directors and executive officers of each of Pioneer and Pioneer Southwest GP prior to the merger are expected to continue as the directors and executive officers of Pioneer and Pioneer Southwest GP, respectively, following the merger, with the exception of the four independent directors of Pioneer Southwest GP who are expected to resign following the merger.

Market Prices of Shares of Pioneer Common Stock and Pioneer Southwest Common Units Before Announcement of the Proposed Merger

Pioneer’s common stock is traded on the NYSE under the ticker symbol “PXD.” Pioneer Southwest’s common units are traded on the NYSE under the ticker symbol “PSE.” The closing price of shares of Pioneer common stock on May 6, 2013 (the last full trading day before Pioneer announced its proposal to acquire all of the Pioneer Southwest common units owned by the public) was $133.54, and the closing price of Pioneer Southwest common units on May 6, 2013 was $26.00.

Pioneer Southwest Special Meeting

Where and When

The Pioneer Southwest special meeting will take place at the offices of Pioneer Southwest, 5205 N. O’Connor Blvd., Suite 200, Irving, Texas 75039, on December 17, 2013, at 9:00 a.m., local time.

What You Are Being Asked to Vote On

At the Pioneer Southwest special meeting, Pioneer Southwest unitholders will vote on the merger proposal and may vote on the adjournment proposal.

Who May Vote

You may vote at the Pioneer Southwest special meeting if you owned Pioneer Southwest common units at the close of business on the record date, October 30, 2013. On that date, there were 35,713,700 Pioneer Southwest common units outstanding. You may cast one vote for each outstanding Pioneer Southwest common unit that you owned on the record date.

What Vote is Needed

The merger proposal will be approved if the holders, as of the record date of the Pioneer Southwest special meeting, of a majority of the outstanding Pioneer Southwest common units vote in favor of the merger proposal at the Pioneer Southwest special meeting. Failures to vote, abstentions and broker non-votes (if any) will have the same effect as a vote against the merger proposal. Pursuant to the voting agreement, Pioneer, Pioneer USA and MergerCo have agreed to vote the Pioneer Southwest common units owned by them in favor of the merger proposal, including the 18,721,200 Pioneer Southwest common units currently held by Pioneer USA, which units represent 52.4% of the outstanding Pioneer Southwest common units and therefore constitute a sufficient number of Pioneer Southwest common units to approve the merger proposal at the Pioneer Southwest special meeting.

The adjournment proposal will be approved if the holders, as of the record date of the Pioneer Southwest special meeting, of a majority of the outstanding Pioneer Southwest common units vote in favor of the adjournment proposal at the Pioneer Southwest special meeting. Failures to vote, abstentions and broker non-votes (if any) will have the same effect as a vote against this proposal.

Of the Pioneer Southwest common units entitled to vote on the proposals at the Pioneer Southwest special meeting, 0.5% of such Pioneer Southwest common units are held, and eligible to be voted, by certain executive officers and directors, and their affiliates, of Pioneer or Pioneer Southwest (not including the parties to the voting agreement).

Recommendation to Pioneer Southwest Unitholders

The members of the Pioneer Southwest Conflicts Committee considered the benefits of the merger agreement and the merger transactions as well as the associated risks and unanimously approved the merger agreement and the merger transactions and determined that the merger agreement and the merger transactions are fair and reasonable to and in the best interests of the Pioneer Southwest unaffiliated unitholders and Pioneer Southwest. This action of the Pioneer Southwest Conflicts Committee constitutes “Special Approval” of the merger agreement and the merger transactions under Pioneer Southwest’s partnership agreement. The Pioneer Southwest Conflicts Committee recommended that the Pioneer Southwest GP Board make the same approval and determination as the Pioneer Southwest Conflicts Committee. Based in part on the Pioneer Southwest Conflicts Committee’s approval and determination, Special Approval and recommendation, the Pioneer Southwest GP Board approved the merger agreement and the merger transactions (such approval being unanimous among the independent directors, with the non-independent directors of Pioneer Southwest GP recusing themselves from the consideration and vote on such approval) and determined that the merger agreement and the merger transactions are fair and reasonable to and in the best interests of the Pioneer Southwest unaffiliated unitholders and Pioneer Southwest. The Pioneer Southwest GP Board caused Pioneer Southwest GP to approve the merger agreement and the merger transactions and directed that the merger agreement and the merger transactions be submitted to the Pioneer Southwest unitholders at the Pioneer Southwest special meeting for approval. The Pioneer Southwest Conflicts Committee and the Pioneer Southwest GP Board recommend that the Pioneer Southwest unitholders vote in favor of the merger proposal.

Pioneer Southwest unitholders are urged to review carefully the background and reasons for the merger described under “The Merger” and the risks associated with the merger described under “Risk Factors.”

Pioneer Southwest’s Reasons for the Merger

The Pioneer Southwest Conflicts Committee considered many factors in determining that the merger agreement and the merger transactions are fair and reasonable to and in the best interests of the Pioneer

Southwest unaffiliated unitholders and Pioneer Southwest. For a discussion of those factors, please read “The Merger — Recommendation of the Pioneer Southwest Conflicts Committee and the Pioneer Southwest GP Board and Reasons for the Merger.”

Opinion of the Pioneer Southwest Conflicts Committee’s Financial Advisor

The Pioneer Southwest Conflicts Committee retained Evercore to act as financial advisor to the Pioneer Southwest Conflicts Committee in connection with the proposal by Pioneer to acquire all of the publicly held Pioneer Southwest common units in exchange for Pioneer common stock. At the Pioneer Southwest Conflicts Committee’s meeting on August 9, 2013, Evercore rendered its oral opinion (subsequently confirmed in writing) that, as of that date, and based upon and subject to the factors, procedures, assumptions, qualifications and limitations set forth in its opinion, the exchange ratio set forth in the merger agreement was fair, from a financial point of view, to the Pioneer Southwest unaffiliated unitholders.

Evercore’s opinion is directed to the Pioneer Southwest Conflicts Committee and the independent directors of the Pioneer Southwest GP Board acting in their capacity as such. It does not address any aspects of the merger other than the exchange ratio and does not constitute a recommendation as to how any Pioneer Southwest unitholder should vote on the merger or any matters related thereto.

The full text of the Evercore written opinion dated as of August 9, 2013, is attached to this proxy statement/prospectus as Annex C. Pioneer Southwest encourages its unitholders to read the opinion carefully and in its entirety. The summary of Evercore’s opinion set forth in this proxy statement/prospectus is qualified in its entirety by reference to the full text of the opinion.

Certain Relationships; Interests of Certain Persons in the Merger

In considering the recommendations of the Pioneer Southwest Conflicts Committee and the Pioneer Southwest GP Board with respect to the merger, Pioneer Southwest unitholders should be aware that certain of the directors and executive officers of Pioneer and Pioneer Southwest GP have interests in the transaction that differ from, or are in addition to, the interests of Pioneer Southwest unitholders generally, including:

The Merger Agreement

The merger agreement and the amendment are attached to this proxy statement/prospectus as Annex A and Annex B, respectively, and are incorporated by reference into this proxy statement/prospectus. You are encouraged to read the merger agreement and the amendment in their entirety because the merger agreement as amended by the amendment is the legal document that governs the merger.

What Needs to Be Done to Complete the Merger

Pioneer and Pioneer Southwest will complete the merger only if the conditions set forth in the merger agreement are satisfied or, in some cases, waived. The obligations of Pioneer and Pioneer Southwest to complete the merger are subject to, among other things, the following conditions:

The obligation of Pioneer to effect the merger is further subject to the satisfaction by Pioneer Southwest, on or prior to the closing date of the merger, of each of the following conditions, or the waiver thereof by Pioneer:

The obligation of Pioneer Southwest to effect the merger is further subject to the satisfaction by Pioneer, on or prior to the closing date of the merger, of each of the following conditions, or the waiver thereof by Pioneer Southwest:

The merger agreement provides that the Pioneer Southwest unitholder voting condition may not be waived. Each of Pioneer and Pioneer Southwest (with the consent of the Pioneer Southwest Conflicts Committee, in the case of Pioneer Southwest) may choose to complete the merger even though any other condition to its obligation has not been satisfied if the necessary Pioneer Southwest unitholder approval has been obtained and the law allows it to do so.

Unitholder Approval; Acquisition Proposal; Change in Recommendation

Subject to the terms and conditions of the merger agreement, and except as described further in “The Merger Agreement — Covenants — Unitholder Approval,” Pioneer Southwest will take, in accordance with applicable law, applicable stock exchange rules and Pioneer Southwest’s partnership agreement, all action necessary to call, hold and convene the Pioneer Southwest special meeting to consider and vote upon the approval of the merger proposal, as promptly as practicable after the registration statement of which this proxy statement/prospectus is a part is declared effective. The Pioneer Southwest Conflicts Committee and the Pioneer Southwest GP Board will recommend approval of the merger proposal to the Pioneer Southwest unitholders, and Pioneer Southwest will take all reasonable lawful action to solicit such approval by the Pioneer Southwest unitholders. Except under certain conditions described in the following paragraph and in “The Merger Agreement — Covenants — Unitholder Approval,” neither the Pioneer Southwest Conflicts Committee nor the Pioneer Southwest GP Board will (A) withdraw, modify or qualify in any manner adverse to Pioneer the recommendation of the Pioneer Southwest Conflicts Committee and the Pioneer Southwest GP Board or (B) publicly approve, adopt or recommend, or publicly propose to approve, adopt or recommend, any “acquisition proposal” (defined and described more fully under “The Merger Agreement — Covenants — Acquisition Proposals”). The actions described in the preceding sentence are referred to in this proxy statement/prospectus as a “Pioneer Southwest Change in Recommendation.” None of Pioneer Southwest GP, Pioneer Southwest or any of their subsidiaries will execute or enter into any letter of intent, memorandum of understanding, agreement in principle, merger agreement, acquisition agreement, option agreement, joint venture agreement, partnership agreement or other similar contract providing for any acquisition proposal.

Notwithstanding the above paragraph, at any time prior to obtaining the Pioneer Southwest unitholder approval, the Pioneer Southwest Conflicts Committee or the Pioneer Southwest GP Board may make a Pioneer Southwest Change in Recommendation if it has determined in good faith, after consultation with its outside legal counsel and financial advisors, that failure to make a Pioneer Southwest Change in Recommendation would be inconsistent with its duties under Pioneer Southwest’s partnership agreement or applicable law; provided, however, that neither the Pioneer Southwest Conflicts Committee nor the Pioneer Southwest GP Board will be entitled to exercise its right to make a Pioneer Southwest Change in Recommendation pursuant to this sentence unless (i) Pioneer Southwest has not engaged in a material breach of its covenant related to acquisition proposals, (ii) Pioneer Southwest has provided to Pioneer three business days prior written notice, advising Pioneer that the Pioneer Southwest Conflicts Committee or the Pioneer Southwest GP Board intends to take such action, specifying the reasons for taking such action in reasonable detail, including, if a reason for the Pioneer Southwest Change in Recommendation is an acquisition proposal, that the Pioneer Southwest Conflicts Committee has determined that the acquisition proposal is a “superior proposal” (defined and described more fully under “The Merger Agreement — Covenants — Acquisition Proposals”) and specifying the terms and conditions of such acquisition proposal and the identity of the person making such acquisition proposal (it being understood that any amendment to the terms of any such acquisition proposal will require a new written notice, as described above, and an additional three business day period), (iii) if a reason for the Pioneer Southwest Change in Recommendation is an acquisition proposal, Pioneer Southwest has provided to Pioneer all materials and information delivered or made available to the person or group of persons making such acquisition proposal (to the extent not previously provided to Pioneer), (iv) each of Pioneer Southwest GP, Pioneer Southwest and the Pioneer Southwest Conflicts Committee has negotiated, and has used its commercially reasonable efforts to cause its representatives to negotiate, in good faith with Pioneer during such notice period to enable Pioneer to revise the terms of the merger agreement such that it would obviate the need for making the Pioneer Southwest Change in Recommendation, and (v) following the end of such notice period, the Pioneer Southwest Conflicts Committee will have considered in good faith any changes to the merger agreement proposed by Pioneer and will have determined that the failure to make a Pioneer Southwest Change in Recommendation would continue to be inconsistent with its duties under Pioneer Southwest’s partnership agreement or applicable law even if such

revisions proposed by Pioneer were to be given effect and, if a reason for the Pioneer Southwest Change in Recommendation is an acquisition proposal, that the acquisition proposal continues to be a superior proposal even if the revisions proposed by Pioneer were to be given effect. Any Pioneer Southwest Change in Recommendation will not invalidate the approval (or “Special Approval,” as defined in Pioneer Southwest’s partnership agreement) of the merger agreement or any other approval of the Pioneer Southwest Conflicts Committee, including in any respect that would have the effect of causing any state (including Delaware) takeover statute or other similar statute to be applicable to the merger transactions.

Pioneer Southwest GP and Pioneer Southwest will, and they will cause their subsidiaries and representatives to, (i) immediately cease and terminate any solicitation, encouragement, discussions or negotiations with any person that may be ongoing with respect to or that may reasonably be expected to lead to an acquisition proposal, and (ii) request such person to promptly return or destroy all confidential information concerning Pioneer Southwest and its subsidiaries.

Neither Pioneer Southwest GP nor Pioneer Southwest will, and they will cause their subsidiaries and use their commercially reasonable efforts to cause their representatives not to, directly or indirectly, (i) initiate, solicit, knowingly encourage or knowingly facilitate (including by way of furnishing information) any inquiries regarding, or the making or submission of any proposal or offer that constitutes, or may reasonably be expected to lead to, an acquisition proposal, (ii) conduct or participate in any discussions or negotiations regarding any acquisition proposal, or (iii) furnish to any person any non-public information or data relating to Pioneer Southwest or any of its subsidiaries or afford access to the business, properties, assets, or, except as required by law or Pioneer Southwest’s partnership agreement, books or records of Pioneer Southwest or any of its subsidiaries. Notwithstanding the foregoing, at any time prior to obtaining the Pioneer Southwest unitholder approval, the Pioneer Southwest Conflicts Committee may take the actions described in clauses (ii) and (iii) above with respect to a third person that makes a bona fide unsolicited acquisition proposal that did not result from a material breach of the provisions of the merger agreement described in this paragraph, if (A) the Pioneer Southwest Conflicts Committee, after consultation with its outside legal counsel and financial advisors, determines in good faith that such acquisition proposal constitutes or could reasonably be expected to result in a superior proposal and that the failure to take such action would be inconsistent with its duties under Pioneer Southwest’s partnership agreement or applicable law, and (B) prior to furnishing any such non-public information to such third person, Pioneer Southwest receives from such third person an executed confidentiality agreement as further described under “The Merger Agreement — Covenants — Acquisition Proposals.”

Termination of the Merger Agreement

Pioneer and Pioneer Southwest can agree to terminate the merger agreement by mutual written consent at any time without completing the merger, even after the Pioneer Southwest unitholders have approved the merger proposal. In addition, either party may terminate the merger agreement on its own upon written notice to the other without completing the merger if:

Federal Income Tax Consequences of the Merger

Tax matters associated with the merger are complicated. Under current law, it is anticipated for U.S. federal income tax purposes that Pioneer Southwest unitholders generally will recognize gain with respect to the exchange of Pioneer Southwest common units for shares of Pioneer common stock in the merger in an amount equal to the excess of (1) each Pioneer Southwest unitholder’s “amount realized” for U.S. federal income tax purposes, which equals the sum of the fair market value of the shares of Pioneer common stock received, plus the unitholder’s allocated share of Pioneer Southwest’s pre-merger liabilities (it being understood that no Pioneer Southwest unitholder bears any personal responsibility or liability in respect of such allocated liabilities), over (2) the unitholder’s aggregate adjusted tax basis in Pioneer Southwest common units (including basis attributable to the unitholder’s share of Pioneer Southwest’s pre-merger liabilities). Pioneer Southwest unitholders generally will recognize loss to the extent that the amount of their basis described in clause (2) above exceeds the amount realized described in clause (1) above. A portion of any amount realized by a unitholder will be treated as income subject to U.S. federal income tax at ordinary rates (up to 39.6% under current law) rather than capital gains rates

due to the operation of the recapture rules applicable to depreciation, depletion and intangible drilling cost deductions allocable to the Pioneer Southwest unitholders, even if the Pioneer Southwest unitholder’s aggregate adjusted basis in Pioneer Southwest common units exceeds the amount realized in the exchange. In addition, certain Pioneer Southwest unitholders may be subjected to the 3.8% Medicare tax on unearned income in respect of any net gain from the exchange. Pioneer Southwest unitholders are urged to consult their tax advisors for a full understanding of the U.S. federal, state, local and foreign tax consequences of the merger that will be applicable to them. Please read “Material U.S. Federal Income Tax Consequences of the Merger” beginning on page 146 for a more complete discussion of the U.S. federal income tax consequences of the merger.

Other Information Related to the Merger

Appraisal Rights

The merger agreement provides for appraisal rights if certain procedures are followed. For a description of these appraisal rights, please read “The Merger Agreement — Appraisal Rights.” The appraisal rights are subject to termination under certain conditions. Please see “Risk Factors — Risks Related to the Merger” and “The Merger Agreement — Appraisal Rights — Termination of the Memorandum of Understanding” for more information about the possible termination of appraisal rights.

Antitrust and Regulatory Matters

No antitrust or other regulatory clearances are required as a condition to the consummation of the merger.

Listing of Pioneer Common Stock to be Issued in the Merger; Delisting and Deregistration of Pioneer Southwest Common Units

Pioneer expects to obtain approval to list on the NYSE the shares of Pioneer common stock to be issued pursuant to the merger agreement, which approval (subject to official notice of issuance) is a condition to the merger. Upon completion of the merger, Pioneer Southwest common units currently listed on the NYSE will cease to be listed on the NYSE and will be subsequently deregistered under the Exchange Act.

Accounting Treatment

The merger will be accounted for in accordance with Financial Accounting Standards Board Accounting Standards Codification 810, Consolidations — Overall — Changes in Parent’s Ownership Interest in a Subsidiary, which is referred to as ASC 810. As Pioneer will control Pioneer Southwest before and after the merger, the changes in Pioneer’s ownership interest in Pioneer Southwest will be accounted for as an equity transaction and no gain or loss on the merger will be recognized in Pioneer’s consolidated statements of operations.

Comparison of the Rights of Pioneer Stockholders and Pioneer Southwest Unitholders

Pioneer Southwest unitholders, other than dissenting unitholders, will own shares of Pioneer common stock following the completion of the merger, and their rights associated with Pioneer common stock will be different from their rights as Pioneer Southwest unitholders due to the differences between the entity forms and governing documents of Pioneer and Pioneer Southwest. See “Comparison of the Rights of Pioneer Stockholders and Pioneer Southwest Unitholders” beginning on page 128 of this proxy statement/prospectus.

Pending Litigation

On May 15, 2013, David Flecker, a purported unitholder of Pioneer Southwest, filed a class action petition on behalf of the Pioneer Southwest unitholders and a derivative suit on behalf of Pioneer Southwest against Pioneer, Pioneer USA, Pioneer Southwest GP and the directors of Pioneer Southwest GP, in the 134th Judicial District of Dallas County, Texas (the “Flecker Lawsuit”). A similar class action petition and derivative suit was filed against the same defendants on May 20, 2013, in the 160th Judicial District of Dallas County, Texas, by purported unitholder Vipul Patel (the “Patel Lawsuit”). On August 27, 2013, the plaintiff in the Flecker Lawsuit filed an amended petition. On September 3, 2013, the court consolidated the Patel Lawsuit into the Flecker Lawsuit (as consolidated, the “Texas State Court Lawsuit”), and the plaintiffs filed a consolidated derivative and class action petition on September 5, 2013.

The Texas State Court Lawsuit alleges, among other things, that the consideration offered by Pioneer is unfair and inadequate and that, by pursuing a transaction that is the result of an allegedly conflicted and unfair process, the defendants have breached their duties under Pioneer Southwest’s partnership agreement as well as the implied covenant of good faith and fair dealing, and are engaging in self-dealing. Specifically, the lawsuit alleges that the director defendants: (i) engaged in self-dealing, failed to act in good faith toward Pioneer Southwest, and breached their duties owed to Pioneer Southwest; (ii) failed to properly value Pioneer Southwest and its various assets and operations and ignored or failed to protect against the numerous conflicts of interest arising out of the proposed transaction; and (iii) breached the implied covenant of good faith and fair dealing by engaging in a flawed merger process. The Texas State Court Lawsuit also alleges that Pioneer, Pioneer USA and Pioneer Southwest GP aided and abetted the director defendants in their purported breach of fiduciary duties.

Based on these allegations, the plaintiffs in the Texas State Court Lawsuit seek to enjoin the defendants from proceeding with or consummating the proposed transaction. To the extent that the merger is implemented before relief is granted, the plaintiffs seek to have the merger rescinded. The plaintiffs also seek money damages and attorneys’ fees. The defendants have filed a motion to dismiss the Texas State Court Lawsuit based on improper forum.

On August 21, 2013, Allan H. Beverly, a purported Pioneer Southwest unitholder, filed a class action complaint against Pioneer Southwest, Pioneer, Pioneer USA, MergerCo and the directors of Pioneer Southwest GP in the United States District Court for the Northern District of Texas (the “Beverly Lawsuit”). The Beverly Lawsuit alleges that the defendants breached their fiduciary duties by agreeing to the merger by means of an unfair process and for an unfair price. Specifically, the lawsuit alleges that the director defendants: (i) failed to maximize the value of Pioneer Southwest to its public unitholders and took steps to avoid competitive bidding; (ii) failed to properly value Pioneer Southwest; and (iii) ignored or failed to protect against the numerous conflicts of interest arising out of the proposed transaction. The Beverly Lawsuit also alleges that Pioneer, Pioneer USA and MergerCo aided and abetted the director defendants in their purported breach of fiduciary duties. On October 15, 2013, the plaintiffs in the Beverly Lawsuit voluntarily dismissed all claims in the lawsuit in accordance with the memorandum of understanding described below.

On September 13, 2013, Douglas Shelton, another purported Pioneer Southwest unitholder, filed a class action complaint against the same defendants in the Beverly Lawsuit (as well as Pioneer Southwest GP) in the same court as the Beverly Lawsuit (the “Shelton Lawsuit”). The Shelton Lawsuit makes similar allegations to the Beverly Lawsuit, and also alleges that Section 7.9 of the Pioneer Southwest partnership agreement fails to alter or eliminate the defendants’ common law fiduciary duties owed to Pioneer Southwest unitholders in the context of the merger. Specifically, the lawsuit alleges: (1) that Pioneer, as controlling unitholder, failed to fulfill its fiduciary duties in connection with the merger because it purportedly cannot establish that the proposed merger is the result of a fair process that will return a fair price to the Pioneer Southwest unaffiliated unitholders; (2) that the director defendants breached their fiduciary duties by failing to exercise due care and diligence in connection with the proposed merger because the proposed merger is purportedly not the result of a fair process that will

return a fair price to the Pioneer Southwest unaffiliated unitholders; and (3) that the non-director defendants aided and abetted the director defendants in their purported breach of fiduciary duties. The plaintiffs in the Beverly Lawsuit and the Shelton Lawsuit (together, the “Federal Lawsuits”) seek the same remedies as the plaintiffs in the Texas State Court Lawsuit. On October 16, 2013, the plaintiffs in the Shelton Lawsuit voluntarily dismissed all claims in the lawsuit in accordance with the memorandum of understanding described below.

On September 23, 2013, Patrick Wilson, another purported Pioneer Southwest unitholder, filed a class action petition on behalf of the Pioneer Southwest unitholders against Pioneer USA, MergerCo, Pioneer Southwest, Pioneer Southwest GP and the directors of Pioneer Southwest GP in the Court of Chancery of the State of Delaware (the “Wilson Lawsuit”). The Wilson Lawsuit alleges that the director defendants breached their purported fiduciary obligations to the Pioneer Southwest unitholders by engaging in a process that undervalued Pioneer Southwest and which allegedly constitutes gross negligence, recklessness, willful misconduct, bad faith or knowing violations of law. Additionally, the Wilson Lawsuit alleges that the non-director defendants aided and abetted the purported breaches of fiduciary duties of the director defendants. The Wilson Lawsuit seeks the same remedies as the plaintiffs in the Texas State Court Lawsuit and the Federal Lawsuits.

Pioneer and Pioneer Southwest cannot predict the outcome of these or any other lawsuits that might be filed subsequent to the date of the filing of this proxy statement/prospectus, nor can Pioneer and Pioneer Southwest predict the amount of time and expense that will be required to resolve these lawsuits.

On September 26, 2013, representatives of the plaintiffs in the Texas State Court Lawsuit and the Federal Lawsuits and representatives of the defendants in such lawsuits entered into the memorandum of understanding to settle the claims and allegations made in such lawsuits. The memorandum of understanding provides the plaintiffs with a period of confirmatory discovery during which the plaintiffs can confirm the fairness and reasonableness of the settlement contemplated by the memorandum of understanding. The parties agreed to use their reasonable best efforts to agree upon, execute and present to the Dallas County, Texas District Court a stipulation of settlement, which will provide for, among other things, a certification, for settlement purposes only, of the applicable class of Pioneer Southwest unitholders to which the settlement will apply; as of the date of this proxy statement/prospectus, execution of the stipulation of settlement is pending. Furthermore, the stipulation of settlement will provide for a full and complete discharge, dismissal with prejudice, settlement and release of all claims, suits and causes of action by the plaintiffs (other than appraisal rights) against the defendants and their representatives arising out of or relating to the allegations made in the Texas State Court Lawsuit and the Federal Lawsuits, the merger transactions or any deliberations, negotiations, disclosures, omissions, press releases, statements or misstatements in connection therewith (including in this proxy statement/prospectus and the registration statement of which this proxy statement/prospectus forms a part, as well as in other filings), any fiduciary or other obligations in respect of the merger or any alternative transaction or under Pioneer Southwest’s partnership agreement, or any costs and expenses associated with settlement other than as provided in the stipulation. All proceedings relating to the allegations made in the Texas State Court Lawsuit other than with respect to the settlement have been stayed. As part of the consideration for the settlement, the merger agreement has been amended by way of the amendment to provide for contractual appraisal rights for the Pioneer Southwest unitholders. For information about these appraisal rights, please read “The Merger Agreement — Appraisal Rights.” The parties to the memorandum of understanding have agreed to use their reasonable best efforts to obtain the agreement of any plaintiffs filing similar lawsuits to the Texas State Court Lawsuit or the Federal Lawsuits (whether filed in any state or federal court) to become party to the memorandum of understanding and the related settlement. As of the date of this proxy statement/prospectus, the plaintiffs in the Wilson Lawsuit have not joined the memorandum of understanding. Furthermore, the plaintiffs in the Federal Lawsuits have voluntarily dismissed all claims in the the lawsuits in accordance with the memorandum of understanding. There can be no assurance that a final settlement will be consummated. See “Risk Factors — Risks Related to the

Merger.” Furthermore, the memorandum of understanding and the settlement contemplated thereby will terminate under certain conditions, described in more detail in “The Merger Agreement — Appraisal Rights — Termination of the Memorandum of Understanding.”

Summary of Risk Factors

You should consider carefully all the risk factors together with all of the other information included in this proxy statement/prospectus before deciding how to vote. The risks related to the merger transactions, Pioneer’s business following the merger, Pioneer’s common stock and Pioneer Southwest’s business and common units if the merger does not occur, and tax related risks are described under the caption “Risk Factors” beginning on page 38 of this proxy statement/prospectus.

Organizational Chart

Before the Merger

The following diagram depicts the organizational structure of Pioneer and Pioneer Southwest as of November 5, 2013, before the consummation of the merger and the other merger transactions.

After the Merger

The following diagram depicts the organizational structure of Pioneer and Pioneer Southwest immediately after giving effect to the merger and the other merger transactions (without giving effect to rounding of fractional shares and assuming no unitholder exercises appraisal rights).

Selected Historical and Pro Forma Financial and Operating Information of Pioneer and Pioneer Southwest

The following tables set forth, for the periods and at the dates indicated, summary historical financial and operating information for Pioneer and Pioneer Southwest and summary unaudited pro forma financial information for Pioneer after giving effect to the proposed merger. The summary historical financial data as of and for each of the years ended December 31, 2008, 2009, 2010, 2011 and 2012, are derived from and should be read in conjunction with the audited financial statements and accompanying footnotes of Pioneer and Pioneer Southwest, respectively. The summary historical financial data as of and for the nine-month periods ended September 30, 2012 and 2013, are derived from and should be read in conjunction with the unaudited financial statements and accompanying footnotes of Pioneer and Pioneer Southwest, respectively. Pioneer’s and Pioneer Southwest’s consolidated balance sheets as of December 31, 2011 and 2012, and as of September 30, 2013, and the related consolidated statements of operations, comprehensive income, cash flows and equity/partners’ capital for each of the three years in the period ended December 31, 2012, and the nine months ended September 30, 2013, are incorporated by reference into this proxy statement/prospectus from Pioneer’s and Pioneer Southwest’s respective Annual Reports on Form 10-K for the year ended December 31, 2012, and their respective Quarterly Reports on Form 10-Q for the quarter ended September 30, 2013.

The summary unaudited pro forma condensed consolidated financial information for Pioneer shows the pro forma effect of the proposed merger. For a complete discussion of the pro forma adjustments underlying the amounts in the table on the following page, please read “Unaudited Pro Forma Condensed Consolidated Financial Statements” beginning on page F-2 of this proxy statement/prospectus.

The proposed merger will be accounted for in accordance with Financial Accounting Standards Board Accounting Standards Codification 810, Consolidations — Overall — Changes in Parent’s Ownership Interest in a Subsidiary, which is referred to as ASC 810. As Pioneer will control Pioneer Southwest before and after the merger, the changes in Pioneer’s ownership interest in Pioneer Southwest will be accounted for as an equity transaction and no gain or loss on the merger will be recognized in Pioneer’s consolidated statements of operations.

The unaudited pro forma condensed consolidated financial statements have been prepared to assist in the analysis of the financial effects of the proposed merger. The unaudited pro forma condensed consolidated statements of operations for the year ended December 31, 2012, and the nine months ended September 30, 2013, have been prepared to give effect to the proposed merger as if it had occurred on January 1, 2012. The unaudited pro forma condensed consolidated balance sheet has been prepared to give effect to the proposed merger as if it had occurred on September 30, 2013. The unaudited pro forma condensed consolidated financial statements are based on assumptions that Pioneer and Pioneer Southwest believe are reasonable under the circumstances and are intended for informational purposes only. They are not necessarily indicative of the financial results that would have occurred if the merger transactions had taken place on the dates indicated, nor are they indicative of the future consolidated results of Pioneer. They also do not reflect non-recurring items arising directly from the merger or any cost savings that the combined entity may achieve.

Summary Historical and Pro Forma Financial Information of Pioneer

Summary Historical Financial Information of Pioneer Southwest

Comparative Per Share and Per Unit Information

The following table sets forth (i) historical per share information of Pioneer, (ii) the unaudited pro forma per share information of Pioneer after giving pro forma effect to the proposed merger and the other merger transactions, including Pioneer’s issuance of 0.2325 of a share of Pioneer common stock for each outstanding Pioneer Southwest common unit (other than Pioneer Southwest common units owned by Pioneer USA and assuming that no unitholder exercises appraisal rights), and (iii) the historical and equivalent pro forma per unit information for Pioneer Southwest.

You should read this information in conjunction with (i) the summary historical financial information included elsewhere in this proxy statement/prospectus, (ii) the historical consolidated financial statements of Pioneer and Pioneer Southwest and related notes that are incorporated by reference into this proxy statement/prospectus, and (iii) the “Unaudited Pro Forma Condensed Consolidated Financial Statements” and related notes included elsewhere in this proxy statement/prospectus. The unaudited pro forma per unit information does not purport to represent what the actual results of operations of Pioneer and Pioneer Southwest would have been had the proposed merger been completed in another period or to project Pioneer’s and Pioneer Southwest’s results of operations that may be achieved if the proposed merger is completed.