QUESTIONS AND ANSWERS ABOUT THE MERGER

         Q:    Please briefly describe the proposed merger and related transactions.

        A:     Helmerich & Payne will consolidate its oil and gas exploration and production and gas marketing business under one company and will spin off that company to Helmerich & Payne's stockholders. The company to be spun off is named Cimarex Energy Co. Key will merge with a newly formed subsidiary of Cimarex and become a new wholly owned subsidiary of Cimarex. When the merger is complete, approximately 34.75% of the Cimarex common stock will be held by former stockholders of Key and approximately 65.25% of Cimarex common stock will be held by stockholders of Helmerich & Payne, each on a diluted basis.

         Q:    What am I being asked to vote upon?

        A:     You are being asked to adopt the merger agreement entered into among Key, Helmerich & Payne and Cimarex.

         Q:    What will I receive in the merger for my Key stock?

        A:     If the merger is completed, as a Key stockholder, you will receive one share of common stock of Cimarex in exchange for each share of Key common stock that you own. You will receive only whole shares of Cimarex common stock. You will receive cash instead of any fractional shares you would otherwise receive in the merger.

         Q:    Does the Key board of directors support the merger?

        A:     Yes. The Key board of directors has determined the merger is fair and in the best interests of Key and its stockholders and the merger agreement is advisable. The Key board of directors has unanimously approved the merger agreement and recommends that the Key stockholders vote " for " the adoption of the merger agreement.

        A more detailed description of the background and reasons for the merger is described under "The Merger" beginning on page     .

         Q:    Are there risks that I should consider in deciding whether to vote on the merger?

        A:     Yes. In evaluating the merger, you should read the "Risk Factors" beginning on page     for a description of various risks you should carefully consider in evaluating the proposed merger.

         Q:    Can I dissent and require appraisal of my shares of Key common stock?

        A:     No. Stockholders are not entitled to dissenters' rights or appraisal rights in connection with the merger.

         Q:    What vote is required to adopt the merger agreement?

        A:     For the merger to occur, the holders of a majority of outstanding Key common stock must adopt the merger agreement. Key stockholders will have one vote for each share of Key common stock owned by them. Helmerich & Payne stockholders are not required to approve the spin-off or the merger.

         Q:    Will Key's shares of common stock continue to be traded on the New York Stock Exchange after the merger is completed?

         A:     No. If the merger is completed, Key's shares of common stock will no longer be listed for trading on the New York Stock Exchange.

         Q:    Where will the Cimarex common stock be listed?

        A:     We will apply to list the shares of Cimarex common stock on the New York Stock Exchange.

1

         Q:    Who will be the senior executive officers of Cimarex?

        A:     Mr. F.H. Merelli, the current Chairman, President and Chief Executive Officer of Key, will become the Chairman, Chief Executive Officer and President of Cimarex. Steven R. Shaw, the current Vice President of Exploration and Production of Helmerich & Payne, will be the Executive Vice President of Cimarex. Paul Korus, the current Chief Financial Officer of Key, will be Vice President and Chief Financial Officer of Cimarex.

         Q:    Who will be the directors of Cimarex?

        A:     If the merger is completed, Key will designate four directors and Helmerich & Payne will designate five directors to the Cimarex board. The Chairman of the Cimarex board will be Mr. F.H. Merelli, the current Chairman, President and Chief Executive Officer of Key.

         Q:    When do you expect to complete the spin-off and the merger?

        A:     If the merger agreement is adopted by the stockholders of Key, then Key, Helmerich & Payne and Cimarex expect to complete the spin-off and the merger as soon as possible after the satisfaction (or waiver, where permissible) of the conditions to the spin-off and the merger, including adoption of the merger agreement by the stockholders of Key and receipt by Helmerich & Payne of its requested rulings from the Internal Revenue Service. We currently anticipate that the merger will be completed during the third calendar quarter of 2002.

         Q:    What should I do now?

        A:    You should mail your signed and dated proxy card(s) in the enclosed envelope or vote via telephone or via the Internet by following the instructions on your proxy card as soon as possible so that your shares of Key common stock will be represented and voted at the Key special meeting. If you plan to attend the special meeting, please be sure to obtain an admission card.

         Q:    Do I need to send in my share certificate(s) now?

        A:    No. Do not send in your share certificate(s) now. Do not send in your share certificate(s) with your proxy card(s). If the merger is completed, Key will send you a letter describing how to exchange your share certificate(s).

         Q:    If I am not going to attend the special meeting, should I return my proxy card(s)?

        A:     Yes. Returning your proxy card(s) ensures that your shares of Key common stock will be represented at the Key special meeting, even if you are unable to or do not attend.

         Q:    How do I vote my shares of Key common stock if they are held in the name of a bank, broker or other fiduciary?

        A:     Your bank, broker or other fiduciary will vote your shares of Key common stock with respect to the merger only if you provide written instructions to them on how to vote, so it is important that you provide them with instructions. If you do not provide them with instructions, under the rules of the New York Stock Exchange, they will not be authorized to vote with respect to the merger. If you wish to vote in person at the meeting and hold your shares of Key common stock in the name of a bank, broker or other fiduciary, you must contact your bank, broker or other fiduciary and request a legal proxy. You must bring this legal proxy to the meeting in order to vote in person. Shares of Key common stock held by a broker, bank or other fiduciary that are not voted because the customer has not provided instructions to the broker, bank or other fiduciary will have the same effect as a vote "against" the proposal.

2

         Q:    Can I change my vote after I mail my proxy card(s)?

        A:     Yes. If you are a record holder, you can change your vote by:

•

completing, signing and dating a new proxy card and returning it by mail to our proxy solicitor so that it is received prior to the special meeting;



•

voting via telephone or via the Internet by following the instructions provided on your proxy card;



•

sending a written notice to the Assistant Corporate Secretary of Key that is received prior to the special meeting stating that you revoke your proxy; or



•

obtaining an admission card, attending the special meeting and voting in person or by legal proxy, if appropriate.

        If your shares of Key common stock are held in the name of a bank, broker or other fiduciary and you have directed such person(s) to vote your shares of Key common stock, you should instruct such person(s) to change your vote or obtain a legal proxy to do so yourself.

         Q:    What if I do not vote, or abstain from voting, or do not instruct my broker to vote my shares of Key common stock?

        A:     If you do not vote, it will have the same effect as a vote against the merger. Abstentions and broker non-votes will also have the effect of votes against the merger.

        If you sign your proxy card but do not indicate how you want to vote, your shares of Key common stock will be voted for the merger.

         Q:    Who can answer my questions?

        A:     If you have any questions regarding the special meeting or need assistance in voting your shares of Key common stock, please contact our proxy solicitor:

         Q:    Where can I find more information about Key?

        A:     You can find more information about Key from various sources described under "Where You Can Find More Information" on page     .

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THE MERGER

         The discussion in this proxy statement/prospectus of the merger and the principal terms of the merger agreement is subject to and qualified in its entirety by reference to the merger agreement, a copy of which is attached to this proxy statement/prospectus as Annex A and is incorporated by reference into this proxy statement/prospectus.

Background of the Merger

        On November 14, 2000, Helmerich & Payne publicly announced that it had retained Petrie Parkman & Co. to act as its financial advisor to identify companies as possible candidates to enter into a business combination transaction with its oil and gas exploration and production business. It was specifically contemplated that a successful transaction could lead, among other things, to the establishment of the Cimarex business as a separate public entity.

        In November 2000, Hans Helmerich, President and Chief Executive Officer of Helmerich & Payne, authorized Petrie Parkman to contact Key to arrange a meeting to discuss a possible business combination of Key and the Cimarex business. On November 22, 2000, a representative of Petrie Parkman contacted Mr. F.H. Merelli, Chairman and Chief Executive Officer of Key, to inquire whether Key would be interested in exploring the possibility of such a transaction and, on that day and the next, representatives of Petrie Parkman engaged in preliminary discussions with Mr. Merelli.

        In January 2001, Key retained Merrill Lynch & Co. as its financial advisor in connection with a possible business combination transaction between Key and the Cimarex business. On January 16, 2001, representatives of Helmerich & Payne and Petrie Parkman met with representatives of Key and Merrill Lynch to provide general information about the Cimarex business and to discuss Key's possible interest in a business combination. The discussions were exploratory in nature. Largely because of then-current capital market conditions, the companies elected not to pursue further negotiations at that time. Over the first nine months of 2001, Helmerich & Payne continued to explore strategic alternatives for the Cimarex business, including, but not limited to, possible combinations with other industry participants. Following the January 2001 meeting, apart from informal inquiries from time to time by Key to Helmerich & Payne regarding its plans for the Cimarex business, there were no contacts between the executive officers of Key and Helmerich & Payne until October 2001.

        On October 25, 2001, a representative of Petrie Parkman contacted Mr. Merelli to propose another meeting between Key and Helmerich & Payne. On October 29, 2001, Mr. Helmerich and other representatives of Helmerich & Payne, together with representatives of Petrie Parkman, met with Mr. Merelli to discuss the two respective companies' business objectives and whether a business combination would be in the best interests of stockholders of the two companies. At this meeting, Mr. Merelli and Mr. Helmerich agreed that a small group of senior management from each of Key and Helmerich & Payne should meet to further consider the fit between the two businesses and possible transaction structures. On November 5, 2001, the parties executed a confidentiality agreement.

        On November 14, 2001, Mr. Helmerich and Douglas E. Fears, Chief Financial Officer of Helmerich & Payne met with Mr. Merelli and Paul Korus, Vice President and Chief Financial Officer of Key, to discuss the potential benefits and the structure of a business combination transaction. Based on those discussions, it was agreed that the parties would begin a due diligence review of, among other things, their respective proved reserves and other assets. Also on November 14, 2001, Skadden, Arps, Slate, Meagher & Flom LLP, Helmerich & Payne's counsel, distributed drafts of the principal agreements relating to the proposed transaction, including a distribution agreement and a merger agreement, to Key, Merrill Lynch, Shearman & Sterling and Holme Roberts & Owen LLP, Key's counsel. Thereafter, the parties and their respective advisors commenced the negotiation of the definitive transaction agreements.

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        On November 28, 2001, Mr. Merelli, Mr. Korus, several other representatives of Key's management and technical teams and representatives of Merrill Lynch met with representatives of Helmerich & Payne's management and representatives of Petrie Parkman to review the proved reserves and other assets of the Cimarex business. On the following day, the same parties met to review Key's proved reserves and other assets and business operations. During the months of December 2001 and January 2002, the parties and their respective advisors continued reciprocal business, engineering, tax, accounting and legal due diligence.

        On December 6, 2001, the board of directors of Key held a special meeting at which Key's management, together with Merrill Lynch and Key's legal advisors, updated the Key board of directors on the proposed transaction and related matters, including the strategic and business considerations relating to the transaction, the ongoing diligence review and the status of discussions between the parties. Following discussion, the Key board of directors authorized Key's management and its advisors to continue discussions regarding the proposed transaction. The Key board of directors also directed Mr. Korus to continue previously initiated discussions with A.G. Edwards & Sons, Inc. regarding the possible adoption of a shareholder rights plan.

        On December 12, 2001, representatives of Merrill Lynch met with representatives of Petrie Parkman to discuss the status of the due diligence reviews and the potential terms of the proposed transaction. Merrill Lynch and Petrie Parkman also held preliminary discussions regarding the exchange ratio for the business combination.

        On January 4, 2002, representatives of Merrill Lynch and representatives of Petrie Parkman held additional discussions regarding the financial and legal terms of the proposed transaction.

        Throughout the month of January 2002, the parties and their respective financial and legal advisors continued their due diligence review and valuation analyses, as well as the negotiation of the terms of the definitive transaction agreements. On February 11 and 12, 2002, Mr. Helmerich and Mr. Merelli held discussions regarding the exchange ratio for the business combination. On February 15, 2002, Mr. Helmerich and Mr. Merelli agreed that, subject to the satisfactory resolution of all remaining issues, a mutually acceptable merger transaction would contemplate the stockholders of Helmerich & Payne owning approximately 65.25% of Cimarex and the stockholders of Key owning approximately 34.75% of Cimarex, each on a diluted basis. Thereafter, negotiations with respect to the financial and legal terms of the transaction and the terms of the transaction documents continued.

        On February 21, 2002, the board of directors of Key held a special meeting to review the proposed transaction. At the meeting, Key's management, together with Key's financial and legal advisors, apprised the Key board of directors of the status of discussions and reviewed the proposed terms of the transaction. Holme Roberts & Owen and KPMG LLP, Key's accountants, discussed the results of their diligence review, and Shearman & Sterling reviewed the terms of the transaction agreements to be entered into by the parties. Merrill Lynch made a presentation regarding the financial terms of the transaction and its valuation analyses of Key and the Cimarex business. In addition, Holme Roberts & Owen and A.G. Edwards made a presentation to the Key board of directors regarding a proposed shareholder rights plan. After extended discussion among the directors, the meeting was adjourned until February 23, 2002.

        On February 23, 2002, the Key board of directors reconvened its special meeting. At that meeting, Merrill Lynch delivered to the board its written opinion that, as of that date, based upon and subject to the factors and assumptions set forth in the Merrill Lynch fairness opinion and taking into account the shares of Cimarex common stock to be distributed to Helmerich & Payne stockholders in the spin-off, the exchange ratio in the merger was fair from a financial point of view to the holders of Key common stock. Following discussion, the Key board of directors determined that the merger and the other transactions contemplated by the merger agreement are fair and in the best interests of Key and its stockholders and declared the merger agreement advisable. The Key board of directors approved the

22

merger and the merger agreement and resolved to recommend that Key's stockholders vote to adopt the merger agreement. The Key board of directors also adopted the shareholders' rights plan. Shortly after the Key board of directors meeting, the merger agreement and ancillary documents were executed by all of the parties to each of the agreements.

        On February 25, 2002, Helmerich & Payne and Key issued a joint press release announcing the approval of the transaction by their respective boards of directors and the execution of the merger agreement.

Reasons for the Merger; Recommendation of the Key Board of Directors

        At its February 23, 2002 meeting, the Key board of directors determined the merger was fair to and in the best interests of Key and its stockholders and the merger agreement is advisable. The Key board of directors unanimously approved the merger agreement and resolved to recommend the adoption of the merger agreement by the Key stockholders.

        In reaching its decision, the Key board of directors considered a number of factors, including the following:

•

the Cimarex business has an inventory of exploratory prospects located inland along the Texas and Louisiana Gulf Coast, where Key has recently been attempting to expand its exploration efforts;



•

Helmerich & Payne's strategy, like that of Key's, is to focus on exploration, rather than the acquisition, of producing properties;



•

the assets being contributed by Helmerich & Payne to Cimarex will be largely debt-free (other than intercompany debt between Helmerich & Payne and the Cimarex business existing as of September 30, 2001 and arising from operations during the period between September 30, 2001 and the effective time of the merger), resulting in a combined company with greater financial flexibility than Key would have had on a stand-alone basis;



•

the increased size of the combined company could allow it to participate in larger scale exploratory and development drilling projects and acquisition opportunities than would be available to Key on a stand-alone basis;



•

the combined company will benefit from the expertise and extensive experience of the production, operations, engineering and geoscientists currently employed by the Cimarex business;



•

the merger will generate increased market visibility and trading liquidity for the combined company, which could enhance the market valuation of Cimarex common stock relative to Key common stock;



•

the merger will be consummated only if approved by the holders of a majority of the Key common stock;



•

the merger is structured as a tax-free reorganization for U.S. federal income tax purposes and, accordingly, will not be taxable either to Key or its stockholders;



•

the potential financial benefits stemming from the enhanced growth prospects of the combined company outweigh the anticipated direct and indirect costs of the merger;



•

the terms of the merger agreement permit Key to terminate the merger agreement at any time before the Key special meeting to accept a superior proposal, subject to its obligation to comply with certain procedural requirements and to pay a termination fee; and

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•

the opinion, dated February 23, 2002, of Merrill Lynch to the Key board of directors that, as of that date, based upon and subject to the factors and assumptions set forth in the opinion and taking into account the shares of Cimarex common stock to be distributed to Helmerich & Payne stockholders in the spin-off, the exchange ratio in the merger was fair from a financial point of view to the Key stockholders.

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

•

the risk that there may be difficulties in combining the business of Key and the Cimarex business;



•

the risk that the potential benefits sought in the merger might not be fully realized;



•

the risk that the merger might not be completed, including the risk that a private letter ruling from the Internal Revenue Service to the effect that the spin-off will generally be tax-free to Helmerich & Payne and its stockholders might not be obtained;



•

the fact that under the merger agreement, Key could be required to pay Helmerich & Payne a termination fee in certain circumstances; and



•

the other matters described under "Risk Factors" on page    .

        In the judgment of the Key board of directors, the potential benefits of the merger outweigh the risks and the potential disadvantages. In view of the variety of factors considered in connection with its evaluation of the proposed merger and the terms of the merger agreement, the Key board of directors did not believe it was practicable to quantify or assign relative weights to the factors considered in reaching its conclusion. Rather, the Key board of directors views its recommendation as being based on the totality of the information presented to and considered by it. In addition, individual Key directors may have given different weights to different factors.

        In considering the recommendation of the Key board of directors with respect to the merger, you should be aware that some officers and directors of Key have interests in the merger that may be different from, or in addition to, the interests of Key stockholders generally. The Key board of directors was aware of these interests and considered them in approving the merger and merger agreement. Please refer to "The Key Special Meeting—Interests of Certain Key and Cimarex Directors and Executive Officers in the Merger" beginning on page     for more information about these interests.

Opinion of Key's Financial Advisor

        Key retained Merrill Lynch to act as its financial advisor in connection with the merger of Key and Cimarex. On February 23, 2002, Merrill Lynch rendered to the Key board of directors its written opinion that, as of that date, based upon and subject to the factors and assumptions set forth in the Merrill Lynch fairness opinion and taking into account the shares of Cimarex common stock to be distributed to Helmerich & Payne stockholders in the spin-off, the exchange ratio in the merger was fair from a financial point of view to the holders of Key common stock.

         The full text of the Merrill Lynch fairness opinion, which sets forth the assumptions made, matters considered and qualifications and limitations on the review undertaken by Merrill Lynch, is attached as Annex B to this proxy statement/prospectus and is incorporated in this proxy statement/prospectus by reference. The summary of the Merrill Lynch fairness opinion set forth in this proxy statement/prospectus is qualified in its entirety by reference to the full text of the opinion. Stockholders of Key are urged to read the opinion in its entirety. The Merrill Lynch fairness opinion was provided to Key's board of directors for its information and is directed only to the fairness from a financial point of view of the exchange ratio with respect to holders of Key common stock and does not

24

address the merits of the underlying decision by Key to engage in the merger and does not constitute a recommendation to Key's stockholders as to how the stockholders should vote on the adoption of the merger agreement. Merrill Lynch has not expressed any opinion as to the prices at which shares of Cimarex common stock will trade following the consummation of the merger.

        Merrill Lynch has consented to the use of Annex B, containing the Merrill Lynch fairness opinion, in this proxy statement/prospectus and to the references to Merrill Lynch under the headings "Summary" and "The Merger" in this proxy statement/prospectus. In giving its consent, Merrill Lynch does not admit and hereby disclaims that it comes within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission promulgated thereunder.

        The summary set forth below does not purport to be a complete description of the financial analyses underlying the Merrill Lynch fairness opinion. The preparation of a fairness opinion is a complex and analytical process involving various determinations as to the most appropriate and relevant methods of financial analysis and the application of those methods to the particular circumstances and, therefore, the opinion is not readily susceptible to partial analysis or summary description. In arriving at its opinion, Merrill Lynch did not attribute any particular weight to any analysis or factor considered by it, but, rather, made qualitative judgments as to the significance and relevance of each analysis and factor. Accordingly, Merrill Lynch believes that its analyses must be considered as a whole and that selecting portions of its analyses, without considering all of its analyses, would create an incomplete view of the process underlying the Merrill Lynch fairness opinion.

        In performing its analyses, numerous assumptions were made with respect to industry performance, general business, economic, market and financial conditions and other matters, many of which are beyond the control of Merrill Lynch, Helmerich & Payne, Cimarex, the Cimarex business or Key. Any estimates contained in the analyses performed by Merrill Lynch are not necessarily indicative of actual values or future results, which may be significantly more or less favorable than suggested by the analyses. Additionally, estimates of the value of businesses or capital securities do not purport to be appraisals or to reflect the prices at which the businesses or capital securities might actually be sold. Accordingly, the analyses and estimates are inherently subject to substantial uncertainty. In addition, the delivery of the Merrill Lynch fairness opinion was among several factors taken into consideration by Key's board of directors in making its determination to approve the merger agreement and the other transactions contemplated by the merger agreement. Consequently, the Merrill Lynch analyses described below should not be viewed as determinative of the decision of Key's board of directors or Key's management with respect to the fairness of the exchange ratio in the merger or value of Key.

        In arriving at its opinion, Merrill Lynch, among other things:

(1)

Reviewed certain publicly available business and financial information relating to Key, Helmerich & Payne and the Cimarex business that Merrill Lynch deemed to be relevant;



(2)

Reviewed certain information, including production forecasts of existing hydrocarbon reserves and financial forecasts, relating to the business, earnings, cash flow, assets, liabilities and prospects of Key and the Cimarex business, furnished to Merrill Lynch by Key and Helmerich & Payne, respectively;



(3)

Reviewed the reserve reports and estimated hydrocarbon volumes for the Cimarex business (a) prepared as of September 30, 2001 by Netherland, Sewell & Associates and (b) prepared as of December 31, 2001 by Helmerich & Payne;



(4)

Reviewed the reserve reports and estimated hydrocarbon volumes for Key (a) prepared as of September 30, 2001 by Key and (b) prepared as of December 31, 2001 by Key and audited by Ryder Scott Company, L.P.;

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(5)

Conducted discussions with members of senior management and representatives of Key and Helmerich & Payne concerning the matters described in clauses (1) and (2) above, as well as the business and prospects of Key and Cimarex before and after giving effect to the spin-off and the merger;



(6)

Reviewed the historical market prices, trading activity and valuation multiples for Key common stock and compared them with those of certain publicly traded companies that Merrill Lynch deemed to be relevant;



(7)

Reviewed the results of operations of Key and the Cimarex business and compared them with those of certain publicly traded companies that Merrill Lynch deemed to be relevant;



(8)

Compared the proposed financial terms of the merger with the financial terms of certain other transactions that Merrill Lynch deemed to be relevant;



(9)

Participated in certain discussions and negotiations among representatives of Key, Helmerich & Payne and the Cimarex business and their financial and legal advisors;



(10)

Reviewed the potential pro forma impact of the merger on Key and Cimarex;



(11)

Reviewed a draft dated February 22, 2002 of the merger agreement;



(12)

Reviewed drafts dated February 22, 2002 of a distribution agreement between Helmerich & Payne and Cimarex and a tax sharing agreement between Cimarex and its affiliates and Helmerich & Payne and its affiliates, in each case related to the merger agreement; and



(13)

Reviewed such other financial studies and analyses and took into account such other matters as Merrill Lynch deemed necessary, including our assessment of general economic, market and monetary conditions.

        In preparing its opinion, Merrill Lynch assumed and relied on the accuracy and completeness of all information supplied or otherwise made available to Merrill Lynch by Key and Helmerich & Payne, discussed with or reviewed by or for Merrill Lynch, or publicly available. Merrill Lynch did not assume any responsibility for independently verifying such information or undertake an independent evaluation or appraisal of any of the assets or liabilities of Key, Helmerich & Payne, or Cimarex and was not furnished with any such evaluation or appraisal, other than the reserve reports referred to above in clauses (3) and (4). In addition, Merrill Lynch did not assume any obligation to conduct any physical inspection of the properties or facilities of Key, Helmerich & Payne or Cimarex. With respect to the reserve reports, hydrocarbon production forecasts and financial forecast information furnished to or discussed with Merrill Lynch by Key or Helmerich & Payne, Merrill Lynch assumed that they had been reasonably prepared and reflected the best available estimates and judgment of Key's or Helmerich & Payne's management as to the expected future financial performance of Key, Helmerich & Payne, the Cimarex business or Cimarex, as the case may be, and their respective petroleum engineers as to their respective reserves, their future hydrocarbon production volumes and associated costs. Merrill Lynch assumed that the spin-off will be tax-free to Key, Helmerich & Payne, Cimarex and their respective stockholders. Merrill Lynch further assumed that the merger will qualify as a tax-free reorganization for U.S. federal income tax purposes. Merrill Lynch also assumed that the final form of each of the merger agreement, the distribution agreement and the tax sharing agreement would be substantially similar to the last draft reviewed by it.

        Merrill Lynch's opinion was necessarily based upon market, economic and other conditions as they existed and could be evaluated on and on the information made available to Merrill Lynch as of the date of its opinion.

        The following is a brief summary of the analyses performed by Merrill Lynch and presented to the Key board of directors in connection with the Merrill Lynch fairness opinion. Some of these summaries

26

of financial analyses include information presented in tabular format. In order to understand fully the financial analyses used by Merrill Lynch, the tables must be read together with the text of each summary. The tables alone do not constitute a complete description of the financial analyses. Considering the data set forth in the tables without considering the full narrative description of the financial analyses, including the methods and assumptions underlying the analyses, could create a misleading or incomplete view of the financial analyses performed by Merrill Lynch.

Financial and Production Forecasts

        Key and Helmerich & Payne provided Merrill Lynch with forecasts of hydrocarbon production from existing proved oil and gas reserves and related financial performance of both Key and the Cimarex business based upon two uniform commodity price scenarios: (i) the November 19, 2001 price scenario based on commodity strip prices of oil and natural gas as of November 19, 2001 and (ii) the February 19, 2002 price scenario based on commodity spot prices from January 1, 2002 to February 19, 2002 and commodity strip prices of oil and natural gas as of February 19, 2002. Both oil price forecasts were based on the price per barrel (Bbl) for West Texas Intermediate crude and the natural gas price forecasts were based on NYMEX (Henry Hub, Louisiana delivery) natural gas prices per million British Thermal Units (MMBTU). Natural gas pricing assumed a standard heating value of one MMBTU per 1,000 cubic feet (Mcf) of natural gas. Adjustments were made by Merrill Lynch to both the crude oil and natural gas price forecasts to reflect location and quality differentials. The following table lists the assumed unadjusted crude oil and natural gas strip prices as of November 19, 2001 and February 19, 2001.

        Key and Helmerich & Payne supplied reserve reports as of two dates: September 30, 2001 and December 31, 2001. Key's and the Cimarex business' September 30, 2001 reserve reports were prepared by Key petroleum engineers and Netherland, Sewell & Associates, respectively, and were adjusted to the November 19, 2001 price scenario by the respective managements. Key's December 31, 2001 reserve report was prepared by Key petroleum engineers and audited by Ryder Scott Company, L.P. and the Cimarex business' December 31, 2001 reserve report was prepared by Helmerich & Payne petroleum engineers. Both Key's and the Cimarex business' December 31, 2001 reserve reports were prepared using flat commodity market prices as of year-end 2001 and then-current operating costs, production and ad valorem taxes and future development costs on an unescalated basis. Oil and gas prices for the December 31, 2001 reserve reports were subsequently adjusted to the February 19, 2002 price scenario by Merrill Lynch. Operating expenses and capital expenditures necessary to lift and produce the proved reserves estimated in each of the reserve reports were based on current market conditions.

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        Forecast financial and production data for Key and the Cimarex business were supplied by Key and Helmerich & Payne managements, respectively, and adjusted to the February 19, 2002 price scenario by Merrill Lynch. Merrill Lynch also made certain adjustments to the assumptions underlying the Cimarex business' 2002 financial and production forecast to make the forecast more comparable to Key's 2002 financial and production forecast. Financial data as of September 30, 2001 and December 31, 2001 and for the twelve months ended September 30, 2001 for Key and the Cimarex business were supplied by Key and Helmerich & Payne managements, respectively. Oil and gas production data for the twelve months and the quarter ended December 31, 2001 for Key and the Cimarex business were supplied by the managements of Key and Helmerich & Payne, respectively.

Net Asset Value Analysis

        Using a discounted cash flow analysis, Merrill Lynch calculated the present value of future cash flows that Key and the Cimarex business could be expected to generate from their existing base of total proved reserves and from their existing base of proved developed reserves (the existing base of proved developed reserves excludes the proved undeveloped reserve classification), based on Key, and the Cimarex business, reserve reports. These cash flows were discounted at 10% and at 12%. Merrill Lynch estimated the Cimarex business' net asset values by adding (i) the discounted cash flows generated by these reserves to, (ii) a range of value determined by Key's management for Helmerich & Payne Energy Services, Inc. (HPESI), (iii) an amount determined by Key's management for working capital and debt and (iv) in the upside adjusted case, the reported book value of the Cimarex business' undeveloped acreage and seismic data. Merrill Lynch estimated Key's net asset values by adding (i) the discounted cash flows generated by these reserves to, (ii) an amount determined by Key's management for working capital and debt and (iii) in the upside adjusted case, the reported book value of undeveloped acreage and seismic data.

        For each of the separate reserve classes that comprise the proved reserves (proved developed producing, proved developed non-producing and proved undeveloped), net cash flow was forecast by Key, generally through December 2020, and discounted back to January 2002. This net cash flow includes all the taxes, capital expenditures and abandonment costs associated with the produced reserves. The cases and results of this analysis are set forth below.

        Based upon the net asset value analysis performed by Merrill Lynch, an implied Key equity market value contribution of 34.0% to 36.5% was derived, yielding an implied exchange ratio of 0.967 to 1.081.

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Contribution Analysis

        Using the financial and operational data described previously for both the Cimarex business and Key for the years ended September 30, 2001 (actual) and December 31, 2002 (projected), Merrill Lynch compared the relative levels of cash flow from operations (CFFO) and earnings before interest, taxes, depreciation, amortization and exploration expense (EBITDE) for each company during this period, as well as the relative level of proved reserves as of September 30, 2001 and December 31, 2001 and actual production levels for the year ending December 31, 2001 and the quarter ending December 31, 2001.

        Relative levels of CFFO, EBITDE, proved reserves and production were used to develop implied equity market value contributions after having been, when applicable, leverage adjusted for debt as of December 31, 2001. The results of these analyses are set forth below.

        The financial metrics contribution analysis implied a Key equity market value contribution of 32.5% to 37.5% or an implied exchange ratio of 0.902 to 1.129. The operational metrics contribution analysis implied a Key equity market value contribution of 33.0% to 35.5% or an implied exchange ratio of 0.924 to 1.034.

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        The selected companies were chosen because they are publicly traded companies with financial and operating characteristics which Merrill Lynch deemed to be similar to those of Key and the Cimarex business, including, among other things, equity market capitalization and natural gas bias. Merrill Lynch calculated various financial ratios for the selected companies and subsequently selected relevant trading ranges for Key and the Cimarex business. The ratios for the selected companies were based on publicly available information, including estimates provided by Merrill Lynch research and other street research. Merrill Lynch calculated the following financial ratios:

•

equity market value multiples of:



•

2002 estimated CFFO; and




•

enterprise value (defined as market value of common equity plus book value of debt,            preferred stock, minority interest less cash) multiples of:



•

2001 estimated EBITDE,



•

2002 estimated EBITDE,



•

proved reserves as of December 31, 2000; and



•

2002 estimated daily production.

        The following table shows the mean and median calculations for the selected companies, as well as the relevant ranges for Key and the Cimarex business.

        From the enterprise value ranges implied by these multiple ranges, Merrill Lynch determined a composite enterprise value range for the Cimarex business under this method of $400 million to $500 million, as well as an equity market value range of $400 million to $500 million. Merrill Lynch determined a composite enterprise value range for Key under this method of $225 million to $300 million or an equity market value range of $200 million to $275 million. The comparable company trading analysis implied a Key equity market value contribution of 32.0% to 34.5% or an implied exchange ratio of 0.882 to 0.989.

        None of the selected companies is identical to Key or the Cimarex business. Accordingly, an analysis of the results of the foregoing is not purely mathematical. Rather, it involves complex considerations and judgments concerning differences in financial and operating characteristics of the selected companies and other factors that could affect the public trading value of the comparable companies or company to which they are being compared.

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Summary

        The following table is a summary of Key's implied ownership percentage in the new company and the implied exchange ratio derived from the net asset value analysis, contribution analysis and the comparable company trading analysis. This table should be read together with the more detailed descriptions set forth above. In particular, in applying the various valuation methodologies to the particular businesses and operations of Key and the Cimarex business and the particular circumstances of the merger between Key and Cimarex, Merrill Lynch made qualitative judgments as to the significance and relevance of each analysis. Accordingly, the methodologies and the implied exchange ratios derived therefrom set forth in the table must be considered as a whole and in the context of the narrative description of the financial analyses, including the assumptions underlying these analyses. Considering the implied exchange ratios set forth in the table without considering the full narrative description of the financial analyses, including the assumptions underlying these analyses, could create a misleading or incomplete view of the process underlying and conclusions represented by Merrill Lynch's opinion.

Merrill Lynch Financial Advisor Fee

        Pursuant to an engagement letter dated January 16, 2001, Key retained Merrill Lynch to act as its financial advisor in connection with the merger. Pursuant to the engagement letter, Key has agreed to pay Merrill Lynch a fee of $50,000 as of the date of the engagement letter and an additional fee of $2,950,000, payable upon closing for services rendered in connection with the merger. Additionally, in the event that Key (including any Key affiliate) receives or becomes entitled to receive any fee or reimbursement of expenses resulting from, or as a result of, the termination of the merger agreement, Key has agreed to pay Merrill Lynch 25% of such fee or reimbursement of expenses, subject to a maximum payment of $2,000,000, payable immediately in cash following receipt by Key of such fee or reimbursement of expenses.

        Key also has agreed to reimburse Merrill Lynch for the expenses reasonably incurred by Merrill Lynch entering into and performing services in connection with its engagement (including reasonable counsel fees) and to indemnify Merrill Lynch and its affiliates and their respective officers, directors, employees, agents and controlling persons against certain expenses, losses, claims, damages or liabilities in connection with its services performed in connection with its engagement, including liabilities under federal securities laws.

        Key retained Merrill Lynch based upon Merrill Lynch's experience and expertise. Merrill Lynch is an internationally recognized investment banking and advisory firm. Merrill Lynch, as part of its investment banking business, is continuously engaged in the valuation of businesses and securities in connection with mergers and acquisitions, negotiated underwritings, competitive biddings, secondary distributions of listed and unlisted securities, private placements and valuations for corporate and other purposes. Merrill Lynch may, in the future, provide financial advisory and financing services to Key, Helmerich & Payne and Cimarex and/or their affiliates and may receive fees for the rendering of such services. In addition, in the ordinary course of our business, Merrill Lynch may actively trade shares of

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Key common stock and other securities of Key, shares of Helmerich & Payne common stock and other securities of Helmerich & Payne, as well as the common stock of Cimarex and other securities of Cimarex, for its own account and for the accounts of customers and, accordingly, may at any time hold a long or short position in such securities.

Stock Exchange Listing

        Cimarex will apply to list its common stock on the New York Stock Exchange.

Material United States Federal Income Tax Consequences of the Merger

        The following accurately describes the opinion of Shearman & Sterling, counsel to Key, subject to the qualifications set forth below and contained herein, as to the material United States federal income tax consequences of the merger to Key and the Key stockholders. Such opinion is not binding on the Internal Revenue Service or any court. It is assumed for purposes of this discussion that the shares of Key common stock are held as "capital assets" within the meaning of section 1221 of the Internal Revenue Code. The tax consequences to each Key stockholder will depend in part upon such stockholder's particular situation. Special tax consequences not described herein may be applicable to particular classes of taxpayers, such as financial institutions, insurance companies, tax-exempt organizations, broker-dealers, traders in securities that elect to mark to market, persons that hold Key common stock as part of a straddle or conversion transaction, persons who are not citizens or residents of the United States and stockholders who acquired their shares of Key common stock through the exercise of an employee stock option or otherwise as compensation. The following disclosure is for general information only and is based upon the Internal Revenue Code, its legislative history, existing and proposed regulations thereunder, published rulings and decisions, all as currently in effect as of the date hereof, and all of which are subject to change, possibly with retroactive effect. Tax consequences under state, local and foreign laws are not addressed herein. Key stockholders are urged to consult with their own tax advisors as to the specific tax consequences to them of the merger and the spin-off, including the application and effect of federal, state, local and foreign income and other tax laws.

        It is a condition to the consummation of the merger that (i) Key receive an opinion from Shearman & Sterling, counsel to Key, dated as of the effective date of the merger, to the effect that the merger will qualify as a reorganization within the meaning of section 368(a) of the Internal Revenue Code and (ii) Helmerich & Payne receive an opinion from Skadden, Arps, Slate, Meagher & Flom LLP, counsel to Helmerich & Payne, dated as of the effective date of the merger, to the effect that the merger will qualify as a reorganization within the meaning of section 368(a) of the Internal Revenue Code. The conditions relating to the receipt of the tax opinions may not be waived by Key or Helmerich & Payne after receipt of the Key stockholder approval unless further stockholder approval is obtained with appropriate disclosure. The opinions will be based on customary assumptions and customary representations made by, among others, Key, Helmerich & Payne, Cimarex and Mountain Acquisition Co. An opinion of counsel represents counsel's best legal judgment and is not binding on the Internal Revenue Service or any court. Accordingly, there can be no assurance that the Internal Revenue Service will agree with the conclusions set forth in the opinion letter, and it is possible that the Internal Revenue Service or another tax authority could assert a position contrary to one or all of those conclusions and that a court could sustain that contrary position.

        In addition, the merger is conditioned on Helmerich & Payne's receipt of a letter ruling from the Internal Revenue Service to the effect that the spin-off will be tax-free to Helmerich & Payne and its stockholders for U.S. federal income tax purposes by reason of its qualification under sections 355 and 368 of the Internal Revenue Code. See "The Merger Agreement—Conditions to the Completion of the Merger" beginning on page    . The letter ruling, while generally binding on the Internal Revenue Service, will be based on certain factual representations and assumptions described in the ruling and set forth in the ruling request. If any assumptions or representations are incorrect or untrue in any

32

material respect, the letter ruling may be rendered invalid with retroactive effect. Under the Tax Sharing Agreement, all, or a portion, of any tax liability resulting from the spin-off being taxable to Helmerich & Payne may be allocated to Cimarex to the extent resulting from certain actions or omissions by Cimarex or its stockholders. If the liability for any such tax is incurred by Cimarex, the amount of such tax would be expected to be material.

        Assuming the merger qualifies as a reorganization within the meaning of section 368(a) of the Internal Revenue Code, as described above, holders of Key common stock who exchange their Key common stock solely for Cimarex common stock in the merger will not recognize gain or loss for United States federal income tax purposes, except with respect to cash, if any, they receive in lieu of a fractional share of Cimarex common stock. Each holder's aggregate tax basis in the Cimarex common stock received in the merger will be the same as his or her aggregate tax basis in the Key common stock surrendered in the merger, decreased by the amount of any tax basis allocable to any fractional share interest for which cash is received. The holding period of the Cimarex common stock received in the merger by a holder of Key common stock will include the holding period of Key common stock that he or she surrendered in the merger.

        A holder of Key common stock who receives cash in lieu of a fractional share of Cimarex common stock will recognize gain or loss equal to the difference between the amount of cash received and his or her tax basis in the Cimarex common stock that is allocable to the fractional share. That gain or loss generally will constitute capital gain or loss. In the case of an individual stockholder, any such capital gain generally will be subject to a maximum United States federal income tax rate of 20% if the individual has held his or her Key common stock for more than 12 months on the date of the merger. The deductibility of capital losses is subject to limitations for both individuals and corporations.

         The preceding discussion does not purport to be a complete analysis or discussion of all potential tax consequences relevant to the merger. Thus, Key stockholders are urged to consult their own tax advisors as to the specific tax consequences to them of the merger, including tax return reporting requirements, the applicability and effect of federal, state, local, and other applicable tax laws and the effect of any proposed changes in the tax laws.

Federal Securities Laws Consequences of the Merger

        The merger agreement requires Key to use all commercially reasonable efforts to cause each of its affiliates to execute a written agreement, substantially in the form attached as an exhibit to the merger agreement, to the effect that such affiliate will not sell, assign, transfer or otherwise dispose of any of the shares of Cimarex common stock issued to such affiliate in exchange for Key common stock in the merger except:

•

pursuant to an effective registration statement under the Securities Act;



•

in conformity with the volume and other limitations of Rule 145 promulgated under the Securities Act; or



•

in a transaction which, in the opinion of independent counsel reasonably satisfactory to Cimarex or as described in a "no-action" or interpretative letter from the Staff of the SEC, is not required to be registered under the Securities Act.

        All shares of Cimarex common stock received by Key stockholders in the merger will be freely transferable, except that shares of Cimarex common stock received by persons who are deemed to be "affiliates" of Key under the Securities Act may resell such stock only in transactions permitted by Rule 145 under the Securities Act, or as otherwise permitted under the Securities Act. Persons who may be affiliates of Key for those purposes generally include individuals or entities that control, are controlled by, or are under common control with, Key, but would not include stockholders who are not officers, directors or principal stockholders of Key.

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Accounting Treatment

        The acquisition of Key by Cimarex would be accounted for under the purchase method of accounting under U.S. generally accepted accounting principles, with Cimarex treated as the acquiror. As a result, the assets and liabilities of the Cimarex business will be recorded at historical amounts, without adjustment to fair values. The assets and liabilities of Key will be recorded at their estimated fair values at the date of merger with the excess of the purchase price over the net amount of such fair values recorded as goodwill. Upon completion of the merger, Cimarex will change its method of accounting for oil and gas properties to "full cost" from the "successful efforts" method currently utilized. Cimarex also plans to change to a December 31 ^st fiscal year end, as opposed to the September 30 ^th fiscal year end presently used by Helmerich & Payne.

Regulatory Matters

        Helmerich & Payne has sought a ruling from the Internal Revenue Service to the effect that the contribution and transfer of the assets and liabilities of the Cimarex business to Cimarex and the spin-off by Helmerich & Payne of all the shares of Cimarex common stock to the holders of Helmerich & Payne common stock generally will be treated as a tax-free transaction for U.S. federal income tax purposes.

        None of the parties is aware of any other material governmental or regulatory approval required for the completion of the merger, other than the effectiveness of the registration statement of which this proxy statement/prospectus is a part and compliance with applicable corporate law of the State of Delaware.

Appraisal and Dissenters' Rights

        In accordance with the Delaware General Corporation Law, there will be no appraisal rights or dissenters' rights available to holders of Key common stock in connection with the merger.

Key Rights Plan

        In connection with the merger, Key entered into a rights agreement with A.G. Edwards & Sons, Inc., as the rights agent. On February 23, 2002, the Key board of directors declared a dividend of one common share purchase right for each outstanding share of Key common stock. The dividend was paid on March 7, 2002 to the stockholders of record on the close of business on that date.

        The rights are not exercisable until the distribution date, which will generally occur upon the earlier of (i) ten business days following (a) a public announcement that a person or group of affiliated or associated persons has acquired beneficial ownership of 15% or more of the outstanding shares of Key common stock (subject to certain exceptions) or (b) an earlier date when a majority of the Key board of directors becomes aware of the existence of such acquiring person and (ii) ten business days following the commencement of, or announcement of an intention to commence, a tender offer or exchange offer, the consummation of which would result in the beneficial ownership by a person or group of 15% or more of the outstanding shares of common stock (subject to certain exceptions). The rights agreement provides that the spin-off and the merger will not trigger a distribution date; and that none of Helmerich & Payne, Cimarex, or Mountain Acquisition will be deemed to be an acquiring person under the rights agreement. A copy of the rights agreement is incorporated by reference to this proxy statement/prospectus.

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THE KEY SPECIAL MEETING

Purpose, Time and Place

        The Key special meeting will be held on [                        ], 2002 at [            ], Mountain Daylight Time, at [                                    ]. The purpose of the Key special meeting is to consider and vote upon a proposal to approve the merger of Mountain Acquisition with and into Key, with Key being the surviving corporation in the merger and becoming a wholly owned subsidiary of Cimarex, by adopting the merger agreement, a copy of which is attached as Annex A to this proxy statement/prospectus, and to consider any other matters that may properly come before the meeting. We currently expect that no other matters will be considered at the Key special meeting.

Recommendation of the Key Board of Directors

        The Key board of directors has determined the merger is fair and in the best interests of Key and its stockholders and the merger agreement is advisable. The Key board of directors has unanimously approved the merger agreement and recommends that the Key stockholders vote " for " the adoption of the merger agreement.

Record Date; Stock Entitled to Vote; Quorum

        Stockholders of record of Key common stock at the close of business on [    ], 2002, the record date for the Key special meeting, are entitled to receive notice of and have the right to vote at, the Key special meeting and any adjournment or postponement of the meeting. On the record date, approximately [    ] shares of Key common stock were issued and outstanding. Stockholders of record of shares of Key common stock on the record date are each entitled to one vote per share on the adoption of the merger agreement.

        A quorum of stockholders is necessary to have a valid meeting of stockholders. A majority of the voting power of the outstanding shares of Key common stock entitled to vote at the meeting must be present in person or by proxy at the Key special meeting in order for a quorum to be established.

        Abstentions and broker "non-votes" count as present for establishing a quorum. A broker "non-vote" occurs on an item when a broker is not permitted to vote on that item without instructions from the beneficial owner of the shares and no instructions are given. We expect, in the event that a quorum is not present at the Key special meeting, that the meeting will be adjourned or postponed to solicit additional proxies.

Votes Required

        Adoption of the merger agreement will require the affirmative vote of the holders of a majority of the shares of Key common stock outstanding on the record date. For purposes of the vote on the merger proposal, abstentions will be counted and have the same effect as a vote "against" the proposal. In addition, failing to vote will have the same effect as a vote "against" the proposal.

        Under the applicable rules of the New York Stock Exchange, a bank, broker or other fiduciary who holds shares for customers who are the beneficial owners of those shares is prohibited from giving a proxy to vote those customers' shares with respect to the proposal to be voted on at the Key special meeting in the absence of specific instructions from the customer. Shares held by a broker, bank or other fiduciary which are not voted because the customer has not provided instructions to the broker, bank or other fiduciary will have the same effect as a vote "against" the proposal.

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Voting by Proxy

Submitting Proxies

        Stockholders of record may vote their stock by attending the Key special meeting and voting their stock in person at the meeting, by completing the enclosed proxy card, signing and dating it and mailing it in the enclosed postage pre-paid envelope or by voting via telephone or via the Internet by following the instructions provided on the enclosed proxy card. If a proxy card is signed by a stockholder of record and returned without specific voting instructions, the stock represented by the proxy will be voted " for " the proposal presented at the Key special meeting.

        Stockholders whose shares of Key common stock are held in the name of a bank, broker or other fiduciary must either direct the record holder of their shares of Key common stock as to how to vote their shares of Key common stock or obtain a proxy from the record holder to vote at the Key special meeting.

Revoking Proxies

        Stockholders of record may revoke their proxies at any time prior to the time their proxies are voted at the Key special meeting. Stockholders can revoke their proxies and change their votes by:

•

completing, signing and dating a new proxy card and returning it by mail to the proxy solicitor so that it is received prior to the special meeting;



•

voting via telephone or via the Internet by following the instructions provided on your proxy card;



•

sending a written notice to the Assistant Corporate Secretary of Key that is received prior to the special meeting stating that you revoke your proxy; or



•

obtaining an admission card, attending the special meeting and voting in person or by legal proxy, if appropriate.

        If your shares of Key common stock are held in the name of a bank, broker or other fiduciary and you have directed such person(s) to vote your shares of Key common stock, you should instruct such person(s) to change your vote or obtain a legal proxy to do so yourself.

        Any written notice of a revocation of a proxy should be sent to the following address:

Key Production Company, Inc.
Attention: Sharon M. Pope, Assistant Corporate Secretary
707 Seventeenth Street, Suite 3300
Denver, Colorado 80202
Facsimile: (303) 295-3494

Other Business; Adjournments

        Key is not aware of any other business to be acted upon at the Key special meeting. If, however, other matters are properly brought before the Key special meeting or any adjourned meeting, your proxies will have discretion to act on those matters or to adjourn the meeting, according to their best judgment.

Proxy Solicitation

        The cost of solicitation of proxies from stockholders will be paid by Key. In addition to solicitation by mail, the directors, officers and employees of Key may also solicit proxies from stockholders by telephone, facsimile or in person. Key will also make arrangements with brokerage houses and other

36

custodians, nominees and fiduciaries to send the proxy materials to beneficial owners. Upon request, we will reimburse those brokerage houses and custodians for their reasonable expenses in so doing.

        Key has retained The Corporate Governance Group of Strategic Stock Surveillance, LLC to provide advice and to aid with the solicitation of proxies from Key stockholders for the Key special meeting. Strategic Stock Surveillance, LLC will receive customary fees as compensation for its services and reimbursement for its related out-of-pocket expenses.

         Do not send any stock certificate(s) with your proxy cards. After the merger is completed, the exchange agent to be selected by Cimarex will send transmittal forms with instructions for the surrender of stock certificate(s) representing shares of Key common stock to former Key stockholders.

Interests of Certain Key and Cimarex Directors and Executive Officers in the Merger

        In considering the recommendation of the Key board of directors to vote for the proposal to adopt the merger agreement, stockholders of Key should be aware that members of the Key and Cimarex board of directors and executive officers of Key and Cimarex have agreements and arrangements that provide them with interests in the merger that differ from, or are in addition to, those of Key stockholders. The Key board of directors was aware of these agreements and arrangements during its deliberations of the merits of the merger and in determining to recommend to the stockholders of Key that they vote for the proposal to adopt the merger agreement. These agreements and arrangements can be summarized as follows:

        Governance Structure.     Under the terms of the merger agreement, the board of directors of Cimarex after completion of the merger will be comprised of nine individuals, four of whom are current directors of Key and one of whom is currently a director of Helmerich & Payne and Cimarex. The remaining four directors will be selected by Helmerich & Payne and will be identified in an amendment to this proxy statement/prospectus.

        Key Employee and Director Stock Options.     At the effective time of the merger, each outstanding Key stock option will be assumed by Cimarex and will be deemed to constitute an option to acquire Cimarex common stock on the same terms and conditions as had been applicable under the historical Key stock option plans. In general, a former Key option holder will be entitled, on exercise of his or her stock option, to purchase that number of shares of Cimarex common stock that could have been purchased under the Key stock option, at an equivalent exercise price per share. Each outstanding option under the Key stock option plans will vest in full upon the effective time of the merger and remain outstanding in accordance with its original terms.

        Helmerich & Payne Stock Options.     At the time of the spin-off, Helmerich & Payne stock options held by Cimarex employees who are former Helmerich & Payne employees (including Steven R. Shaw and other executive officers of Cimarex) will be converted into Cimarex stock options, using a conversion ratio based on a comparison of the trading price of Helmerich & Payne common stock for the five trading days immediately prior to the fifth trading date immediately prior to the record date for the spin-off with the trading price of Cimarex common stock for the first five trading days following the spin-off.

        Directors' and Officers' Indemnification.     From and after the effective time of the merger, Cimarex will indemnify any persons who are or were officers or directors of Key prior to the effective time of the merger for losses in connection with any action arising out of or pertaining to acts or omissions, or alleged acts or omissions, by them in their capacities as such, whether commenced, asserted or claimed before or after the effective time of the merger. Cimarex will maintain existing, or provide comparable, directors' and officers' liability insurance policies for a period of six years following the effective time of the merger.

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Employment and Severance Agreements and Other Compensation

        Income Continuance Plan.     Each Key executive officer (including Key executive officers who are party to agreements described below) participates in the Key Income Continuance Plan under which each officer is entitled to receive 24 months continuation of his base compensation (as defined in the plan) and health and life insurance benefits if his employment is terminated without cause or he resigns as a result of changed circumstances following the effective time of the merger. The plan may not be terminated during the 30-month period following the effective time of the merger. See "Management of Cimarex—Employment Agreements and Change in Control Agreements with Named Executive Officers" on page    .

Agreements with Employees

        F.H. Merelli.     Key has an employment agreement with F.H. Merelli, the chairman, president and chief executive officer of Key and, following the merger, Cimarex. The agreement will be assumed by Cimarex following the merger. Under the agreement, upon (i) Mr. Merelli's termination of employment without cause or due to his death or disability or (ii) Mr. Merelli's resignation for good reason (each as defined in the agreement), he is entitled to receive his base salary for two years and the maximum incentive compensation payable pursuant to any plan or program established by Cimarex. Mr. Merelli also participates in the Income Continuance Plan. Any payment made to Mr. Merelli pursuant to his employment agreement will be deducted from the benefits to which he is otherwise entitled pursuant to the Income Continuance Plan. See "Management of Cimarex—Employment Agreements and Change in Control Arrangements with Named Executive Officers" on page    .

        Steven R. Shaw.     Mr. Shaw, who will become executive vice president of Cimarex following the spin-off, is party to an agreement with Helmerich & Payne that will be assumed by Cimarex upon the spin-off. The agreement provides that, if Mr. Shaw is terminated by Cimarex within 24 months after the spin-off other than for cause, disability, death or the occurrence of a substantial downturn, or if Mr. Shaw terminates his employment for good reason within 24 months after the spin-off (as such terms are defined in the agreement), (i) Cimarex will pay Mr. Shaw a pro-rated annual bonus and an additional lump-sum payment equal to two times his base salary and annual bonus, (ii) any options granted to Mr. Shaw will become fully exercisable and any restricted stock granted to Mr. Shaw will become nonforfeitable, and (iii) Cimarex will provide Mr. Shaw with 24 months of continued health insurance. In addition, if Mr. Shaw voluntarily terminates his employment during the 30-day period beginning 18 months after the spin-off, any options granted to Mr. Shaw before the spin-off will become fully exercisable and any restricted stock granted to Mr. Shaw before the effective time of the merger will become nonforfeitable. See "Management of Cimarex—Employment Agreements and Change in Control Agreements with Named Executive Officers" on page    .

        Messrs. Korus, Jorden, Albi and Bell.     Cimarex will also assume the employment agreements with Paul Korus, Thomas E. Jorden, Joseph R. Albi and Stephen P. Bell, who will become executive officers of Cimarex following the merger. These agreements provide for a lump-sum payment equal to two times the executive's base salary at the time of the merger if the executive is terminated without cause at any time following the merger (as defined in the agreement). Any payments made to these executives pursuant to these agreements will be deducted from the benefits to which the executive is otherwise entitled pursuant to the Income Continuance Plan described above. See "Management of Cimarex—Employment Agreements and Change in Control Agreements with Named Executive Officers" on page    .

        Messrs. Nagel, Burau and McLaughlin.     Each of Messrs. Nagel, Burau and McLaughlin, who will become vice presidents of Cimarex following the spin-off, is party to an agreement with Helmerich & Payne that will be assumed by Cimarex. The agreement provides that, if the executive is terminated by Cimarex within 24 months after the spin-off other than for cause, disability, death or the occurrence of

38

a substantial downturn, or if the executive terminates his employment for good reason within 24 months after the spin-off (as such terms are defined in the agreement), Cimarex will pay the executive a pro-rated annual bonus and an additional lump-sum payment equal to two times his base salary and annual bonus, any options granted to the executive will become fully exercisable and any restricted stock granted to the executive will become nonforfeitable, and Cimarex will provide the executive with 24 months of continued health insurance.

        Restricted Stock Agreement with Paul Korus.     Mr. Korus is party to a restricted stock agreement pursuant to which he received 10,000 restricted shares of Key common stock. At the effective time of the merger, the restricted shares of Key common stock will automatically convert into restricted shares of Cimarex common stock, and Cimarex will assume the restricted stock agreement. The restricted shares generally vest in full on September 20, 2002; however, if Mr. Korus' employment is terminated without cause or he resigns for good reason (each as defined in the agreement) during the six-month period following the effective time of the merger, the restricted shares will immediately vest.

Stock Ownership of Management and Certain Stockholders

        As of the close of business on March 31, 2002, directors and executive officers of Key and their affiliates as a group beneficially owned and were entitled to vote approximately 1,255,181 shares of Key common stock (including options exercisable for shares of common stock), representing approximately 8.44% of the shares of Key common stock outstanding on that date (including shares represented by such options).

        All of the directors and executive officers of Key that are entitled to vote at the Key special meeting have indicated that they intend to vote their shares of Key common stock in favor of adoption of the merger agreement.

Appraisal and Dissenters' Rights

        In accordance with the Delaware General Corporation Law, there will be no appraisal rights or dissenters' rights available to holders of Key common stock in connection with the merger.

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CIMAREX

        Helmerich & Payne, through Cimarex, engages in the following activities:

•

the origination of prospects;



•

the identification, acquisition, exploration and development of prospective and proved oil and gas properties;



•

the production and sale of crude oil, condensate and natural gas; and



•

the marketing of natural gas.

        For simplicity, we refer also in this section to Cimarex, rather than to Helmerich & Payne's oil and gas exploration and production and gas marketing businesses, as though the historical oil and gas exploration and production and gas marketing business of Helmerich & Payne had always been owned by Cimarex. Cimarex considers itself a medium-sized independent exploration and production company.

        Cimarex was incorporated on February 14, 2002 under the laws of the State of Delaware. It was intended that Cimarex would become the successor to the Cimarex business operated by Helmerich & Payne.

        All of Cimarex's oil and gas operations are conducted in the United States. Most of the current exploration and drilling effort is concentrated in Oklahoma, Kansas, Texas and Louisiana. Cimarex also explores from time to time in New Mexico, Alabama, Michigan, Mississippi and the Rocky Mountain area.

        Helmerich & Payne reorganized the Cimarex business in 1995. This action resulted in the consolidation or elimination of several management positions and the formation of geographical exploitation/exploration teams comprised of geological, engineering and land personnel. These personnel primarily develop in-house oil and gas prospects as well as review outside prospects and acquisitions for their respective geographical areas. Cimarex believes that this structure allows each team to gain greater expertise in its respective geographical area and reduces risk in the development of prospects.

        Cimarex has been focusing on developing prospects using 3-D seismic technology. Currently, Cimarex is involved in 3-D seismic surveys covering more than 1,480 square miles, of which approximately 1,180 square miles are proprietary. Approximately 1,100 square miles of land covered by such surveys is located near the Texas and Louisiana onshore Gulf Coast. Cimarex's exploration and development program has covered a range of prospects, from shallow "bread and butter" programs to deep, expensive, high risk/high return wells.

        Cimarex continued its drilling program in Oklahoma, Kansas, west Texas, south Texas and south Louisiana, participating in a total of 123 wells during fiscal 2001. Of the 123-well total, 47 wells were development wells drilled in areas where reserves were previously booked and 29 wells were dry holes. Cimarex increased its development of proved undeveloped reserves in fiscal 2001 as the result of high natural gas prices during the last half of calendar 2000. The focus of this drilling was the Redfork play in western Oklahoma, additional development of Ashland Field in southeastern Oklahoma and the Hugoton Field in Kansas, as well as additional drilling in the panhandle of Texas and in southern Louisiana. Cimarex's participation in these 47 development wells resulted in the addition of approximately 15.7 Bcf of gas and 75,826 Bbls of oil previously classified as proved undeveloped. Of the remaining 76 wells drilled during the year, 40 were exploratory wells, 20 of which were successfully completed. These drilling efforts resulted in new discoveries of approximately 12.8 Bcf of gas and 1,145,195 Bbls of oil and condensate.

        A total of $80,040,769 was spent in Cimarex's exploration and development program during fiscal 2001. This figure includes $7,838,770 of geophysical expense, but is exclusive of expenditures for

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acreage and acquisition of proved oil and gas reserves. Cimarex's total acquisition cost for acreage in fiscal 2001 was $18,611,957. Cimarex also spent $737,500 for the acquisition of proved oil and gas reserves during fiscal 2001. The reserves associated with these acquisitions were 495,888 Mcf of gas and 434 barrels of oil.

        Cimarex's fiscal 2002 exploration and production budget has been reduced to approximately $50 million due to lower product prices, higher service company costs and high-grading of existing prospects in order to reduce finding costs. This is a 47.6% reduction from actual exploration and production expenditures in fiscal 2001.

Market for Oil and Gas

        Cimarex does not refine any of its production. The availability of a ready market for such production depends upon a number of factors, including the availability of other domestic production, price, crude oil imports, the proximity and capacity of oil and gas pipelines and general fluctuations in supply and demand. Cimarex does not anticipate any unusual difficulty in contracting to sell its production of crude oil and natural gas to purchasers and end-users at prevailing market prices and under arrangements that are usual and customary in the industry. Cimarex and its subsidiary, Helmerich & Payne Energy Services, Inc., or HPESI, have successfully developed markets with end-users, local distribution companies and natural gas brokers for gas produced from successful exploratory wells and development wells.

        Substantially all of Cimarex's gas production is sold to and resold by HPESI. During fiscal 2001, the price that Cimarex received for the sale of its natural gas has fluctuated. Cimarex's average per Mcf natural gas sales price in fiscal 2001 for each of the first through fourth quarters was $4.73, $6.49, $4.27 and $2.66, respectively.

        Last year's record high natural gas prices spawned an increase of productive capacity and a dramatic increase in drilling. In the long-term, natural gas prices will be impacted by the following factors:

•

decline in deliverability of domestic supply;



•

increased use of natural gas for electrical generation;



•

a recovery of U.S. economic growth;



•

the increased usage and better management of natural gas storage;



•

seasonal usage;



•

fuel switching;



•

usage of gas as a feed stock; and



•

importation of gas from Canada and Mexico.

        All these factors will continue to influence the cyclical nature of the supply/demand balance and will continue to occur as drilling activity and productive capacity respond to the changing prices.

        Historically, Cimarex has had no long-term sales contracts for its crude oil and condensate production. Cimarex continues its practice of contracting for the sale of its Kansas, Oklahoma and portions of its west Texas crude oil for terms of six to twelve months in an attempt to assure itself of the best price in the area for crude oil production. During fiscal 2001, the price that Cimarex received for the sale of its crude oil has steadily decreased. The average per barrel crude oil sales price received by Cimarex in fiscal 2001 for each of the first through fourth quarters was $31.44, $28.09, $26.12 and $25.33, respectively.

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        Mid-East tensions, disputes among OPEC and non-OPEC countries over production quotas and sluggish economies have created a continued mixed market in crude oil trading.