8-K

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d) OF

THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): December 12, 2011

MARKWEST ENERGY PARTNERS, L.P.

(Exact name of registrant as specified in its charter)

Delaware (State or other jurisdiction of incorporation or organization) 001-31239 (Commission File Number) 27-0005456 (I.R.S. Employer Identification Number)

1515 Arapahoe Street, Tower 1, Suite 1600, Denver CO 80202

(Address of principal executive offices)

Registrant’s telephone number, including area code: 303-925-9200

Not Applicable.

(Former name or former address, if changed since last report.)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

oWritten Communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

oSoliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

oPre-Commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

oPre-Commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

ITEM 8.01.Other Events.

Potential Acquisition of Minority Interest in MarkWest Liberty Midstream

Overview

On December 12, 2011, MarkWest Energy Partners, L.P. (the “Partnership”) executed a non-binding letter of intent (the “LOI”) with affiliates of The Energy & Minerals Group (“EMG”) regarding the potential acquisition (the “MarkWest Liberty Midstream Acquisition”) by the Partnership of EMG’s interest in MarkWest Liberty Midstream & Resources, L.L.C (“MarkWest Liberty Midstream”) held by M&R MWE Liberty, LLC, an affiliate of EMG (“EMG Liberty”).  Under the LOI, the Partnership, EMG and EMG Liberty have agreed to negotiate in good faith towards the execution of mutually acceptable definitive, written agreements containing material terms consistent with those described below.  The Partnership, EMG, and EMG Liberty currently anticipate executing such agreements and closing the MarkWest Liberty Midstream Acquisition on or about December 31, 2011.   The remainder of this Form 8-K describes the provisions of the definitive agreements that the Partnership, EMG and EMG Liberty expect to execute.  There can be no assurance that the Partnership, EMG or EMG Liberty will be able to agree to terms for the potential MarkWest Liberty Midstream Acquisition on the same basis as those described herein or that they will be able to reach any agreements regarding the MarkWest Liberty Midstream Acquisition at all.

In exchange for EMG Liberty’s interest in MarkWest Liberty Midstream, the Partnership would (1) pay to EMG Liberty $1.0 billion in cash and (2) issue to EMG Liberty 19.95 million unregistered units of a new class (“Class B Units”) of the Partnership.  Subject to limited exceptions described below under “Change of Control,” the Class B Units would convert into common units (“Common Units”) of the Partnership on a one-for-one basis (the “Converted Units”) in five equal installments.  The first conversion of 20% of the Class B Units would occur on July 1, 2013 and the subsequent four conversions, each of 20% of the Class B Units, would occur on the first four anniversaries of July 1, 2013.  The Class B Units would not be entitled to participate in any distributions of available cash of the Partnership prior to their conversion into Common Units.  Following the closing of the MarkWest Liberty Midstream Acquisition, the Partnership would own 100% of MarkWest Liberty Midstream.

The Partnership expects the MarkWest Liberty Midstream Acquisition will be accounted for as an equity transaction, and no gain or loss will be recognized through the statement of operations in accordance with Financial Accounting Standards Board Accounting Standards Codification 810, Consolidations — Overall — Changes in Parent’s Ownership Interest in a Subsidiary.  The Partnership’s historical financial statements include 100% of the assets, liabilities, and results of operations of MarkWest Liberty Midstream. Historical net income is not expected to change as a result of the MarkWest Liberty Midstream Acquisition, however, the portion of net income attributable to the Partnership is expected to increase due to the acquisition of the non-controlling interest.  Giving effect to the MarkWest Liberty Midstream Acquisition, for the year ended December 31, 2010, the net income attributable to non-controlling interest of $30.6 million would be reduced to $2.7 million and net income attributable to the Partnership would increase from $0.5 million to $28.4 million.  Giving effect to the MarkWest Liberty Midstream Acquisition, for the nine months ended September 30, 2011, the net income attributable to non-controlling interest of $33.2 million would be reduced to $0.9 million and net income attributable to the Partnership would increase from $134.8 million to $167.1 million.  The Partnership expects that the MarkWest Liberty Midstream Acquisition will also change net income attributable to the Partnership’s common unitholders per Common Unit due to the change in income attributable to the Partnership and the number of Common Units and Class B Units outstanding.

Registration Rights

The LOI provides that the Partnership would provide EMG Liberty with certain registration rights related to the Class B Units following their conversion into Converted Units.  The Partnership would be obligated to use commercially reasonable efforts to (1) file a resale registration statement registering the resale of the Converted Units; (2) have such registration statement declared effective by July 1, 2013 and (3) maintain the effectiveness of such registration statement for a period of up to six years.  Furthermore, following the effectiveness of such registration statement, EMG Liberty would be granted piggyback registration rights permitting EMG Liberty to sell its Converted Units, in an amount representing up to 20% of the total number of Common Units to be offered in any firm commitment, underwritten offering effected by the Partnership (including no more than an equal percentage of any Common Units subject to any over-allotment option requested by the managing underwriters in such an offering).  The LOI further provides that the Partnership would provide EMG Liberty comparable 20% participation and sale rights in the event that the Partnership was to adopt a continuous equity or similar program in the future.

The LOI also provides that, beginning January 1, 2017, EMG Liberty would have the right to demand that the Partnership conduct up to three underwritten offerings of any Converted Units still held by EMG Liberty.  EMG Liberty would be restricted from exercising more than one demand right in any twelve-month period.  The Partnership would bear all reasonable expenses (excluding any underwriting discounts and commissions and fees for more than one counsel for EMG Liberty) in connection with the registration and offering of EMG Liberty’s Converted Units.

Transfer Restrictions

The LOI provides that, except for EMG Liberty’s ability to (1) participate in underwritten offerings or any continuous offering program as described above and (2) make distributions, beginning on or after January 1, 2016, to EMG Liberty’s limited partners in an aggregate amount not exceeding 2.5 million Converted Units (provided that such limited partners agree to a customary one-year lock-up arrangement), EMG Liberty would not be permitted to transfer its Class B Units or Converted Units, without the prior written consent of the Partnership.

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Voting and Governance

The LOI provides that, prior to the conversion of the Class B Units into Converted Units, the Class B Units would not be entitled to vote on any matters on which the holders of Common Units are entitled to vote, other than those matters that disproportionately and adversely affect the rights and preferences of the Class B Units in relation to other class of interests in the Partnership.  In such cases, the holders of Class B Units would vote together as a single class.  Unless otherwise determined by our general partner, upon conversion of the Class B Units, EMG Liberty would be entitled to vote Converted Units that represent in aggregate no more than 5% of the Partnership’s outstanding Common Units; any Converted Units in excess of such 5% threshold would not be entitled to vote.  Holders of Class B Units would not have any contractual right to designate representatives to the board of directors of the Partnership’s general partner.

Change of Control

The LOI provides in connection with a Change of Control (as defined below) where at least a majority of the outstanding Common Units are converted or exchanged into cash or a combination of cash and securities of a third party, either the Partnership, on the one hand, or the holders of any then-outstanding Class B Units, on the other hand, would be entitled to elect to cause each Class B Unit to be converted into Common Units based on the Discounted Conversion Formula (as defined below) and receive the consideration that would have been payable with respect to such Converted Units in the Change of Control transaction.  In a Change of Control transaction, where the Partnership is not the surviving entity, and neither the Partnership, on the one hand, nor the holders of the Class B Units, on the other hand, elect such a conversion, then the Class B Units would convert into a class of securities of the surviving entity with substantially similar rights to the Class B Units.  In the case of any Change of Control of the Partnership not accompanied by a conversion or exchange involving at least a majority of the then outstanding Common Units of the Partnership, then either the Partnership, on the one hand, or the holders of the then outstanding Class B Units, on the other hand, would be entitled to elect to convert Class B Units into Common Units based on the Discounted Conversion Formula.

“Discounted Conversion Formula” means an adjustment to the one-to-one initial conversion rate at which a Class B Unit would otherwise convert into a Common Unit by applying a discount rate based on the period in which the Change of Control occurs.  Such discount rate ranges from a conversion discount of 19.7% at December 31, 2011 to no discount at June 30, 2017.

“Change of Control” means (1) the direct or indirect sale, lease, transfer, conveyance or other disposition (other than by way of merger or consolidation), in one or a series of related transactions, of all or substantially all of the properties or assets of the Partnership to any third party or (2) the consummation of any transaction (including, without limitation, any merger or consolidation) the result of which any third party becomes the beneficial owner, directly or indirectly, of more than 50% of the voting securities of the Partnership in a transaction in which acquired securities are not subject to any voting limitation under the Partnership’s partnership agreement (or, if the Partnership does not survive such transaction, the voting securities of the surviving entity).

Potential MarkWest Liberty Utica Joint Venture

Pursuant to the LOI, the Partnership and EMG have also agreed to negotiate in good faith regarding a new midstream joint venture covering various development activities in the Utica Shale area of Ohio, including the construction of significant natural gas processing and NGL fractionation, transportation, and marketing infrastructure, beginning in 2012 (such joint venture, the “Utica JV”).  The Utica JV would be similar to the parties’ existing MarkWest Liberty Midstream joint venture, with EMG funding a majority of the initial capital expenditures required to develop the Utica JV midstream infrastructure. There can be no assurance that the Partnership and EMG will be able to agree to terms for the potential Utica JV on the same basis as those described herein or that they will be able to reach any agreements regarding the Utica JV at all.

Amendment to Credit Facility

In anticipation of the transactions described above, the Partnership has entered into discussions with the lenders under our revolving credit facility (the “Credit Facility”) to, among other things, increase the borrowing capacity under the Credit Facility by $150 million to $900 million. There can be no assurances, however, that the borrowing capacity will increase or that, if increased, the funds available under the amended Credit Facility will continue to be made available on the same or similar terms as under the existing Credit Facility.

Forward Looking Statements

Information contained in this Current Report on Form 8-K may include certain statements concerning expectations for the future that are forward-looking statements as defined by federal law.  Actual results could vary

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significantly from those expressed or implied in such statements and are subject to a variety of known and unknown risks, uncertainties, and other factors that are difficult to predict and many of which are beyond management’s control.  Among those are (i) the risk that the Partnership may not enter into or consummate the MarkWest Liberty Midstream Acquisition or the Utica JV and (ii) the risk that either the MarkWest Liberty Midstream Acquisition or the Utica JV is consummated, but that the anticipated benefits such transaction cannot be fully realized.  Among the factors that could cause results to differ materially are those risks discussed in the periodic reports we file with the Securities and Exchange Commission, including our Annual Report on Form 10-K for the year ended December 31, 2010, and our Quarterly Report on Form 10-Q for the quarters ended June 30, 2011 and September 30, 2011. You are urged to carefully review and consider the cautionary statements and other disclosures made in those filings, specifically those under the heading “Risk Factors.”  We do not undertake any duty to update any forward-looking statement except as required by law.

ITEM 7.01.Regulation FD Disclosure

On December 12, 2011, the Partnership issued a press release announcing the potential MarkWest Liberty Midstream Acquisition and the potential Utica JV.  A copy of the press release is furnished as Exhibit 99.1 hereto.

The information contained in this Item 7.01 shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or incorporated by reference in any filing under the Securities Act or the Exchange Act, except as shall be expressly set forth by specific reference in such filing.

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ITEM 9.01.Financial Statements and Exhibits.

(d)Exhibits.

Exhibit No.	Description of Exhibit
99.1	Press release dated December 12, 2011, announcing the MarkWest Liberty Midstream Acquisition and the Utica JV.

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SIGNATURE

Pursuant to the requirements of the Securities and Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

	MARKWEST ENERGY PARTNERS, L.P.
	(Registrant)
	By:	MarkWest Energy GP, L.L.C.,
		Its General Partner
Date:  December 12, 2011	By:	/s/ NANCY K. BUESE
		Nancy K. Buese
		Senior Vice President and Chief Financial Officer

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EX-99.1

Exhibit 99.1

Contact:	Frank Semple, Chairman, President and CEO	Contact:	John Raymond, Managing Partner &CEO
	Nancy Buese, Senior VP and CFO		Jeff Rawls, Managing Director
	Dan Campbell, VP of Finance/Treasurer		Patrick Wade, Managing Director
Phone:	(866) 858-0482	Phone	(713) 579-5000
E-mail	investorrelations@markwest.com

MarkWest Energy Partners Announces Intent to Acquire
Remaining 49% Interest in Liberty Joint Venture,

Increases 2012 DCF and Growth Capital Expenditure Guidance,

and Announces Utica Joint Venture

DENVER—December 12, 2011— MarkWest Energy Partners, L.P. (NYSE: MWE) and The Energy & Minerals Group (EMG) today announced that MarkWest and EMG have executed a letter of intent regarding the acquisition by MarkWest of EMG’s interest in MarkWest Liberty Midstream & Resources, LLC. Under the letter of intent, MarkWest and EMG have agreed to negotiate towards the execution of definitive agreements containing material terms consistent with those described in this release. MarkWest and EMG currently anticipate executing definitive agreements and closing the acquisition on December 31, 2011.

The acquisition consideration includes $1.0 billion of cash and the issuance of 19.95 million unregistered MWE Class B Units to EMG. The acquisition of EMG’s interest by MarkWest is conditioned on the parties’ ability to negotiate and execute a definitive purchase agreement as well as to satisfy any required closing conditions.

MarkWest expects that on a DCF per unit basis, the acquisition is immediately accretive in 2012 and would be up to 6 percent accretive in 2013 and beyond.  MarkWest has sufficient available liquidity to fund the cash portion of the acquisition consideration.

The Class B Units that would be issued to EMG would convert into MWE Common Units on a one-for-one basis in five equal annual installments beginning on July 1, 2013 and each of the four annual anniversaries thereafter.  The Class B Units would not be entitled to participate in any distributions of available cash from MarkWest prior to their conversion and would not have voting rights, other than with respect to matters that disproportionately and adversely affect the rights and preferences of the Class B Units in relation to other classes of partnership interests in MarkWest.  Upon conversion of the Class B Units, EMG’s right to vote as a common unitholder of MarkWest would be limited to a maximum of 5 percent of MarkWest’s outstanding Common Units.

The Class B Units would not be transferrable prior to their conversion into Common Units.  Once converted, EMG would have the right to participate in underwritten offerings undertaken by MarkWest up to 20 percent of the total number of Common Units offered.  EMG would also have limited rights to distribute Common Units to its limited partners beginning in 2016 and the right to three underwritten offerings beginning in 2017, but restricted to no more than one offering in any twelve-month period.

As a result of the acquisition, and subject to its closing, MarkWest is increasing its 2012 DCF guidance to a range of $480 million to $540 million and increasing its 2012 capital expenditure guidance to a range of $900 million to $1.3 billion.  In addition, MarkWest expects the acquisition to increase its fee-based net operating margin by up to 6 percent annually beginning in 2012.

Pursuant to the letter of intent, MarkWest and EMG would create a new Utica Shale midstream joint venture, which would develop significant natural gas processing and NGL fractionation, transportation, and marketing infrastructure in eastern Ohio beginning in 2012.  EMG would fund a majority of the initial capital expenditures required to develop the Utica midstream infrastructure. MarkWest and EMG expect to issue a press release in early January 2012 with additional details regarding the Utica joint venture and its planned activities in the Utica Shale.

“Our Liberty joint venture with EMG has made significant capital investments to develop world-class midstream infrastructure that has been critical to the development of the liquids-rich area of the Marcellus,” said Frank Semple, Chairman, President and Chief Executive Officer of MarkWest.  “The joint venture has been very successful and we believe this acquisition represents a significant achievement for both parties. The creative acquisition structure includes up-front cash and deferred issuance of MWE common units.  This structure provides immediate and future accretion and reflects the strength of our relationship with EMG as well as their confidence in the future value of MWE’s common units.  Looking ahead, we are very pleased to work closely with EMG in a new joint venture to leverage a similar operational and financial platform to develop integrated NGL transportation, fractionation, storage, and marketing services in the liquids-rich corridor of the Utica Shale.”

“Since inception a mere three years ago, the Liberty JV has adapted to the success and needs of the producer community — as demonstrated by record drilling activity levels in the basin — via multiple iterations of geographic and functional expansions that has manifested the development of a large scale, world-class integrated midstream system that allows the producers to fully develop and maximize the value of their underlying reserves,” stated John Raymond, Managing Partner and CEO of EMG. “This is the direct result of the strong relationship between the teams at EMG and MarkWest.  To that end, MarkWest has been an exceptional partner and, as operator of the JV, has consistently delivered best-in-class execution in the development of these critical assets.  The JV has also exercised considerable commercial and financial flexibility via structural solutions and scalable capital needs to support the development of this multi-billion dollar project.  Via the newly formed Utica JV, we look forward to again working with the dedicated and talented group of people at MarkWest to continue to meet the dynamic needs of the producer community in one of the most promising basins in North America over the years to come.  We look forward to long-term participation in the continued growth of the value of MarkWest via our substantial unit position and the newly formed Utica JV.”

Morgan Stanley & Co. LLC is acting as the exclusive financial advisor to MarkWest in connection with the proposed transaction.

Citigroup is acting as the exclusive financial advisor to EMG in connection with the proposed transaction.

MarkWest will host a conference call and webcast on Monday, December 12, 2011, at 5:00 p.m. Eastern Time to discuss today’s announcements.  Interested parties can participate in the call by dialing (800) 475-0218 (passcode “MarkWest”) approximately ten minutes prior to the scheduled start time. To access the webcast, please visit the Investor Relations section of MarkWest’s website at www.markwest.com.

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About MarkWest Energy Partners

MarkWest Energy Partners, L.P. is a master limited partnership engaged in the gathering, transportation, and processing of natural gas; the transportation, fractionation, marketing, and storage of natural gas liquids; and the gathering and transportation of crude oil. MarkWest has extensive natural gas gathering, processing, and transmission operations in the southwest, Gulf Coast, and northeast regions of the United States, including the Marcellus Shale, and is the largest natural gas processor and fractionator in the Appalachian region.

About The Energy & Minerals Group

The Energy & Minerals Group is the management company for a series of private equity funds totaling in excess of $3.3 billion of commitments.  EMG focuses exclusively on making direct investments across the natural resources industry in conjunction with experienced management teams focused on hard assets that are integral to existing and growing markets.  For additional information on EMG, please contact John Raymond at 713-579-5000.

This press release includes “forward-looking statements.”  All statements other than statements of historical facts included or incorporated herein may constitute forward-looking statements. Actual results could vary significantly from those expressed or implied in such statements and are subject to a number of known and unknown risks, uncertainties, and other factors that are difficult to predict and many of which are beyond management’s control. Among those are (i) the risk that MarkWest may not enter into or consummate a transaction involving the acquisition of all of the interests in MarkWest Liberty Midstream & Resources, L.L.C. held by EMG or its affiliates or a transaction with EMG regarding a new midstream joint venture in the Utica Shale area of Ohio and (ii) the risk that one or both of these transactions is consummated, but that the anticipated benefits from the transactions cannot be fully realized.  Although we believe that the expectations reflected in the forward-looking statements are reasonable, we can give no assurance that such expectations will prove to be correct. The forward-looking statements involve risks and uncertainties that affect our operations, financial performance, and other factors as discussed in our filings with the Securities and Exchange Commission (SEC).  Among the factors that could cause results to differ materially are those risks discussed in the periodic reports we file with the SEC, including our Annual Report on Form 10-K for the year ended December 31, 2010, and our Quarterly Report on Form 10-Q for the quarters ended June 30, 2011 and September 30, 2011. You are urged to carefully review and consider the cautionary statements and other disclosures made in those filings, specifically those under the heading “Risk Factors.”  We do not undertake any duty to update any forward-looking statement except as required by law.