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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): November 10, 2015

MARKWEST ENERGY PARTNERS, L.P. (Exact name of registrant as specified in its charter)

Delaware	001-31239	27-0005456
(State or other jurisdiction of incorporation or organization)	(Commission File Number)	(IRS Employer Identification No.)

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

(Address of principal executive officers)

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:

xWritten 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 1.01. Entry into a Material Definitive Agreement.

On November 10, 2015, MarkWest Energy Partners, L.P., a Delaware limited partnership (the “Partnership”), MPLX LP, a Delaware limited partnership (“MPLX”), MPLX GP LLC, a Delaware limited liability company and the general partner of MPLX (“MPLX GP”), Sapphire Holdco LLC, a Delaware limited liability company and wholly owned subsidiary of MPLX (“Merger Sub”), and, for certain limited purposes set forth in the Amendment, Marathon Petroleum Corporation, a Delaware corporation and the parent of MPLX GP (“MPC”), entered into an Amendment (the “Amendment”) to the Agreement and Plan of Merger dated as of July 11, 2015 (the “Merger Agreement” and together with the Amendment, the “Amended Merger Agreement”), among the Partnership, MPLX, MPLX GP, Merger Sub and, for certain limited purposes set forth in the Merger Agreement, MPC, pursuant to which Merger Sub will be merged with and into the Partnership (the “Merger”), with the Partnership surviving the Merger as a wholly owned subsidiary of MPLX.

The Amendment provides for an increase in the total cash consideration to be received by the Partnership’s unitholders from $675 million in cash to $1,075 million in cash.  As a result, under the Amended Merger Agreement, MPC will contribute $1,075 million to MPLX, without receiving any new equity in exchange, and, at the effective time of the Merger (the “Effective Time”), (a) each outstanding Partnership common unit (the “Common Units”) will be converted into the right to receive 1.09 MPLX common units (such consideration, the “Common Equity Consideration”) and an amount in cash obtained by dividing (i) $1,075 million by (ii) the number of Common Units plus the number of Canceled Awards (as defined below) plus the number of Partnership class B units (the “Class B Units”), in each case outstanding as of immediately prior to the Effective Time (together with the Common Equity Consideration, the “Common Merger Consideration”) and (b) each outstanding Class B Unit will be converted into the right to receive 1 MPLX class B unit. Under the Amended Merger Agreement, at the Effective Time, the Partnership class A units, all of which are owned by wholly owned subsidiaries of the Partnership, will be converted into a specified number of MPLX class A units, as more fully described in the Amended Merger Agreement.

As a result of the Merger, each phantom unit under the Partnership’s equity plans that is outstanding immediately prior to the Effective Time will become fully vested and converted into an equivalent number of Common Units, which will be canceled and converted into the right to receive the Common Merger Consideration (each Phantom Unit, as so converted, a “Canceled Award”).

Other than as expressly modified pursuant the Amendment, the Merger Agreement, which was filed as Exhibit 2.1 to the Current Report on Form 8-K filed with the Securities and Exchange Commission (the “SEC”) by the Partnership on July 13, 2015, remains in full force and effect as originally executed on July 11, 2015. The foregoing description of the Amendment and the transactions contemplated thereby does not purport to be complete and is subject to, and qualified in its entirety by, the full text of the Amendment, which is attached hereto as Exhibit 2.1 to this Current Report on Form 8-K and is incorporated by reference in this Item 1.01.

Item 8.01. Other Events.

On November 10, 2015, the Partnership issued a joint press release with MPLX announcing the execution of the Amendment.  A copy of the press release issued is filed as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated by reference in this Item 8.01.

Item 9.01. Financial Statements and Exhibits.

(d)Exhibits

Exhibit No.	Description
2.1	Amendment to Agreement and Plan of Merger, dated as of November 10, 2015, by and among MarkWest Energy Partners, L.P., MPLX LP, MPLX GP LLC, Marathon Petroleum Corporation and Sapphire Holdco LLC
99.1	Press Release dated November 10, 2015, issued by MarkWest Energy Partners, L.P. and MPLX LP

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Cautionary Statement Regarding Forward-Looking Statements

This communication includes “forward-looking statements.” All statements other than statements of historical facts included or incorporated herein may constitute forward-looking statements that involve a number of risks and uncertainties. These statements may include statements regarding the proposed acquisition of the Partnership by MPLX, the expected timetable for completing the transaction, benefits and synergies of the transaction, future opportunities for the combined company and any other statements regarding the Partnership’s and MPLX’s future operations, anticipated business levels, future earnings and distributions, planned activities, anticipated growth, market opportunities, strategies and competition. All such forward-looking statements involve estimates and assumptions that are subject to a number of risks, uncertainties and other factors that could cause actual results to differ materially from those expressed or implied in such statements. Factors that could cause or contribute to such differences include: factors relating to the satisfaction of the conditions to the proposed transaction, including regulatory approvals and the required approval of the Partnership’s unitholders; the parties’ ability to meet expectations regarding the timing and tax treatment of the proposed transaction; the possibility that the combined company may be unable to achieve expected synergies and operating efficiencies in connection with the transaction within the expected time-frames or at all; the integration of the Partnership being more difficult, time-consuming or costly than expected; the effect of any changes resulting from the proposed transaction in customer, supplier and other business relationships; general market perception of the proposed transaction; exposure to lawsuits and contingencies associated with MPLX; the ability to attract and retain key personnel; prevailing market conditions; changes in the economic and financial conditions of the Partnership and MPLX; uncertainties and matters beyond the control of management; and the other risks discussed in the periodic reports filed with the SEC, including the Partnership’s and MPLX’s Annual Reports on Form 10-K for the year ended December 31, 2014 and the Partnership’s and MPLX’s subsequent reports on Form 10-Q. These risks, as well as other risks associated with the Partnership, MPLX and the proposed transaction, are also more fully discussed in the definitive proxy statement/prospectus referred to below.  The forward-looking statements should be considered in light of all these factors. In addition, other risks and uncertainties not presently known to the Partnership or MPLX or that the Partnership or MPLX considers immaterial could affect the accuracy of the forward-looking statements. The reader is cautioned not to rely unduly on these forward-looking statements. The Partnership and MPLX do not undertake any duty to update any forward-looking statement except as required by law.

Additional Information and Where to Find It

This communication may be deemed to be solicitation material in respect of the proposed acquisition of the Partnership by MPLX. In connection with the proposed acquisition, the Partnership and MPLX have filed relevant materials with the SEC, including MPLX’s registration statement on Form S-4 which included a definitive proxy statement/prospectus and was declared effective by the SEC on October 29, 2015, and the Partnership’s definitive proxy statement/proxy statement which was filed with the SEC on October 30, 2015 and mailed to the Partnership’s common unitholders on or about October 30, 2015.  Investors and security holders are urged to read all relevant documents filed with the SEC, including the definitive proxy statement/prospectus, because they contain important information about the proposed transaction. Investors and security holders are able to obtain the documents free of charge at the SEC’s website, http://www.sec.gov, or for free from the Partnership by contacting Investor Relations by phone at 1-(866) 858-0482 or by email at investorrelations@markwest.com or for free from MPLX LP at its website, http://ir.mplx.com, or in writing at 200 E. Hardin Street, Findlay, Ohio 45840, Attention: Corporate Secretary.

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Participants in the Solicitation

This communication is not a solicitation of a proxy from any investor or securityholder. However, the Partnership and its directors and executive officers and certain employees may be deemed to be participants in the solicitation of proxies from the holders of Partnership common units with respect to the proposed transaction. Information about the Partnership’s directors and executive officers is set forth in the proxy statement for the Partnership’s 2015 Annual Meeting of Common Unitholders, which was filed with the SEC on April 23, 2015 and the Partnership’s current reports on Form 8-K, as filed with the SEC on May 5, 2015, May 19, 2015 and June 8, 2015. Information about MPLX’s directors and executive officers is available in MPLX’s Annual Report on Form 10-K filed with the SEC on February 27, 2015 and MPLX’s current report on Form 8-K, as filed with the SEC on March 9, 2015. To the extent holdings of Partnership securities have changed since the amounts contained in the proxy statement for the Partnership’s 2015 Annual Meeting of stockholders, such changes have been or will be reflected on Statements of Change in Ownership on Form 4 filed with the SEC. Additional information regarding those persons and other persons who may be deemed to be participants in the solicitation of proxies from the holders of Partnership common units with respect to the proposed transaction, including their respective direct and indirect interests in the proposed transaction, by security holdings or otherwise, is contained in the definitive proxy statement/prospectus and other relevant materials filed with the SEC referred to above. These documents may be obtained free of charge from the SEC’s website http://www.sec.gov, or from the Partnership and MPLX, as applicable, using the contact information above.

Non-Solicitation

This communication shall not constitute an offer to sell or the solicitation of an offer to sell or the solicitation of an offer to buy any securities, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offer of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended.

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SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this Report to be signed on its behalf by the undersigned hereunto duly authorized.

	MarkWest Energy Partners, L.P.
	By: MarkWest Energy GP, L.L.C., its general partner
Date: November 10, 2015	By:	/s/ Nancy K. Buese
		Name: Nancy K. Buese
		Title:   Executive Vice President and Chief Financial Officer

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EXHIBIT INDEX

Exhibit No.	Description
2.1	Amendment to Agreement and Plan of Merger, dated as of November 10, 2015, by and among MarkWest Energy Partners, L.P., MPLX LP, MPLX GP LLC, Marathon Petroleum Corporation and Sapphire Holdco LLC
99.1	Press Release dated November 10, 2015, issued by MarkWest Energy Partners, L.P. and MPLX LP

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EX-2.1

Exhibit 2.1

EXECUTION VERSION

AMENDMENT TO AGREEMENT AND PLAN OF MERGER

THIS AMENDMENT TO AGREEMENT AND PLAN OF MERGER, dated as of November 10, 2015  (this “Amendment”), is made by and among MarkWest Energy Partners, L.P., a Delaware limited partnership (the “Partnership”), MPLX LP, a Delaware limited partnership (“Parent”), MPLX GP LLC, a Delaware limited liability company and the general partner of Parent (“Parent GP”), solely for purposes of Section 5.15 of the Merger Agreement (defined below), Marathon Petroleum Corporation, a Delaware corporation and the ultimate parent of Parent GP (“MPC”), and Sapphire Holdco LLC, a Delaware limited liability company and a wholly owned Subsidiary of Parent (“Merger Sub” and, with MPC, Parent and Parent GP, the “Parent Entities”).

RECITALS

1.The Parent Entities and the Partnership are parties to that certain Agreement and Plan of Merger, dated as of July 11, 2015 (the “Merger Agreement”).

2.Pursuant to Section 8.2 of the Merger Agreement, the Parent Entities and the Partnership desire to amend the Merger Agreement as set forth herein.

3.All capitalized terms used, but not defined, in this Amendment shall have the meanings ascribed thereto in the Merger Agreement.

4.In consideration of the foregoing, the representations, warranties, covenants and agreements set forth in this Amendment and the Merger Agreement, and other good and valuable consideration, the adequacy and receipt of which are hereby acknowledged, the parties, intending to be legally bound, hereby agree as follows:

SECTION 1.  Amendments To The Merger Agreement.  The parties hereby agree to amend the Merger Agreement as follows:

(a)        Section 2.1(a) of the Merger Agreement is hereby amended by deleting the reference to $675,000,000 in the first and last sentences of Section 2.1(a) and replacing it with $1,075,000,000.

SECTION 2.  Effectiveness of Amendment.  Upon the execution and delivery of this Amendment, the Merger Agreement will thereupon be deemed to be amended as hereinabove set forth as fully and with the same effect as if the amendments made hereby were originally set forth in the Merger Agreement, and this Amendment and the Merger Agreement will henceforth respectively be read, taken and construed as one and the same instrument.

SECTION 3. Reaffirmation by the Parent Entities.  Parent hereby makes and reaffirms the representations and warranties contained in Section 4.2(e) and Section 4.17 of the Merger Agreement, and MPC hereby reaffirms the covenants set forth in

Section 5.15(b) of the Merger Agreement, after giving effect to the amendments effected pursuant to Section 1 hereof.

SECTION 4. Partnership Authority Relative to Amendment.  The Partnership has all requisite power and authority to execute and deliver this Amendment and to consummate the transactions contemplated hereby, subject to obtaining the Partnership Unitholder Approval.  The execution, delivery and performance by the Partnership of this Amendment, and the transactions contemplated hereby, have been duly authorized and approved by the General Partner, which, at a meeting duly called and held, has (i) approved and declared advisable this Amendment and the transactions contemplated hereby and (ii) resolved to submit the Merger Agreement, as amended by this Amendment, to a vote of the Limited Partners of the Partnership and to recommend approval of the Merger Agreement, as amended by this Amendment, by the Limited Partners of the Partnership, and except for obtaining the Partnership Unitholder Approval for the approval of the Merger Agreement, as amended by this Amendment, and consummation of the transactions contemplated hereby, no other partnership action on the part of the Partnership is necessary to authorize the execution, delivery and performance by the Partnership of this Amendment and the consummation of the transactions contemplated hereby.  This Amendment has been duly executed and delivered by the Partnership and, assuming due authorization, execution and delivery, of this Amendment by the other parties hereto, constitutes a legal, valid and binding obligation of the Partnership, enforceable against it in accordance with its terms.

SECTION 5. Parent Entities Authority Relative to Amendment.  Each of the Parent Entities has all requisite power and authority to execute and deliver this Amendment and to consummate the transactions contemplated hereby.  The execution, delivery and performance by the Parent Entities of this Amendment, and the consummation of the transactions contemplated hereby, have been duly authorized and approved by Merger Sub and Parent, as its sole member, by Parent GP, for itself and on behalf of Parent, and by MPC and no other entity action on the part of the Parent Entities is necessary to authorize the execution, delivery and performance by the Parent Entities of this Amendment and the consummation of the transactions contemplated hereby.  This Amendment has been duly executed and delivered by the Parent Entities and, assuming due authorization, execution and delivery of this Amendment by the Partnership, constitutes a legal, valid and binding obligation of each of the Parent Entities, enforceable against each of them in accordance with its terms.

SECTION 6. References to the Merger Agreement.  After giving effect to this Amendment, each reference in the Merger Agreement to “this Agreement”, “hereof”, “hereunder”, “herein” or words of like import referring to the Merger Agreement shall refer to the Merger Agreement as amended by this Amendment, and all references in the Parent Disclosure Letter and the Partnership Disclosure Letter to “the Agreement” shall refer to the Merger Agreement as amended by this Amendment.

SECTION 7. Construction.  Except as expressly provided in this Amendment, all references in the Merger Agreement, the Parent Disclosure Letter and the Partnership Disclosure Letter to “the date hereof” and “the date of this Agreement” or words of like import shall refer to July 11, 2015.

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SECTION 8.  General Provisions.

(a)        Miscellaneous.  Sections 8.1 through 8.10 and Section 8.12 of the Merger Agreement are incorporated by reference into this Amendment and will apply to the Parent Entities and the Partnership mutatis mutandis.

(b)        Agreement in Effect.  Except as specifically provided for in this Amendment, the Merger Agreement will remain unmodified and in full force and effect.

[Signature Pages Follow]

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IN WITNESS WHEREOF, the parties have caused this Amendment to be duly executed and delivered as of the date first above written.

PARENT:
MPLX LP
By: MPLX GP LLC,
its general partner
By:	/s/ G.R. Heminger
Name: G.R. Heminger
Title: Chairman and Chief Executive Officer
PARENT GP:
MPLX GP LLC
By:	/s/ G.R. Heminger
Name: G.R. Heminger
Title: Chairman and Chief Executive Officer
MERGER SUB:
SAPPHIRE HOLDCO LLC
By:	/s/ P.K.M. Beall
Name: P.K.M. Beall
Title: President
MPC:
MARATHON PETROLEUM CORPORATION
By:	/s/ G.R. Heminger
Name: G.R. Heminger
Title: President and Chief Executive Officer

[Signature Page to Amendment to Merger Agreement]

PARTNERSHIP:
MARKWEST ENERGY PARTNERS, L.P.,
By:	MARKWEST ENERGY GP, L.L.C.,
	its general partner
By:	/s/ Frank M. Semple
Name:	Frank M. Semple
Title:	Chairman, President and Chief Executive Officer

[Signature Page to Amendment to Merger Agreement]

EX-99.1

Exhibit 99.1

Marathon Petroleum Corp. increases cash consideration for MPLX/MarkWest merger

·One-time cash payment from MPC raised to $1.075 billion from $675 million

·Total cash consideration of ~$5.21 per unit substantially enhances value to MarkWest unitholders

·MarkWest board and executive management affirm their support for the transaction and its revised terms

·Supplemental proxy cards being mailed to unitholders

·Special unitholder meeting scheduled for Dec. 1, 2015

FINDLAY, Ohio, and DENVER, Nov. 10, 2015 - MPLX LP (NYSE: MPLX) and MarkWest Energy Partners, L.P. (NYSE: MWE) (MarkWest) today announced that in connection with the merger of MPLX and MarkWest, MPLX has agreed to increase the amount of the cash consideration payable to MarkWest common unitholders by $400 million, from $675 million to $1.075 billion. Under the revised terms of the merger agreement, MarkWest common unitholders will receive 1.09 MPLX common units plus a one-time cash payment of approximately $5.21 per MarkWest common unit, for a total consideration of approximately $52.93 per MarkWest common unit, based on fully diluted units currently outstanding and the closing price of MPLX’s common units on Nov. 10, 2015. The increase in cash merger consideration is being contributed to MPLX by its sponsor, Marathon Petroleum Corporation (NYSE: MPC), under which no new equity units will be issued to MPC. In addition, as part of the original transaction, MPC will contribute approximately $225 million, based on the price of MPLX’s common units on Nov. 10, 2015, to maintain its 2 percent general partner interest in MPLX. These proceeds will be retained by the partnership to support its growth.

All other terms of the merger agreement announced on July 13, 2015, remain the same. The merger is recommended by the boards of directors of MPC, MPLX and MarkWest, and the executive management of both partnerships also strongly support the transaction and its revised terms.

The proposed transaction will combine MarkWest, the second-largest processor of natural gas in the United States and largest processor and fractionator in the Marcellus and Utica shale plays, with MPLX, a rapidly growing crude oil and refined products logistics partnership sponsored by MPC. The combination will create one of the largest master limited partnerships (MLPs) and is expected to generate a mid-20 percent compound annual distribution growth rate through 2019.

“The enhancement to the terms of our agreement reflects the commitment of MPLX and its sponsor, MPC, to the combination with MarkWest and our conviction that the transaction will create significant benefits for the unitholders, customers and employees of both partnerships,” said Gary R. Heminger, MPLX chairman and chief executive officer. “This increase substantially enhances the transaction value for MarkWest unitholders, who will not only benefit from significant distribution growth, but also a substantially lower equity yield, investment-grade debt funding costs, enhanced access to capital and liquidity, and a strong general partner prepared to provide support and financial flexibility.”

Even in a challenging environment of lower MLP valuations and higher yields, and in continued support of the combination, MPLX recently affirmed its guidance that creates a mid-20-percent compound annual distribution growth profile through 2019, including a 25 percent distribution growth rate in 2016. Heminger noted that MPC, as the sponsor and general partner, has extensive options available to support the growth profile of the combined partnership, including dropping down the large and growing $1.6 billion inventory of MLP-qualifying earnings before interest, taxes, depreciation and amortization.

“The increased cash consideration and ownership of a higher growth MLP with a strong sponsor provides even more compelling value for MarkWest unitholders, who will own approximately 73 percent of the

combined partnership when the transaction is completed,” said Heminger. “Both partnerships’ unitholders can share in the strong upside potential of a combined partnership with multiple operational platforms as well as significant growth and commercial synergy opportunities. Coupled with MarkWest’s $1.5 billion average annual organic capital growth program over the next five years, our substantial resources and strong sponsor will support the combined partnership in unlocking an incremental $6 billion to $9 billion of potential organic growth projects,” said Heminger.

“Our Board continues to support the combination with MPLX and recommends that MarkWest unitholders vote in favor of the merger proposal,” said Frank M. Semple, MarkWest chairman, president and chief executive officer. “The long-term strategic value of the combination with MPLX and the support from Marathon Petroleum is compelling. The increased cash contribution of $400 million substantially enhances the value of the transaction for our unitholders. We are excited to complete the transaction and continue the important work of developing critical midstream projects for our producer customers, and delivering significant unitholder value over the long term.”

The transaction is subject to approval by MarkWest unitholders and other customary closing conditions and, subject to the satisfaction of those conditions, is expected to close in December 2015. The date of the special meeting of MarkWest common unitholders is Dec. 1, 2015. MarkWest unitholders of record as of Oct. 5, 2015, will be entitled to vote on approval of the merger and the associated proposals.

MarkWest unitholders are urged to vote “FOR” the merger and related matters and submit their proxy as promptly as possible, either by telephone, via the internet or by marking, signing and dating the proxy card that was provided to unitholders along with the proxy statement and prospectus.

MarkWest expects to mail supplemental proxy materials to its unitholders in the near future.

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About MPLX LP

MPLX is a fee-based, growth-oriented master limited partnership formed in 2012 by Marathon Petroleum Corporation to own, operate, develop and acquire pipelines and other midstream assets related to the transportation and storage of crude oil, refined products and other hydrocarbon-based products. Headquartered in Findlay, Ohio, MPLX’s assets consist of a 99.5 percent equity interest in a network of common carrier crude oil and products pipeline assets located in the Midwest and Gulf Coast regions of the United States and a 100 percent interest in a butane storage cavern located in West Virginia with approximately 1 million barrels of natural gas liquids storage capacity.

About MarkWest Energy Partners

MarkWest Energy Partners, L.P. is a master limited partnership that owns and operates midstream service businesses. MarkWest has a leading presence in many natural gas resource plays including the Marcellus Shale, Utica Shale, Huron/Berea Shale, Haynesville Shale, Woodford Shale and Granite Wash formation.

MPLX Investor Relations Contacts:

Geri Ewing (419) 421-2071

Teresa Homan (419) 421-2965

MPLX Media Contacts:

Chuck Rice (419) 421-2521

Jamal Kheiry (419) 421-3312

MarkWest Investor Relations and Media Contact:

Joshua Hallenbeck (866) 858-0482

This press release contains forward-looking statements within the meaning of federal securities laws regarding MPLX LP (“MPLX”), Marathon Petroleum Corporation (“MPC”), and MarkWest Energy Partners,

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L.P. (“MWE”),. These forward-looking statements relate to, among other things, expectations, estimates and projections concerning the business and operations of MPLX, MPC, and MWE . You can identify forward-looking statements by words such as “anticipate,” “believe,” “estimate,” “objective,” “expect,” “forecast,” “guidance,” “imply,” “plan,” “project,” “potential,” “could,” “may,” “should,” “would,” “will” or other similar expressions that convey the uncertainty of future events or outcomes. Such forward-looking statements are not guarantees of future performance and are subject to risks, uncertainties and other factors, some of which are beyond the companies’ control and are difficult to predict. In addition to other factors described herein that could cause MPLX’s or MWE’s actual results to differ materially from those implied in these forward-looking statements, negative capital market conditions, including a persistence or increase of the current yield on common units, which is higher than historical yields, could adversely affect MPLX’s ability to meet its distribution growth guidance, particularly with respect to the later years of such guidance. Factors that could cause MPLX’s or MWE’s actual results to differ materially from those implied in the forward-looking statements include: the ability to complete the proposed merger of MPLX and MWE on anticipated terms and timetable; the ability to obtain approval of the transaction by the unitholders of MWE and satisfy other conditions to the closing of the transaction contemplated by the merger agreement; risk that the synergies from the MPLX/MWE transaction may not be fully realized or may take longer to realize than expected; disruption from the MPLX/MWE transaction making it more difficult to maintain relationships with customers, employees or suppliers; risks relating to any unforeseen liabilities of MWE or MPLX, as applicable; the adequacy of MPLX’s and MWE’s respective capital resources and liquidity, including, but not limited to, availability of sufficient cash flow to pay distributions, and the ability to successfully execute their business plans and implement their growth strategies; the timing and extent of changes in commodity prices and demand for crude oil, refined products, feedstocks or other hydrocarbon-based products; volatility in and/or degradation of market and industry conditions; completion of pipeline capacity by competitors; disruptions due to equipment interruption or failure, including electrical shortages and power grid failures; the suspension, reduction or termination of MPC’s obligations under MPLX’s commercial agreements; each company’s ability to successfully implement its growth plan, whether through organic growth or acquisitions; modifications to earnings and distribution growth objectives; federal and state environmental, economic, health and safety, energy and other policies and regulations; changes to MPLX’s capital budget; other risk factors inherent to MPLX or MWE’s industry; and the factors set forth under the heading “Risk Factors” in MPLX’s Annual Report on Form 10-K for the year ended Dec. 31, 2014, filed with the Securities and Exchange Commission (SEC); and the factors set forth under the heading “Risk Factors” in MWE’s Annual Report on Form 10-K for the year ended Dec. 31, 2014, and Quarterly Report on Form 10-Q for the quarter ended September 30, 2015, filed with the SEC. These risks, as well as other risks associated with MPLX, MWE and the proposed transaction are also more fully discussed in the joint proxy statement and prospectus included in the registration statement on Form S-4 filed by MPLX and declared effective by the SEC on October 29, 2015. Factors that could cause MPC’s actual results to differ materially from those implied in the forward-looking statements include: risks described above relating to the MPLX/MWE proposed merger; changes to the expected construction costs and timing of pipeline projects; volatility in and/or degradation of market and industry conditions; the availability and pricing of crude oil and other feedstocks; slower growth in domestic and Canadian crude supply; an easing or lifting of the U.S. crude oil export ban; completion of pipeline capacity to areas outside the U.S. Midwest; consumer demand for refined products; transportation logistics; the reliability of processing units and other equipment; MPC’s ability to successfully implement growth opportunities; modifications to MPLX earnings and distribution growth objectives; federal and state environmental, economic, health and safety, energy and other policies and regulations; MPC’s ability to successfully integrate the acquired Hess retail operations and achieve the strategic and other expected objectives relating to the acquisition; changes to MPC’s capital budget; other risk factors inherent to MPC’s industry; and the factors set forth under the heading “Risk Factors” in MPC’s Annual Report on Form 10-K for the year ended Dec. 31, 2014, filed with SEC. In addition, the forward-looking statements included herein could be affected by general domestic and international economic and political conditions. Unpredictable or unknown factors not discussed here, in MPLX’s Form 10-K, in MPC’s Form 10-K, or in MWE’s Form 10-K and Form 10-Qs could also have material adverse effects on forward-looking statements. Copies of MPLX’s Form 10-K are available on the SEC website, MPLX’s website at http://ir.mplx.com or by contacting MPLX’s Investor Relations office. Copies of MPC’s Form 10-K are available on the SEC website, MPC’s website at http://ir.marathonpetroleum.com or by contacting MPC’s Investor Relations office. Copies of MWE’s Form 10-K and Form 10-Qs are available

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on the SEC website, MWE’s website at http://investor.markwest.com or by contacting MWE’s Investor Relations office.

Additional Information and Where to Find It

This communication may be deemed to be solicitation material in respect of the proposed acquisition of MWE by MPLX. In connection with the proposed acquisition, MWE and MPLX have filed relevant materials with the SEC, including MPLX’s registration statement on Form S-4 that includes a definitive joint proxy statement and a prospectus and was declared effective by the SEC on October 29, 2015. Investors and security holders are urged to read all relevant documents filed with the SEC, including the definitive joint proxy statement and prospectus, because they contain important information about the proposed transaction. Investors and security holders are able to obtain the documents free of charge at the SEC’s website, http://www.sec.gov, or for free from MPLX LP at its website, http://ir.mplx.com, or in writing at 200 E. Hardin Street, Findlay, Ohio 45840, Attention: Corporate Secretary, or for free from MWE by contacting Investor Relations by phone at 1-(866) 858-0482 or by email at investorrelations@markwest.com.

Participants in the Solicitation

This communication is not a solicitation of a proxy from any investor or securityholder. However MPLX and MWE and their respective directors and executive officers and certain employees may be deemed to be participants in the solicitation of proxies from the holders of MWE common units with respect to the proposed transaction. Information about MPLX’s directors and executive officers is available in MPLX’s Annual Report on Form 10-K filed with the SEC on February 27, 2015 and MPLX’s current report on Form 8-K, as filed with the SEC on March 9, 2015. Information about MWE’s directors and executive officers is set forth in the proxy statement for MWE’s 2015 Annual Meeting of Common Unitholders, which was filed with the SEC on April 23, 2015 and MWE’s current reports on Form 8-K, as filed with the SEC on May 5, 2015, May 19, 2015 and June 8, 2015, and in the definitive joint proxy statement filed by MPLX, which was declared effective by the SEC on October 29, 2015. To the extent holdings of MWE securities have changed since the amounts contained in the definitive joint proxy statement filed by MPLX, which was declared effective by the SEC on October 29, 2015, such changes have been or will be reflected on Statements of Change in Ownership on Form 4 filed with the SEC. Investors may obtain additional information regarding the interest of such participants by reading the definitive joint proxy statement and prospectus regarding the acquisition. These documents may be obtained free of charge from the SEC’s website http://www.sec.gov, or from MWE and MPLX using the contact information above.

Non-Solicitation

This communication shall not constitute an offer to sell or the solicitation of an offer to sell or the solicitation of an offer to buy any securities, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offer of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended.

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