(Reuters) - Refiner Marathon Petroleum Corp (>> Marathon Petroleum Corp) will expand into natural gas processing with its $15.6 billion (10 billion pounds) acquisition of MarkWest Energy Partners LP (>> Markwest Energy Partners LP).

Marathon Petroleum is buying MarkWest through its pipeline unit, MPLX LP (>> MPLX LP). The cash-and-stock deal will create the fourth-largest master limited partnership - a tax efficient structure - valued at $21 billion.

Shares of Marathon Petroleum, which will control the combined company through MPLX's general partner, rose as much as 11 percent to a four-year high of $60.30. MPLX shares fell as much as 18 percent to $56.86.

MarkWest unitholders will get 1.09 MPLX units and $3.37 in cash per unit, translating to $78.64 per unit - a 32 percent premium to MarkWest's Friday close.

MarkWest's shares rose as much as 16 percent to $69.15, but were short of the offer price, reflecting the drop in MPLX shares, analysts said.

MarkWest, the second-largest U.S. natural gas processor, has plants across the country, including Pennsylvania's Marcellus shale and Ohio's Utica shale. It also has over 4,000 miles of pipelines, mostly natural gas and natgas liquids, and one crude oil pipeline.

"Strategically, this brings MPC/MPLX a major Northeast natural gas gathering and processing footprint, which complements MPC's nearby refining footprint ...," said

Raymond James analyst Cory Garcia, referring to Marathon's refineries in Kentucky, Michigan and Ohio.

The deal will also help Marathon Petroleum add condensate storage and stabilization plants at a time when companies are betting on the export of processed condensate, or super-light crude, following a relaxation of the U.S. crude export ban.

MPLX, which operates a network of crude oil and refined product pipelines in the U.S. Midwest and Gulf Coast regions, is building a condensate pipeline in Ohio to move output from the Utica shale field.

MarkWest will operate as a unit of MPLX after the deal closes, expected in the fourth quarter.

Marathon Petroleum, which set up MPLX in 2012, will contribute $675 million to fund the cash component of the deal.

MPLX will also assume $4.2 billion in MarkWest's debt, implying an enterprise value of $20 billion for MarkWest.

MPLX also said on Monday it would "indefinitely" defer its acquisition of Marathon Petroleum's marine transportation assets.

UBS Investment Bank advised MPLX and Jones Day was its legal adviser. Jefferies advised MarkWest, while Cravath, Swaine & Moore LLP was its legal adviser.

(Writing by Swetha Gopinath; Editing by Kirti Pandey and Sayantani Ghosh)

By Shubhankar Chakravorty and Amrutha Gayathri