MarkWest Energy Partners, L.P. (MarkWest Energy) is a master limited partnership engaged in the gathering, processing and transportation of natural gas; the transportation, fractionation, storage and marketing of natural gas liquids (NGLs), and the gathering and transportation of crude oil. It provides services in the midstream sector of the natural gas industry. The Company also provides processing and fractionation services to crude oil refineries in the Corpus Christi, Texas area through its Javelina gas processing and fractionation facility. As of December 31, 2011, the Company operated in four segments: Southwest, Northeast, Liberty and Gulf Coast. Effective December 31, 2011, the Company acquired the remaining 49% interest in MarkWest Liberty Midstream. On February 1, 2011, the Company acquired Langley processing plant.