MarkWest Energy Partners, L.P. is a natural gas gathering, processing and transportation company master limited partnership of the United States. The company is also engaged in transporting and fractionating the natural gas liquids. The company divides the business into four parts: Southwest, Northeast, Liberty and Utica. Since 2007, the company develops natural gas supplies in emerging resource plays, now it has become the majority of its growth. In 1988, the company was founded in Denver, Colorado.