EVANS CITY — MarkWest Energy Partners announced Monday it will invest at least $500 million in Butler County in the next five years.

This involves buying Keystone Midstream Services, which owns and operates two cryogenic gas processing plants near Evans City, a gas gathering system and an associated field compression station.

MarkWest is buying Keystone for $512 million, according to a Rex Energy news release.

Rex Energy, which owns 28 percent of Keystone, and is one of the top Marcellus Shale natural gas drillers in Butler County, expects to receive about $120 million from the sale.

Rex Energy, and its nonoperating partner, Sumitomo, have leases on about 64,000 acres in the county. The two also will enter an agreement with MarkWest, which is a natural gas gathering, processing and transportation company based in Denver with offices in Houston, Pa.

This agreement will have MarkWest storing, processing and moving natural gas that Rex Energy produces from its Marcellus Shale wells in Butler County and the surrounding area.

The economic scale of this deal dwarfs other recent investments in the county.

Here are three examples:

• The 2010 expansion of Butler Memorial Hospital cost $161 million

• Since 2000, renovations and construction at Slippery Rock University cost $250 million

• Westinghouse Electric’s headquarters in Cranberry Township cost $236 million for the construction of four buildings.

Additionally, MarkWest is planning to extend a planned natural gas liquids pipeline from Houston, Pa., to the compression stations in the county.

The pipeline is expected to be completed in the first half of 2014, which will allow Rex Energy to move ethane from those processing plants.

The sale of Keystone to MarkWest must now move through regulatory approval, but is expected to be completed by this summer.