Pioneer Natural Resources Company is a large independent exploration and production company with operations in the United States, Tunisia and South Africa. It was formed in 1997 as a result of a merger between Parker & Parsley Petroleum Company and MESA Inc. The company is one of the largest operators in the Hugoton gas field in Kansas and the West Panhandle gas field in the Texas Panhandle. Pioneer Natural Resources Company is one of the most active drillers in the South Texas Pawnee and Edwards Reef fields. The company is a Fortune 500 company and is included in S&P 400 Index.