But it will be around for a long time." Woods explained that Exxon and Pioneer will be able to use their combined capabilities to drive down emissions and produce lower carbon intensity oil and gas. Exxon purchased XTO Energy in 2009 for approximately $36 billion. In the late 1990s, the merger between Exxon and Mobil was valued around $80 billion. The deal with Pioneer Natural vastly expands Exxon's presence in the Permian Basin, a massive oilfield that straddles the border between Texas and New Mexico. Drilling the Permian accounted for 18% of all U.S. natural gas production last year, according to the U.S. Energy Information Administration.Pioneer's more than 850,000 net acres in the Midland Basin will be combined with Exxon's 570,000 net acres in the Delaware and Midland Basin, nearly contiguous fields that will allow the combined company to trim costs.