As oil and gas production from conventional sources continues to decline, companies are concentrating on unconventional sources such as shale formations. The United States has significant technically recoverable deposits of shale oil and gas. Natural gas from proven and unproven shale resources accounts for 542 trillion cubic feet (Tcf) or around a quarter of the total natural gas resource estimate of 2,214 Tcf as of January 1, 2010.In addition to the recoverable shale gas deposits, there are large shale oil formations in Monterey/ Santos, Southern California, estimated to hold 15.4 billion barrels or 64 percent of the total shale oil resources as well as in the Bakken and Eagle Ford.