The Barnett Shale 

The Barnett Shale is a geological formation of economic significance. It consists of sedimentary rocks of Mississippian age (354–323 million years ago) in the U.S. State of Texas. The formation is estimated to stretch from the city of Dallas to west of the city of Fort Worth and south, covering 5,000 square miles (13,000 km²) and at least 17 counties.

Some experts have suggested the Barnett Shale may be the largest onshore natural gas field in the United States. The field is proven to have 2.5 trillion cubic feet (59 km³) of natural gas and is widely estimated to contain as much as 30 trillion cubic feet (8.5×1011 m³) of natural gas resources. Oil also has been found in lesser quantities, but sufficient enough (with recent high oil prices) to be commercially viable.