The Barnett Shale is a geological formation located in the Bend Arch-Fort Worth Basin. It consists of sedimentary rocks of Mississippian age (354–323 million years ago) in Texas. The formation underlies the city of Fort Worth and underlies 5,000 mi² (13,000 km²) and at least 17 counties.
Some experts have suggested the Barnett Shale may have the largest producible reserves of any onshore natural gas field in the United States. The field is proven to have 2.5×1012 cu ft (71 km3) of natural gas, and is widely estimated to contain as much as 30×1012 cu ft (850 km3) of natural gas resources. Oil also has been found in lesser quantities, but sufficient (with recent high oil prices) to be commercially viable.
The Barnett Shale is known as a "tight" gas reservoir, indicating that the gas is not easily extracted. The shale is very hard, and it was virtually impossible to produce gas in commercial quantities from this formation until recent improvements were made in hydraulic fracturing technology and horizontal drilling, and there was an upturn in the natural gas price.