The Barnett Shale is a geological formation located in the Bend Arch-Fort Worth Basin. It consists of sedimentary rocks dating from the Mississippian period (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.
As of 2007, some experts suggested that the Barnett Shale might have the largest producible reserves of any onshore natural gas field in the United States. The field is thought to have 2.5×10^12 cu ft (71 km3) of recoverable natural gas, and 30×10^12 cu ft (850 km3) of natural gas in place. Oil also has been found in lesser quantities, but sufficient (with 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 impermeable, and it was virtually impossible to produce gas in commercial quantities from this formation until oil and gas companies learned how to effectively use massive hydraulic fracturing in the formation. The use of horizontal drilling further improved the economics, and made it easier to extract gas from under developed areas.
Future development of the field will be hampered in part by the fact that major portions of the field are in urban areas, including the rapidly growing Dallas-Fort Worth Metroplex. Some local governments are researching means by which they can drill on existing public land (e.g., parks) without disrupting other activities so they may obtain royalties on any minerals found, whereas others are seeking compensation from drilling companies for damage to roads caused by overweight vehicles (many of the roads are rural and not designed for use by heavy equipment). In addition, drilling and exploration have generated significant controversy because of environmental damage including contamination to the ground water sources.