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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 (850,000,000,000 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.
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.
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).
The Barnette Shale field may increase our known reserves by as much as 15%. (Tim)