The Barnett Shale is a hydrocarbon-producing geological formation which consists of sedimentary rocks and the productive part of the formation is estimated to stretch from the city of Dallas west and south, covering 13,000 km² and is estimated by some as the largest onshore natural gas field in the United States. The Barnett Shale has acted as an important source and sealing cap rock for conventional oil and gas reservoirs in the area. It was thought that only a few of the thicker sections close to Fort Worth would support economic drilling. It was not until the 1980’s with new advances in horizontal drilling and well fracturing technology used by Mitchell Energy, a small independent, that the potential of the Barnett Shale was realized. Significant drilling activity did not begin until gas prices increased in the late 1990’s. Devon Energy acquired Mitchell Energy in 2002, and has established itself as the leading producer from the Barnett Shale. Natural gas production in the US increased by 8.8% during the first half of 2008 compared to the previous year with most production stemming from the Barnett layer in Texas, which is proof that new reserves can be extracted thanks to new technology