The Hugoton-Panhandle gas field, in parts of Kansas, Texas, and Oklahoma, is an area of almost 8,500 sq mi and contains one of the world's largest known gas reserves. The field was discovered in 1918 by drilling on a surface structural feature and now includes almost 5.5 million acres which are being drained by 10,500 wells. Production is found in Permian and Pennsylvanian granite wash and carbonate rocks in the south and in Permian dolomite and limestone in the north. The primary controlling mechanisms for the accumulations are, in the south, a compaction anticline over the buried Amarillo ridge and, in the north, a hydrodynamic trap caused by a slight reduction in permeability at the western (updip) edge of the field. In the Kansas part production is limited both updip nd downdip by water. The field is a volumetric reservoir and has produced a total of 24.5 trillion cu ft of gas. It now is producing at the rate of 1.4 trillion cu ft a year. Remaining reserve is estimated to be about 28 trillion cu ft.