The Oklahoma City field, located near the geographical center of the state, is a large faulted anticline in the pre-Pennsylvanian rocks the structure of which is represented by anticlinal folding in the Pennsylvanian and Permian sediments. It was discovered by geological mapping on the Garber sandstone (a surface formation). The main reservoir rocks are massive magnesian limestone of Cambro-Ordovician age and sandstones in the Simpson group of Ordovician age. Oil is obtained at depths ranging from 6,100 to 6,500 feet under pressures of 2,300 pounds per square inch. The total estimated productive area is 12,800 acres, approximately 60 per cent of which has been developed. Fifty-thousand-barrel wells are not uncommon. Many records have been established for the drilling and ompletion of large wells. Folding and faulting of the pre-Pennsylvanian sediments occurred in late Mississippian or early Pennsylvanian.