The Middle-Ordovician Simpson group, deposited in a shallow epicontinental sea, is composed of a sequence of interbedded limestone, shale, and smaller amounts of sandstone. By means of electric and radioactivity logs the group is divided into eight zones traceable across Pecos County, except for its absence, over the Fort Stockton uplift area, caused by post-Simpson erosion. Each zone thins persistently eastward toward the Texas Peninsula, which separated the West Texas basin from the Oklahoma basin of deposition. A northwestward increase in sand content of each zone indicates that the source area for the sand was to the northwest. In Pecos County oil is produced from the Simpson at present in only a small north-central area. / text

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