In the South Permian Basin the Permian rests on an irregular surface of folded older rocks. On the east side of the basin the exposed Permian is sparsely fossiliferous. The lower part is largely limestone and the upper is red clastic sediment with layers of gypsum. In the mountains west of the Pecos River the Permian is fossiliferous limestone, sandstone, and shale except for the highest formations, which are largely gypsum and redbeds. In the subsurface the Permian strata pass through a succession of changes in lithologic facies, which are related to the regional structure. Two broad negative areas subsided during the Permian more rapidly than the platforms bordering them. Limestone reefs and banks on the margins of the platforms grade into clastic rocks in the intervening negative areas. On the platforms, between the marginal reefs, are lagoonal dolomites, evaporites, and clastic rocks. The Permian Basin was probably connected with the open sea by way of a geosyncline crossing northern Mexico. Because of the narrowness of the entrance to the basin and the barriers to free circulation of ocean water within the basin, the salinity of the water probably varied greatly from place to place. Evaporites were deposited in areas where the salinity of the water was abnormally high.
Permian oil reservoirs are in porous dolomites, and to a lesser extent in sandstone layers. Production is obtained from several of the pre-Permian formations.