The Permian Central Basin platform consists in large part of a great carbonate build-up of Wolfcamp to upper Guadalupe age, and within it may be recognized a great many individual but related reefs. One of the best revealed of these reefs is one of late San Andres age which extends for more than 70 mi along the east flank of the Central Basin platform from the Penwell-Jordan pool of Crane and Ector Counties, to the Means pool of northern Andrews County. Isopach and facies studies demonstrate that during late San Andres time this reef grew in relatively shallow water along the then eastern margin of the platform. To the east lay somewhat deeper water of the Midland basin where rocks of the same age are sandstone, limestone, and dolomite. On the opposite or western side of he reef the water was nearly as shallow as over the reef itself, but slightly more saline, and the rock is dolomite with traces of anhydrite.

Growth of the upper San Andres reef ceased at the close of San Andres time, but the reef was affected by subsequent post-San Andres crustal movements so that now the southern part of the reef is 600 ft higher than the northern end.

During reef growth, porosity developed widely throughout its extent and can now be traced almost continuously from the southern to the northern extremity, but beyond the reef to the east and to the west, relatively little porosity developed. By the beginning of post-San Andres time the voids had been filled with fluids--gas, oil, and water. These fluids, according to their specific gravities, responded to subsequent structural movements, within the limits of available porosity, so that now the oil is concentrated in the several pools which occupy much of the trend.