Caves and Other Features of Permian Karst in San Andres Dolomite, Yates Field Reservoir, West Texas

Abstract

Cave cements, breccias, and several kinds of internal sediment are lithologic features of a Late Permian (Guadalupian, Kazanian) karst recognized in cores from the San Andres Formation in the reservoir of the Yates oil field in the Permian basin of west Texas. Other indications of a paleokarst system at Yates have been bit drops, sudden rushes of oil during drilling, extremely high flow rates recorded for some early field wells, and fragments of dissolution rubble produced with the oil from some wells. Karst is also indicated by 285 unfilled caves in 142 of the 898 wells drilled within the boundaries of the field to the end of year 1983. The caves range in height from 1 foot (0.3 m) to 21 feet (6.4 m), average 2.9 feet (0.9 m), and have a modal height of 2 feet (0.6 m). They are most numerous in the eastern part of the field, where permeable shelf-edge skeletal sands in the upper San Andres made the rock more susceptible to meteoric infiltration and solution. The agents of karstification were probably dynamic freshwater lenses which developed beneath a cluster of low-relief limestone islands produced by tectonic uplift of the field or by one or more low stands of the Late Permian seas. Support for this island hydrologic model comes from well data on the heights of the San Andres caves and their spatial distribution in the Late Permian. The patterns of cave abundance and height closely fit patterns of physical and chemical dynamics believed to be characteristic of freshwater lenses beneath limestone islands.