Crude oil production in Texas totals more than 47 billion bbl; however, the rate of production has been declining since 1972. The most rapid decline was 6% in 1979, but subsequent decline rates have diminished to 2.5% in 1983. Production of second-crop oil (attributable to infill drilling and enhanced recovery methods) from Permian basin reservoirs has been responsible for a large part of this improvement. Second-crop oil will likely be instrumental in arresting the rate of production decline for the state.

Production from San Andres and Grayburg reservoirs played a major role in establishing the Permian basin as a premier oil province in Texas and the United States. Since the first commercial production at Westbrook field, Mitchell County, in 1921, these reservoirs have accounted for more than 40% of the oil produced from the Permian basin and more than 15% of the oil produced in Texas. Researchers at the Bureau of Economic Geology have subdivided these reservoirs into several plays on the basis of geographic association and similarities in depositional controls, trapping styles, and drive mechanisms. The reservoirs consist of dolomitized carbonates interpreted as restricted-shelf deposits on the northern and eastern shelves of the Midland basin and restricted-platform carbonates on the Ozona platform, Yates area, and the Central Basin platform. The San Andres and Grayburg exhibits widespread reservoir heterogeneity owing to complex depositional and diagenetic facies relationships. This internal complexity, along with a less efficient solution-gas drive, accounts for the large volume of unrecovered movable oil. Innovative infill-drilling programs based on geologic concepts of facies-controlled reservoir development provide an opportunity for further reserve growth as well as increased production.

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