Natural Gas in Devonian and Silurian Rocks of Permian Basin, West Texas and Southeast New Mexico

Abstract

Abstract Gas and oil fields producing from Devonian reservoirs in the Permian basin of West Texas and New Mexico form a remarkably uniform pattern—fields producing only gas on the south and west, fields with high gas/oil ratios and gas-condensate fields near the center of the region, and fields which produce only oil on the north. The methane content of gases in these fields has a similar regional variation from more than 90 per cent methane on the south and west to less than 30 per cent on the northeast. Analysis of variation of other gas components and crude oil suggests that the methane variation is independent of both other gases and crude oil composition. Relation of methane variation to variation in gas volume suggests that variation in gas distribution is primarily a function of methane distribution. Limited data suggest that carbon dioxide may be anticipated in increasing volumes along the southern margin of the region. Uniformity of the observed pattern indicates that only gas will be found in Silurian-Devonian reservoirs in the Delaware and Val Verde basins. The gas-distribution pattern bears no consistent relation to Silurian-Devonian lithofacies or structural relief. There is simply an increase in the amount of methane toward the Marathon thrust belt. It is proposed that the gas pattern may have been effected by distribution of methane in solution in formation water migrating away from the Marathon thrust belt under the influence of supernormal pressures resulting from overthrusting. Carbon dioxide may have been distributed similarly at a later time and in smaller volumes than methane. Differential distribution of both gases may have been a function of volume of methane or carbon dioxide in solution in a given area and of the volume of gas-bearing water reaching the area.