Porous limestones and dolomites in the San Andres Formation (Middle Permian) serve as reservoirs for many oil and gas accumulations found in the Permian basin of southeast New Mexico and West Texas. The mature state of field discovery, development, and exploitation associated with exploratory effort directed toward the San Andres, together with the wide range of factors which have been found to characterize its hydrocarbon traps, allow it to serve as an example which illustrates many of the basic truths and concepts of modern petroleum geology.

San Andres carbonate and evaporite rocks, together with their lateral formational and lithofacies equivalents, were deposited during a gross cycle of marine transgression and regression. Reservoir porosity is restricted generally to rocks deposited in shelf-margin reef, shallow-marine and intertidal environments. Many of these environments may be identified on the basis of characteristic lithology, depositional features, and fauna. A knowledge of environmental-lithologic patterns is essential to understand the stratigraphic controls affecting hydrocarbon accumulation.

Regional San Andres structure consists of a broad south-plunging syncline characterized by gentle easterly dips from outcrops in central New Mexico and westerly dips from outcrops in central Texas toward an axis near the New Mexico-Texas line. Local closed anticlines, noses, and flexures, which overlie either basement uplifts and faults or older shelf-margin reefs, interrupt the regional structural grain and significantly influence hydrocarbon accumulation.

Potentiometric data indicate that fresh water entering the San Andres in the high mountainous New Mexico outcrop area flows easterly through porous units and becomes highly saline before reaching a discharge area along the lower lying outcrop belt in central Texas. Groundwater movement has caused east-tilting oil-water contacts in many of the oil fields.

Examination of the productive fields indicates hydrocarbon accumulation is controlled by highly variable combinations of (1) structural, (2) stratigraphic,

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and (3) hydrodynamic elements. Trap types recognized range from large closed anticlines with apparent horizontal hydrocarbon-water contacts to more obscure and subtle traps of unclosed structural/stratigraphic or stratigraphic type with notably nonhorizontal hydrocarbon-water contacts.

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