Facies and stratigraphy of the San Andres formation, northern and northwestern shelves of the Midland Basin, Texas and New Mexico

The San Andres Formation on the Northern and Northwestern Shelves of the Midland Basin is a progradational stratigraphic unit consisting predominantly of carbonate facies. Lithofacies include dolomite, laminated anhydrite and dolomite, massive bedded anhydrite, limestone, salt, and red beds. These lithofacies represent depositional environments that include deep-water outer shelf, shallow-water inner shelf, shallow-water to emergent shoals, and a sabkha complex that comprises intertidal to supratidal algal mud flats, hypersaline lagoons or brine pans, and terrigenous mud flats. Deposition was cyclic; a cycle began with a transgression followed by a gradual shoaling-upward sequence. Cycles commonly terminated with subaerial exposure before renewed transgression initiated a new cycle. Much of the dolomitization probably occurred during periods of subaerial exposure in schizohaline environments. Likewise, porosity was probably also developed during subaerial exposure. Surface topography probably exerted considerable control on dolomitization and porosity development. Additional diagenetic alteration of carbonates may have occurred as a result of an influx of hypersaline brine. San Andres reservoirs of the Northern and Northwestern Shelves yielded 12.7 percent of the total oil production for the State of Texas in 1980. Trapping mechanisms for the oil are both structural and stratigraphic. Maps and cross sections in this report document the nature of these mechanisms. Large volumes of oil are trapped in a discontinuous, structurally high, and stratigraphically thin belt that rims the deep northern Midland Basin and that overlies older shelf margins. Porosity zones thin updip from this belt; source rocks are subjacent to this belt of porosity. Additional oil is trapped in a series of steplike, updip porosity pinch-outs exhibiting little or nonstructural control. Regional porosity pinch-outs control the northern limits of oil production in the Northern and Northwestern Shelves of Texas and eastern New Mexico.