The Oakville Formation consists of sediments deposited by several major fluvial systems that traversed the Texas Coastal Plain during the Miocene Epoch. Facies geometry and composition, together with superimposed structure and topography, are important determinants of ground-water flow, aquifer transmissivity, and regional hydrochemical evolution.

San Andres oil constitutes more than 80 percent of the total production from the Northern Shelf of the Midland Basin, Texas. The San Andres and Clear Fork carbonate rocks of the Northern Shelf contain sufficient amounts of lipid-rich organic material to rank them as potential petroleum source beds. Organic maturation of these rocks, as revealed from vitrinite reflectance and kerogen color, however, is not sufficient to have initiated catagenesis. Therefore, oil within Northern Shelf reservoirs was derived mostly from other sources.

Eocene stratigraphic units in the East Texas Basin are composed of a thick sedimentary sequence (approximately 2,000 ft [600 m]) of fresh-water aquifers and aquitards covering an area of approximately 15,000 mi2 (51,000 km2). Analysis of abundant data on hydraulic head, pressure, and water chemistry from water wells tapping the Wilcox, Carrizo, and Queen City aquifers reveals the regional ground-water circulation patterns, locations and mechanisms of major recharge and discharge areas, and evolution of ground-water chemistry.

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.