The Olmos Formation is a Late Cretaceous formation in the easternmost portion of the Rio Grande Embayment of the Gulf Coast Basin in Texas. It is the uppermost unit in the Campanian Taylor Group of Texas (Indest and McPherson, 1985), unconformably overlain by the Escondido Formation and underlain by San Miguel Formation (Stephenson, 1942). Both of the Escondido and San Miguel Formations are sequences of mudstone and sandstone. The formation consists of greenish-gray shaly clay and finely sandy clay, interbedded with greenish-gray, cross-bedded soft to hard sandstone, with seams of coal and lignite (Stephenson, 1927). The Olmos wedge thickens in a southward direction starting from southern Medina and Uvalde County, reaching 1,000 feet depth in Webb County with two depocenters (Tyler, 1987) with the principal axis of deposition east-west trending (Indest and McPherson, 1985). Six lithofacies are defined by Indest and McPherson (1985), with Lithofacies F and C having the greatest reservoir potential for stratigraphically-trapped hydrocarbons, or inversely, carbon dioxide. A large portion of oil production in the Olmos is from low-permeability delta-front, shoreface, and shelf sandstones (Tyler, 1987) which could be explored for carbon dioxide injection in areas where the permeability is sufficiently high. In Central and South Texas, many hydrocarbon traps are created by submarine volcanic extrusives creating mounds that affects the formation, specifically in Zavala County where the Olmos is locally affect by more than 30 volcanic bodies. The resulting differential compaction has created thinning and draping, and northeast-southwest trending horsts and grabens over volcanic mounds (Indest and McPherson, 1985).
Indest, D.J., Mcpherson, J.G. 1985. "Depositional Systems In The Olmos Formation And Their Relationship To Volcanic Highs And Hydrocarbon Emplacements, Zavala County, Texas", Habitat Of Oil And Gas In The Gulf Coast, Bob F. Perkins, Gene B. Martin. doi: 10.5724/gcs.85.04.0191.
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Stephenson, L.W. 1927. Notes on the stratigraphy of the Upper Cretaceous formations of Texas and Arkansas: American Association of Petroleum Geologists Bulletin, v. 11, no. 1, p. 1-17.
Stephenson, L.W., King, P.B., Monroe, W.H., and Imlay, R.W., 1942, Correlation of the outcropping Cretaceous formations of the Atlantic and Gulf Coastal Plain and Trans-Pecos Texas: Geological Society of America Bulletin, v. 53, no. 3, p. 435-448.
Tyler, N., 1987, Depositional Setting and Production Characteristics of Low Permeability Sandstones, Upper Cretaceous Olmos Formation, South Texas, SPE/DOE Joint Symposium on Low Permeability Reservoirs, 18-19 May, Denver, Colorado, Society of Petroleum Engineers, doi: 10.2118/16424-MS.
Prepared by Emily Moskal, 2020.