The Ellenburger Group in central and west Texas is part of a vast sheet of Lower Ordovician carbonate sediments deposited on the southern edge of the North American craton. In the west Texas area the Ellenburger, a 1,000 to 2,000-ft (300 to 600 m) section of dolomite, is extremely important as an oil and gas reservoir. Since the first discovery in 1928, the Ellenburger has produced more than 500 million bbl of oil in the Permian basin and, more recently, huge gas reserves have been established in the deep Delaware basin.

During deposition of the Ellenburger carbonate sequence, the predominant environment was probably similar to the area of carbonate mud and pelletal carbonate mud deposition on the Great Bahama Bank. Today the Delaware basin section is largely microcrystalline to coarsely crystalline dolomite with many sedimentary structures indicative of shallow-water to supratidal deposition. On the basis of textures, fabrics, and insoluble residues, the Ellenburger Group in the Delaware basin may be divided into three units. Generally, porosity is confined to the middle and lower units and is related to subaerial solution and associated brecciation. Tectonic fracturing, related to late Paleozoic deformation, apparently is responsible for greatly increasing permeability.

Several Ellenburger fields produce up to 50% carbon dioxide with methane. The carbon dioxide content increases in the west and south toward low-salinity formation water.

Ellenburger hydrocarbons probably were derived from the overlying Simpson (Middle Ordovician) shale.

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