The Ellenburger Group (Lower Ordovician) of Texas is a laterally extensive peritidal carbonate shelf sequence. It forms a major deep oil reservoir, having estimated reserves of 1.15 billion barrels of oil, and it also contains an estimated 2.2 billion barrels of oil equivalent.

 4 oversized sheets, Lambert Conformal Conic projection based on standard parallel 33 degrees and 45 degrees. Accompanied by a text booklet, The Tectonic Framework of Texas.

Horizontal drilling in a mature hydrocarbon province such as Texas can increase recovery from reservoirs in which a significant proportion of oil or gas in place remains unrecovered by conventional means.

The East Penwell San Andres Unit produces from a depth of approximately 3,400 ft (1,040 m) on the east flank of a broad, low-relief anticline on the east side of the Central Basin Platform of the Permian Basin in Ector and Crane Counties, West Texas.