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.

This study is a product of ongoing Bureau of Economic Geology investigations, by means of geologic and petrophysical modeling, into the nature of reservoir compartmentalization or heterogeneity.

The San Saba Member of the Wilberns Formation of central Texas is mostly coarse-grained, trilobitic limestone that grades eastward to dolomite and contains sandstone intervals in its westernmost exposures.