Reservoirs in the Permian Basin of Texas are estimated to have contained a total of 105.7 billion barrels (Bbbl) of oil at discovery. As of January 1985, these reservoirs had produced a cumulative volume of 25.3 Bbbl of oil, and proved reserves were calculated at 5.9 Bbbl. Thus, of original oil in place, and given a projected recovery efficiency of less than 30 percent at current development and technology, 74.5 Bbbl of oil will remain in Permian Basin reservoirs at depletion. Eighty-eight percent of this unrecovered oil lies at depths of less than 8,000 ft.
Reservoirs of Guadalupian age, primarily the San Andres and Grayburg Formations (and to a much lesser extent the Yates and Seven Rivers Formations), dominate the oil-resource base, containing more than 50 percent of the original oil in place and 56 percent of the estimated ultimate recovery. In contrast, Leonardian Clear Fork platform carbonates and heterogeneous, low-recovery, Spraberry-Dean submarine-fan sandstones account for nearly 25 percent of the in-place oil but only 13 percent of the ultimate recovery. Of the original oil in place resource, 80.4 Bbbl remains. Together, mobile and residual oil account for 93 percent of the remaining oil; proved reserves total 7 percent. The cumulative volume of unrecovered mobile oil in excess of proved reserves in the Permian Basin is estimated to be 30 Bbbl, concentrated on the Central Basin Platform. Plays on the fringing shelves and within the interior subbasins are also good prospects for the extension of conventional recovery strategies. Restricted-platform carbonates, containing 44 percent of the remaining mobile oil, are the dominant reservoir rocks. They also contain 40 percent of the residual oil; thus, platform carbonates are a primary target for extended conventional and tertiary oil recovery strategies in the Permian Basin of Texas.