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. Since discovery in 1927 the unit has produced 43 million barrels of primary and waterflood oil of an estimated 164 million barrels of original oil in place. Approximately 30 million barrels of mobile oil remains in the main reservoir of this unit. This report presents the results of the integration of geologic and engineering studies of the reservoir to locate this remaining mobile oil.The upper San Andres Formation [Permian, Guadalupian) at Penwell field is composed of an upward-shoaling suite of shallow-marine, carbonate-ramp facies that are now thoroughly dolomitized and that contain sulfates as nodules and cements. The lower, subtidal part of the stratigraphic section is composed primarily of pellet grainstone/packstone containing varying amounts of fusulinids and bivalves and thin zones of sponge-algal boundstone. These facies are overlain by, and/or interbedded with, thin zones of dasycladacean algae grainstone. The upper, peritidal part of the stratgraphic section is composed of nonporous mudstone and pisolite packstone, which form the reservoir seal. Pisolite packstone facies exhibit abundant subaerial-exposure features such as caliche, low-relief karst-collapse structures, and desiccation cracks. The main reservoir facies, pellet grainstone/packstone, has two diagenetically controlled textures that give the rock a mottled appearance. The darker colored, unaltered texture was found to have permeabilities of approximately 1 md or less and oxygen-isotope compositions in the range of 3 to 5.5%0 (PDB). The lighter colored, altered texture was found to have permeabilities of approximately 10 md or more and oxygen-isotope compositions of 1 to 4%0 (PDB). Alteration was determined to have resulted from late-stage leaching of dolomite, which increased permeability by widening intercrystalline pore throats and preferentially removing early, isotopically enriched parts of dolomite rhombs. Significantly, this diagenetic mottling occurs so minutely and randomly that conventional core-analysis techniques are insufficient to detect it.Maps of production averaged over time (to remove production/drilling-strategy artifacts such as early drilling along lease lines) and values of calculated remaimng mobile oil revealed production patterns resulting from lateral reservoir heterogeneity. The south part of the unit, where nonreservoir peritidal rocks are thick, was found to have low production and low volumes of remaining mobile oil. In contrast, the north part of the unit was found to contain higher volumes of remaining mobile oil related to inferred east-west-trending tidal channels. These tidal-channel features are targets for infill development drilling.
Major, R. P., Vander Stoep, G. W., and Holtz, M. H., 1990, Delineation of Unrecovered Mobile Oil in a Mature Dolomite Reservoir: East Penwell San Andres Unit, University Lands, West Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 194, 52 p.