San Andres and Grayburg reservoirs have yielded about 42 percent (9.8 billion barrels) of the total cumulative production of oil from the Permian Basin of West Texas. However, recovery efficiencies have been low, and significant quantities of mobile oil remain after primary and conventional secondary recovery. Major San Andres and Grayburg reservoirs, each of which has produced more than 10 million barrels, are estimated to contain 8.7 billion barrels of unrecovered mobile oil. About 900 million barrels of unrecovered mobile oil lies within a northwest-trending group of fields including McElroy, Dune, Waddell, Jordan, and Penwell on the eastern margin of the Central Basin Platform. This oil is trapped within reservoir heterogeneities inherited from original depositional systems and modified by diagenesis during subsequent burial. Current rates of production decline in these old fields can be moderated or reversed through selective recompletion and infill-drilling programs guided by a knowledge of the distribution of remaining mobile oil. This distribution can be defined only through integrated geological and engineering studies of the reservoirs.Such a study of part of the Dune field has resulted in the delineation of more than 10 million barrels of unrecovered mobile oil in an area of only 1 square mile. Much of this oil occurs in a single reservoir interval (the middle unit) that includes a high-permeability grainstone bar facies. The low recovery of hydrocarbons from this zone is a result of limited reservoir contact during primary production and waterflooding because of pronounced permeability stratification. Detailed characterization of carbonate rock types and diagenetic history, calibration of well-log data to low-temperature core analysis, and mapping of remaining oil-saturated thickness and permeability trends are key aspects of this study. Successful application of these approaches at the Dune field suggests that wider use of integrated methods offers major economic potential in an otherwise mature hydrocarbon province.