CONTROLS ON RESERVOIR HETEROGENEITY IN PERMIAN SHALLOW-WATER PLATFORMCARBONATE RESERVOIRS, USA - IMPLICATIONS FOR SECONDARY RECOVERY

Shallow-water platform carbonate reservoirs of Permian age have accoun ted for more than half of the oil production in the Permian Basin, one of the largest oil producing regions in the United States. Despite mo re than 70 years of production activities, including advanced primary and secondary development, however, these reservoirs still contain as much as two-thirds of the original oil in place. This low recovery eff iciency is linked to an incomplete understanding of the geological and petrophysical heterogeneities that characterize these reservoirs and control fluid flow in the subsurface. Integrated outcrop and subsurfac e studies of Permian reservoir successions in the Permian Basin illust rate that heterogeneity is significantly the product of the combined e ffects of original depositional patterns and subsequent diagenesis. Im portantly the relative contribution of each of these to the developmen t of reservoir porosity and permeability varies systematically across the depositional platform and through the stratigraphic succession in concert with topography, depositional environment, a relative changes in sea level. Accurate modeling, and simulation of such reservoirs dem ands detailed characterization of subtle but significant changes in de positional and diagenetic facies, which can best be accomplished by ap plying integrated models of geological and petrophysical architecture best obtained through a synthesis of outcrop and subsurface studies. T he paper describes the styles and causes of heterogeneity in Permian r eservoirs of the Permian Basin and illustrates the importance of such investigations in designing optimum strategies for recovery of remaini ng oil resources. Experience from the Permian Basin suggests that maxi mum recovery of the original oil resource depends upon application of these types of studies early in the reservoir exploitation history and preferably before secondary recovery operations have begun.