Styles of heterogeneity in dolomitized platform carbonate reservoirs: Examples from the central basin platform of the permian basin, southwestern USA

Abstract

The Permian Basin of West Texas and southeastern New Mexico, southwestern U.S.A., is the premiere oil basin of the United States. At discovery, reservoiors in this prolific province contained more than 100 billion barrels of oil, almost a quarter of all the discovered in the U.S. Almost half of this resource (43%) was contained in a single reservoir type, dolomitized platform carbonate. Dolomitized platform carbonates were deposited on shallow shelves fringing the basin and on a horst block, the Central Basin Platform, that divides the basin and separates sites of deep-water siliciclastic sedimentation in the adjacent subbasins. The Central Basin Platform hosts many large combined structural/stratigraphic trap reservoirs in dolomitized platform carbonates. These range in size up to four billion barrels of original oil in place. Integrated geoscience and engineering characterization of four of these fields; Dune, Emma, Penwell, and Taylor-Link, allows comparison of the styles and scales of heterogeneities that influence recovery in this reservoir type. Facies composition and architecture exert fundamental controls on paths of fluid movement during production. Principal facies are extensive, deep subtidal fusulinid wackestones that shoal upward into shallow-subtidal and intertidal packstones and grainstones in which the dominant productive facies are grainstone bars and shoals, and shorezone systems that are dissected by dip-oriented tidal systems. Rapid lateral facies changes together with highly cyclic shoaling sequences result in pronounced permeability variations both laterally and vertically in the section. Superimposed on the depositional framework is a multi-event diagenetic overprint. These carbonate reservoirs are thoroughly dolomitized and partly cemented by sulfates. A post-burial leaching event increased permeability in some parts of these rocks. Karst processes have a large affect on reservoir quality in the southern part of the Central Basin Platform. Even though these carbonate reservoirs have undergone substantial diagenetic modification depositional facies still exert the primary control on remaining, and in particular, mobile, oil saturation.