Petrophysical Characterization and Distribution of Remaining Mobile Oil: South Cowden Grayburg Reservoir, Ector County, Texas

This report uses the rock fabric method to construct a reservoir model of a Permian Basin Grayburg dolomite reservoir. Rock fabrics are related to porosity, permeability, and capillary pressure and are distributed within a sequence stratigraphic framework. Key findings are that 1) remaining oil saturation is found in intervals with between 0.1 and 1 md, below perforated intervals, and between injection wells when no producer is present; 2) the best production comes from areas where anhydrite has been dissolved, creating a touching-vug reservoir; and 3) the presence of a 100-ft ROZ indicates that this reservoir is in imbibition mode, not drainage. The South Cowden reservoir, a large Permian Basin oil field in Ector County, West Texas, produces from a dolomitized carbonate-ramp succession in the Grayburg (Permian) Formation. The field can be divided into a western area, characterized by interparticle porosity and by diagenetic products that conform moderately well to depositional textures, and an eastern area, characterized by interparticle and touching vug porosity and by poor conformance of diagenetic products to depositional textures. Thin-section analysis demonstrates that the reservoir interval can be characterized by grain-dominated dolopackstones, medium crystalline mud-dominated dolostones, and a touching-vug pore system composed of dissolved-anhydrite nodules, fusumolds, and microfractures. Two rock-fabric-specific porosity-permeability transforms are defined, one for the grain-dominated and medium crystalline, mud-dominated dolostones and one for the touching-vug pore system. In the western area the porous and permeable intervals conform to high-frequency cycles (HFC), and flow barriers occur at some cycle boundaries. In the eastern area, late dissolution of anhydrite has occurred, and no evidence has been found to relate the removal of anhydrite to HFC's. As a result, there is little conformance of petrophysical properties to HFC's, horizontal flow barriers are few, and pillars of impermeable anhydritic dolomite are scattered throughout the area. The presence of a 100-ft residual-oil zone (ROZ) at the base of the reservoir downdip indicates that the downdip part of the reservoir is in imbibition rather than drainage capillary-pressure mode. Imbibition curves have lower water saturations in the productive interval and significantly smaller transition zones than predicted by drainage curves. The initial water-saturation profile can be approximated by assuming that the base of the ROZ is the zero capillary-pressure level (zcpl). However, making this assumption will overestimate the volume of oil in the ROZ and transition zone. If the zcpl is placed above the base of the ROZ, the original oil in place will be understated. The ROZ is thought to have formed by remigration of oil as a result of eastward tilting during the Tertiary because a ROZ does not appear to be in the updip part of the field. The three-dimensional distribution of remaining oil can be described by integrating permeability maps, vertical conformance of waterflooding from log analysis, and operational history. Intervals that have been invaded by floodwater are identified by comparing original-water-saturation estimates from capillary-pressure models with water saturation calculated from resistivity logs according to the Archie equation. Flooded intervals have more than 1 md permeability, and considerable volumes of movable hydrocarbons are located in zones having 1 to 0.1 md permeability. Not all of the greater-than-1-md rock is flooded, however. Unflooded high permeability is found (1) below perforated intervals, (2) between injection wells when injection and producing wells are located within a large area of high permeability, and (3) between producing wells and low-kh areas, even if an injector is in the low-kh area.