Karst-Controlled Reservoir Heterogeneity and an Example from the Ellenburger (Lower Ordovician) of West Texas

This study is a product of ongoing Bureau of Economic Geology investigations, by means of geologic and petrophysical modeling, into the nature of reservoir compartmentalization or heterogeneity. The model has been designed to provide a basis for subsequent infill drilling in the reservoirs in question along trends of greatest geologic potential, rather than according to geometrically defined well spacings.The Lower Ordovician Ellenburger Group was the site of the investigations from which more than 10,000 ft (3,050 m) of core from 63 wells was logged. Identification of six facies assemblages resulted: (1) lithic arenite, (2) mixed siliciclastic-carbonate packstone, (3) ooid-peloid grainstone, (4) mottled mudstone, (5) laminated mudstone, and (6) gastropod/peloid packstone. These facies assemblages record initial transgression and subsequent progradation and aggradation, indicating that paleoslope was generally southerly and easterly from the Texas Arch toward the Ouachita and Marathon orogenic belts. It was also found that all facies assemblages, with the local exception of the ooid-peloid grainstone assemblage, have low intergranular and intercrystalline porosity.The karst model, constructed by means of cross sections and then an isometric projection of the Ellenburger surface at Emma field hung on the cross-section data, illustrates that porosity development in Ellenburger Group carbonates is directly related to a prolonged period of subaerial exposure that coincided with a Middle Ordovician eustatic lowstand. This latter period occurred before transgression of Simpson Group siliciclastics, during which a widespread system of caves, sinkholes, joint-controlled solution features, and collapse breccias and related fractures developed. Of particular importance to reservoir development was the formation of a regionally extensive cave system between 100 and 300 ft (30 and 90 m) beneath the exposed Ellenburger surface. Infill of this cave system by Simpson Group sand and clay segmented the upper Ellenburger into three karst facies, which are, in descending order, (1) cave-roof dolostones (fracture and mosaic breccias), (2) laterally persistent cave-fill facies (siliciclastic-matrix-supported and carbonate-matrix-supported breccias), and (3) lower collapse facies (chaotic clast-supported breccias) of the cave floor.Pronounced vertical segregation of permeable zones defined by the three karst facies is evident in the Emma, Andector, Martin, and University Block 13 fields and in several other major Ellenburger reservoirs. Lateral reservoir heterogeneities formed by localized, vertically restricted collapse structures, such as those that appear in the Shafter Lake Ellenburger reservoir, also contribute to compartmentalization of producing zones within the upper Ellenburger Group. Secondary and tertiary recovery programs in these Ellenburger reservoirs can be improved by the integration of concepts of lateral and vertical heterogeneity inferred by the karst model.