Clear Fork reservoirs in the Permian Basin typically display a wide range of geologic and petrophysical properties that make the efficient recovery of hydrocarbons difficult. A key step in improving recovery efficiency is defining the patterns of variability in these rocks. The critical elements of variability that must be defined are facies, groupings of rocklike properties; and sequence architecture, the framework of facies variability. As in all carbonate reservoirs, rock-based studies must form a fundamental basis for characterizing and modeling facies and sequence architecture variability through the reservoir. Combined with wireline-log data, they provide a basis for defining both rock attribute distributions and reservoir framework.
At Fullerton field, we used 29 cores (>14,000 ft [>4270 m]), well logs from approximately 800 wells, three-dimensional seismic data, and outcrop data to define facies (rock attributes) and sequence stratigraphy (reservoir framework). The Fullerton reservoir section averages 500 ft (152 m) that can be subdivided into three stratigraphic units (Abo, Wichita, and Lower Clear Fork) and parts of two composite and six high-frequency sequences. At the base of the reservoir section, Abo rocks (sequences L1.1 and L1.2) consist of clinoformal, outer-platform, subtidal, fusulinid-crinoid packstones that exhibit locally excellent porosity and permeability characteristics but are highly variable in continuity. Wichita rocks were deposited in peritidal tidal-flat settings and consist of mud-rich facies that generally display poor continuity and commonly very low permeability and oil saturation despite locally high porosity. Wichita rocks (sequences L1.2 and L2.0) are updip inner-platform equivalents of both partly underlying Abo and overlying Lower Clear Fork facies. Lower Clear Fork rocks (sequences L2.1 and L2.2) are dominantly middle-platform subtidal, grain-rich ooid-peloid packstone and grainstone facies that exhibit the best permeability and oil saturation properties.
Although basic facies distributions are defined by high-frequency sequence architecture, the reservoir framework must be based on the correlation of higher resolution depositional cycles. Because gamma-ray logs showed little or no relationship to facies and cyclicity, we calibrated porosity logs to cyclicity and used them to define 10 to 15 ft (3 to 5 m) cycles throughout the reservoir.