We are employing a rock-fabric methodology to calculate permeability in the Fullerton Clear Fork reservoir. This methodology is based on the observation that different rock fabrics display distinct porosity/permeability interrelationships. To use this approach, one must define (1) the number and types of rock fabrics on the basis of their fabric, pore type (Figure 8), lithology, and crystal size, and (2) the distribution of these rock fabrics within the stratigraphic framework. Relationships between porosity and permeability can then be used to define and apply transforms for each rock-fabric class. To achieve this goal, we assembled a complete and unbiased calibration data set of core-plug samples and matched thin sections for the entire lower Clear Fork-Wichita reservoir succession at Fullerton. Carefully prepared and analyzed core-plug data were then compared with thin-section descriptions of each plug to define rock-fabric (RF) classes.

Initial interpretations from this work demonstrate that the Wichita consists dominantly of tidal-flat fabrics (Figure 9) including RF Class 3 mud-dominated limestones, and RF Class 3 fine-crystalline mud-dominated dolostones (Figure 10). Porosity/permeability data from these samples plot as expected in the RF Class 3 field. The lower Clear Fork high-frequency sequence L2.1 contains mostly RF Class 2 grain-dominated dolopackstones (Figure 11), medium-crystalline mud-dominated dolostones, and RF Class 1 lime grainstones. However, because these lime grainstones contain significant separate-vug porosity (Figure 12), they exhibit Class 2 or 3 porosity/permeability relationships. Lower Clear Fork high-frequency sequence L2.2 rocks contain similar rock fabrics to those in L2.1, but many RF Class 2 dolostones display petrophysical Class 1 porosity/permeability relationships. We think this inconsistency, which has been observed in other Clear Fork successions (but not in other Permian carbonate rocks), is due to the presence of poikilotopic anhydrite (Figure 13); the presence of anhydrite in patches causes a decrease in porosity but little corresponding decrease in permeability.

Although reservoir cycles (Figure 6) typically display muddy, low-porosity bases and more porous grain-dominated tops, these facies do not necessarily correspond to changes in RF class. Major changes in RF class, which seem to be controlled by a combination of diagenesis and facies, instead are more apparent at the scale of cycle sets or high-frequency sequences.