Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
West Texas Geological Society Fall Symposium, Midland, Texas, October 25-26, 2001
South Wasson Clear Fork Reservoir Model
F. Jerry Lucia, Stephen C. Ruppel, James W. Jennings,Jr., and Stephen E. Laubach
A reservoir model for the South Wasson Clear Fork (SWCF) field, Gaines County, Texas has been constructed using information from an outcrop analog in Apache Canyon, Sierra Diablo Mountains, and from the subsurface. The SWCF field contains two reservoirs, the lower Clear Fork and the middle Clear Fork. There is a seal between the middle and upper Clear Fork, and the upper Clear Fork apparently contains residual oil resulted from oil remigration. Permeability and initial water saturation are calculated using simple relationships to total porosity despite the wide diversity in rock fabrics. The large volume and patchy distribution of anhydrite apparently reduce porosity with little change in pore size, resulting in a grouping of the various rock fabrics into one rock-fabric petrophysical class. Porosity values were calculated from wireline logs, and vertical profiles of permeability and initial water saturation were calculated using a single porosity-permeability transform.
A sequence stratigraphic framework was used to distribute porosity and permeability throughout the reservoir. The framework was based on outcrop studies, seismic response, and core descriptions. The cyclic nature of the Clear Fork is illustrated by outcrop descriptions, and this character has been applied to the subsurface using seismic data and core descriptions. The basic correlation unit is the high-frequency cycle (HFC). The HFC's are subtidal cycles defined by caps of grain-dominated fabrics. One group has silt bases and can be easily correlated using acoustic and porosity logs. The second group is correlated on the basis of statistical data that shows that grain-dominated dolostones tend to be more porous than mud-dominated dolostones. Each HFC is divided into a grain-dominated and a mud-dominated flow layer for reservoir simulation studies. Statistical analysis of porosity and permeability data from the outcrop shows poor small-scale spatial correlation within one rock-fabric flow layer but a larger scale of variability that may have more spatial correlation and impact on fluid flow.
The rock-fabric reservoir model contains 44 layers in the middle Clear Fork and 44 layers in the lower Clear Fork. Permeability is distributed within these layers using various statistical methods, and simulation results are compared. The results are also compared with a proportional method of building the reservoir model. The impact of fractures on performance is examined.
Lucia, Senior Research Fellow