South Wasson Clear Fork Reservoir Modeling: The Rock Fabric Method for Constructing Flow Layers for Fluid Flow Simulation

F. Jerry Lucia, James W. Jennings, Jr., Stephen C. Ruppel
Bureau of Economic Geology
John A. and Katherine G. Jackson School of Geosciences
The University of Texas at Austin

The Rock Fabric method for characterizing carbonate reservoirs provides a fundamental link between petrophysical properties and geologic descriptions by relating pore-size distribution to particle size, sorting, and vug types. Flow layers are defined as stratigraphic layers composed of the same fundamental rock fabric because outcrop data have shown that petrophysical properties are highly variable but near randomly distributed within rock-fabric layers. This method has been applied to the South Wasson Clear Fork reservoir with some interesting results. In this reservoir, rock fabrics include large and medium crystalline grain-dominated dolopackstones and medium crystalline mud-dominated dolostone of petrophysical class 1 and 2. Crossplots of porosity and permeability demonstrate that the fabrics tend to plot in the petrophysical class 1 field, whether classified as class 1 or class 2. It is suggested that the class 2 fabrics shift into the class 1 field as a result of the presence of unusually large volumes of poikilotopic anhydrite. Adding the patches of anhydrite reduces the porosity, but the pore sizes, and thus permeability, remain nearly constant. As a result, a single porosity-permeability transform and a single saturation model was used to calculate permeability and initial water saturation from well logs.

Rock-fabric flow layers are the basic elements for constructing a fluid flow model. Three basic rock fabrics are found in this reservoir; silty mud-dominated dolostone, mud-dominated fabric, and grain-dominated dolopackstone. These fabrics are systematically distributed within two types of high-frequency cycles (HFC’s): silt-based cycles and carbonate cycles. Both types of cycles typically have bases composed of mud-dominated fabrics and caps of grain-dominated dolopackstone. Therefore, each HFC contains two rock-fabric flow layers. HFC’s and flow layers were mapped using the acoustic, resistivity, and porosity logs. The gamma ray log was of little use because of the large amount of diagenetic uranium present. Acoustic, porosity, and resistivity logs were used to identify silt-based cycles. Porosity logs were used to identify silt-free carbonate cycles on the basis of a statistical study of porosity and rock fabrics that shows that mud-dominated fabrics tend to have lower porosity than grain-dominated fabrics. The resulting rock-fabric maps together with the calculated permeability and initial water saturation comprise the basic information from which the flow simulation model is constructed.