From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

2003 West Texas Geological Society Fall Symposium, Midland, Texas, October 8–9, 2003

Better Than a Porosity Cutoff: The Rock-Fabric Approach to Understanding Porosity and Permeability in the Lower Clear Fork and Wichita Reservoirs, Fullerton Field, West Texas

Rebecca H. Jones, F. Jerry Lucia, Stephen C. Ruppel, and Jeffrey A. Kane

Abstract

Porosity cutoffs have traditionally been used to subdivide reservoirs, but using this practice alone fails to yield adequate information about the connectivity of pore space. To account for permeability variation in the mixed limestone and dolostone reservoir at Fullerton field, we employed the rock-fabric approach in an initial study area as a pilot for field-wide flow modeling. In this method, samples are assigned a petrophysical class on the basis of fabric, pore type, lithology, and crystal size and then class-specific transforms are used to calculate permeability from wireline log porosity. For this technique to be successful it is necessary to obtain a complete and unbiased calibration sample set (i.e., foot-by-foot samples) with high-quality core analyses and thin sections made from each sample. To create a continuous record of the lower Clear Fork and Wichita reservoirs, we sampled two cores using this technique, described the matching thin sections, and assigned a petrophysical class to each sample. We then considered vertical petrophysical class variation in the context of our core-based sequence stratigraphic framework.

Whereas cycles typically contain a muddy, low-porosity base and more porous grain-dominated top, there is not necessarily a corresponding change in petrophysical class. Most major petrophysical class changes occur between sets of cycles or high-frequency sequences because of the overprint of dolomitization and other types of diagenesis. We mapped these stratigraphically keyed vertical changes in petrophysical class throughout the study area and calculated permeability from wireline log porosity using rock-fabric-number transforms to create a 3-D permeability model. Core analysis permeability at other cored wells in the study area show this model to be accurate. We anticipate that our permeability model will be useful for modeling saturation and ultimately identifying unswept areas for enhanced recovery at Fullerton field.