From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, Friday, April 8, 2005
Distribution of Reservoir Rock Fabrics and Petrophysical Classes: Fullerton Clear Fork field, West Texas
A major task in building a reservoir model is quantifying the geologic framework with petrophysical properties. Porosity and water saturation values can be obtained from wireline logs, but permeability cannot. The traditional engineering approach is to construct a single porosity-permeability transform from core data to estimate permeability from porosity logs. In a recently completed study of Fullerton Clear Fork field in West Texas, geologists and engineers worked together to define porosity-permeability transforms specific to both stratigraphic intervals and regions of the field. These transforms are defined on the basis of rock fabrics, which include carbonate texture, pore type, and dolomite crystal size. The numerous rock fabrics are grouped into three petrophysical classes—each having its unique transform—and linked to stratigraphy and region through the study of high-resolution thin sections and core descriptions.
The reservoir at Fullerton field is Permian System and Leonardian Series in age. Formations include the Abo, Wichita, and Lower Clear Fork, which describe various facies composing Leonardian sequences 1 (L1) and 2 (L2). The most striking petrophysical differences occur between peritidal and subtidal facies. Observed peritidal facies include cemented lime mudstones and wackestones and fine-crystalline dolostones. These fabrics are petrophysical class 3 and have low permeability, even given high porosity, because of the small size of interparticle pores and the abundance of separate, fenestral pores. Subtidal facies include lime packstones and grainstones and medium- to coarse-crystalline dolostones. These fabrics are petrophysical class 2 and 1 and have comparatively higher permeability for a given porosity because of the dominance of larger, well-connected interparticle pores.