Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
2004 West Texas Geological Society Fall Symposium, Midland, Texas, October 2729.
How to Generate a Field-Wide Rock-Fabric Model in a Carbonate Reservoir: Fullerton Clear Fork Field, West Texas
Rebecca H. Jones, F. Jerry Lucia, and Stephen C. Ruppel
A detailed understanding of permeability would greatly improve enhanced recovery strategies at many Permian Basin fields. However, unlike porosity, permeability cannot be measured by wireline tools and therefore is not widely available. As such, porosity is often used as a substitute for permeability, but this substitution can be very misleading because the relationship between porosity and permeability in carbonate reservoirs is typically complex. In order to accurately calculate permeability at Fullerton field, we used a rock-fabric approach that first determines, then applies, rock-fabric-specific porosity-permeability transforms throughout the reservoir.
The rock-fabric approach (described in Lucia, 1995) is based first on identifying rock fabrics and then grouping them into petrophysically similar classes. Petrophysical class 1 rock fabrics include grainstones and large crystalline dolostones; class 2 fabrics include grain-dominated packstones and medium crystalline dolostones; and class 3 fabrics include mud-dominated packstones, wackestones, and mudstones and fine crystalline dolostones. Previous studies have shown that each petrophysical class has a different porosity-permeability transform. Ideally, rock fabrics are described and petrophysical classes assigned using paired thin sections and high-quality core-plug analysis data collected on a foot-by-foot basis. However, these data are not always available. Herein, we describe how new petrophysical data from representative cores can be used to leverage preexisting data and model rock fabrics throughout a field.
To create a continuous rock-fabric profile of the Fullerton reservoir (Leonardian Lower Clear Fork and Wichita), we cut nearly 500 new plugs from two cores. Plugs were cut from each foot of selected core and a set of matching high-quality core analyses and thin sections obtained. Thin sections were described using the rock-fabric approach (Lucia, 1995) and assigned a petrophysical class that was then compared with the petrophysical class indicated from the porosity-permeability cross-plot (detailed results described in Jones et al., 2003). Samples displaying disparate petrophysical class values were reexamined to determine the causes of the inconsistency and their stratigraphic position noted. Once rock-fabric changes were established in this continuous vertical profile, we related their distribution to cycles, systems tracts, and high-frequency and composite sequences. We observed that most major rock-fabric changes occur at the sequence or systems tract boundaries. However, some rock-fabric changes are diagenetic; in these cases, rock-fabric boundaries coincided with boundaries between limestone and dolostone.
In a shallow-water carbonate platform reservoir, such as that at Fullerton Clear Fork field, associations between rock fabrics and sequence stratigraphic elements can be expected to be relatively consistent throughout the reservoir. To test this assumption, we examined all other available thin sections and core analyses from the field. Once we were satisfied that our predictions of rock-fabric relationships were correct, we created porosity-permeability cross-plots from existing whole core analyses, making separate plots for each rock-fabric/sequence stratigraphic interval observed in the continuous rock-fabric profile. Because our continuous vertical rock-fabric profile demonstrated that several rock-fabric changes were coincident with mineralogy changes, we employed wireline logs (photoelectric factor, neutron and density) to map dolostone vs. limestone.
By integrating new, high-quality petrophysical data and a detailed sequence stratigraphic framework in a setting having relatively limited internal variation, we were able to maximize the utility of preexisting petrophysical data and map changes in rock fabrics on a field-wide basis. Our method of calculating permeability using rock-fabric-specific porosity-permeability transforms honors the widely varying pore types and degrees of pore connectivity present in Permian Basin carbonate reservoirs and thereby results in a more realistic 3-D estimation of matrix permeability than many single-transform techniques.
Jones, R.H., Lucia, F.J., Ruppel, S.C., and Kane, J.A., 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: West Texas Geological Society Publication 03-112, p. 47–66
Lucia, F.J., 1995, Rock fabric/petrophysical classification of carbonate pore space for reservoir characterization: American Association of Petroleum Geologists Bulletin, 79(9), p.1275–1300