From Bureau of Economic Geology, The University of Texas at Austin (
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Bureau Seminar, April 1, 2005

Surprising Lessons from Multidisciplinary Characterization of a Permian Carbonate Platform Reservoir

Stephen Ruppel


Geological characterization of Fullerton Clear Fork field, a 30,000-acre (12,000-hectare), 1.5-billion-barrel, shallow-water platform reservoir in West Texas, has revealed a broad spectrum of geological, petrophysical, and geophysical relationships that provide new insights into controls of reservoir development and the interpretation of subsurface data sets.

Among the surprising, nonintuitive lessons learned from this study are (1) major seismic reflectors may define facies changes instead of time lines or sequence boundaries; (2) permeability is a function of depositional facies, whereas porosity is a function of diagenesis; (3) porosity development is a response to early diagenesis and stabilization: in peritidal rocks, by dolomitization and in subtidal rocks by calcite lithification; (4) limestones in this dominantly dolostone reservoir are low-permeability flow baffles in peritidal rocks, but permeable, high-flow layers in subtidal rocks; (5) borehole geophysical data can provide almost as much geological information as cores; (6) properly calibrated porosity logs can be better tools for identifying and correlating depositional cycles than gamma-ray logs; (7) 3D seismic data can be a more robust measure of interwell and extrawell porosity distribution than well log data; and (8) diagenesis and porosity development are locally the result of reactivation of deep structures. An important key to these insights is the integration of both outcrop and subsurface data.

Although all of the observations cited above derive from specific circumstances associated with Fullerton field, each provides important insights into the interpretation of subsurface data sets in carbonate reservoirs worldwide.