Integration of Rock Fabrics and Stratigraphy for Petrophysical Quantification of Reservoir Framework
Rebecca R. Harrington, F. Jerry Lucia, 2012. "Integration of Rock Fabrics and Stratigraphy for Petrophysical Quantification of Reservoir Framework", Anatomy of a Giant Carbonate Reservoir: Fullerton Clear Fork (Lower Permian) Field, Permian Basin, Texas, Stephen C. Ruppel
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A major task in building a reservoir model is quantifying the geologic framework using petrophysical properties. Porosity and water saturation values can be obtained from wireline logs, but permeability is a rock property that cannot be obtained directly from logs. Commonly, a single porosity-permeability transform is used to estimate permeability from porosity logs. However, such an approach fails to account for variations in porosity-permeability relationships that are common in carbonates. In this study, rock-fabric-specific porosity-permeability transforms are used together with log porosity to calculate permeability. Although the reservoir at Fullerton field contains several rock fabrics, they can all be grouped into three petrophysical groups, each having a unique porosity-permeability relationship. These petrophysical groups can be linked to facies and stratigraphy using an integrated study of thin sections and cores.
The lowermost stratigraphic unit in the reservoir (the Wichita of sequences L1 and L2) is composed dominantly of peritidal facies consisting of fine-crystalline mud-dominated dolostones and mud-dominated limestones, all of which can be assigned to a single petrophysical group (class 3). In contrast, rocks of the overlying Lower Clear Fork (sequences L2.1–L2.3) display far more petrophysical variability, both stratigraphically and geographically. Sequence L2.1 comprises an upper late highstand peritidal succession and a lower, transgressive, and early highstand systems tract succession of subtidal facies. The peritidal rocks such as those of the Wichita are fine-crystalline mud-dominated fabrics of petrophysical class 3. The subtidal rocks, which contain both limestone and dolostone, are more variable. Dolostones are mostly medium-crystalline, subtidal grain-dominated dolopackstones and medium-crystalline, mud-dominated petrophysical class 2 rocks. Limestones are composed of oomoldic grainstone. Although grainstones normally possess petrophysical class 1 rock fabrics, they display class 2 petrophysical relationships because they are moldic. Lower Clear Fork sequence L2.2 is also dominated by class 2 medium-crystalline dolostone fabrics. However, these rocks display class 1 porosity-permeability relationships because of abundant poikilotopic anhydrite. Oomoldic limestone, such as that in sequence L2.1, is also locally common. The uppermost L2.3 reservoir sequence contains fine-crystalline dolostone, class 3 peritidal facies.
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Anatomy of a Giant Carbonate Reservoir: Fullerton Clear Fork (Lower Permian) Field, Permian Basin, Texas
Despite declining production rates, existing reservoirs in the United States contain large quantities of remaining oil and gas that constitute an enormous target for improved diagnosis and imaging of reservoir properties. The resource target is especially large in carbonate reservoirs, where con entional data and methodologies are normally insufficient to resolve critical scales of reservoir heterogeneity. The objectives of the research described in this volume were to develop and test such methodologies for improvedimaging, measurement, modeling, and prediction of reservoir properties in carbonate hydrocarbon reservoirs. The focus of the study is the Permian Fullerton Clear Fork reservoir of the Permian Basin of west Texas. This reservoir is an especially appropriate choice because the Permian Basin is the la gest oil-bearing basin in the United States and, as a play, Clear Fork reservoirs have exhibited the lowest recovery efficiencies of all carbonate reservoirs in the Permian Basin.