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RUPPEL, Stephen C.
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Abstract: Identifying Facies, Cyclicity, and Rock Fabrics in ShallowPlatform Carbonates with Wireline Logs: Permian Basin, Texas

Carbonate reservoirs are characterized by complex variations in facies and rock fabrics that result in considerable heterogeneity. Resolution and mapping of these variations are fundamental to accurate definition of petrophysical properties and construction of reservoir models necessary for efficient exploitation. For the most part, adequate characterization of carbonate facies and rock fabric variability requires large numbers of cores. However, some modern wireline logs, when properly calibrated with cores, can provide surprisingly robust resolution of carbonate reservoir attributes that can rival that obtainable from core.

Spectral gamma-ray logs, which differentiate potassium/thorium gamma-ray response from uranium response, are inexpensive wireline logs that can, at the same time, delineate facies and demarcate cycle boundaries. In the Permian Clear Fork Group, for example, siliciclastic-bearing, cycle-top, tidal-flat deposits are readily differentiated from cycle-base, uranium-rich subtidal rocks. Because of the marked differences in petrophysical properties between these two facies, the spectral log can provide a rigorous basis for detailed petrophysical characterization.

Although generally considered to be a tool for fracture identification, calibrated borehole imaging logs can resolve facies successions and cycle boundaries with nearly the same accuracy as cores. In typical Permian successions, tidal-flat rocks are readily characterized by a thinly laminated pattern, deeper subtidal fusulinid-bearing rocks by a vermiform character, and shallow subtidal rocks by a granular pattern on image logs. Grikes, solution pits, and collapse breccias, commonly associated with karst horizons in Wolfcampian and Pennsylvanian rocks, are also readily definable.

AAPG Search and Discovery Article #90918©1999 AAPG Southwest Section Meeting, Abilene, Texas