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GRUBB, Jubal G., and RUPPEL, Stephen, C.
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Abstract: Styles of Heterogeneity in a Shallow-Water-Platform Carbonate Reservoir, Block 9 Wolfcamp Unit, Andrews County, Texas

Shallow-water Wolfcamp (lower Permian) carbonate reservoirs are productive along the outermost platform in eastern Andrews County, Texas. Waterflood efforts in the Block 9 Wolfcamp unit need to be optimized. To improve secondary recovery efficiency, we are investigating the causes and styles of heterogeneity in this reservoir.

The reservoir is composed of parts of two major sequences. The lower succession, middle Wolfcamp, consists of cycles (8-20 feet thick) composed of basal reworked phylloid algal graindominated packstones that grade upward into diverse skeletal / peloidal packstones with abundant encrusted skeletal grains, capped by grain-dominated skeletal packstones. Cycle-top graindominated packstones display the highest porosity values. Porosity at the sequence scale increases upward, owing to platform aggradation to shallow subtidal environments and greater subaeral exposure and meteoric dissolution at cycle tops. The lower portion of the upper sequence is composed of transgressive cyclic successions (5-11 feet thick) of outer platform fusulinid-rich skeletal wackestones /packstones that grade upward into coated grain / peloidal grain-dominated packstones. Porosity and permeability are restricted to cycle tops because of better sorted grain-rich fabrics with primary interparticle porosity. Correlation and mapping of laterally discontinuous paleo-karst dissolution features in the lower sequence also are crucial to understanding reservoir heterogeneity. Core and borehole image logs reveal significant porosity heterogeneity related to karst processes that have locally enhanced dissolution porosity or reduced porosity by shale infilling in karst dissolution features.

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