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Porosity Characterization Utilizing Petrographic Image Analysis: Implications for Rapid Identification and Ranking of Reservoir Flow Units, Happy Spraberry Field, Garza County, Texas

M. John M. Layman II1 and Wayne M. Ahr2
1 Amerada Hess Corporation, Houston, Texas
2 Texas A&M University, College Station, Texas

Carbonate reservoirs are commonly heterogeneous and exhibit lateral and vertical variations in porosity and permeability. As a result, it becomes necessary to use new tools and techniques to characterize such reservoirs and to determine flow-unit architecture. Petrographic image analysis (PIA) was used to analyze the carbonate porosity of the reservoir at Happy field, Garza County, Texas. The producing interval consists of lower Permian oolitic grainstones and packstones with associated floatstones, rudstones, and in situ Tubiphytes bindstones.

Reservoir pore characteristics and their corresponding degrees of connectivity were determined using standard petrography, PIA, core analyses, and mercury injection capillary pressures. The PIA method enables rapid measurements of pore size, shape, frequency of occurrence, and abundance. Pores are also interpreted for genetic origin. Common pore characteristics, or “porosity fingerprints,” were used to identify stratigraphic and diagenetically similar intervals, within which four pore facies were defined. Pore facies were defined by evaluating the PIA data for trends and patterns and subsequently ranked for quality by comparing the PIA data with measured porosity, permeability, and median pore throat diameters. Pore facies exhibiting oomoldic and solution-enhanced interparticle porosity ranked best in quality. Rocks with incomplete molds and dispersed interparticle pores ranked second, rocks with mainly separate molds ranked third, and rudstones-floatstones-bindstones with dispersed separate vugs and matrix porosity ranked fourth. The PIA technique is a viable and fast alternative to standard petrographic methods. It yields data that compares with petrophysical measurements and is a valuable tool for reservoir characterization in heterogeneous carbonate pore systems.