Abstract

Carbonate reservoirs may be heterogeneous with both lateral and vertical variations in porosity and permeability. New technology and improved understanding of carbonate reservoir rocks has led to improved reservoir description, flow unit delineation, and flow unit ranking. Petrographic image analysis (PIA), a relatively new method, was used to analyze the carbonate porosity of the reservoir interval at Happy field, Garza County, Texas. The reservoir produces from depths of –4900 to –5100 feet and consists of lower Permian oolitic grainstones and packstones, floatstones, rudstones, and in situ Tubiphytes bindstones.

Reservoir pore characteristics and their corresponding degrees of connectivity (“quality”) 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. Common pore characteristics were used to identify stratigraphic and diagenetically similar intervals, within which four pore facies were observed. Pore facies were defined and ranked as to quality by comparing PIA data with measured porosity, permeability, and, in a limited number of samples, 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, and bindstones with dispersed separate vugs and matrix porosity ranked fourth. The PIA technique is a viable and fast alternative to standard petrography. It yields data that compares with petrophysical measurements and, when properly used, is a valuable method for reservoir characterization in heterogeneous carbonate pore systems.