The complexity of pore networks in fine-grained low-permeability sandstones makes accurate modeling of fluid-flow properties difficult owing to the lack of quantitative information concerning the pore structure. Many such sandstones in the Uinta basin, Utah, are reservoirs for large amounts of natural gas. These sandstones, most of which are Tertiary and Cretaceous in age, commonly contain pores that vary greatly in size. Variation in pore size is partly due to the dissolution of mineral grains and pore-filling cement; howwever, many of the secondary pore spaces contain authigenic clay, principally illite and kaolinite, which has served to create micropore space.

We have developed a method to digitize and quantify pore networks of fine-grained rocks using the apparent pore space observed in photomicrographs of thin sections. By digitizing numerous photographs, statistical data were generated, thereby making it possible to address the problem of pore structure. Pore structure data include such parameters as the pore size and shape, anisotropy of the pore arrangement within the rock matrix, and pore connectivity.

Specimens obtained from CIG Exploration, Inc., Natural Buttes 21 cores (Sec. 15, T10S, R22E) were used to determine

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pore anisotropies of tight sandstones. Several analysis methods were used; each indicated that little anisotropy was present in the pore structure.

Statistical-shape parameters, derived from the measured perimeters and areas of the pores, suggest that most of the pores within each sample are tabular rather than tubular in shape. Knowledge of the pore structure suggests that the pores should be treated as oblate rather than prolate spheroids in modeling the electromagnetic or fluid-flow properties of these rocks.

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