Evaluation of Pore Geometry of Some Low-Permeability Sandstones-Uinta Basin
Authors: Keighin C. William , Sampath K. 
Published: December 2007

Summary
A study of the permeability/porosity characteristics and petrographic properties of some low-permeability sand-stones indicates that the permeability reduction in these sandstones due to confining pressure is much more significant than porosity reduction. The sandstones were fine grained, and most pores were secondary and small; the few large pores present usually were filled with authigenic kaolinite.

Introduction
Natural gas occurs in some low-permeability (k usually less than 0.1 md at formation pressure and S ), fine-grained to very fine-grained sandstones of Tertiary and Cretaceous age in the Uinta basin in Utah. Samples on which engineering and petrographic studies were conducted were taken from cores from the Tuscher formation in the Mesaverde group of late Cretaceous age in the Colorado Interstate Gas CO.'s Natural Buttes 21 (Sec. 15, T10S, R22E) and C-K GeoEnergy Corp's GC-1 (Sec. 16, T17S, R24E) in the Uinta basin. The lithology of this formation includes conglomaratic sandstones, very fine-grained to medium-grained sandstones, carbonaceous shales, and some thick coal beds. The sediments were deposited in alluvial-fan, flood-plain, interdeltaic, and deltaic settings with well-developed swamps. Even though similarities between samples were noted, porosity-and probably permeability-characteristics of the sandstones often change drastically within a few centimeters. These changes axe caused by variation in amount of carbonate cement, grain size, and the presence of silt, clay or organic-rich lamina parallel to bedding surfaces. The fine-grained to very fine-grained sandstones resemble other low-permeability sandstones of the same age in the Uinta basin that have been studied petrographically. Great caution should be used before generalizing about vast quantities of rock from limited data, especially in the absence of data pertaining to lithologic characteristics and variations. Understanding the pore structure of real or imagined reservoir rock is basic to its economic evaluation. Pores in low-permeability sandstones are typically small [0.1 to 10 microns (um)], have a high surface area because of authigenic clays, and have a very tortuous path for fluid flow (Figs. 1 through 3). Although measured surface area varies with grain size, ionic species of clay minerals, and method of determination, representative surface areas are shown in Table 1. Even if "large" pores exist, their effectiveness is controlled by the size of the pore throats that limit access (or egress). The combination of numerous small pores and their common clay linings, with attendant high surface areas, have a marked influence on formation resistivity factors, capillary and flow phenomena, and the retention of connate water.

Analytical Methods
Mineral composition was determined by X-ray diffraction on whole-rock samples; modal analyses of 300 points per thin section were made to define mineral distribution. The only clay minerals revealed by X-ray diffraction were kaolinite and illite, present in approximately equal amounts. Neither chlorite nor swelling clays were detected by X-ray diffraction, but traces of chlorite were seen in thin section. Pore characteristics were studied through their appearance in thin section impregnated with blue-stained epoxy, through their response to mercury injection, and with the scanning electron microscope (SEM)-both as rock chips and as pore casts. The response of permeability and porosity to confining pressure were studied in the laboratory. Permeability measurements were conducted on dry core samples about 1 in. (2.5 cm) in diameter and 2 in. (5 cm) long. Core samples were dried to a constant weight in a vacuum oven at approximately 194 deg. F (90 deg. C).