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SCHENK, CHRISTOPHER J., and RICHARD M. POLLASTRO, U. S. Geological Survey, Denver, CO

ABSTRACT: Reservoir Characterization of Heavy Oil and Tar Sand Reservoirs -- an Example from the Tertiary of the Uinta Basin, Utah

Significant quantities of heavy hydrocarbons occur in marginal lacustrine and fluvial sandstones of the Tertiary Green River and Colton formations, Uinta Basin, Utah. The largest of the these deposits, the 6 billion-barrel in place Sunnyside deposit, was examined to determine if reservoir properties would hamper recovery operations. The sandstones are mainly fluvial channel and marginal lacustrine deposits with variable hydrocarbon saturations, based on visual examination. Spatial variability in hydrocarbon saturation is due mainly to control by types of preserved stratification and diagenesis. Diagenetically, the hydrocarbon-bearing sandstones are complex. Petrographic and SEM studies have shown that the framework grains are commonly coated with a variety of clay minerals, including early pure smectite, illite, illite/smectite, and chlorite. Kaolinite occurs as a pore-lining cement and as a replacement of detrital feldspars. Other authigenic minerals include several zeolites (analcime, chabazite, erionite) and carbonates (calcite, dolomite, ankerite). The variability in hydrocarbon saturation caused by sedimentologic and diagenetic heterogeneity, the diversity in pore morphology caused by authigenic mineralization, and the presence of smectite combine to indicate that recovery of heavy hydrocarbons from these sandstones by in-situ processes would be difficult. Recovery fluids could cause the translocation of clays and other authigenic minerals, resulting in decreased permeability and recovery; swelling and further growth of authigenic smectite during in-situ recover likewise could result in significant permeability loss.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.