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Abstract

Tertiary and Cretaceous nonmarine sandstones are reservoirs for large amounts of natural gas at Natural Buttes field in the eastern part of the Uinta basin, Utah. A cored interval in the Upper Cretaceous Tuscher Formation dominantly comprises fine- to medium-grained, moderately to well-sorted sandstones and less abundant carbonaceous and coaly shale beds. These rocks represent sedimentation on the lower part of an alluvial braidplain. The Paleocene and Eocene Wasatch Formation unconformably overlies Cretaceous rocks and intertongues with marginal lacustrine strata of the Green River Formation. The cored interval in the upper part of the Wasatch consists of fine-grained lenticular sandstones with small-scale cross-bedding, argillaceous siltstones, and variegated mudstones, all of which were deposited in lower delta plain settings along the margin of Lake Uinta.

Cored sandstones in the Tuscher and Wasatch formations have been extensively modified by minor quartz overgrowths; by the precipitation and subsequent dissolution of a carbonate mineral assemblage comprising iron-free calcite, ferroan calcite, dolomite, and ankerite; by local occurrences of anhydrite and barite; and by the formation of authigenic illite, mixed- layer illite-smectite, kaolinite, chlorite, and corrensite. Most authigenic carbonate formed during early burial before significant compaction. During later stages of diagenesis, anhydrite and barite precipitated locally, replacing detrital grains and mineral cements such as carbonate. Porosity and permeability have been significantly reduced in the sandstones owing to clay mineral development and the formation of carbonate cement.

Large amounts of natural gas are stratigraphically trapped in these lenticular, diagenetically modified low-permeability sandstones. Potential source rocks in the Tuscher Formation may have generated thermogenic gas even though they are only moderately mature with respect to liquid hydrocarbon generation.

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