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Natural gas is produced from diagenetically created pore space developed in stratigraphic traps in nonmarine sandstones of the Upper Cretaceous Neslen, Farrer, and Tuscher Formations of the Mesaverde Group in the eastern part of the Uinta Basin. Porosity and permeability in these rocks are quite low and highly variable. For this reason, the reservoir rocks are commonly termed “tight” or “unconventional.”

Although core is limited, our comparison of the physical and biological constituents in the rocks in the subsurface with those in temporally equivalent beds exposed along the Book Cliffs at the southeast margin of the basin indicates the reservoir units were formed by subaerial streams. Channel-form beds in the Neslen were formed in small, meandering streams on a coastal plain, and those of the overlying lower part of the Farrer represent more numerous and larger meandering streams with common straight segments. The Tuscher and the upper part of the Farrer form the uppermost Cretaceous rocks in the eastern part of the basin, and they were formed as part of an anastamosing complex of mixed braided and meandering streams that combined to preserve a thick sequence of sandstone units. Boundaries between these formations are gradational, as are the boundaries between depositional settings.

The sandstones, studied in thin section and by X-ray diffraction and scanning electron microscopy (SEM), are predominantly moderately well sorted very fine to fine-grained sublitharenites and litharenites. Rock fragments, primarily of chert and fine-grained sedimentary rocks, are commonly abundant. Deformation of labile rock fragments occurred, but the effects of compaction are relatively minor. No fractures, either open or healed were seen in thin section, but mineralized fractures were seen in core.

Chemical diagenesis had a greater influence on the sandstones than did mechanical compaction. Intergranular carbonate cements, dolomite/ankerite and calcite, are common; fine-grained authigenic(?) siderite occurs only in deeper (i.e., Neslen Formation) samples. Calcite content appears to decrease (to zero?) in the deeper samples, but ankerite becomes more abundant. Authigenic kaolinite commonly fills pores, partially to completely, but evidently has not replaced feldspars. Authigenic illite usually lines micropores (5-10 /μm) in partially leached chert grains; in some cases, it partially replaces kaolinite. Development of authigenic clays significantly modified pore geometry and fluid flow and retention characteristics of the sandstones. Probably more than one episode of leaching removed, partially to completely, carbonate cements, chert grains, and feldspars. Dissolution formed secondary porosity, which appears to be responsible for the favorable reservoir characteristics of some of the sandstones.

Sandstone units containing the coarsest grains and the largest scale of crossbedding are commonly preferentially and pervasively cemented with carbonate minerals. Therefore, porosity and permeability may be very low in units that would otherwise be preferred exploration targets.

We suggest that the principal hydrocarbon reservoir rocks in the nonmarine part of the Mesaverde Group developed in diagenetically enhanced stratigraphic traps.

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