Depositional setting and diagenetic evolution of some Tertiary unconventional reservoir rocks, Uinta Basin, Utah
The Douglas Creek Member of the Tertiary Green River Formation underlies much of the Uinta basin, Utah, and contains large volumes of oil and gas trapped in a complex of fractured low-permeability sandstone reservoirs. In the southeastern part of the basin at Pariette Bench, the Eocene Douglas Creek Member is a thick sequence of fine-grained alluvial sandstone complexly intercalated with lacustrine claystone and carbonate rock. Sediments were deposited in a subsiding intermontane basin along the shallow fluctuating margin of ancient Lake Uinta. Although the Uinta basin has undergone postdepositional uplift and erosion, the deepest cored rocks at Pariette Bench have never been buried more than 9,800 ft (3,000 m).
The sandstones, dominantly lithic arkoses and feldspathic litharenites, were derived from source terranes south of the Uinta basin. Secondary silica and several generations of authigenic calcite [Ca1.8-1.9(Mg0.02-0.06Fe0.02-0.06)(CO3)2], dolomite [Ca1.3-1.4(Mg0.6-0.7Fe0.02-0.04)(CO3)2], and ankerite [Ca1.2-1.3(Mg0.2-0.3Fe0.4-0.6)(CO3)2] form a replacive cement in the sandstones. Commonly, syntaxial overgrowths of late iron-bearing carbonate occur on detrital grains and preexisting relicts of iron-free carbonate cement. In sandstone where carbonate has been partly dissolved, abundant authigenic illite, partly ordered mixed- ayer illite-smectite, and small amounts of chlorite partly to completely fill secondary pores. Isotopic composition of carbonate cement and grain-supported rock range from -0.39 to -6.18 ^pmil for ^dgr13C and -7.80 to -13.98 ^pmil for ^dgr18O, indicating that authigenic carbonate formed at low temperatures in the presence of meteoric waters by a process of solution-precipitation. Enrichment of carbon and oxygen in early diagenetic calcite and fossiliferous rock relative to late diagenetic ankerite indicates a trend toward lighter isotopic carbonate compositions with increasing diagenesis.
Kerogenous rocks at Pariette Bench are thermochemically immature and therefore are not the source of oil produced in the field. Hydrocarbons are compositionally similar to some of the oils produced from the Green River Formation in the Bluebell-Altamont field and are interpreted to have migrated from mature Green River source rocks through a network of open fractures. The occurrence of small amounts of hydrocarbon in secondary pores indicates that its emplacement postdated carbonate dissolution.
|Depositional setting and diagenetic evolution of some Tertiary unconventional reservoir rocks, Uinta Basin, Utah
|American Association of Petroleum Geologists Bulletin
|American Association of Petroleum Geologists
|Eastern Energy Resources Science Center
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