Descriptions, petrology, photographs, and photomicrographs of core from the Green River Formation, south-central Uinta Basin, Utah (original) (raw)
Anastamosing, low-gradient, distributary channel deposits in the lower and middle members of the Eocene Green River Formation are the primary oil reservoirs in the south-central portion of the Uinta Basin, Utah. The Eocene depocenter was situated along the fluctuating southern shoreline of Lake Uinta, where complex deposits of the marginal-lacustrine environment were commonly laid down. The Green River Formation contains several fining-upward sequences that can be recognized in outcrop, core, and well logs. Each sequence is about 60 to 120 feet (18-36 m) thick and consists of strata typically 30 to 35 feet (9-11 m) thick that were deposited during multiple lake-level fluctuations. The subaerial to subaqueous channels commonly possess an erosional base and exhibit a fining-upward character. Bedding features commonly range from large-scale trough and planar cross-bedded (or lamination) facies at the base, to a climbing ripple facies near the uppermost bed boundary. The best reservoir quality is typically within the laminated to cross-stratified facies, and the poorest reservoir quality is within the climbing ripple facies which usually possesses more deleterious micas and/or detrital clays. Diagenesis exerts a major control on reservoir quality. Some sandstone beds were cemented early by an iron-poor calcite cement, which was often later leached resulting in good reservoir quality rock with secondary intergranular porosity (up to 20 percent) and permeability (10 to 100 millidarcies [md]). Without early calcite cementation, reservoir quality was often reduced by intense compaction, silicic and ironrich carbonate cements, and authigenic clays.
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