Distribution and characterization of source rocks in the upper black shale facies, Green River Formation, northern Uinta Basin, Utah




Wells, Stephen N.

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The black shale facies occurs throughout the subsurface in the Altamont-Bluebell-Cedar Rim fields of northeastern Utah. The objective of this investigation was to define, delineate, and characterize the source rock potential of the basal unit within the upper black shale facies in the northern Uinta Basin. This stratigraphic interval was informally referred to as the carbonate marker unit, and is upper and lower bound by the carbonate marker and Long Point Bed, respectively. This study uses contour maps, along with applied source rock characterization of this specific interval within the upper black shale facies of the Green River Formation.

In the northern Uinta Basin, depth to the carbonate marker ranges from approximately 4,500 ft in the southern Cedar Rim field to more than 13,000 ft in the northern Altamont-Bluebell field. Thickness obtains a maximum of approximately 450 ft to the north, and thins to a minimum of approximately 100 ft as you move south into the basin. The carbonate marker unit consists predominantly of source facies containing kerogen-rich calcareous shales and marlstones with present day TOC averages ranging 0.5 to 4.6 wt.%. Original, pre-maturation TOC averaged a range of 1.7 to 6.8%. As expected, both original and present-day TOC increase southward into the basin and decrease to the north. The kerogen fraction of the carbonate marker unit is dominated by type I amorphous material. An increase in proportions of herbaceous, woody and inertinitic types occurs in the southern portion of the study area.

A number of direct and indirect methods were used to assess the degree of thermal maturity for the carbonate marker unit. Methods include vitrinite reflectance (Ro), Rock-Eval Tmax, Rock-Eval transformation ratio (PI), and HI-derived transformation ratio (TRHI) used to assess relative volumes of hydrocarbons generated.

The carbonate marker unit is shown to be in the mature oil window throughout the majority of its extent in the northern Uinta Basin. Maximum thermal maturity occurs in a northeast-southwest trend along the interplay of depth, thickness, and source rock quality. Maturity decreases to the far north and south-southeast as this interplay becomes limited by one of these factors.


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Earth and Environmental Science