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Abstract

EPNOTES
Precambrian Chuar Source Rock Play: 
An Exploration Case History in Southern Utah

Thomas L. Uphoff1

ABSTRACT

Source rock potential of the Upper Proterozoic Chuar Group, specifically the Walcott Member of the Kwagunt Formation, provides the basis for a petroleum play in southern Utah. Analyses of Chuar black shales from outcrops in the Grand Canyon show total organic carbon values of 3-9%, hydrogen indices up to 255, and maximum maturity within the oil window. Modeling indicates a potential 150 mi2 (400 km2) area with a minimum generative potential of 2700 MBO*. Chuar source rocks are proposed as one part of a petroleum system that includes reservoirs in the Cambrian Tapeats Sandstone and seal in the overlying Bright Angel Shale. Prospective structures include anticlines of Laramide age not drilled through the Tapeats interval. One such structure is the Circle Cliffs uplift, which exhibits an area under closure of 900 mi2 (2300 km2) at the top of the Devonian. In 1994, BHP Petroleum drilled the 28-1 Federal well on the Circle Cliffs structure. The well logged 142 ft (43 m) of Tapeats porosity (>7%) and flowed CO2 gas at rates up to 5.0 Mcf* per day. Analysis of bitumen in the reservoir indicated an earlier hydrocarbon charge and suggested a new oil type for the region.

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