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 mi 2 (400 km 2 ) 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 mi 2 (2300 km 2 ) 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 CO 2 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. MBO = million barrels of oil; Mcf = million cubic feet.