Preliminary Petroleum Source Rock Assessment of Upper Proterozoic Chuar Group, Grand Canyon, Arizona

James G. Palacas, Mitchell W. Reynolds

Strata in the Chuar Group, Grand Canyon, Arizona, are potential petroleum source rocks. This group, divided into the Galeros Formation below and the Kwagunt Formation above, consists predominantly of very fine-grained siliciclastic rocks and thin sequences of sandstones and stromatolites and cryptalgal carbonate rocks. Over half the succession consists of organic-rich, gray to black mudstone and siltstone.

Geochemical analyses indicate that the 281-m thick Walcott Member, the uppermost unit of the Kwagunt, has good to excellent petroleum source rock potential. The lower half of the Walcott is characterized by total organic carbon (TOC) contents as much as 7.0% (average ~ 3.0%), hydrogen indices as much as 204 mg HC/g TOC (average 135 mg HC/g TOC), genetic potentials (S₁ + S₂) of nearly 16,000 ppm (average ~ 6,000 ppm), and extractable organic matter (EOM) as much as 4,000 ppm. Data for the upper Walcott are incomplete but suggest that these rocks are as rich or richer than the lower Walcott. Maturity assessment indicates that source rocks of the Walcott are within the oil generation window.

Strata of the thermally mature underlying Awatubi Member of the Kwagunt and the thermally mature to overmature Galeros Formation are, in general, rated as poor oil sources with genetic potentials generally less than 1,000 ppm, but they are possible gas sources. Several thin sequences in these units, however, display good oil source characteristics, with EOM nearly 2,000 ppm and genetic potentials nearly 7,000 ppm.

Chuar Group strata may be potential sources for economic accumulations of petroleum in upper Proterozoic or lower Paleozoic reservoir rocks in northwest Arizona and southwest Utah.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.