--> --> ABSTRACT: Potential for Precambrian Source Rock in Utah, by Thomas C. Chidsey, M. Lee Allison, James G. Palacas; #91002 (1990).

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ABSTRACT: Potential for Precambrian Source Rock in Utah

Thomas C. Chidsey, M. Lee Allison, James G. Palacas

A recent study by the U.S. Geological Survey indicates that mudstones and siltstones of the late Proterozoic Chuar Group in the Grand Canyon, Arizona, are potential petroleum source rocks. Geochemical analyses indicate that these rocks are organic rich, containing probably as much as 10% organic carbon, and are within the oil-generating window. Preliminary evaluations suggest that the Chuar or equivalent units underlie some areas of the Colorado Plateau in Utah, representing a significant hydrocarbon potential for the region.

The Chuar Group in the Grand Canyon dips north under the Kaibab Plateau of southwest Utah and northern Arizona and is truncated to the east by a pre-Phanerozoic normal fault. The Tidewater No. 1 Kaibab Gulch well in southern Utah penetrated rocks assigned to the Chuar, consisting of dark-gray shales with possible plantlike spores and variable amounts of carbonaceous material. Other deep wells in Utah also have provided evidence that the Chuar Group extends into some parts of the Utah portion of the Colorado Plateau. Strata of the Chuar Group may be correlative with the Big Cottonwood Formation near Salt Lake City and the Red Pine Shale of the Uinta Mountain Group. Preservation could occur in pre-Phanerozoic grabens (tectonically related to similar faulting found in the Grand Canyon) a d in areas that experienced subsidence or on the flanks of older Precambrian highs.

Geochemical analyses of crude oils from various oil fields, subsurface oil shows, seeps, and tar sand samples from the southern portion of Utah are being used to determine if any such occurrences may have had a Precambrian source. Such information can help define the areal extent of Precambrian source rocks where deep penetrations are lacking. Seismic surveys might help to locate possible pre-Phanerozoic grabens, regional fault patterns, and stratigraphic pinchouts.

Exploration targets include lower Paleozoic (or younger) reservoir rocks and fractured Precambrian rocks. Well penetrations of these rocks are sparse over much of the Colorado Plateau, so the possibility of large new fields could be significant.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990