ABSTRACT: Regional Geology and Exploration Strategy for Central Utah

Douglas A. Sprinkel

Central Utah (eastern Juab, eastern Millard, southern Utah, Sanpete, and western Sevier Counties) has continually lured petroleum explorationists into the area for the past 40 years because of its regional geologic setting, large untested areas, and high potential for giant fields. Past efforts have tested surface anticlines, thrust belt-style structures, and deep subsurface structures identified from seismic reflection data. Although most exploration tests have ended in failures, many wells have yielded several shows of oil and gas, including recovery of oil

from drillstem tests and production tests in three wells. These wells coupled with additional detailed surface investigations provide a better understanding of the regional structural geometries, timing structural events, stratigraphy, and source rock potential.

The regional geology of central Utah is generally understood, but the extent of potential stratigraphic units and the structural evolution is still debated. Unraveling and understanding these structural and stratigraphic complexities will guide future successful exploration efforts. It's critical to understand the regional relationship between the Arapien Shale and Twin Creek Limestone, both of Jurassic age; the lateral and vertical extent of the Cretaceous Mancos Shale; and the role and extent of thrusting, diapirism, and Basin and Range normal faulting in the structural evolution of central Utah.

Three belts, generally trending north-south, with specific exploration targets dominate central Utah. The western belt occurs in eastern Juab and Millard Counties where deep (18,000-25,000 ft) Paleozoic (mostly Mississippian) and Proterozoic potential exists below a regional detachment. The eastern belt occurs in Sanpete and Sevier Counties where the Cretaceous Mancos Shale is buried at depths of 10,000 to 13,000 ft and has been structurally deformed by duplex-style thrusting and later modified by diapirism. The third belt occurs in southern Utah County where the Jurassic Twin Creek Limestone and Jurassic Navajo Sandstone has been thrusted eastward over the Cretaceous Mancos Shale.

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