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The AAPG/Datapages Combined Publications Database

Four Corners Geological Society

Abstract


Canyonlands Country, Eighth Field Conference, 1975
Pages 263-275

Petroleum Geology of East-Central Utah and Suggested Approaches to Exploration

Charles W. Spencer

Abstract

The Paradox fold and fault belt dominates the central part of east-central Utah. The Uncompahgre Uplift borders the area on the northeast, the San Rafael Swell on the northwest, the Henry Mountains basin on the west, and the Monument Upwarp is on the south edge.

During Cambrian, Devonian, and Mississippian time the area was a shallow shelf marginal to the Cordilleran geosyncline to the west. The Cambrian and Devonian rocks are predominantly sandstone, shale, and carbonate, whereas the Mississippian strata are chiefly dolomite and limestone. Paleotectonism had an influence on both Devonian and Mississippian stratigraphy and petroleum entrapment.

During Pennsylvanian time the ancestral Uncompahgre Uplift shed coarse arkosic clastics to the southwest into the northwest-trending Paradox Basin. Saline rocks were deposited in this trough in response to eustatic changes of sea level coupled with an arid climate. Subsequent salt flowage resulted in the formation of numerous northwest-trending salt anticlines. Pennsylvanian algal, bioclastic, and oolitic carbonate reservoirs were deposited along the shelf margins of this basin. Marine carbonates and sandstones of Permian age intertongue with continental red beds to the east. Triassic and Jurassic strata are dominantly continental red beds with a few eastward-terminating marine tongues. A thin sequence of Lower Cretaceous continental clastics was deposited, followed by a thick sequence of Upper Cretaceous marginal-marine sandstone and open-marine shale.

Potential hydrocarbon source beds are present in east-central Utah in Paleozoic and Mesozoic marine deposits. Mississippian and Permian organic-rich shales are present west of the study area and may have supplied petroleum to Mississippian and Permian reservoirs in the area by long-range migration.

Only 2.2 percent of the area has been explored by drilling to the Mississippian and Devonian. Most of the production from the six oil and gas fields in the area is from Mississippian carbonate reservoirs located on paleostructure beneath a disconformable cover of Pennsylvanian saline deposits.

The area is an underachiever in terms of the abundance of oil shows in relation to the petroleum reserves discovered. This abundance of shows is related to regional paleotectonic tilt that varied both in degree and direction throughout geologic time. Generated oil moved back and forth in reservoirs leaving residual shows. The suggested exploration approach is the careful mapping of paleomigration paths after typing oils to source beds and determining the time of hydrocarbon generation. The most easily defined prospects are those where isolated reservoir porosity and permeability are developed in proximity to source beds. The Pennsylvanian provides the best potential for this type of prospect. East-central Utah has potential for major reserves of petroleum and is only sparsely explored. Stratigraphic traps have the greatest potential.


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