ALLISON, M. LEE, and THOMAS C. CHIDSEY, JR., Utah Geological Survey, Salt Lake City, UT

ABSTRACT: Exploration Targets in the Great Basin of Utah

Three types of petroleum exploration targets are present in the Great Basin of Utah: structural traps in Tertiary sedimentary and volcanic rocks, unconformity traps of buried hills of Paleozoic rocks draped by Tertiary deposits, and structural traps related to thrusting where a wide variety of potential reservoir rocks are juxtaposed against Paleozoic source rocks.

Tertiary targets are delineated by seismic surveys and consist of tilted fault blocks and faulted anticlines. The only success to date is Amoco's West Rozel field, in Great Salt Lake, which has in-place reserves estimated at 100 million to 1 billion bbl of oil, but is presently uneconomic. The oil is low gravity (4 degrees API) with an extremely high sulfur content (12.5%). Little exploration has been done for these targets since the early 1980s when Amoco decided not to develop the field due to high water-cut and costs for offshore development.

Buried-hill targets are delineated by closely spaced gravity surveys and consist of gravity highs representing Paleozoic reservoir rocks sealed by Tertiary sediments. A 2108-ft dry hole, drilled in 1989 on a gravity high in Skull Valley, encountered a buried hill of Pennsylvanian-Permian Oquirrh Formation at 211 ft. Another well is planned to test a gravity high in Tooele Valley west of Salt Lake City.

Thrust-related targets are delineated using a combination of deep-penetrating geophysical techniques and consist of faulted ramp anticlines. These structures may offer the best opportunity for a large discovery. Interpretation of a recent magnetotelluric (MT) line over the Wah Wah Mountains delineated an anticlinal structure defined by probable Chainman Shale at a depth of about 6000 ft. A follow-up seismic reflection survey was completed over the MT-defined structure in preparation for a possible test well.

Approximately 220 wells have been drilled in Utah's Great Basin for a density of one well per 155 sq mi or fewer than one well per four townships. The presence of many untested targets in this sparsely drilled region, coupled with at least one large oil accumulation, should encourage aggressive exploration efforts.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.