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Abstract: Structural Geometry and Petroleum Potential of the Sanpete-Sevier Valley Antiform, Central Utah Thrust Belt

D. A. Sprinkel, T. F. Lawton, M. P. Weiss

The Sanpete-Sevier Valley antiform has been described as an anticline cored by mobile Jurassic Arapien Shale. Our work indicates that the antiform is instead a belt of dominantly west-verging, imbricate backthrusts and tight overturned folds in Jurassic to Cretaceous rocks. Thus, we use the term antiform to describe this 75-mile-long belt of deformed rocks. The antiform is bounded on the north and south by northeast-trending structures and on the west by the west-verging Wales thrust system. The east boundary is uncertain, but probably unfolds above imbricate thirst faults that merged into the blind, east-verging Gunnison thrust. Development of the backthrusts and growth of the antiform took place during late Sevier thrusting, evidenced by progressive unconformities in Ca panian to Paleogene strata. Neogene extensional faults formed within the Wales thrust system dropped the antiform down to the east, offset Tertiary and Quaternary strata at the surface, and attenuated Triassic and Jurassic strata at depth. The attenuation fault separates the Sanpete-Sevier Valley antiform from a larger footwall anticline.

About 23 exploratory wells have penetrated the antiform. Wells drilled along the east limb of the antiform provided numerous petroleum shows. One well produced oil during production tests, but was eventually abandoned because of mechanical problems. That well also penetrated the attenuation fault within Mesozoic rocks and the top of the footwall anticline; however, potential upper Paleozoic rocks were not drilled. Possible exploration targets include: (1) unconformity truncations, thrust imbrications, or hydrodynamic traps within both Upper Cretaceous shoreface reservoirs and the fractured, organic-rich marine mudstones along the east flank of the antiform at 7,000 to 13,000 feet; and (2) footwall anticline culminations, below the attenuation fault, within Paleozoic reservoirs sourced by Mississipian or Permian rocks at 17,000 to 22,000 feet.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada