The Western Utah Fold-Thrust Belt: A Frontier Petroleum Province

Herring, Donna M.; Greene, David C.

A newly recognized Sevier-age fold-thrust belt extends for more than 130 km along strike in what has been considered the hinterland of the Sevier Orogeny. The Confusion Range in western Millard County, Utah exposes Ordovician through Triassic strata in what was previously interpreted as a broad structural trough or synclinorium with minimal overall shortening. However, the range is more accurately characterized as an east-vergent, fold-thrust belt with significant (~10 km) horizontal shortening during the late Jurassic to Eocene Sevier Orogeny. To detail the structural architecture and style of deformation in this fold-thrust belt, we completed a series of four balanced and restorable cross sections across the Confusion Range and adjacent Tule Valley, with a fifth, strike-parallel tie section; together these five sections total approximately 290 km in map length. There are few deep drillholes in the range, and published seismic data in the area is optimized for deep reflections and therefore of limited usefulness in interpreting upper crustal structure; however, the well-exposed surface geology plus the requirements of consistent bed length, area balance, and retrodeformability impose significant constraints on interpretation. The temporal, stratigraphic, and structural correlation of the Confusion Range fold-thrust belt with the Sevier frontal thrust belt in central Utah has significant implications for petroleum potential of western Millard County. The recent discovery of the Covenant field in Sevier County, Utah - with an estimated 100 MMBOIP - has proved the petroleum system concept for the central Utah salient of the Sevier fold-thrust belt. These cross sections provide basis for seismic interpretation and identification of similar potential plays in the Confusion Range and its southern continuation, in western Utah. The fold-thrust model includes a mechanism by which the source rocks of the Pilot Shale and Chainman Formation were returned to oil-window depths prior to the Basin and Range extension, generating hydrocarbons to fill Sevier-age traps. The cross sections show trapping opportunities in multi-leveled Sevier structures and potential for preserved fold-thrust trap oilfields in the Confusion Range.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013