Paleozoic Source and Reservoir Rocks in Unbreached Thrust Ramp Anticlines, Millard County, Utah

GARRISON, PAUL B., and BRADLEY R. LARSEN, Geoman Geological Services, Billings, MT

Surface geology, source rock geochemistry, and seismic data indicate that substantial hydrocarbon reserves may occur beneath a regional detachment fault underlying Tule Valley and the Confusion Range in northern Millard County, west-central Utah. Paleozoic hydrocarbon source and reservoir rocks in Millard County are laterally equivalent to highly productive rocks in Railroad Valley, Nevada, oil fields. However, the volume of hydrocarbons trapped in thrust ramp duplex anticlines beneath a regional detachment fault is potentially much greater than that in established Nevada fields.

The Devonian Guilmette Formation, which consists of interstratified brown, sucrosic dolomite and gray limestone, and the Mississippian Chainman Shale are exposed in the folded and thrusted Confusion Range. Regional geochemical analysis confirms that the Chainman Shale contains enough total organic carbon (TOC) to serve as an effective hydrocarbon source rock. Some surface samples exceed 3% TOC; average TOC is in excess of 1.5%. Thermal maturity of these source rock surface samples indicates that these rocks were subjected to deep burial during their geologic history and that they have generated the maximum amount of hydrocarbons. In addition, thermal maturity of these samples is consistent with hydrocarbon preservation at the "floor" of the oil window and within the area of peak wet g s generation. Petrographic examination of potential reservoir facies in the Guilmette Formation confirms that liquid hydrocarbons were contained in porous, permeable dolomite. Petrographic examination of kerogen from these same facies also confirms the presence of solid bitumen (dead oil) in the surface samples.

Migrated seismic reflection data acquired by COCORP in 1981 displays a regional detachment fault underlying the Tule Valley and Confusion Range. Geometry along the profile of this detachment fault is similar to that along thrust fault ramps, such as the Lewis thrust in northwest Montana and southwest Alberta, Canada. Cenozoic extension occurred along this detachment surface, reactivating the thrust in a reverse sense, from east to west. However, listric normal (Basin-and-Range) faults did not disrupt the footwall beneath the detachment surface. Thus, imbricate anticlines (duplex structure) composed in part of Guilmette Formation and Chainman Shale, may reside unbreached and as yet untapped beneath the reactivated decollement. Additional evidence for thrust ramp structural geometry ben ath the Confusion Range is found in its surface expression. The Confusion Range exhibits an eastward convexity in plan view that is identical with that of the Lewis thrust in the vicinity of Waterton field, a giant gas accumulation in duplex anticlinal structures. The potential exists in Millard County, Utah, for a Waterton-size hydrocarbon accumulation.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)