Abstract: Exploration and Evaluation of Roosevelt KGRA, Utah

C. W. Berge, G. W. Crosby, R. C. Lenzer

The Roosevelt prospect is on the boundary between the Mineral Range and Milford graben. Valley-fill sediments in the graben are approximately 5,000 ft (1,500 m) thick. Bedrock is stepped up along several normal faults to the west flank of the Mineral Range, where the westernmost exposures of Precambrian(?) gneissic crystalline rocks are invaded by upper Cenozoic granodiorites. Paleozoic sedimentary rocks, exposed on the west side of the valley, terminate by erosion in the graben.

The Precambrian crystalline rocks are in an uplifted block that was present prior to intrusion of the granodiorites. The east side of the horst block is obscured in the zone of injection. Rhyolitic flows from seven or more eruptive centers cap much of the granodiorites east of the prospect. These flows range in age between 0.4 and 2.8 m.y. B.P. Magma additions to the chamber feeding the eruptive centers supply the heat beneath Roosevelt prospect.

Fresh scarps in alluvium and the cutting and displacement of hot-spring deposits indicate recent faulting. The Dome fault, relatively unimportant in terms of displacement, is a major controlling structure in the hydrologic regime in the subsurface, but many additional faults are indicated by the geophysical data.

Investigations indicate 1,000 acres (400 ha.) of Roosevelt prospect may be underlain by a commercial steam reservoir; another 1,200 acres (480 ha.) have good potential, and the total thermal anomaly covers approximately 8,000 acres (3,200 ha.). Not all of the anomaly is potentially productive, inasmuch as production is controlled by the presence of fracture zones as well as by heat and fluid availability. Tentative assessment is between 2,200 and 8,000 productive acres (880 and 3,200 ha.).

Silicic crystalline rocks, at the surface or at shallow depths, are present throughout the thermal anomaly. The fracture system acts as the reservoir. Its depth is less than 3,000 ft (900 m) in places. The fracture zones have high effective permeability, yielding flow rate approaching one million pounds (450,000 kg) per hour total mass flow from a reservoir in excess of 200°C; pressures are near hydrostatic, and fluids show less than 8,000 ppm total dissolved solids. The data strongly favor a water-dominated reservoir and there is evidence of a second, deeper reservoir.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC