Abstract: Geothermal Exploration of Roosevelt KGRA, Utah

Charles W. Berge, Gary W. Crosby, R. C. Lenzer

The Phillips Petroleum Co. exploration program combining geologic, geochemical, and geophysical methods has resulted in discovery of a high-temperature, low-salinity, liquid-dominated geothermal system. Testing to determine commercial production of the system is now in progress.

The Roosevelt prospect is at the boundary between the Mineral Range and Milford graben in eastern Beaver County, Utah. Valley-fill sediments in the graben are approximately 1,500 m thick in the center of the valley. Bedrock is stepped up along several normal faults to the west flank of the range, where the westernmost exposures consist of Precambrian(?) gneissic rocks. These are invaded in a zone of injection by late Cenozoic granite and related silicic differentiates. Paleozoic sedimentary rocks, exposed on the west side of the valley, terminate by erosion somewhere in the graben.

The westernmost exposures of Precambrian crystalline rocks appear to be in a horst block which is bounded on the east by the Dome fault. Rhyolitic flows, from seven or more eruptive centers, cap much of the granite east of the prospect. Magma additions to the chamber, feeding these eruptive centers, are thought to be supplying the heat beneath Roosevelt prospect.

Recent faulting in the vicinity of the prospect is indicated by fresh scarps in alluvium and the cutting and displacement of hot-spring deposits. Faults appear to be major controlling structures in the subsurface hydrologic regime.

The investigations undertaken prior to the July 30, 1974, lease sale included hydrochemistry, chemical geothermometry, petrography, gravity, magnetics, resistivity (dipole mapping and MT), groundnoise, and thermal-gradient drilling. A model assimilating the data was formulated. Additional resistivity, thermal-gradient drilling, and deep drilling resulted in modifications of the existing model.

Of all tools used, shallow temperature gradients appear to outline best the potential reservoir area as we now know it or perceive it to be.

The thermal anomaly, as mapped by Phillips, covers approximately 8,000 acres. All of this, however, should not be thought of as potentially productive, inasmuch as production is controlled not only by heat and fluid availability, but by the presence of fracture zones.

The thermal anomaly is underlain by intermediate and silicic crystalline rocks, at the surface or at shallow depths. The fracture system is the reservoir. The depth to its top is less than 900 m over a significant part of the anomaly. The fracture zones have extraordinarily high effective permeability locally, yielding up to 113,000 kg/hr flashed steam from a reservoir in excess of 200°C, pressures near hydrostatic, and fluids with less than 10,000 ppm total dissolved solids. Predicted reservoir temperatures from analysis of surface waters are remarkably similar to reservoir temperatures encountered in deep drilling operations.

AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana