Investigating Geothermal Resources in the Black Rock Desert, Utah, using MT and Gravity
Christian Hardwick, Rick Allis, and David Chapman
Utah Geological Survey
Magnetotellurics (MT) and gravity, complemented by regional borehole information are the basis for assessing three geothermal areas: Crater Bench, Pavant Butte and, Thermo, located in the Sevier Thermal Anomaly within the Basin and Range Province of western North America. Recent geothermal studies on sedimentary basins in western Utah suggest the possibility of significant geothermal reservoirs at depths of 3 to 5 km. Our study areas are centered in or near such basins. Since 2010, we have added 263 MT stations and 371 gravity stations to the existing data coverage. Gravity anomalies and electrical resistivities provide estimates of depth to basement and buried structure geometry that may control upwelling of geothermal fluids. Resistivity models also help assess thermal characteristics in regards to the presence of hot fluids and/or clay-rich sediments within the basins. Depth-to-basement values vary from 1.5 to 3.4 km for the Crater Bench are and have a maximum basin depth of 3 km for the Pavant Butte area, including a region over 15 km wide with more than 2 km of sediments overlying basement rock. Independent 2D modeling of MT data are consistent with gravity anomaly models and structural interpretations. The Pavant Butte area displays resistivity values of less than 10 Ohm?m in regions within 1 km of the surface and extending to known basement depths. We attribute the large, hot-spring system (89?C) adjacent to Crater Bench to the upwelling of deep hydrothermal fluids along the margin of a buried horst block. Corrected temperatures encountered at the bottom of an oil and gas well near Pavant Butte were in excess of 220?C, whereas other wells in the area were significantly cooler (~100?C) at similar depths. Preliminary findings from a 2012 drilling program and geophysical surveys at Pavant Butte make for an attractive geothermal target, and resource potential appears to be significant.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013