DETAILED CONVENTIONAL AND INNOVATIVE 3D GEOLOGIC MAPS OF GEOTHERMAL SYSTEMS IN THE GREAT BASIN REGION, WESTERN USA: CRITICAL COST-EFFECTIVE TOOLS FOR GEOTHERMAL EXPLORATION

We have applied detailed mapping of bedrock and Quaternary deposits as a cost-effective tool in assessing controls on geothermal systems in the Great Basin. Delineating stratigraphy is important in this region of interfingering Neogene volcanics and sediments, as it facilitates geophysical modeling, accurate well-logging, fault recognition, and reservoir definition. Our detailed mapping and regional studies also show that four types of structural settings host nearly 90% of known geothermal systems: 1) fault tips, 2) fault stepovers (SOs), 3) fault intersections, and 4) accommodation zones (AZs). Where oriented favorably in the current stress field, these settings comprise critically stressed belts of closely spaced faults (e.g. horsetailing fault tips and breached relay ramps). Mid-segments of major faults are relatively devoid of activity due to impermeable clay gouge and periodic release of stress in major earthquakes.

Geothermal potential increases with structural complexity. Most robust, higher-temperature fields with operating power plants are hybrid systems (>1 favorable setting). Also, AZs are less common than fault tips but disproportionately host power plants (e.g. Steamboat, McGinness Hills, Bradys, Salt Wells), as they consist of multiple overlapping, intersecting, and terminating faults. The detailed maps further show that geothermal activity generally occupies the most complex parts of individual settings. Examples include 1) a small AZ in a broad SO at Tuscarora; 2) small SO and fault intersection in an AZ at McGinness; and 3) major fault tip in an AZ at Salt Wells. These findings can guide exploration, especially for blind systems (i.e. no surface hot springs), which are common.

Combined with available geophysical data, the surface maps foster production of 3D models and 3D geologic maps. Our 3D models show that production wells typically penetrate faults with high slip-dilation tendency in steeply plunging, pipe-like bodies of high fault intersection and/or tip density. Wells outside these pipes may be hot but lack permeability. The 3D models define fluid flow conduits and are thus critical for minimizing drilling risk. We conclude that where exposures are sufficient, detailed surface and 3D maps (both relatively low budget) should precede and guide expensive geothermal drilling.