APPLICATIONS OF STRUCTURAL GEOLOGY TO ELUCIDATING PROCESSES OF CRUSTAL FLUID FLOW, REDUCING RISKS IN GEOTHERMAL EXPLORATION, AND FACILITATING GEOTHERMAL DEVELOPMENT IN THE GREAT BASIN REGION, WESTERN USA: IT’S ALL ABOUT PERMEABILITY (Invited Presentation)

Due to a high geothermal gradient and active extensional to transtensional tectonics, the Great Basin region is richly endowed in geothermal resources. However, the region is capable of producing much more than the current capacity of ~720 MW from ~25 power plants. Sufficient permeability is more difficult to find than suitable temperature. Moreover, most geothermal resources are blind (no hot springs or steam vents), so exploration requires elucidating controls on subsurface fluid flow.

Because Quaternary faults are the main control on geothermal activity, we initially characterized favorable structural settings of known systems. Major normal faults are generally devoid of systems. Nearly 90% of systems reside in step-overs (relay ramps) in normal faults, normal fault terminations, fault intersections, and accommodation zones. Pull-aparts host some systems in transtensional areas. High fault density in these settings facilitates high permeability and fluid flow. The paucity of systems on mid-segments of major faults results from reduced permeability in clay gouge and periodic stress release in major earthquakes. Step-overs, terminations, intersections, and accommodation zones are critically stressed areas, where fluid pathways more likely remain open due to breccia-dominated faults and more abundant micro-seismicity that hinders fracture healing.

These findings were incorporated into a play fairway analysis, whereby 9 parameters were synthesized to produce a geothermal potential map of 96,000 km² of Nevada. Parameters were grouped in subsets and weighted to delineate rankings for heat and local, intermediate, and regional scale permeability, which collectively defined likely areas for geothermal fluid flow or play fairways. Structural features, including structural settings, location, age, slip rates, and slip/dilation tendency of Quaternary faults, geodetic strain rates, horizontal gravity gradients, and earthquake density were used to assess permeability. Dozens of prospective areas were identified. Two sites with particularly high potential were selected for temperature-gradient drilling, with results indicating discovery of two blind systems and initially validating the technique. This work shows the applicability of structural geology in reducing geothermal exploration risk.