This study seeks to better understand geothermal energy development risk in Washington State. In this region, crustal stress is dominated by the complex tectonics of the Cascade volcanic arc, and active faulting promotes and sustains geothermal reservoir permeability and provides connection to the postulated heat source. Three prospect-scale sites were selected for Phase 1 of a geothermal play-fairway analysis (PFA); Mount St. Helens seismic zone (MSHSZ), Wind River valley (WRV), and Mount Baker (MB). In Phase 1 of the PFA, heat and permeability potential was modeled from existing and publicly available data which are integrated into a map for each site of geothermal development potential using weighting derived from a multiple experts-opinion approach using an analytical hierarchy process. A heat potential model was created based on the locations of Quaternary volcanic vents, hot springs, Quaternary intrusive rocks, geothermometry, and temperature gradient data. Permeability potential was estimated using three dimensional modeled fault geometries in an elastic half space that slips in response to tectonic crustal stresses estimated from regional strain rates modeled from publicly available Global Positioning System (GPS) velocities. Volcanic deformation at MSHSZ and MB are modeled as Mogi sources of deformation. The resulting permeability potential analysis reveals 1) if faults are acting as fluid conduits or barriers, 2) the portions of faults likely to host fluid flow where slip is promoted and large slip gradients imply damage, and 3) the geometry of adjacent rock volumes that have dense fracture networks due to locally concentrated stresses that provide the porosity and permeability to host a commercially viable reservoir. Geometric fault location uncertainty is explored to determine where improved constraints would significantly alter predicted geothermal potential and thus target new data acquisition planned in Phase 2 of the project.
Geothermal resources require heat, fluid, and permeability. Active faulting and accompanying fractures can supply this permeability, allowing sustained deep circulation of hot water to shallow reservoirs accessible to wells and the high flowrates necessary for commercial electricity production.