Update on the Geoscientific Understanding of the Utah FORGE Site

The EGS reservoir at Utah FORGE is entirely hosted by fractured Tertiary granitoid that forms the stratigraphic basement in the north Milford valley and that is exposed throughout the core of the nearby Mineral Mountains. The top of basement is defined by a gentle westdipping surface that originated as a tectonic feature, which below the FORGE site was buried beneath that 3000 feet of alluvial basin fill. Fracture patterns in the central Mineral Mountains formed when the maximum compressive stress was vertical, and then rotated, tilting ~40° of eastward, during uplift and exhumation more than 6-10 million years ago. The basement surface probably thus began as a steep west-dipping fracture plane that evolved into a range front fault and rotated into its modern position. Based on the FMI log from well 5832, fractures in the vicinity of the EGS reservoir are densely spaced and have a wide range of orientations similar to the Mineral Mountains. Multiple independent data sets indicate that the modern stress regime remains extensional, characterized by normal faulting and a maximum horizontal compressive stress oriented approximately N25°E. These are likely responsible for subsidiary faults that have formed outside the EGS reservoir, including the Opal Mound fault, which forms the west boundary of the Roosevelt Hot Springs hydrothermal system, and the Mineral Mountains West fault system, which runs south of the FORGE site. By contrast, the Negro Mag fault runs roughly east west, intersecting the Opal Mound fault and coinciding with the northern boundary of the Roosevelt Hot Springs hydrothermal system. Importantly, no major fault structures transect the FORGE site. Anomalous heat flow comprises localized hydrothermal convection east of the Opal Mound fault and regional conduction (~70°C/km, well 58-32) west of the Opal Mound fault. Shallow groundwater resources comprise westward outflow from Roosevelt Hot Springs that occupies basin fill aquifers. Soil gas CO2 and He isotope anomalies correlate with Roosevelt Hot Springs, confirming the absence of detectable hydrothermal upflow beneath the Utah FORGE site.