| Title | Update on the Geological and Geochemical Understanding of the Utah FORGE Site |
|---|---|
| Authors | Stuart SIMMONS, Stefan KIRBY, John BARTLEY, Clay JONES, Emily KLEBER, Tyler KNUDSEN, John MILLER, Kristen RAHILLY, John McLENNAN, Tobias FISCHER, Joseph MOORE, Rick ALLIS |
| Year | 2019 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Utah FORGE, structural geology, fault systems, hydrothermal fluid flow, EGS |
| Abstract | This report focusses on fault-fracture patterns and soil gas analyses in regard to understanding fluid flow and permeability controls within and surrounding the Utah FORGE EGS reservoir. The EGS reservoir is entirely hosted by fractured granitoid that forms the stratigraphic basement and that is exposed throughout the core of the nearby Mineral Mountains. The top of basement is defined by a gentle west-dipping 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 have lengths that range from 20 to 200 m, and fracture spacings that range from 5-15 m. Fracture azimuths range widely, with about half the population being randomly oriented. The remaining fracture azimuths fall into two predominant populations of 80 to 120° and 0 to 30°. The E-W trending distribution dips steeply to the north, whereas the NNE-SSW population dips steeply and gently west to form a conjugate set. The geometry of the conjugate NNE-SSW fracture set reflects early development when the maximum compressive stress was vertical followed by ~40° of eastward tilt, 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 ultimately, it accommodated a large amount of extension across the north Milford valley before locking up due to cohesion as the slip plane rotated into its modern position. Based on the FMI log from well 58-32, fractures in the vicinity of the EGS reservoir are densely spaced, and they have a wide range of orientations that reflect the history of 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. Within and around the FORGE site, CO2 and He isotope soil gas surveys confirm the absence of detectable hydrothermal upflow beneath the FORGE site. Instead and as expected, anomalously high CO2 fluxes and mantle helium were detected east of the Opal Mound fault in the area of known hydrothermal activity associated with Roosevelt Hot Springs. |