The Geologic and Structural Framework of the Fallon FORGE Site

Abstract

The working geologic framework model of the Fallon Frontier Observatory for Research in Geothermal Energy (FORGE) site has been updated with new data. These new data have been collected in order to fill gaps in an already robust data set, and contribute significantly to our evolving understanding of the geologic framework at the Fallon site. Our previous analyses has confirmed that the Fallon FORGE site consists of Mesozoic crystalline metamorphic and plutonic basement rocks, with low permeability, and conductive temperatures in excess of 175°C at ~1.7 km depth. Data from multiple deep wells within the FORGE site confirm that the target reservoir for EGS experiments has sub-commercial fluid productivity and injectivity and does not support a convective hydrothermal system. Image log data show that the natural fracture system is appropriately oriented with respect to local tectonic stresses for reactivation and propagation of fractures during EGS stimulation. Gravity and magnetic data, along with 2D and 3D modeling and inversion of these data, re-processed and re-interpreted seismic reflection data, geologic data from analogous Mesozoic basement units, and temperature data have been integrated with the existing data. The combined data set has been used to build the working geologic framework model of the Fallon FORGE site. This updated geologic framework represents a significant step forward in the characterization of the Fallon site, especially the crystalline basement. These data and analyses demonstrate that the Mesozoic crystalline basement units are well-suited to serve as EGS stimulation targets and are representative of many potential EGS localities throughout the United States.