Geologic Setting of the West Flank: A FORGE Site Adjacent to the Coso Geothermal System

Abstract

The West Flank FORGE (Frontier Observatory for Research in Geothermal Energy) site is located just west of the Coso geothermal field, eastern California. Coso is a fluid-dominated, high temperature ( more than 350¢ªC) geothermal system that has been producing power continuously since 1987. The reservoir is composed of highly faulted, fractured and hydrothermally altered Cretaceous and Jurassic plutonic basement rocks. The heat source is a shallow, silicic magma chamber associated with overlying, Quaternary rhyolites and basalts of the Coso volcanic field. Over 30 years of development drilling and associated investigations demonstrates that several well-defined boundaries exist at Coso beyond which there is no commercial, hydrothermal geothermal potential. The West Flank is just went of one of these boundaries and it meets FORGE temperature (175-225¢ªC), host rock (crystalline rock), and depth (FORGE temperature window within 1.5-4.0 km) criteria. It has been hypothesized that the Coso volcanic field exists within a right-releasing step-over between two NW-striking, dextral fault systems. Two distinct fault populations (WNW-trending and antithetical, NE-trending strike-slip faults and N- to NNE-trending normal faults) yield high permeability drilling targets in the geothermal field. The West Flank appears to be segmented from the geothermal field by one such northerly trending fault bounded by a dense, felsic dike swarm. The West Flank's 83-11 well demonstrates that the West Flank is hot (83-11 BHT of 300¢ªC), low permeability and is comprised of weakly altered, leucogranite and diorite basement rocks. Drilling induced borehole features analyzed in 83-11 indicate that the West Flank stress field has been rotated to 081¢ª in contrast to the minimum principal stress orientations within the geothermal field which range from 103¢ª to 108¢ª. Well logs from within and outside of the geothermal field, mapping, geophysics (e.g., MT, seismic reflection, gravity, aeromagnetics), thermal models, a more than 20 yr Meq catalogue, and other data sets are being assessed to develop a working, 3d conceptual geologic model of the West Flank.