| Title | The Geothermal Geodometer: A Geochemical Approach to Targeting Upflow in Long Valley |
|---|---|
| Authors | Adam JOHNSON, Robert SELWOOD, Daniel FEUCHT |
| Year | 2022 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Long Valley geothermal field, Casa Diablo, Basalt Canyon, geothermometry, geodometer, upflow, outflow, lidar, magnetotellurics |
| Abstract | The Long Valley Caldera complex located on the eastern slope of the Sierra Nevada hosts a productive geothermal system on the southwest margin. Power generation from the field harnesses 150-190°C fluids from outflow in the Basalt Canyon and Casa Diablo areas. This outflow extends a further 10km to the east of Casa Diablo and is expressed by surface manifestations such as fumaroles, mud pots, and hot springs with indicated resource upflow temperatures approaching 240°C based on chemical geothermometers. These estimated temperatures greatly exceed maximum measured temperatures and conjunction with historic drilling suggest a higher temperature upflow to the west. Chemical geothermometers from several wells and surface features along the outflow path were used to back-calculate a distance-to-upflow from the current Basalt Canyon production wells. General trends show an overall linear decrease in temperatures from west to east and a more rapid re-equilibration of silica relative to cation geothermometers, both of which are consistent with previous studies and our understanding of reaction kinetics. When quartz geotemperature is plotted as a function of distance from the westernmost, highest temperature sample (production well 14-25), extrapolation to inferred resource temperatures of 230-240°C suggests that upflow occurs within a 5-7.5 km radius from that point. Previous geophysical and structural studies have suggested upflow may be associated with permeability along high angle faults that form the western moat, which falls within the 5-7 km target area. Upflow in this area is further supported by resistivity structures in 3D modelled magnetotelluric data and lidar data that indicates a WNW structural pathway linking this area with production in Basalt Canyon. This chemical “geodometer†approach shows the utility of applying this technique to understanding geothermal systems that produce exclusively from outflow and can provide insight into additional exploration drilling or geophysical studies to further develop higher temperature resources. |