Geothermometry Mapping of Deep Hydrothermal Reservoirs in Southeastern Idaho: Final Report
The Eastern Snake River Plain (ESRP) in southern Idaho is a region of high heat flow. Sustained volcanic activities in the wake of the passage of Yellowstone Hotspot have turned this region into an area with great potential for geothermal resources. Numerous hot springs with temperatures up to 75 ºC are scattered along the margins of the plain. Similarly, several hot-water producing wells and few hot springs are also present within the plain. The geothermal reservoirs in the area are likely to be hosted at depth in the felsic volcanic rocks underneath the thick sequences of basalts within the plain and the Paleozoic rocks underneath both basalts and felsic volcanic rocks along the margins. The heat source to these geothermal resources is thought to be the mid-crustal sill complex which sustains high heat flow in the ESRP. Several thermal anomaly areas are believed to be associated with the local thermal perturbation because of the presence of favorable structural settings. However, it is hypothesized that the pervasive presence of an overlying groundwater aquifer in the region effectively masks thermal signatures of deep-seated geothermal resources. The dilution of deeper thermal water and re-equilibration at lower temperatures are significant challenges for the evaluation of potential resource areas in the ESRP. To address this issue, this project, led by the Idaho National Laboratory (INL), aimed at applying advanced geothermometry tools including temperature-dependent mineral and isotopic equilibria with mixing models that account for processes such as boiling and dilution with shallow groundwater that could affect calculated temperatures of underlying deep thermal waters. Over the past several years, we collected approximately 100 water samples from springs/wells for chemical analysis as well as assembled existing water chemistry data from literature. We applied several geothermometric and geochemical modeling tools to the compositions of ESRP water samples. Geothermometric calculations based on principle of multicomponent equilibrium geothermometry with inverse geochemical modeling capability (e.g., Reservoir Temperature Estimator, RTEst) have been useful for evaluation of reservoir temperatures. Similarly, sulfate-water oxygen isotope geothermometry was also applied to several samples in tandem with RTEst. In summary, geothermometric calculations of ESRP thermal water samples indicated numerous potential geothermal areas with elevated reservoir temperatures. Specifically, areas around southern/southwestern side of the Mount Bennet Hills and within the Camas Prairie in the southwestern portion of the ESRP suggest temperatures 140-190 °C. In the northern portion of the ESRP, Lidy Hot Springs, Ashton, Newdale, and areas east of Idaho Falls have expected reservoir temperature ?140 °C. In the southern ERSP, areas near Buhl and Twin Falls are found to have elevated temperatures as high as 160 °C. These areas are likely to host potentially economic geothermal resources; however, further detailed study is warranted to each site to evaluate hydrothermal suitability for economic use.