GEOTHERMAL POTENTIAL OF THE FOLD AND THRUST BELT OF SOUTHEAST IDAHO

Allis et al. (2013, 2015) and others have made the case that hot sedimentary geothermal reservoirs may have considerable economic potential within a temperature-depth window of 150 to 200 ^oC at 2 to 4 km. One such area in Idaho’s fold and thrust belt (Welhan et al., 2014; Welhan, 2016) has been evaluated to provide a first-order estimate of stored thermal energy and electric power-generating potential. Thermal data from more than 30 oil and gas exploration wells drilled in Idaho’s thrust belt, indicate elevated heat flows ranging from 90 to >120 mW/m² in the most favorable areas east of the Blackfoot volcanic field, where four boreholes encountered the 150 ^oC isotherm at depths of 2.5 to 3.3 km with corrected bottom-hole temperatures of 185 to 240 ^oC at 4 km. Potential reservoir rocks known to be oil and gas producers in the Utah and Wyoming thrust belt, several of which also host productive aquifers in the study area, were encountered in all wells drilled in the high-heat flow areas. The Triassic Nugget Sandstone and Jurassic Twin Creek Limestone are two of the most likely high-temperature reservoir candidates, with permeabilities in the former ranging up to hundreds of mD to several Darcies.

A Monte Carlo analysis based on the volume of stored heat in these hot reservoir rocks and a range of thermal recovery and energy conversion factors documented in operating geothermal power plants worldwide suggests the median electric power-generating potential of this prospect is in excess of 1 GW_e. The best near-term development potential may exist in an area defined by a single high heat-flow well with reservoir temperatures ranging from 150-195 ^oC at depths between 3.1 and 4 km that is located within 25 km of the phosphate ore processing center of Soda Springs and less than 5 km from a regional high-voltage transmission corridor. The power-generating potential in the area around this well is conservatively estimated at 160 to 200 MW_e but may be much higher, depending on the size of the reservoir that is eventually delineated.