STRUCTURAL AND GEOMECHANICAL ANALYSIS OF A POTENTIAL GEOTHERMAL RESOURCE IN BASALTS, WESTERN SNAKE RIVER PLAIN, IDAHO

The Snake River Plain is a zone of high heat flow but little geothermal energy exploration has occurred. Project Hotspot, the Snake River Scientific Drilling Project, tested for deep geothermal resources and drilled the MH-2 on the Mountain Home Air Force Base by drilling into fractured crystalline and hydrothermally altered basalt to a total depth of 1,821 m. At 1,745 m depth the drill hole encountered flowing artesian hydrothermal fluids of at least 150°C. We integrate geological analyses of core, in situ stress analyses, image log, and borehole geophysical analyses, to describe the structural environment that produces permeability for artesian flow. The rocks in the lower 540 m of the drill hole consist of basalt flows up to 30 m thick, altered basalt, and thin sedimentary horizons. The mechanical stratigraphy is defined by 9 mechanical horizons that fall into one of 3 ranges of rock strengthon the basis of experimentally determined strength data, core logging, and geophysical logs. Hydrothermal alteration products and mineralization in the core are associated with 3 highly faulted sections, with the lowermost section, associated with the zone of flowing thermal water. Shear slip indicators on faults observed in core indicate bimodal strike-slip and normal failure mechanisms. The state of stress determined from drilling induced tensile fractures and borehole breakouts indicate the maximum horizontal stress, SH, is oriented 047° ± 7°. The in situ stress orientations exhibit little variation over the depth of the measured interval, but the SH magnitude varies with depth, and is best explained by an oblique normal fault stress regime. The geomechanical model indicates that if pore pressures at depth are elevated above the normal hydrostatic gradient, as is common in geothermal reservoirs, the differential stress and horizontal stress anisotropy are low and the system has the potential to have mixed normal and strike-slip failure mechanisms. Our observations and interpretations suggest the MH-2 borehole was drilled into a dilational step-over between the tips of oblique normal faults along the boundary of a buried fault block. Such structural complexities may enhance the potential for geothermal resources by creating interconnected fracture permeability at depth.