ROCK PROPERTY DESCRIPTIONS INTERPRETED FROM BOREHOLE GEOPHYSICAL DATA COLLECTED IN SLIMHOLES DRILLED FOR PROJECT HOTSPOT: THE SNAKE RIVER GEOTHERMAL DRILLING PROJECT

The Snake River Geothermal Drilling Project is an effort to determine the potential for geothermal energy development in the central Snake River Plain, Idaho. Few data are available in the central Snake River Plain so sampling by drilling provides data in the vertical profile to help determine the geothermal potential. Two slimhole cores are being drilled at the Kimama and Kimberly sites to depths around 2000 m. Wireline borehole geophysical tools were run down the boreholes to collect data about the rock properties. The data include gamma ray, neutron, electrical resistivity (Elog), sonic, magnetic susceptibility logs and televiewer images. The tools were run the length of the borehole to collect data to depths of around 1850 m. At Kimama, the majority of core is basalt with few intervals of fine-grained sediments. Core from the Kimberly site shows basalt and rhyolite with intervals of fine-grained sedimentary layers. Wireline logs can help us understand basic rock properties and the relationships between fracture distribution and rock type. Gamma ray logs can tell us the amount of clay alteration in the volcanic rocks, neutron logs and sonic logs tell us about the porosity of the formations, Elogs can tell us about the level of saturation in the rocks, magnetic susceptibility logs can show anomalies in the mineralogy and televiewer data give us visual information about the rocks, including saturated zones and fracture zones. Critical to the development of geothermal systems is the amount of porosity and the potential for fluid flow in the subsurface. The volcanic nature of the formations means there is little primary porosity to contribute to fluid flow. Fractures are the primary mode of fluid transport so the analysis of fractures and the level of saturation in fracture zones are necessary to evaluate the geothermal potential. Preliminary analyses of the core show that fractures exhibit a series of alteration phases, including amygduloidal fine-grained calcite. Despite the nearly zero porosity of the matrix in the rocks, mineralizing fluids appear to form connected pathways in the rock. The borehole geophysical data show where fracture zones exist and whether the porosity produced by the fractures is sufficient for geothermal development.