Project Hotspot: Insight into the Subsurface Stratigraphy and Geothermal Potential of the Snake River Plain

Abstract

Preliminary stratigraphic sections for Project Hotspot’s Kimama drill hole based on gamma and neutron logs correlate well with lithologic observations. The gamma logs document the depth and thickness of sedimentary interbeds, whereas the neutron logs respond to the contrast between massive flow interiors and more porous flow tops. These data show the presence of at least 550 basalt flows (0.1-50 m thick), grouped into at least 30 flow groups, 13 m to 170 m thick (most 20-100 m thick). Flows are designated based upon morphological characteristics, including the presence of rubbly, highly fractured flow tops, massive to vesicular flow interiors, and rubbly flow bases; flow groups are commonly separated by sedimentary interbeds, which range in thickness from 0.2 m to 50 m. Increased conductivity along basalt flow unit contacts and associated fractures is a well-documented trend in the subsurface stratigraphy of the SRP. Determining the location of permeable basalt flow unit contacts and impermeable sediment layers is necessary to understand the flow of groundwater and potentially, geothermal fluid. The compilation of well log data shows an apparent agreement between lithologic and geophysical stratigraphy, with observed basalt flow unit contacts and sediment interbeds at least roughly mirrored by spikes and dips in natural gamma and neutron log signals. Natural gamma logs have the additional capability of identifying more evolved, high potassium basalt compositions through signal peaks that are unrelated to lithologic flow boundaries. These observations demonstrate the usefulness of wireline geophysical data to determine basalt flow group boundaries, flow compositions, and probable fluid transport avenues.