FRACTURE ZONE IDENTIFICATION FROM CORE AND BOREHOLE GEOPHYSICAL DATA IN SLIMHOLE WELLS DRILLED FOR PROJECT HOTSPOT: THE SNAKE RIVER GEOTHERMAL DRILLING PROJECT

The Snake River Geothermal Drilling Project (Project Hotspot) is drilling three deep slimhole wells at the Kimama, Kimberly, and Mountain Home sites in the central Snake River Plain (SRP), Idaho. The Kimama and Kimberly wells are complete and the Mountain Home well is in progress. Geophysical data have been collected at both Kimama and Kimberly sites. Full core is recovered and complete suites of wireline borehole geophysical data are collected along with vertical seismic profiles. The objective of the project is to assess the potential for geothermal energy development in the SRP. Part of that assessment includes evaluating the changes in the nature of fractures with depth through the study of physical core samples and analysis of the wireline geophysical data. Techniques exist to evaluate physical properties (lithology, porosity) and to identify fracture zones and saturated zones through the analysis of wireline log data. Most experience with these techniques has been in sedimentary rocks but the SRP is a volcanic province and the rock types at Kimama and Kimberly are primarily basalt and rhyolite, respectively, with interbedded thin sedimentary layers. This project seeks to develop techniques to identify anomalies in the physical properties of igneous rocks using porosity logs (neutron and acoustic), lithology logs (gamma ray and magnetic susceptibility) and fracture/saturation logs (televiewer and electrical resistivity). The core will be used to constrain the geophysical data and confirm the ability to identify permeability in fracture zones and saturated zones through analysis of the wireline log data. The fracture inventory is complete for the Kimama hole and preliminary analyses indicate that fracture zones are related to basaltic flow boundaries. The matrix porosity of these igneous lithologies is near zero aside from porosity from vugs and vesicles. However, open and sealed fractures indicate that mineralizing fluids form connected pathways in the rock. Core samples show a series of alteration phases, including amygdaloidal fine-grained calcite and secondary clays. The geophysical data will be used to predict anomalies in lithology and identify open fractures and saturated zones with high permeability.