Analysis of Natural and Drilling Induced Fractures From Newly Acquired Borehole Image Logs at the Star Peak Geothermal Field, NV: Implications for Localized Stress Field Rotation Within a Normal Fault Step-Over

The orientations of open, partially open, and closed features were interpreted in three acoustic borehole image (ABI) logs recently collected at the Star Peak Geothermal Field. The most prevalent population of features across the logs have moderate to steep westerly dips, with secondary populations of higher angle east and southeast dipping features. These orientations are broadly consistent with the structural variation along the Rye Patch fault zone. Drilling induced fractures were observed in two of the logs, providing a proxy for the maximum stress orientation within the geothermal field. Drilling induced fractures (DIFs) measured in 46-28 at the south end of the field exhibit a dominant north-northwest trending maximum stress field orientation, while DIFs in 52-28 in the center of the field display a west-southwest trend. Both of these vary from the regional stress field orientation, which is oriented north-northeast/south-southwest for the maximum horizontal stress orientation. The Star Peak geothermal system is associated with permeability along a highly fractured portion of a step-over between fault splays in the hanging wall of the main trace of the Rye Patch Fault Zone. Based on analysis of the DIFs, this step-over may also be associated with a vertical-axis rotation in the stress field.