Washington State Geothermal Play-Fairway Analysis: Preliminary TCH Drilling Results

The three phase Washington State Geothermal Play-Fairway Analysis was designed to overcome the challenges of dense vegetation, glacial veneers, and extreme precipitation. Phase 1 was as a desktop study and gap analysis of past data sets. In Phase 2, geologic (mapping, geochronology) and geophysical (LiDAR, MT, passive seismic, gravity, and magnetics) data were collected, synthesized and modelled. In Phase 3, two temperature core holes (TCH) designed and sited to test the PFA methodology, were drilled in the summer of 2019. The first TCH, MB 76-31, was drilled along Little Park Creek, 11 km west-southwest of the summit of Mount Baker. The drill site was the closest accessible location along a trend -- defined by a strong magnetic gradient and lidar lineament -- from the Baker Hot Springs (BHS). The borehole was rotary drilled to 321m and then cored at a TD of 450 m. Water samples, 127 m of rock core, repeat temperature logs, and a geophysical image log were obtained to probe for heat and fluid flow in fracture systems. Geochemical analysis of the water samples indicate a clear geothermal influence but a low geothermometer (105 C) compared to BHS (150-170C), likely evidence of mixing with groundwater near the surface, and/or loss of SiO2 due to precipitation and re-equilibration in shallower depths. The equilibrated measured temperature gradient of 64ºC/km and calculated heat flow of 145 mW/m^2 is more than twice the regional average, also indicating local influence of the Mt Baker magmatic system at the Little Park Creek TCH site. The second TCH, MSH 17-24, was drilled near Schultz Creek, 16 km north-north east of Mt St Helens. This site was chosen based on ambient noise tomography maps which indicated a local anomaly of slow shear wave velocities (Vs), both absolute and relative to Vp. This borehole had been rotary drilled to 141 m in 2018 and then deepened with a core rig to 321 m in 2019.