| Abstract |
Sierra Geothermal Power (SGP) has been exploring the Reese River geothermal prospect in north-central Nevada since 2006. The Reese River prospect is a “blind” resource that lies within a region of NE-trending faults called the Humboldt structural zone. As of May 2009, exploration conducted by SGP at Reese River includes: geological mapping (with a focus on structural geology), chemical geothermometry of well waters, and a shallow (2 m) temperature study, as well as 2D seismic, gravity and magnetotelluric surveys. In addition, SGP has drilled a total of five wells that range in depth from 487 m to 1580 m (~1500 feet to ~5200 feet). The Reese River prospect area is geologically complex with areas of intense subsurface alteration including advanced argillic and silicified zones. Depth-to-basement estimates have been derived from modeled gravity profiles revealing the structure of fault blocks which match well with drill log-inferred fault zones (i.e. lost circulation). Magnetotelluric data indicate a widespread zone of alteration at relatively shallow depths. Shallow drilling from the 1970’s and 1980’s has mapped a broad geothermal anomaly with a surface expression greater than 10 km long by 3 km wide. Deeper drilling has confirmed commercial grade temperatures with well 13-4 reaching a bottomhole temperature of ~150 °C (302 °F). Geothermometry results were obtained for water samples collected from several wells. In three of the wells, silica geothermometry is in agreement with local source rock temperatures and cation geothermometry (up to 155 °C) is in agreement with deeper bottomhole temperatures. Based on Na/K and Na-K-Ca geothermometry from an 85 °C farmer’s well, reservoir temperatures at depth may exceed 200 °C. The chemistry of the Reese River geothermal fluids are similar to that being produced at the Beowawe power plant in Nevada. Cross-correlation of the magnetotelluric data, modeled gravity, and drillhole data allow for new interpretations of the subsurface structure and permeability, thereby aiding in identifying new drill targets. |