| Keywords |
geothermal, exploration, Nevada, geophysics, gravity, magnetotellurics, drilling, temperature gradient, geothermometry |
| Abstract |
Sierra Geothermal Power (SGP) is actively exploring four geothermal properties in Nevada; Reese River, Silver Peak, Alum and Barren Hills. Each prospect is at a different stage of exploration. The exploration techniques used have included analysis of historic temperature gradient hole (TGH) data, the drilling of slim wells, gravity, surface geology and structure mapping, remote sensing, temperature surveys at conducted at a depth of 2 m, magnetotellurics (MT), reflection seismic and airborne electromagnetic methods. Independent resource assessments were commissioned from GeothermEx and their total resource assessment of the four properties is a 90% confidence of 136 MW or greater and a 50% confidence of 307 MW or greater. Although SGP has been exploring all four prospects since 2006, the majority of the exploration activities have been conducted at the Reese River geothermal prospect in north-central Nevada. The Reese River prospect is a “blind” resource that lies within a region of NE-trending faults called the Humboldt structural zone. SGP has drilled a total of five wells that range in depth from 487 m to 1580 m and conducted geochemical and geophysical surveys. These have detected areas of intense subsurface hydrothermal alteration including advanced argillic and silicified zones. Shallow TGH 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 at the Reese River prospect. Deeper drilling has confirmed commercial grade temperatures with well 13-4 reaching a bottom-hole temperature of ~150 °C. The chemistry of the Reese River geothermal fluids are similar to that being produced at the Beowawe reservoir in Nevada. Compatible geothermometry of up to 155 °C were obtained for water samples from three wells and cation geothermometry from a 85 °C shallow water well suggested that reservoir temperatures at depth may exceed 200 °C. Depth-to-basement has been estimated from modeled gravity profiles, with offsets fitting fault zones inferred from lost circulation zones in wells. MT data indicate a widespread zone of low resistivity smectite alteration at <200 m depth with an underlying relatively resistive ~70 °C clastic aquifer. This is sourced from deeper resistive aquifers, probably fractured carbonates, igneous intrusives and metamorphic rocks at 125 to 150 °C that are less well resolved below a 3D distribution of low resistivity smectite clay 300 to 1800 m thick. A revised 3D MT correlation of the MT data, modeled gravity, and drillhole data allow for new interpretations of the subsurface structure and permeability, thereby aiding in identifying new drill targets. |