NPGS Dinner Meeting: Thursday, April 3, 2014—RSVP by Tuesday April 1
Speaker: Dr. James E. Faulds, NBMG Director/State Geologist/Professor, Nevada Bureau of Mines and Geology, University of Nevada, Reno
Topic: Why is Nevada in Hot Water? New Approaches to Understanding and Harnessing Nevada’s Vast Geothermal Resources
Abstract: Although conventional geothermal systems have been successfully exploited for electrical production and district heating in many areas, exploration and development of new systems is commonly hampered by the risk of unsuccessful drilling. A major problem in selecting the best sites for geothermal development has been a lack of characterization of structural-stratigraphic controls for known systems. We have therefore completed a comprehensive inventory of the structural settings of all known geothermal systems (426 total) in the Great Basin region of the Basin and Range province (western USA), combining conventional tools of detailed mapping and structural analysis with innovative techniques of 3D modelling and slip and dilation tendency analysis. Characterization of known fields is especially important for discovering blind or hidden systems (i.e., with no surface expressions such as hot springs), which probably comprise the bulk of geothermal resources in any region.
Abundant geothermal activity in the Great Basin region is intimately related to the tectonic setting. Interestingly, volcanism plays only a minor role, as it generally ceased in late Miocene time. Thus, upper crustal magma chambers generally do not supply heat for the geothermal activity. Instead, most geothermal systems are fault controlled. Geothermal activity is concentrated in areas with the highest strain rates along or proximal to the eastern and western margins of the Great Basin, with high temperature systems clustering in transtensional areas of highest strain in the NW Great Basin. In the western Great Basin, a system of NW-striking dextral faults known as the Walker Lane accommodates ~20% of the North American-Pacific plate motion and is kinematically linked to N- to NNE-striking normal fault systems throughout the region. Enhanced extension in the NW Great Basin probably results from the NW termination of the Walker Lane and the related transfer of dextral shear into WNW-directed extension. The capacity of geothermal power plants correlates with strain rates, with the largest (hundreds of MW) along the Walker Lane and San Andreas fault systems, where strain rates are ≥1 cm/yr. Lesser systems (tens of MW) reside east of the Walker Lane, where strain rates are much less. Our inventory of structural settings of known geothermal systems in the Great Basin shows that most reside in 1) step-overs (or relay ramps) in normal fault zones (32%), which are characterized by overlapping fault strands, increased fracture density, and thus enhanced permeability; 2) normal fault terminations (25%), where horse-tailing generates a myriad of closely spaced faults and thus increased permeability; 3) fault intersections (22%), where multiple minor faults typically connect major faults and fluids can flow readily through highly fractured, dilational quadrants; and 4) accommodation zones (9%), which contain multiple fault tips and intersecting faults. Our 3D models of several systems indicate that geothermal upwellings in these settings are focused in pipe-like bodies of higher fault density oriented approximately perpendicular to the least principal stress. Further, many higher enthalpy systems are actually hybrids and contain more than one type of favorable setting (e.g. fault tip in accommodation zone). Quaternary faults lie within or near most geothermal systems. Geothermal fields are rare along major range-front faults due to reduced permeability in thick zones of clay gouge and periodic release of stress in major earthquakes. The favorable settings correspond to long-term, critically stressed areas, where fluid pathways are more likely to remain open in breccia-dominated fracture networks.
Similar findings in other extensional settings (e.g., western Turkey) bodes well for developing systematic exploration strategies in continental rifts worldwide. This also demonstrates that relatively inexpensive geologic studies should be conducted and integrated with geophysical data to define the structural setting prior to expensive drilling. Systematic workflow can reduce the risks in geothermal exploration and ultimately facilitate extensive development of what may be the world’s most readily available form of energy.
Where: Ramada Reno Hotel; 6:30 PM
1000 East 6th Street, Reno, NV 89512
When: Cocktail Reception 6:30, Skyline Bar, 14th Floor
HOSTED BY: Timberline Drilling Inc.
Dinner Served at 7:00 PM
Cost: NPGS Members $20; Non-Members $23; Students $10
Please RSVP by Tuesday Apr 1with the following link: https://docs.google.com/forms/d/1XaQ8bJu5Ktmd6DZH0D2XdgJ9pAEWbPeJPSlJDuQnFgE/edit#
If you have questions, please contact:
Vicki D. Ehni
Ehni Enterprises, Inc.