Resource Examples

  Fault Age versus Heat Flow in Nevada


Preliminary results of the analysis of the database were described by Wisian et al. (1999).  They found that 95% of the geothermal systems with temperatures over 150°C occur in areas where the regional heat flow is greater than 80 m/Wm2.  This result can be attributed to two factors.  Young magma chambers will be found in areas with high regional heat flow as high temperature conditions are required at depth to generate magmas. Secondly, for geothermal systems related to deep circulation of water rather than localized magma systems, there seems to be a “maximum” depth of circulation of about 6 km. Thus the higher the heat flow in an area, the higher will be the temperature experienced by the deeply circulated water.

While it has been recognized that there is a frequent association of geothermal systems in the Basin and Range with faulting, the degree of correlation was qualitative.  Merging the geothermal well database with a compilation of geologically recent faulting (Raines et al., 1996) shows the degree of correlation between the two.  Combining the two databases shows that 90% or more of major known geothermal systems are within 3 km of late Pleistocene or younger faults.  This correlation suggests that future exploration in the region should include young faults as a key indicator.  One of the better examples of the close association of historic faulting and geothermal systems is Dixie Valley, Nevada where there is geothermal production immediately to the north of a 1954 surface rupture zone (Bell and Katzer, 1987).  The close association between recent faulting and the existence of geothermal systems suggests that recent faulting is a requirement for system development (due to the effect of self-sealing) and that future exploration in the region should include young faults as a key indicator.
References: 
Bell, J.W., and T. Katzer, Surficial geology, hydrology and late Quaternary tectonics of the IXL Canyon Area, Nevada, Nevada Bureau of Mines and Geology Bulletin, 102, 52pp., 1987. 
Raines, Gary L. Don L. Sawatzky, and Katherine A. Connors, U.S. Geological Survey Digital Data Series DDS-041, Great Basin  Geoscience Data Base, 1996. 
Wisian, K. W., D. D. Blackwell, and M. Richards, Heat flow in the western U. S. and extensional geothermal systems, Proceedings, 24th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, Ca., January 25-27, vol 24, 219-226, 1999.

 

EXAMPLES OF HOW TO USE THE DATA BASE INFORMATION
The western United States has multiple areas where high and low temperature geothermal energy is available. Both temperature types are being used for small and large scale energy production in many of the Western states. The following examples of Geothermal Areas are shown to give ideas of what is possible with the Western Geothermal Area Data Base. The geothermal areas represented are Desert Peak, Brady Hot Springs, Black Rock Desert, Gerlach, Fly Ranch, McCoy, Leach Hot Springs, and Grass Valley.  The gradient maps were produced using the gradient data (at multiple depths to view possible over-turns in the gradient) and the program Surfer by Golden Software. The geology maps were scanned from the Nevada Bureau of Mines Bulletins. The surface topography maps are from the U.S.G.S. GeoData homepage, specifically the 1:250,000 maps. There are now available from the U.S.G.S. digital maps at a scale of 1:24,000. The images below can be saved for more detailed viewing. In examining the gradients, we have displayed the gradient contours for the "deep", "shallow" and "combined" well depths. Deep gradients are usually 150 meters and deeper. In general there  are many more well gradients for shallow depths than deep. A deep well location could have both a deep and shallow gradient if the gradient changed over the depth intervals. This often occurs when the well gradient is high in the shallow depths and then becomes isothermal or negative for the deeper gradient interval. The gradient maps below have not been limited to the aerial extent of the actual well locations. The areas of gridding away from the well locations do not necessarily depict the proper gradient values for these areas. 

The next step for understanding the data is to include the temperature-depth curves for the wells examined.  In order to determine a well's thermal significance, the well temperature-depth curve is studied in comparison to regional and local conditions. A series of example temperature-depth curves with descriptions of the factors that effect their character are found at the Temperature Log page. Any discussion the following graphics develop is welcome.
Locations of geothermal gradient examples in Nevada
Desert Peak And Brady Hot Springs Geothermal Areas, Nevada
Black Rock Desert, Gerlach and Fly Ranch Geothermal Area, Nevada
Black Rock Desert, Gerlach, and Fly Ranch, Nevada, gradient contours and well locations
McCoy Geothermal Area, Nevada
Leach Hot Springs and Grass Valley Geothermal Area, Nevada