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.
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|