EXPLORATION AND DEVELOPMENT AT DIXIE VALLEY, NEVADA: SUMMARY OF DOE STUDIES

摘要

Dixie Valley is the hottest (>285 deg C at 3 km) and one of the largest geothermal systems (63 MW power plant operating for almost 20 years) in the Basin and Range province. The heat source is deep circulation in a high heat flow, highly fractured upper crust without a significant magmatic thermal input. Many hot springs in the Basin and Range Province share many of the characteristics of the Dixie Valley system. Thus major geothermal resource questions are how significant are these systems, what determines their location, and what are the best ways to evaluate and develop them. The USDOE sponsored extensive research associated with the Dixie Valley system in the period from approximately 1995 to 2002. These studies have been summarized in an extensive report to be published by the Nevada Bureau of Mines and Geology (Blackwell et al., 2007). This paper briefly summarizes the contents of that report including the main studies, their results, and interpretation of their significance as related to Basin and Range fault reservoir definition and development. Techniques applied as part of the research activities on a regional basis (Dixie Valley, the Stillwater Range and the Clan Alpine/Augusta Range) included but are not restricted to geology, geophysics, hydrology, and hydrogeochemistry. Within the producing area there were studies of the subsurface geology, thermal regime, fluid geochemistry, seismic reflection characteristics, and potential field structural mapping. In particular a gravity study, an EM survey, and a low level, high resolution magnetic survey were completed. In conjunction with extensive older geophysical data the combined data sets were used to develop a geological model of the system. A number of geochemically focused studies were carried out. These included initial pre-production composition measurements, non-condensable gas and water isotope composition measurements, regional He isotope studies, chemical evolution-time series (evidence for compartmentalization) measurements, tracer tests (reservoir connectivity), and dating of sinters and travertine. Remote sensing studies included air photo interpretation, hyperspectral studies of Dixie Meadows, INSAR Synthetic Aperture Radar Interferograms for ground subsidence, and infared measurements. Finally numerical modeling of generic natural state Basin and Range flow systems and specific applications to the Dixie Valley geometry were used to develop constraints on the deeper aspects of the flow system. These studies help in the evaluation of other Basin and Range systems by confirming that the Dixie Valley system must be in a transient state to reach the high temperatures observed. The results of these studies are summarized and a model of a Basin and Range geothermal system like Dixie Valley is described. The system is not a simple fault plane, but a complicated flow system with much character related both to the Basin and Range normal faulting and the permeability of the country rocks. The fluid flow paths are complicated and vary on a small scale leading to a complex reservoir system. The complexity indicates a much larger "reservoir" than would be the case if the system was a simple planar fault zone. The system is probably in a transient condition related to events on a 10,000 to 100,000 yr time frame. Much of what has been learned in Dixie Valley is transferable to other Basin and Range geothermal systems.