Why Basin and Range Systems are Hard to Find II: Structural Model of the Producing Geothermal System in Dixie Valley, Nevada

Abstract

Dixie Valley is the hottest (> 285 °C at 3 km) and one of the largest geothermal systems (63 MW power plant operated for over 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 the characteristics of the Dixie Valley system. Major geothermal resource questions are: what determines their location; and what are the best ways to evaluate and to develop them. The Dixie Valley system occurs in a complicated fracture network dominated by steep dips leading to a flow system both by 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 explains the longevity of production by the existence of a much larger “reservoir” than would be the case if the system was controlled by a simple planar fault zone. Much of what has been learned in Dixie Valley is transferable to other Basin and Range geothermal systems. The USDOE sponsored extensive research associated with the Dixie Valley system in the period from approximately 1995 to 2002. These studies will be summarized in an extensive report to be published by the Nevada Bureau of Mines and Geology (Blackwell et al., 2010). This paper briefly summarizes the conclusions of that report about Basin and Range fault structure as related to geothermal reservoir definition and development.