Update on the Stratigraphic and Structural Framework of the Proposed Fallon FORGE Site, Nevada

摘要

The Fallon FORGE site lies in the SE part of the Carson Sink basin in west-central Nevada. The structural framework of the Carson Sink is dominated by Miocene to recent extensional features, including systems of N- to NNE-striking normal faults. Seismic reflection and gravity data reveal that the Carson Sink consists of a series of half grabens, including a gently west-tilted half graben in the Fallon FORGE area. The Carson Sink region also contains a series of extensional anticlines and synclines, resulting from reversals in the predominant dip direction of normal fault systems. The west-tilted half graben at the FORGE site occupies the west limb of a northerly trending extensional anticline, the axis of which lies directly east of the FORGE site. Cenozoic normal faults are relatively widely spaced in the area (0.5-2 km) in contrast to closely spaced faults in nearby geothermal systems, such as Salt Wells, Bradys, and Desert Peak. Multiple features at the FORGE site indicate the absence of an active hydrothermal system, including low permeability encountered in wells, lack of convective temperature profdes, lack of recent faulting, lack of a favorable structural setting for geothermal activity, and lack of surface geothermal features. However, borehole imaging and modeling of the stress regime suggest that the basement contains abundant N- to NNE-striking preexisting fractures, which approximately parallel σ_(Hma)x and are therefore favorably oriented for stimulation. High extensional strain rates in the area are conducive to increasing permeability through reactivation of shear fractures during hydraulic stimulation. Relatively competent target formations for EGS experiments include Triassic to Jurassic felsic metavolcanic rocks, Jurassic quartzite, and Jurassic to Cretaceous granitic intrusions. The combination of both crystalline metamorphic and granitic rocks offers an exceptional opportunity to conduct EGS experiments in multiple rock types at one site.