BASIN SCALE 3D MODELLING OF DIXIE VALLEY GEOTHERMAL SYSTEM

Non-magmatic Basin & Range geothermal systems seem to require a very specific, enigmatic combination of structural and flow conditions, the prime example being Dixie Valley, NV. Previous 2D models of this system (oriented perpendicular to range-bounding fault) can match some of the observed temperatures and fluxes, however at late times the models cool excessively and inadequately characterize distal effects. Using TOUGH2-Eos1sc (for super-critical fluid conditions), a 3D basin scale model calibrated to field observations at Dixie Valley addresses effects of along-strike heterogeneity and structural controls on fluid flow.

The model considers a 40x24x12km block with a range bounding fault zone extending to 7.5 km depth, with a basal heat flow of 90 mWm2, and bulk rock and fault permeabilities of 10-17 m³ and 10-16 m³ respectively. Bulk rock permeabilities higher than this cause convecting fluids to excessively cool the entire system. After reaching a convective equilibrium, a vertical "conduit" is opened in the fault plane, simulating a rupture resulting from seismic events. This triggers transient upflow of thermal fluids along the conduit lasting a few hundred to several thousand years, depending on the conduit permeability. A wide conduit allows downflow from the surface immediately adjacent to the upflow zone (counterflow) cooling parts of the shallow fault zone, contrary to field observations. Adding a deep (4 km) permeable zone (of limited lateral extent to avoid excessive cooling) with permeability 10^-14 to 10^-15 m² discharging into the base of the fault zone conduit, and narrowing the discharge conduit at the surface eliminates the counterflow. This suggests that a delicate balance is required between the conductance of the deep fluid source and shallow upflow zone. A suitable deep fluid supply region may be fault-bounded Jurassic basalt beds in the hanging wall of the Stillwater Fault.

Supplying sufficient deep fluids to the conduit without causing excessive cooling from counterflow or deep convection may require periodic re-establishment of deep permeability by seismic events, with permeability declining between events.