Three-Dimensional Heat and Mass Balance Modelling of Basin and Range Geothermal Systems Initiated by Opening of Structurally Controlled Conduits

The Basin and Range province of the Western US hosts numerous geothermal systems with observed temperatures > 200 – 285 °C at 2-3 km depth, most of which were blind discoveries with no known magmatic heat source. At the Dixie Valley Power Plant system in Nevada, a “Hot Rock” anomaly, with temperatures > 200° C, at depths shallower than 3 km, extends >20 km parallel to the Stillwater Fault zone indicating a heat source (above conductive background) below or adjacent to the holes, actively heating the rock for >20 ka. To study such systems, using heat and mass balance software (TOUGH2) with an equation of state module designed to include supercritical H2O phase transition and properties, the temperature evolution and fluid flow in steeply dipping conduits (representative of a fault stepover or intersection) extending to depths of 7.9 and 11.5 km, opened into a 3 dimensional (40 x 18 x 12 km) polygonal model was investigated. Various conduit geometries and rock properties for “bulk” rock, basin fill, and damage zone domains were tested to determine their influence on these transient systems. The hottest observed Basin and Range temperatures (285 ºC) are matched in rapidly evolving geothermal systems after opening a high conductance or deep (11.5 km) conduit, into a bedrock domain where mostly conductive heat transfer dominates. Heat is mined from depth by convecting fluids from within and immediately adjacent to the conduit. When the conduit is “open” to infiltrating surface fluids, these systems are short lived – lasting not more than a few hundred years. But if surface fluids are limited, either by the geometry of the conduit, or by “sealing” the top portion of the conduit – these systems can maintain elevated temperatures for > 100 ka, where the bulk of heat and fluids supplying these systems are locally sourced. This suggests that multiple such convecting systems can exist in close proximity – both spatially and temporally. Small increases in bedrock permeability (but low enough for conductive heat transfer to dominate) leads to systems with higher temperature and greater fluid outflow. When the top of the conduit is sealed, adjacent rock units are heated by the conduit, especially where shallow permeable reservoirs allow fluids to move laterally. Comparison of temperaturedepth (T-D) profiles of and volumes of “Hot Rock” anomalies from model results for time intervals >20ka, conduits “sealed” to 2 km, and with shallow reservoir rocks, closely match observations for the Dixie Valley system. Surface geothermal features (e.g., sinters, travertine, and pre-production outflow) may represent periodic “tapping” of this reservoir by tectonic rupture or some other process, or discrete systems that develop in response to rupturing. These models suggest a possible mechanism for the origin of these enigmatic systems, providing a conceptual model for the distribution and volumes of hot rock available for energy production, and potential clues for finding additional occurrences of these enigmatic systems.