| Abstract |
A 2D reactive transport model of the Dixie Valley, Nevada, geothermal area was developed using TOUGHREACT [1] to assess fluid flow pathways and fluid rock interaction processes. The model domain was adapted from a geological and structural cross-section and corresponds to the north-western part of Dixie Valley including half of the Stillwater Range and half of the valley floor. Setting up the model included specification of the mineralogy of the different rock units, the formulation of the corresponding mineral dissolution and precipitation reactions, the explicit definition of two major normal faults and the specification of a dual continuum domain along the uppermost 1 km of the eastern one of these normal faults. The use of a dual continuum domain allowed simulating the presence of a small-scale thermal spring system being fed by a highly permeable but narrow fracture zone. The model was run for various permeabilities of the dual continuum fracture, whereas bulk rock fluid flow and thermal parameters were defined according to a previous flow simulation study performed by McKenna and Blackwell [2]. Model results were tested against available field data such as chemical analysis of thermal springs, isotherms inferred from geothermal wells, and results of the previous modeling study [2]. Moreover, simulated chemical compositions for the geothermal spring were combined with multicomponent geothermometry to assess whether the model reflects the observation that geothermal springs often are out of chemical equilibrium while the chemical signature of the fluid reservoir at depth is inherited [3]. Simulation results reveal that a minimum permeability of 10-12 m2 for the dual continuum fracture is needed to preserve the geochemical signature of the reservoir. The simulations also suggest that the presence of small-scale spring-feeding fractures having an elevated permeability when compared to large-scale fault systems can significantly alter the shallow fluid flow regime of geothermal systems. For the Dixie Valley case, the model infers that such elevated permeabilities lead to a shallow (less than 1 km) convection cell where superficial water infiltrates along the range front normal fault and connects the small-scale geothermal spring through basin filling alluvial sediments. References: [1] Xu et al. (2011) Comp. & Geoscience. 37, 763-774 [2] McKenna J.R. and Blackwell D.D. (2004), Geothermics. 33, 457-476 [3] Reed, M.H., Spycher, N.F. (1984), Geochim. Cosmochim. Acta. 48, 1479-1492. |