The Development of a 3D Structural-Geological Model as part of the Geothermal Exploration Strategy – a Case Study from the Brady’s Geothermal System, Nevada, USA

Abstract

In the framework of geothermal exploration campaigns 3D structural-geological modeling plays an important role in the understanding of geothermal systems. The focus for the Brady’s geothermal system located in the Basin and Range province is on the identification of structural controls on fluid flow and permeability anisotropy. In addition to 3D structural-geological modeling, the applied exploration strategy also includes stress field analysis and surface geochemical surveys. We have used 1) detailed geological maps, 2) borehole data, 3) processed 2D seismic and gravity data, and 4) a digital elevation model as input parameters of the 3D model. Based on these data, four representative cross sections have been developed as a major input for a preliminary 3D geological model. Well logs are used to verify the stratigraphic structure between the cross sections. The major strike direction of the faults is NNE. Normal faulting is the dominant stress regime. Dip angles range from 45° to 80°. The 3D model consists of eight geological units. The Mesozoic basement consists of granites and metamorphic rocks. Above, a sequence of Tertiary ash-flow tuffs, lacustrine sediments, and lava flows of different composition has been encountered. 3D structural models populated with geomechanical and stress data can help to delineate between dilational and shear zone both being prone for channeling fluids. In a later stage, stress data derived from fault plane analysis shall be integrated into the 3D structural-geological model applying the slip and dilation tendency technique to estimate hydraulically active fault zones. These results shall be verified by surface gas measurements to understand the impact of individual faults on fluid flow.