Analysis of Favorable Structural Settings and Step-Overs in Normal Fault Systems in the Great Basin Region: Implications for Geothermal Exploration and Development
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Authors
Giddens, Mary Hannah
Issue Date
2024
Type
Thesis
Language
en_US
Keywords
Favorable structural setting , Geothermal , Hidden systems , Relay ramps , Step-over , Structure
Alternative Title
Abstract
The Great Basin region (GBR) in the western U.S. may host vast amounts (~10 GWe) of undiscovered geothermal resources. Although ~39% of the known geothermal systems in the GBR lack visible hot springs or fumaroles, up to 75% of the geothermal resources in the area may be blind or hidden. Nearly all geothermal systems in the GBR reside in six types of favorable structural settings, including normal fault terminations, step-overs (i.e., relay ramps) in normal faults, fault intersections, accommodation zones, displacement transfer zones whereby strike-slip faults end in arrays of normal faults, and pull-aparts in strike-slip fault systems. Although the affinity between geothermal systems and these structural settings is widely known, little research has been conducted to distinguish which geometries of a particular structural setting are more conducive for geothermal activity. Distinguishing the most favorable geometries is crucial to improving exploration strategies for hidden systems, selecting optimal drilling targets, and increasing the efficiency of existing power plants. Through field observations, Quaternary fault data, LiDAR, and regional geophysical surveys (e.g., gravity, magnetics, and magnetotellurics), a detailed inventory identified 1,087 favorable structural settings within the GBR. Step-overs in Quaternary normal fault zones are the most common favorable structural setting identified in the GBR, accounting for ~42% (454) of all settings. Complex fault geometries within the step-overs, including multiple minor faults connecting the major overlapping fault strands, enhance permeability and generate efficient pathways for hydrothermal fluid flow. Step-overs are common in normal fault zones, and thus it is difficult to distinguish which step-overs might host a blind geothermal system. However, step-overs come in a variety of geometries depending on the relative overlap, underlap, and spacing between the major fault strands. In addition, the geometry of the minor faults that breach the step-over can vary from relatively major oblique-slip faults that directly connect the major fault strands to systems of en échelon faults that parallel the major fault strands. Step-overs were therefore further classified based on orientation, sense of stepping (right vs. left), amount of overlap and spacing between main fault strands, and linkage style (hard vs. soft). Of the step-overs identified, ~54% are right stepping, ~51% hard linked, and ~50% are underlapping. Relay ramp width ranged from 0.1-14.6 km, with an average of 2.8 km. Step-overs associated with higher-temperature (>120°C) geothermal systems are ~56% right stepping, ~70% hard-linked, ~56% overlapping, and ~74% lie between fault strands oriented north to north-northeast. Average relay ramp width for these higher-temperature step-overs is 3.3 km. Producing systems (i.e., contain operating power plants) in step-overs preferentially step left (~64%), are hard-linked (~73%), overlap (~55%), and have an average relay ramp width of 3.4 km. These data suggest that higher-temperature systems favor left-stepping, overlapping, hard-linked geometries in the GBR. Step-overs with hard-linked and overlapping fault strands have relative high densities of fractures, faults, and fault intersections, all of which serve to enhance permeability. Additional structural complexity provides subvertical conduits of enhanced permeability that facilitates transport of hot fluids from greater depths. These attributes combined with faults optimally oriented to experience dilation in the current stress field provide long-lived permeable pathways that can host higher-temperature geothermal systems. This information will aid in more efficient exploration of hidden geothermal systems across the GBR.