Characterizing Structural Controls of EGS-Candidate and Conventional Geothermal Reservoirs in the Great Basin: Developing Successful Exploration Strategies in Extended Terranes

Project PI: Dr. James Faulds, University of Nevada, Reno

Project duration: 1 February 2010 to 31 January 2013.

Funding agency: U.S. Department of Energy

Total project funding: $935,505 (federal) + $235,000 (UNR including cost-share)

Award number: DE- EE0002748

Project goals and activities:

We conducted a comprehensive analysis of the structural controls of geothermal systems within the Great Basin and adjacent regions.  Our main objectives were to:

  1. Produce a catalog of favorable structural environments and models for geothermal systems.
  2. Improve site‐specific targeting of geothermal resources through detailed studies of representative sites, which included innovative techniques of slip tendency analysis of faults and 3D modeling.
  3. Compare and contrast the structural controls and models in different tectonic settings.
  4. Synthesize data and develop methodologies for enhancement of exploration strategies for conventional and EGS systems, reduction in the risk of drilling non‐productive wells, and selecting the best EGS sites.

Phase I (Year 1) involved a broad inventory of structural settings of geothermal systems in the Great Basin, Walker Lane, and southern Cascades, with the aim of developing conceptual structural models and a structural catalog of the most favorable structural environments. This overview permitted selection of 5‐6 representative sites for more detailed studies in Years 2 and 3. Sites were selected on the basis of quality of exposure, potential for development, availability of subsurface data, and type of system, so that major types of systems can be evaluated and compared.  The detailed investigations included geologic mapping, kinematic analysis, stress determinations, gravity surveys, integration of available geophysical data, slip tendency analysis, and for some areas 3D modeling.  In Year 3, the detailed studies were completed and data synthesized to a) compare structural controls in various tectonic settings, b) complete the structural catalogue, and c) apply knowledge to exploration strategies and selection of drilling sites.

Key Project Findings

Structural controls of 426 geothermal systems were analyzed with literature research, air photos, Google Earth imagery, and/or field reviews. Of the systems analyzed, we were able to determine the structural settings of more than 240 sites. However, we found that many “systems” consisted of little more than a warm or hot well in the central part of a basin. Such “systems” were difficult to evaluate in terms of structural setting in areas lacking in geophysical data.

Of the 426 fields analyzed, major findings include:

  • 39% of the known systems are blind or hidden with no surface hot springs or fumaroles.
  • The structural setting for ~25% of the systems could not be determined. In most cases, undetermined systems reside in the central part of a basin (commonly warm wells) that lacks adequate geophysical data for elucidating the subsurface structure.
  • Step-overs or relay ramps in normal faults are the most favorable setting, hosting ~32% of the systems analyzed. Such areas are characterized by multiple, commonly overlapping fault strands, increased fracture density, and thus enhanced permeability.
  • Normal fault terminations host 25%.
  • Intersections of normal faults and strike-slip or oblique-slip faults host 22%.
  • Accommodation zones (belts of intermeshing oppositely dipping normal faults) host 9%.
  • Displacement transfer zones host 5%.
  • Pull-aparts in strike-slip faults host 3%.
  • Bends in major normal faults host 2%.
  • Major range-front faults host 1% of the known systems.

Some of the more robust systems contain more than one type of structural setting. These include Steamboat, Coso, and Dixie Valley. About 21% of the fields are hybrid or compound systems.

Project outputs and publications:

DOE Peer Review presentation, May 2010

Conference paper: Faulds et al., 2012, New Zealand Geothermal Workshop

Final report to the DOE, February 2014.

Structural inventory dataset (MS Excel)

Faulds et al., 2021,  Inventory of structural settings for active geothermal systems and late Miocene (~8 Ma) to Quaternary epithermal mineral deposits in the Basin and Range province of Nevada. https://pubs.nbmg.unr.edu/Inventory-structural-settings-p/r058.htm

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