Linking Structural Geology and Tectonics Research to the Future of Utility-scale Geothermal Energy

Benjamin Surpless, Trinity University
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Abstract

As a structural geologist focused on fault-related fracture networks and permeability generation, I include geothermal energy potential as an implication of my results. However, after recent interactions with those in the geothermal industry, I recognize the challenges involved in gauging that potential, but I am encouraged by the acknowledged importance of structural analysis for geothermal exploration.

Fault damage zones commonly control hydrothermal circulation, and structural discontinuities, like changes in fault orientation or complex fault interactions, generate fracturing that elevates permeability. Thus, the Basin and Range province, with high heat flow and active normal faults, holds immense geothermal production potential. In this presentation, I will discuss my own research then link my findings to different types of utility-scale geothermal systems, including conventional geothermal plants and enhanced geothermal systems (EGS), which require hydraulic fracturing.

The west-dipping Sevier normal fault zone of Utah accommodates extension on the eastern margin of the Basin and Range province. Near Orderville, Utah, Sevier fault segments display a range of displacements and geometric interactions. We focused on outcrops of the Jurassic Navajo Sandstone near the tip of the low-displacement (< 3 m) Spencer Bench fault segment. There, erosion has exposed a fault cross-section, which we analyzed using scanline analysis, virtual outcrop modeling, model-based fracture network characterization, and network topologic analysis.

Results reveal asymmetric accommodation of strain across the fault, with a hanging wall that is more intensely and complexly fractured than the footwall. The footwall displays long, isolated fractures sub-parallel to the Spencer Bench fault plane. In the hanging wall, fracture intensity decreases with increasing distance from the fault, but fracture orientations and cumulative frequency analysis suggest that fracturing in the hanging wall was also affected by stresses associated with the high-displacement Mt. Carmel fault segment, 400 meters to the west, likely both increasing the width of fracturing and introducing strike variability across the damage zone. Network topologic analysis reveals a more connected fracture network in the hanging wall and decreasing connectivity with increasing distance from the fault plane.

Because connectivity is crucial for the circulation of hydrothermal fluids, if this system were in the subsurface and the fault remained active, the linking zone between faults would be a possible target for conventional geothermal assessment. However, EGS development relies on predictable flowpaths generated almost exclusively by hydraulic fracturing, so this linking zone between segments would not be considered for those systems.

Session

Societal relevance of structural geology and tectonics