Stress and Fault Rock Controls on Fault Zone Hydrology, Coso Geothermal Field, CA

Davatzes, Nicholas¹, Stephen Hickman¹ (1) U.S. Geological Survey, Menlo Park, CA

In crystalline rock of the Coso Geothermal Field, CA, fractures are the primary source of permeability. At reservoir depths, borehole image, temperature, and mud logs indicate fluid flow is concentrated in extensively fractured damage zones of large faults well-oriented for slip. In contrast, fault cores often function as hydrologic barriers separating regions of distinct fluid inclusion chemistry and temperature gradient. Distributed fracture networks play only a minor role in fluid flow despite locally high fracture density and some fractures well-oriented for slip. At the surface, hydrothermal activity is concentrated along active normal fault traces where borehole stress measurements and focal mechanism inversions indicate high deviatoric stress but relatively low mean stress conducive to normal slip.

Borehole measurements, surface mapping and mineralogical and micro-structural observations on core and outcrop samples indicate that these variations in fault zone hydrology result from: (1) recurrent slip driven by the remote stress; (2) changing fault zone mineralogy resulting from chemical alteration and healing; (3) fracture connectivity. Brittle fracture and frictional slip in low porosity crystalline rocks produce dilation owing to surface roughness along fracture walls, brecciation, and micro-cracking. Yet, active precipitation and alteration in the geothermal system implies rapid sealing of fractures. Precipitated calcite and silica retain the brittle dilatant behavior responsible for permeability generation as revealed by crack-seal textures and brecciated cements. Conversely, fault rocks enriched in phyllosilicates demonstrate ductile behavior and reduced frictional strength that minimize dilation accompanying slip and mitigate permeability regeneration. Over time, a fault core enriched in phyllosilicates acts as a persistent barrier to cross-fault flow, whereas continued brittle fracture in the damage zone produces an episodically well-connected fracture network conducive to fault-parallel fluid low.