THE EFFECT OF FAULT GOUGE ON FLUID FLOW, LISBON VALLEY, UT

The existence of authigenic and reworked clays in fault zones is well understood to control the hydrologic and mechanical properties of faults. The net effect these clays have on fluid conductivity, reservoir quality, and fluid derived mineralization is dependent on the origin of the gouge. To address this topic, we collected samples along four high-angle normal faults exposed within a copper mine in Lisbon Valley of Southeast Utah. The GTO, Lisbon Valley, Keystone, and Centennial faults were all sampled in gouge zones ranging from ~0.1 to ~5.5 meters across. Fault displacement ranged from 5 meters to 400 meters. Complementing field work was XRD analysis of all suitable samples to determine the quantity and type of clay in each gouge. We related the hydrologic properties affected by fault gouge with shale gouge ratio (SGR) modeling using T7 computer software. SGR quantitatively estimates clay amounts in fault gouge that allow fault zone permeability estimates to be derived.

Field interpretations suggest gouge zones are dominated by either maroon or green clays with most sites having lenses of the non-dominate component. Sampled gouge zones exhibited grain sizes from clay to fine sand and poorly to moderately lithified. Color of gouge samples does not seem to correlate with offset of faults, however, there is a correlation between juxtaposed stratigraphy and color. Preliminary XRD data shows high concentrations of illite in nearly all samples, chlorite is present in samples with green/yellow hues, and halloysite existing in samples with maroon hues. Along with clays, copper minerals such as Pseudomalachite were identified in some samples, suggesting copper mineralization occurred before major fault displacement.

Understanding the effect on hydrologic dynamics will ultimately assist the team of geologists at the Lisbon Valley Mine further understand and predict copper mineralization along fault zones in the area along with constraints on fault-seal capacity when considering in-situ recovery.