Structure and hydraulics of brittle faults in sandstone

A class of faults formed in porous sandstone by shearing across joint zones are different from previously described deformation band faults which also form in sandstone. These faults are associated with outcrop scale fracturing and fragmentation whereas deformation bands form in sandstone by grain scale deformation processes. The purpose of this study is to document the deformation mechanisms and evolution of faults formed from shearing across joint zones and determine their hydraulic properties. Lateral and normal faults with slip magnitudes from several millimeters to about 150 meters are examined using detailed field mapping in the Valley of Fire State Park, Nevada, USA.;The initial fracturing in these fault systems occurs as a result of shearing first along the preexisting joints and later across successively formed new joints. Strain localization in the span between adjacent sheared joints results in progressive fragmentation of the weakened sandstone and formation of a series of gouge pockets along the zone. With increasing slip a through-going gouge zone is formed within the damaged rock. The damage distribution around faults is evaluated through kilometer scale scanline measurements and compared to faults in the subsurface using borehole data. There is a systematic widening of the fault damage zones with increasing slip though the most extensively fractured zones occur near fault intersections or segment stepovers. Petrophysical image analyses of fault gouges indicate that permeability may be reduced by up to 2--4 orders of magnitude in these materials; fracture networks developed around fault zones may increase permeability. These combined effects are evaluated through permeability upscaling using power averaging. Results indicate that faults formed by shearing of joint zones introduce a significant permeability anisotropy. Fault parallel permeability may be increased up to two orders of magnitude relative to the host rock as a function of joint density and aperture. Fault normal permeability may be reduced by up to 3 orders of magnitude and is a function of the width and continuity of fault gouge zones. Fault zone permeability varies with slip magnitude: in general increasing slip is correlated to decreasing fault normal permeability and increasing fault parallel permeability. permeability and increasing fault parallel permeability.