Abstract

The Big Hole fault, a dip-slip normal fault cutting the Jurassic Navajo Sandstone in the northern San Rafael Swell of east-central Utah, is characterized by a network of anastomosing deformation bands. Hydraulic tests were conducted along the Big Hole fault to determine how deformation-band faulting affects fluid flow, both perpendicular and parallel to the fault. Six tests were conducted: three at each of two locations along the fault, one near the tip, and the other in a wash nearby where there is greater displacement. The footwall, hanging wall, and fault core were isolated using pneumatic packers in one injection well at each location and one (at the wash site) or two (at the tip site) observation wells. One test was conducted in each zone at each location by injecting water into one of the zones and monitoring pressure changes in all three zones of the observation wells.

The hydraulic test results demonstrate that the Big Hole fault can and does act as a barrier to flow perpendicular to the fault plane, but not for all test configurations. No definite conclusion can be drawn from these tests on whether there is enhanced flow parallel to the fault plane. We saw no correlation between fault displacement and the hydraulic properties of the fault. The lack of consistent results demonstrates that the hydraulic properties of the fault are highly variable, indicating that it is both heterogeneous and extremely anisotropic, possibly due to changes in fault-core thickness and slip-surface density over small distances. Since deformation-band faulting is the dominant faulting style in many other high-porosity, reservoir-quality sandstones in the Colorado Plateau region of central and southeastern Utah, these hydraulic characteristics would also likely apply to similar sandstone units where they are cut by other deformation-band faults.