Abstract Deformation-band faults in high-porosity reservoir sandstones commonly contain a fault core of intensely crushed rock surrounding the main slip surfaces. The fault core has a substantially reduced porosity and permeability with respect to both the host rock and individual deformation bands. Although fault core thickness is a large uncertainty in calculations of transmissibility multipliers used to represent faults in single-phase reservoir flow models, few data exist on fault core thickness in deformation-band fault zones. To provide accurate estimates of deformation-band fault petrophysical properties, we measured fault core thickness at six sites (each 415 m [13–49 ft] along strike) along the Big Hole fault in the Navajo Sandstone, central Utah. These data show that the thickness is highly variable and does not correlate with either the amount of slip or the number of slip surfaces. The thickness of the fault core is likely to be dependent on local growth processes, specifically the linkage of fault segments. This suggests that correlations of fault permeability with throw may not apply to deformation-band faults. Simple calculations of two-phase flow properties based on measured porosity and permeability values suggest that deformation-band faults containing fault core are likely barriers to two-phase flow. More data on the variability of fault core thickness and its petrophysical properties need to be collected to characterize population statistics for models of deformation-band fault fluid-flow properties.

Abstract The numerous CO 2 reservoirs in the Colorado Plateau region of the United States are natural analogues for potential geological CO 2 sequestration repositories. To understand better the risk of leakage from reservoirs used for long-term underground CO 2 storage, we examine evidence for CO 2 migration along two normal faults that cut a reservoir in east-central Utah. CO 2 -charged springs, geysers, and a hydrocarbon seep are localized along these faults. These include natural springs that have been active for long periods of time, and springs that were induced by recent drilling. The CO 2 -charged spring waters have deposited travertine mounds and carbonate veins. The faults cut siltstones, shales, and sandstones and the fault rocks are fine-grained, clay-rich gouge, generally thought to be barriers to fluid flow. The geological and geochemical data are consistent with these faults being conduits for CO 2 moving to the surface. Consequently, the injection of CO 2 into faulted geological reservoirs, including faults with clay gouge, must be carefully designed and monitored to avoid slow seepage or fast rupture to the biosphere.