The Geometry and Thickness of Deformation-band Fault Core and its Influence on Sealing Characteristics of Deformation-band Fault Zones

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 4–15 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.