STRAIN LOCALIZATION & FLUID FLOW ACROSS FAULTS

Localization is a mechanical phenomenon that allows large strains to accumulate in limited rock volumes. Variability in localization processes in heterolithic clastic sedimentary rocks results in a wide range of internal fault geometries that have a profound impact on subsurface groundwater and petroleum flow. First, we assert that deformation processes that incorporate shale into faults are more efficient than deformation processes in sandstone alone. The degree of this efficiency varies as a function of stress state and the relative and absolute physical properties of each faulted lithology at the time of deformation. Localization is best developed when contrasting strong, brittle sandstones and weak, ductile shales are faulted, resulting in a shale-dominated fault gouge between discrete bounding faults. This type of localization has a large impact on subsurface groundwater and petroleum flow because of the ca 106-108 difference in permeability between sandstone and shale in the upper crust. A fault zone comprising a number of primary and secondary slip surfaces that displace a combination of strong and weak layers evolves toward a network of connected shale gouge bodies with increasing slip, approaching the permeability of the shale endmember. Simultaneously, the shale progresses to dominate the mechanical behavior of the fault. The purpose of this presentation is thus to illustrate: • how fault nucleation and evolution of a heterogeneous, porous, layered Earth is intimately related to the resulting hydraulic properties of faults • how the nature of localization can have minimal hydraulic impact when mechanical contrasts are small and localization favors no particular lithology but tremendous hydraulic impact when localization occurs in a weak clay gouge.