CROSS-FAULT FLOW IN SEDIMENTARY ENVIRONMENTS
Available methodologies used to evaluate cross-fault flow properties (e.g., SGR or CSP) are predicated on the definition of a fault as a single surface as defined by seismic reflection data and do a poor job of accounting for the variability in fault zone structure observed in nature. Over the past decade we have undertaken a series of novel sandbox experiments to define primary controls on fault zone structure that determine cross-fault flow in simple sand-clay systems. We have observed how ‘soft’ clays become volumetrically concentrated in fault zones, how ‘hard’ clays bend, break, and sometimes shear-soften, and how multiple clay beds become amalgamated and form significant cross-fault barriers. We have also discovered that holes develop in the gouge that overcome the permeability-reducing effects of the clay smear and that these holes may be associated with the intricate relay structure of the fault zone. Moreover, the type of fault relay complexity may be associated with the boundary condition of fault propagation dip with respect to the material properties of the faulted materials (i.e. friction angle).
Taking these experiments in the context of other field and subsurface examples, we have learned that fault processes that deform the offset and entrained sedimentary lithologies play an important role in determining the hydrologic properties of faults as they determine the distribution and continuity of those lithologies, and counteracting those processes are those that thin and create holes in fault gouge lithologies.