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Quantifying Fault Damage Zone Permeability in Crystalline Rocks

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Determining fault zone permeability is essential for understanding a number of geological and geophysical processes. These include earthquake rupture, crustal strength, sub-surface fluid flow in hydrocarbon reservoirs and around waste repositories, and fault-hosted base metal ore deposition. In nature, permeability is enhanced in the damage zone of faults, where fracturing occurs on a wide range of scales. Here we analyze the contribution of microfracture damage on the permeability of faults that cut through low porosity, crystalline rocks by combining field and laboratory measurements. Microfracture densities surrounding strike-slip faults with well-constrained displacements ranging over 3 orders of magnitude (~0.12 m – 5000 m) have been analyzed.

The faults studied are excellently exposed within the Atacama Fault Zone, where exhumation from 6-10 km has occurred. Microfractures in the form of fluid inclusion planes (FIPs) show a log-linear decrease in fracture density with perpendicular distance from the fault core. Damage zone widths defined by the density of FIPs scale with fault displacement, and an empirical relationship for microfracture density distribution throughout the damage zone with displacement is derived. Damage zone rocks will have experienced differential stresses that were less than, but some proportion of, the failure stress.

As such, permeability data from progressively loaded, initially intact laboratory samples, in the pre-failure region provide useful insights into fluid flow properties of various parts of the damage zone. The permeability evolution of initially intact crystalline rocks under increasing differential load leading to macroscopic failure was determined at water pore pressures of 50 MPa and effective pressure of 10 MPa.

Permeability is seen to increase by up to, and over, two orders of magnitude prior to macroscopic failure. Further experiments were stopped at various points in the loading history in order to correlate microfracture density within the samples with permeability. By combining empirical relationships determined from both quantitative fieldwork and experiments we present a model that allows microfracture permeability distribution throughout the damage zone to be determined as function of increasing fault displacement.

Some relevant references:

Mitchell, T.M. and Faulkner, D.R. Experimental measurements of permeability evolution during triaxial compression of initially intact crystalline rocks and implications for fluid flow in fault zones. In press: Journal of Geophysical Research, July 2008

Faulkner, D.R., Mitchell, T.M. and Rutter, E.H and Cembrano, J. 2008. On the structure and mechanical properties of large strike-slip fault zones. In: Wibberley C. A. J., Kurtz W., Imber J., Holdsworth R. E. & Collettini C. (eds) The Internal Structure of Fault Zones: Implications for Mechanical and Fluid-Flow Properties. Geological Society of London Special Publication 299, 139-150, doi: 10.1144/SP299.9.

Faulkner, D.R., Mitchell, T.M., Healy, D. and Heap, M.J. 2006. Slip on ‘weak’ faults by the rotation of regional stress in the fracture damage zone. Nature, 444, 922-925. doi:10.1038/nature05353.

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