Experimental Investigation of Permeability and Mechanical Evolution of Cataclastic Shear Bands

Abstract:

Cataclastic shear bands in unconsolidated sands are typically stronger and lower permeability zones than the surrounding matrix material, and act as barriers to fluid flow in aquifers and reservoirs. Characterization of the mechanical and hydraulic properties of shear bands is especially important for basin and petroleum studies due to their role as barriers to fluid flow in reservoirs. We investigated the nature of permeability and the structural evolution of shear bands and proximal host material collected from the footwall of the McKinleyville thrust fault in Humboldt County, California, and from the footwall of the Moab Fault in Moab, Utah. We report on a suite of laboratory experiments that included measurement of permeability, porosity, and acoustic wave velocity measurements over a range of effective stresses from 5 – 65 MPa.

 The permeability-porosity trends are similar for all samples, with permeability decreasing systematically with increasing effective stress and decreasing porosity. The permeability of the unconsolidated host sand decreases from 8.2×10^-16 m² at 5 MPa effective stress to 3.1×10^-17 m² at 65 MPa, while the permeability of shear bands decreases from 5.9×10^-17 m² to 3.0×10^-18 m² over this stress range. Elastic wave speeds increase systematically with increasing effective stress. P-wave velocities of the host sand increase from 1125 m/s at 5 MPa to 3180 m/s at 65 MPa, and velocities of the cataclastic shear bands increase from 1616 m/s to 3058 m/s. Ongoing and future work will investigate time dependent changes in stiffness, strength, and permeability, and will expand this dataset to include shear bands from a range of geologic settings.