LABORATORY MEASUREMENTS OF PERMEABILITY REDUCTION IN NATURALLY OCCURRING SHEAR BANDS FORMED IN UNLITHIFIED SANDS

Strain localization in porous sands leads to the formation of shear bands, which typically accommodate a few mm to tens of cm of slip, through grain fracturing, grain rotation and grain boundary sliding. Reduced bulk permeability due to grain size reduction, porosity loss, and cementation in shear bands can lead to decreased reservoir quality and compartmentalization. Previous studies have mainly focused on characterization of band geometry, grain size, structure, and origin. Permeability measurements for shear bands are rare, and have generally been limited to shear bands created in a laboratory or to field measurements under atmospheric pressure conditions. The purpose of this study is to quantify permeability reduction in shear bands, and investigate links between permeability and other shear band properties. We present a comprehensive suite of data obtained for naturally occurring shear bands from an outcrop of unlithified sand in the footwall of the McKinleyville thrust fault in northern California. These data include laboratory permeability, porosity, and grain size measurements, as well as SEM analyses. We measured shear band and host sand permeability under isostatic stress conditions, for confining pressures ranging from 0.1 - 5 MPa. At each confining pressure, we applied a constant flow rate across the sample, measured the resulting pressure gradient using a differential pressure transducer, and determined permeability by Darcy’s Law.

We find that the mean grain size for host sand is 208 µm, whereas the mean grain size for shear bands ranges from 31 – 185 µm, with the mean grain size decreasing with increased shear band thickness. Host sand porosity ranges from 42 - 45%, and shear band porosity ranges from 32 - 39%. SEM images indicate that shear bands are weakly cemented; there is no evidence of cementation in the host sand. Host sand permeability decreases from 1x10^-14 m² (10 mD) to 2x10^-15 m² (2 mD) as confining pressure is increased from 0.1 to 5 MPa. Results for six shear bands illustrate a decrease from 1x10^-14 m² to 1x10^-17 m² over the same stress range. Preliminary results suggest a clear and systematic relationship between larger shear band thickness and decreased grain size, decreased porosity, and decreased permeability.