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

Open Access
Author:
Perez, Enrique
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 21, 2010
Committee Members:
  • Demian Saffer, Thesis Advisor
Keywords:
  • Cataclasis
  • Permeability
  • Shear bands
  • Deformation bands
Abstract:
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 physical properties. I 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 and XRD analyses. I measured shear band and host sand permeability under isostatic stress conditions, for confining pressures ranging from 0.05 - 5 MPa. At each confining pressure, I 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. I 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 as a function of increasing band thickness. Host sand porosity ranges from 42 - 45%, and shear band porosity ranges from 32 - 39%. SEM images indicate that shear bands have higher abundance of phyllosilicates but the there is no evidence of phyllosilicates clogging pore space or coating grains in the host sand. Host sand permeability decreases from ~1x10-14 m2 (10 mD) to ~2x10-15 m2 (2 mD) as effective stress is increased from 0.4 to 5 MPa. Results for five shear bands illustrate a decrease from ~8x10-15 m2 to ~1x10-17 m2 over a similar stress range (0.05 to 5 MPa). Results suggest a clear, systematic relationship between larger shear band thickness and decreased grain size, decreased porosity, increased phyllosilicate content, and decreased permeability.