An experimental investigation of the development and permeability of clay smears along faults in uncemented sediments

Abstract

The generation of clay smears along faults in uncemented sediments has been studied through laboratory experiments in a newly developed high stress ring shear apparatus. The main objective is to investigate basic mechanisms involved in the deformation process of sediments during faulting and formation of clay smears. The experimental test program comprises ring shear tests on sand with embedded clay segments (sand–clay sequence) under constant effective normal stress. Visual inspection of the samples after testing, analyses of thin sections and permeability measurements across the shear zone are used to characterise geometrical continuity, thickness and sealing potential of the smear. Deformation processes such as grain reorientation, clay smear and cataclasis are identified from the tests. The complexity of the shear zone is observed to increase with the effective normal stress applied to the specimen and the number of clay segments used in the ring (multilayered sand–clay sequences). At low effective normal stress, in clay-rich sediments, clay smear is the most efficient mechanism for permeability reduction. The permeability across the smear decreases with ring rotation (or shear displacement) and effective normal stress. A maximum decrease of two orders of magnitude compared to the permeability of the surrounding sand is observed after 90° rotation under 10.5 MPa effective normal stress. Sand–sand juxtaposition shear is dominated by grain rolling causing only minor permeability reduction. At high effective normal stress, permeability measurements across clay smear and sand–sand juxtaposition yield similar values indicating that the permeability reduction is dominated by grain size reduction in the sand.