Permeability change during experimental fault smearing

Abstract

Smear sourced from shale or clay is thought to provide an across‐fault barrier to fluid flow in sedimentary rocks. Permeability changes of this phenomenon were investigated using a triaxial testing machine. Experimental specimens consisting of interlayered siltstone (low initial permeability, ∼10−16 m2) and sandstone (high initial permeability, ∼10−13 m2) were subjected to 20, 30, and 40 MPa of effective normal stresses on a precut surface, dividing each specimen at a 30° angle to its axis, at axial shortening velocities between 0.14 and 1.41 μm s−1. Permeability was measured by the oscillation method, and permeability changes categorized in three distinct regimes that corresponded to progressively increasing rock deformation: regime 1, rapid reduction due to compaction of the siltstone layer prior to fault movement; regime 2, constant and minimum permeability while the smear developed; and regime 3, permeability recovery caused by smear thinning and loss of smear continuity. The duration of regime 2 and of smear continuity recorded provide measures of the sealing potential produced by the smear. The shale smear factor SSF, defined as (fault throw)/(thickness of low‐permeability layer), shows that there may be a relation between seal potential and effective normal stress. Both factors in SSF can reach higher values when effective normal stress is 40 MPa than when that is ≥30 MPa.