Experiments on clay smear formation along faults

Abstract

A ring shear apparatus was used to investigate the development of clay smear along faults in sand–clay sequences. Experiments were performed, using six different clay types, different stress conditions (σ n =6–500 kPa) and different amounts of clay (4 and 12.5%). The development of clay smear seems to depend on the competence contrast between the clay and the surrounding sand. Clay, when it is less competent than sand, behaves in a ductile manner, resulting in the development of clay smears along the fault. Clay which is more competent than sand behaves in a brittle manner, resulting in the formation of angular fragments. Whether the clay is more or less competent than the sand is dependent on the stress conditions, the initial porosity of the sand and the mechanical properties of the clay. The results suggest that stress conditions allowing the sample to contract will result in the formation of fluid flow barriers, whereas dilation results in the formation of conduits.

Permeability change during experimental fault smearing

“…To reproduce faults in artificial sand/clay successions, ring samples were assembled that consisted of a block of clay sandwiched by a ringed block of sand [Weber et al, 1978;Sperrevik et al, 2000;Clausen and Gabrielsen, 2002]. These experiments were successful in forming clay smears that thinned with increasing throw.…”

“…As Sperrevik et al [2000] and Clausen and Gabrielsen [2002] pointed out, the competency contrast between the sandstone and the low-permeability layer is crucial to smear production. In this study, even though the Berea sandstone was much lithified compared to the siltstone, changing the effective normal pressure s neff would change the contact area (asperity) of the sandstone smear on the precut surface.…”

Shale smear along large faults: continuity of smear and the fault seal capacity

“…The continuity of smears may be influenced by factors such as fault throw, thickness of source layer, fault geometry (extensional v. contractional dip relay), rock properties at the time of deformation, and contrast in mechanical strength between shale and non-shale lithologies. Experiments carried out using a ring shear apparatus have shown that the competence contrast between the clay and the surrounding lithology (sand in the experiments) is one major control on the continuity of the smear (Sperrevik et al 2000). In experiments where the sand had low shear strength, the clay behaved in a brittle manner and produced no smear.…”

Simulating the effect of subseismic fault tails and process zones in a siliciclastic reservoir analogue: Implications for aquifer support and trap definition

“…Aydin and Johnson,1978;Fossen and Hesthammer, 1998), clay/shale smearing (e.g. Lindsay et al, 1993;Yielding et al, 1997;Sperrevik et al, 2000;Faerseth, 2006), cementation (e.g. Knipe, 1993;Sverdrup and Bjørlykke, 1997;Hesthammer et al, 2002;Fisher et al, 2003), and pressure solution (e.g.…”