Effects of clay content on the frictional strength and fluid transport property of faults

We examined two effects of the presence of clay in brittle deformations: the reduction in frictional strength and the impediment of across-fault fluid flow. Permeability was monitored during the sliding deformation of a gouge of various mixes of Na-montmorillonite powder and granular quartz along a 30 degrees precut surface of Berea sandstone under 80 MPa of normal stress, 5 MPa of pore water pressure, and room temperature. The decrease in the friction coefficient of a gouge with increasing clay content was not simple, but showed a sharp drop at 50 vol.% clay content. The reduction in permeability due to deformation increased with increasing clay content from 0 to 24 vol.%, and a dramatic reduction of similar to 2.5 orders of magnitude occurred at 18 and 24 vol.% clay content. However, in a gouge with more than 29 vol.% clay content, deformation reduced the permeability by only 0.5 orders of magnitude. Thus the transitional clay contents at which the clay dominated the properties of fault strength and fluid transport were 50 and 29 vol.%, respectively. These values almost agree well with values obtained using a model with an equal-sized clast ( quartz) framework. The clay matrix can completely fill the pores sustained by a closest-packed quartz framework when the clay content reaches 29 vol.%, whereas the content required to fill the pores of a framework that can barely sustain quartz-quartz contact is 50 vol.%.