Effects of pressure on pore characteristics and permeability of porous rocks as estimated from seismic wave velocities in cores from TCDP Hole-A

Changes in V_p/V_s (Poisson's ratio) around a fault are related to changes in the fluid transport properties of rocks, which play a significant role in seismogenic processes. Here we report a notable relationship between V_p/V_s and the permeability of porous fault-related rocks (Chelungpu fault, Taiwan) by direct and simultaneous measurement of elastic wave velocities (V_p and V_s) and permeability under increasing effective confining pressure (P_eff) up to 25 MPa. V_p and V_s for all samples increased with P_eff in the range up to 20 MPa, then were nearly constant as P_eff increased to 25 MPa. Most silty sandstones with large proportions of fine-grained material showed positive correlations between V_p/V_s and permeability with rising pressure. On the other hand, well-sorted sandstones showed only slight changes in permeability with respect to V_p/V_s with rising pressure. We infer that grain size distributions, in particular the amount of silt- and clay-size grains, are responsible for the change in permeability with pressure as small particles clog pore networks with increasing P_eff, causing the decrease in permeability. These findings may be useful to explain changes in permeability and pore pressure in the deep crust.