How rock mechanical properties affect fault permeability in Neogene mudstone?

Abstract

We investigate the relationship of permeability of fault developing in Neogene mudstone layer with mechanical properties of the host rock and stress condition (or depth) at the faulting, focusing on the Wakkanai Formation (middle to late Miocene, siliceous mudstone) and the Koetoi Formation (late Miocene to Pliocene, diatomaceous mudstone) at the Horonobe area, northern Hokkaido. Based on the previous studies, including laboratory experiments to measure permeability evolution along a shear zone induced in mudstone specimen. We constructed the model of fault permeability evolution for these Neogene siliceous formations; in the case of Koetoi Formation, where opal A to opal CT transition in diagenesis is not clearly observed, failure envelope is influenced by maximum burial depth, and when fault is created at the depth close to the maximum burial depth, fault becomes compact and the permeability is not high comparing to the host rock. On the other hand, in the case of Wakkanai Fm, pore collapse condition of failure envelope is much larger than the stress condition expected from maximum burial depth because hardening in diagenesis is strong, mainly by opal A to opal CT transition in diagenesis, and therefore faults induced at the depth shallower or similar to the maximum burial depth is dilative and high permeable. This model can be applied to general Neogene siliceous mudstone for assessment of seal properties of mudstone formations.