Permeability, Salinity, and Nanoscale Processes in Low Porosity Clay-Rich Sediments

This study reports on constant rate of strain and incremental loading testing of illite shale and Nankai accretionary wedge sediments from the NanTroSEIZE deep sea drilling program (IODP Leg 348). The Nankai megathrust fault is a recurring source of great earthquakes and tsunami with seismogenesis occurring below 3 - 5 km. Typical one-dimensional soil consolidation tests are limited by effective stresses well below those present at seismogenic depths. A high capacity consolidometer was utilized here to study fluid flow in clay-rich sediments approaching ~200 MPa stress states equivalent to ~8 - 12 km depth. The resulting consolidation state leads to nanofilm dominated porosity where the Electric Double Layer (EDL) significantly influences permeability and fluid chemistry evolution. Our results indicate ultrafiltration (i.e. osmotic filtration of ions and dissolved isotopes) should initiate and intensify below ~1.5 km depth at Nankai. Ultrafiltration removed up to ~40% Na+ and ~70% K+ from fluids expelled during laboratory consolidation, increasing salinity in residual pore fluids. Ultra-low permeabilities were reached with the lowest value reported as K = 2.0x10-14 m/s. A distinct change in slope of permeability trends was also observed at low porosities. The onset of ultrafiltration and deflection in permeability trends both initiate very near where opposing EDLs begin to interact during consolidation, and bulk pore water is nearly absent below void ratios of ~0.40. I propose that condensing electrically influenced monofilms dominate porosity and flow properties at depths >~1.5 km at Nankai and similar conditions should exist globally in thick clay-rich marine sedimentary sequences.