Depth dependent spatial variation of pore size and permeability for sediments at the Nankai accretionary prism

Abstract:

Permeability structure at accretionary prism controls spatial variation of pore pressure, thus, plays an important role on earthquake processes. Sediment porosity at accretionary prism decreases with burial depth due to mechanical compaction and tectonic compression. Elastic wave velocity and thermal conductivity of porous media that are primary controlled by porosity thus increase with depth. On the other hand, influence of depth and compaction on pore size distribution is not well known. Permeability of porous sediment is basically related to porosity and pore size; therefore, depth variation of permeability is probably estimated from pore size data. In this study, to approximate a permeability-depth profile at accretionary prism, porosity and pore size distribution were measured on cuttings and core samples obtained from NantroSEIZE project, IODP Expedition 348 (Site C0002).

Initially, samples were washed with sea water, then dried using a vacuum pump. Grain volume and grain density were measured by using a gas pycnometer. After gas pycnometer measurement, pore size distribution was measured by a mercury intrusion porosimeter using same samples. Pore volume was estimated from the difference between wet and dry weights and also by a mercury intrusion porosimeter. Therefore, porosity was calculated by two methods.

Our results show porosities measured by a mercury porosimetry decreases gradually from 40 to 20 % at a depth from 1000 to 3000 mbsf. The deviation of porosity from a normal consolidation curve is observed at depth of 2200 mbsf and 3000 mbsf. On the other hand, porosity calculated by wet and dry weights scattered significantly, and the porosities are greatly higher than those measured by mercury intrusion porosimetry. Mean and modal pore sizes decreased with depth, but the deviation of pore size from the linear reduction curve is observed at depth of 2200 mbsf and 3000 mbsf. Permeability-depth profile predicted by porosity and pore size data shows permeability slightly decreases with depth but relative permeable at depth of 2200 mbsf. Lower permeable layer is also formed at 3000 mbsf. The permeable layer at 2200 mbsf is explained by the sand rich composition. Our results suggest that permeability depth profile is influenced by mechanical compaction and lithological variation.