Abstract
Porosity-depth relationships in the Neogene argillaceous and arenaceous sediments at the deeply-drilled representative wells of Japan were studied. The relationships both in the Neogene argillaceous sediments of Hokkaido and Niigata areas are closely resembled. In average argillaceous sediments of Japan, about 1200m in burial depth corresponds to 50 per cent in absolute porosity; 1600m to 40 per cent, 2100m to 30 per cent, 2700m to 20 per cent, and 3700m to 10 per cent. On the contrary, porosity-depth relationships in the Neogene arenaceous sediments in both areas are considerably differed. Porosities in arenaceous sediments of Niigata are always higher than those recognized at the same burial depth of Hokkaido. ‘Porosity Window’, which was newly termed in the paper, in the Neogene sediments of Niigata is much bigger than those of Hokkaido. 10 per cent porosity in argillaceous sediments and 20 per cent porosity in arenaceous sediments are very important to define the process of diagenesis in both sediments. Argillaceous rock type based on porosity range and mineral facies in the rock indicate the paleo-temperature and burial depth and can utilize to petroleum exploration.
