POROSITY-DEPTH RELATIONSHIPS IN THE NEOGENE ARGILLACEOUS AND ARENACEOUS SEDIMENTS OF JAPAN

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; 1600 m 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 ismuch 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. Introduction Various phases of diagenesis in argillaceous and arenaceous sediments and rocks have been reviewed bymany authors (LARSEN and CHIL INGAR, 1967, 1979; RIEKE and CHILINGARIAN, 1974; and CHILINGARIAN and WOLF, 1975, 1976). These publications indicate that pressure, temperature and reaction time are the physical factors, and pH, Eh and chemical composition of pore-fluid are the chemical factors controlling diagenesis. All of these factors should be considered inmonitoring the process of diagenesis. Practically, however, it is very difficult to identify the stage of diagenesis by using all of the above factors. Thusmany researchers used various parameters to define the process of diagenesis. One of these parameters in argillaceous sediments is the change of absolute porosity with burial depth, since the porosity in sediments generally decrease only due to the increase of overburden pressure and its value is irreversible, especially in the case of argillaceous sediments. In other words, it is possible to say that the porosity is a physical parameter to define the process of diagenesis in argillaceous sedimentsmainly controlled by compaction. From the discussion on the change of porosity with depth, HEDBERG (1936) firstly classified the compaction process in argillaceous sediments intomechanical rearrangement, dehydration,mechanical deformation and recrystallization stages. Later, WELLER (1959) and BURST (1969) reported their original compactionmodels. Recently, from the results of the experimental compaction study of Namontmorillonite clay (AOYAGI et al., 1976), AOYAGI and ASAKAWA (1977) recognized the following process of diagenesis in argillaceous sediments: early compaction (80-30 per cent porosity), late compaction (30-10 per cent), and recrystallization or transformation (less than 10 per cent) stages. From the study of relationship between compressibility and porosity of argillaceous rocks of Japan (INAMI and HoSHINO, (1974), HOSHINO and INAMI (1977) reached almost the same conclusion on compactionmodel. * Technology Research Center , Japan National Oil Corp. 2-2-2 Uchisaiwaicho, Chiyoda-ku, Tokyo 100, Japan.