Diagenetic transformation of clay minerals, zeolites and silica minerals in Cretaceous and Tertiary argillaceous rocks from deeply drilled wells in Japan were studied. Transformations of these minerals during diagenesis were as follows: in clay minerals, montmorillonite → montmorillonite‐illite mixed‐layer mineral → illite; in zeolites, volcanic glass → clinoptilolite → heulandite and/or analcite → laumontite and/or albite; in silica minerals, amorphous silica → low‐cristobalite → low‐quartz. Maximum overburden pressures and geothermal temperatures corresponding to these transformations in each well studied were calculated. For clay minerals, a pressure of approximately 900 kg cm−2 and a temperature of about 100°C are necessary for the transformation from montmorillonite to mixed‐layer mineral and 920 kg cm−2 and 140°C for mixed‐layer mineral to illite. Transformation from kaolinite to other minerals requires much higher pressures and temperatures than from montmorillonite to mixed‐layer mineral. For zeolites, 330 kg cm−2 and 60°C are required for the transformation from volcanic glass to clinoptilolite, 860 kg cm−2 and 120°C for clinoptilolite to heulandite and/or analcite, and 930 kg cm−2 and 140°C for heulandite and/or analcite to laumontite and/or albite. For silica minerals, 250 kg cm−2 and 50°C are necessary for the transformation from amorphous silica to low‐cristobalite and 660 kg cm−2 and 70°C for low‐cristobalite to low‐quartz. Based on these diagenetic mineral transformations, seven mineral zones are recognized in argillaceous sediments. On the other hand, from the porosity studies of argillaceous sediments in Japan, the process of diagenesis is classified into the following three stages. The early compaction stage is marked by shallow burial and viscous rocks with more than 30% porosity. The late compaction stage is characterized by intermediate burial and plastic rocks with 30‐10% porosities. The transformation stage is marked by deep burial and elastic rocks with less than 10% porosity.

No abstract

No abstract

No abstract

Various siliceous rocks are found in the Ohdoji, Akaishi and Maido Formations from the western Aomori basin, and the Yotsuzawa and Wadagawa Formations from the eastern Aomori basin of northern Honshu, Japan. These rocks are classified into diatomite, siliceous shale and chert.Diatomite is composed of abundant amorphous silica and has porosity between 50 and 65%. Siliceous shale is composed of a large amount of quartz, and has porosity ranging from 25 to 35%. Chert is chiefly composed of cristobalite or quartz, and has porosity between 20 to 30%.Average contents of total organic carbon, S,, and S, generally increase from argillaceous rocksdiatomitessiliceous shalescherts. Maturation of organic matter in these rocks is generally lower than that in average source rocks. Diatoms, which appeared in the late Cretaceous and became increasingly important in the Miocene, are the principal primary producers of organic matter in the marine environment during the Cenozoic. Excellent organic components and higher biological productivity show that diatoms might be the most important source of petroleum during the Neogene in Japan. Proteins, carbohydrates and lipids in diatoms have been transformed into fulvic acids, humic acids and humin by polycondensation and polymerization. Later, these humin materials could be changed into insoluble kerogen under the effect of mild temperature and pressure. A part of the lipids would transform to geochemical fossils (biomarkers). Amorphous silica in cells of diatoms would change to low-cristobalite and low-quartz by the increase of geothermal temperature.