Oil and natural gas deposits exist in the sedimentary basins having various sedimentary environments. Some Soviet scientists use the following lithogenetic stages; diagenesis, proto-catagenesis, meso-catagenesis, apo-catagenesis and metamorphism. These classified stages are convenient to the geochemical study of hydrocarbon deposits.
Table 1 shows the relation among the lithogenetic stages and other geologic characteristics, such as geostatic pressure, temperature, rank of coal, absolute age, clay mineral, bituminoids, iodine, hydrocarbons and chemical type of groundwaters.
After the geochemical examinations of hydrocarbon deposits in Japan and other regions in the world, the author noticed that at the later apo-catagenesis stage the characters of clay minerals, bituminoids and groundwaters were changed greatly. The approximate absolute ages of the stage are n x 107 years in Japan and n x 108 years in the platform areas in the world.
These phenomena are important for the geochemical exploration of natural gas and oil deposits as well as for the geochemical research works.
