Major oil and gas fields in Japan are located in the area from the central Hokkaido through northeast Honshu. Most productive horizons are generally found in formations of the late Middle Miocene (approximately 12-10 Ma) Onnagawa provincial stage of Japan. These formations are composed mainly of hard mudstone, siliceous shale, diatomaceous mudstone, marlite, and acidic pyroclastic rocks. Source rock potentials in argillaceous rocks of the Miocene Ohdoji Formation in Aomori, the Onnagawa Formation in Akita, and the Lower Teradomari Formation in Niigata show the highest values as compared with other formations in these areas. Average contents of organic carbon and hydrocarbons of siliceous sediments such as diato-mite, siliceous shale, and chert in the Aomori basin indicate the higher values as compared with other argillaceous sediments. Diatoms, which appeared in the late Cretaceous, are the principal primary producers of organic matter in the marine environment during the Cenozoic. Organic components and biological productivity show that diatoms have been the most important source of petroleum during the Neogene in Japan.