PETROLEUM SOURCE ROCKS IN THE TSUGARU BASIN, NORTH HONSHU, JAPAN

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This study investigates the petroleum source rock potential of the Neogene Tertiary muddy sediments of the Tsugaru Basin of western Aomori. More than 100 kerogens included in these sediments and 7 kerogens comprised in the Neogene Tertiary sediments of the Oga oil field for reference were examined by elemental analysis, visual kerogen method, nuclear magnetic resonance and vitrinite reflectance. The major part of the Neogene Tertiary sediments in the Thugaru Basin contain low-potential type III or II-III kerogens. But the Upper Miocene Odoji and the Lower Pliocene Akaishi Formation in the central part of the basin include high-potential type I or II kerogens. These formations have suffered burial diagenesis depper than 2,000 meters and their organic matter should be matured to exceed the threshold of intense oil generation (vitrinite reflectance value of 0.5%). The kerogen contents of sediments in the Odoji and the Akaishi Formation are mostly more than 1%. These organic geochemical characters of the formations compare with those of the Onnagawa and the Funakawa formation accepted as source rocks in the Oga oil field. Therefore it is concluded from total viewpoint of the organic content, type and maturity that the Upper Miocene to the Lower Pliocene strate, namely the Odoji and the Akaishi Formation, in the central part of the Tsugaru Basin have good potential for petroleum generation.