Formation of Onnagawa siliceous source rocks and tight oil

The incorporation of sulfur into kerogen in the early stage of diagenesis and the migration of silica during silica mineral transformation play important roles in the formation of Onnagawa siliceous source rocks. The S/C atomic ratio of Onnagawa kerogen is variable, affecting the timing of oil generation. Organic matter in the siliceous source rock is concentrated or diluted by the migration of dissolved silica accompanying silica mineral diagenesis, resulting in the formation of organic-rich and organic-poor siliceous rocks. Oil generation of Onnagawa siliceous source rocks rich in organic sulfur starts at low maturity level of vitrinite reflectance（VR）＝ 0.4 ％, that nearly corresponds to the transformation stage from Opal-CT to authigenic quartz. The volume reduction due to quartz crystallization leads to the effective porosity increase and fracture formation, resulting in the formation of siliceous reservoir rocks. Oil generation in the siliceous source rocks proceeds significantly at VR ＝ 0.4 to 0.6 ％, the similar timing of siliceous reservoir rock formation, leading to the formation of tight oil deposits consisting of low-mature oils（VR<0.6 ％）. Compaction and thermal maturation of siliceous rocks proceed further with increasing burial depth, inducing the oil expulsion. The oil expelled from the siliceous rocks at VR＝ 0.6 to 0.7％ has been accumulated to form conventional oil deposits in the Akita basin. Compared to the siliceous rocks and crude oils of Monterey Formation, those of Onnagawa Formation are characterized by lower organic sulfur concentration and smaller δ34S value. Monterey siliceous source rocks significantly richer in organic sulfur were probably deposited in the super-reducing（euxinic）conditions. The diverse organic sulfur concentrations and oil generation kinetic parameters of Onnagawa siliceous source rocks are due to the diverse primary productivity and reducibility in the paleo-ocean of Onnagawa Formation.