We re-discovered tight/shale oil accumulations through a successful acid treatment in the Onnagawa Formation, the main petroleum source rock in Japan, which is Monterey-like biosiliceous shale of the Middle Miocene age. Its productivity is fairly high, comparable with that of the Bakken shale, and is supported by matrix pores of quartzporcelanite and carbonate-cemented natural fractures which is stimulated effectively through acidizing. The tight oil horizon corresponds to the best source and

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... e best reservoir rocks. It is the most siliceous part of the formation, which is reasonably understood as a result of an increase in primary productivity with diatom blooms in the Miocene age. We think the tight oil is generated in situ and a part of continuous accumulation, while it is in a structural trap so far. The above exploration concept will be applicable to the Monterey Formation of California and the Dae-Khurie Formation of Sakhalin, which are siliceous rocks of same type, and re-evaluation of non-commercial hydrocarbon ows or shows in exploratory wells is recommended.