Tight oil exploration in the Onnagawa formation, Miocene biosiliceous rocks of Japan: what we have learned and its implication for Monterey-like siliceous rocks in the Circum-North Pacific

Abstract

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 quartz-porcelanite and carbonate-cemented natural fractures which is stimulated effectively through acidizing. The tight oil horizon corresponds to the best source and the 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 flows or shows in exploratory wells is recommended.