Pacific Section AAPG Convention

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Tight Oil Potential In Neogene Monterey-Like Biosiliceous Shale Of Japan


We have proposed a diagenetic hydrocarbon trap model in Monterey-like biosiliceous shale formation called Onnagawa (Tsuji et al, 2011), which is the main source rock of Japan and has similar potential as that of Upper Monterey. We however have much and continuous oil shows in the Onnagawa shale far below the trap, which is indicative of a shale oil system. We have reviewed the Yuihara-Ayukawa field in Akita of Japan mainly from geochemical aspect, where commercial oil & gas production from the Onnagawa Shale with a conventional trap is ongoing. In the field, various producing reservoirs in the Onnagawa shale seem to have a common gas-oil system. And the maturity of the hydrocarbon is much higher than that of source rocks close to these reservoirs, which indicates normal migration process from the kicthen. We also have non-commercial oil production or intense oil shows from tight reservoirs, typically porcelanites, and the maturity of oil, however, is lower than that of commercial oils and close to that of early-matured source rocks around. In summary, we have two different hydrocarbon accumulations in one filed. One is productive, mature and probably migrated, and another is non-commercial so far, less mature and indigenous. Now we think the latter is a part of a shale oil or tight oil system. Although not enough data are available so far, we expect basin-wide shale oil potential with the order of 100 million bbl without risk factor, where our fields are located in the upper transition zone of oil generation window. To confirm the existence of shale oil system, we started The Onnagawa Tight Oil Project with support of JOGMEC in 2012. Firstly, we tried acid treatment on naturally but weakly fractured shale reservoir of an existing deviated-well of the Yuihara-Ayukawa field to testify the productivity of Onnagawa tight oil. We had a success with over 50 times improvement in terms of Productivity Index. Starting from with rate of over 300bbl/D, the oil production is ongoing with over 100bbl/D after 10-month production, and its cumulative production is getting over 60,000bbl. We also have started the first multi-stage fracturing project in Japan to optimize fracturing operation in Onnagawa shale. The operation was completed for a newly drilled horizontal well of another oil field last year, and we are waiting for flow-back.. As the next step to the current fracturing project, a pilot test in Yurihara-Ayukawa field with higher potential is being planned.