Shale Oil Potential in Neogene Siliceous Shales of Japan

Yokoi, Satoru¹; Waseda, Amane²; Tsuji, Takashi²
¹Domestic Project Division, Japan Petroleum Exploration Co., Tokyo, Japan.
²Research Center, Japan Petroleum Exploration Co., Tokyo, Japan.

We have proposed a hydrocarbon trap model in Monterey-like biosiliceous shale formation called Onnagawa(Tsuji et al 2011, AAPG ). The trap is associated with diagenetic transformation of silica mineral from opal-CT to quartz, and across the boundary, the overlying opal-CT porcelanite layer forms a seal and underlying clay-poor quartzose porcelanite forms a reservoir. Meantime, the Onnagawa Shale is the main source rock in Japan, particularly Akita basin, with fairly high potential. It is 500m thick in average and has TOC of 2% in average,up to 5% in part, and H.I. of 500 typically. The Onnagawa is under review in the light of Shale oil/gas, as in the Monterey shale. As the first step, we have reviewed the Yuihara-Ayukawa field, where the largest commercial oil & gas production from the Onnagawa Shale is ongoing. In the field, various reservoirs, including Dolerite, Tuff and quartzose porcelanite probably fractured, seem to have a common gas-oil contact. In other words, they constitute one pressure system. And the maturity of the hydrocarbon in this system is much higher than that of source rocks close to the reservoirs, which indicates normal migration process. We also have sub-commercial production or intense oil show from relatively tight reservoirs that is typically alternation of porcelanite and tuff, 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 sub-commercial or non-productive so far, relatively immature and indigenous. Now we think the latter is a candidate for shale oil or tight oil exploration with horizontal drilling and massive stimulation treatments and the first pilot test is being planned. In addition, we have other similar examples in this basin, although not enough data are available to confirm. We also expect much more potential with an order of 100 million bbl in deeper and matured area, following the Bakken-type continuous shale oil accumulation model, where our field is located in the transition zone just above oil generation window.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012