The exploration works for the Nishkruosawa formation, the lowest oil bearing horizon in the Akita area, are still much inferior to the above-situated formations. It is expected, however, that the Nishikurosawa formation could be the most important target for exploration in the future.
Known surface and subsurface geological evidences show the following lithological facts. The mudstone distributes dominantly in Yuri oil producing area and Uyashinai area. The tuff, mainly composed of basaltic pyroclastics abundantly distributes in the area of western side of Lake Hachieogata and the Northern Akita area.
On the other hand, the facies of fossil Foraminifera included in these strata show some differences between areas. The deep water benthonic species and warm water planktonic ones are dominant in Yuri and Uyashinai area. On the contrary, shallow water benthonic species are dominant and planktonics are rare in the western side of Lake Hachirogata area.
That is to say, Yuri oil producing area seems to be a subsiding zone forming a kind of trough, and showing a remarkable contrast compared with the Northern Akita province which seems to be a kind of uplifting zone.
A well named Miyama AK-1 was drilled at the Northern part of Yuri oil producing area in 1971. This well penetrated the Nishikurosawa formation to the extent of 1, 000 meters, and remarkable development of acidic volcanic rocks were found in the formation. As a result of Schlumberger logging, it was recognized that these acidic rocks have high porocities and will become a good hydrocarbon reservoir.
In the last decade, many big oil and gas fields were discovered in acidic volcanic reservoir of Nanatani formation, which is assigned to the corresponding horizon with Nishikurosawa formation, in Niigata oil producing area. It is abvious that the acidic rocks in this horizon are making a remarkable contribution for acumulation of hydrocarbons.
From the above mentioned facts, the writers consider that the trough-like zone where mudstone are dominant should be the important objects for tracing the reservoir of Nishikurosawa formation. Namely, Yuri oil producing area, Akita area and Noshiro area would be the interesting areas for the oil exploration in the near future.
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