Abstract Over 14 million cu m of oil and 2 billion cu m of gas have been produced from the formations of the Akita basin which is one of the subbasins of the back-arc basin. Neogene and post-Neogene volcanic activity was important in the history of this basin. Hydrocarbons have accumulated in anticlines mainly formed by block faulting of the basement. Tuffaceous rocks are important reservoir-rock facies, and sandy tuff is especially good in reservoir-rock quality.

Relatively large oil fields lie in areas where the late Miocene shales of both the Onnagawa (stage IV) and Funakawa (stage V) formations are thickly developed. These formations were deposited in a stagnant environment which is characteristic of a closed marginal sea; therefore, organic matter must have been satisfactorily preserved.

The terrestrial heat flow in the back-arc side is three or four times as high as that in the fore-arc side (Kitakami basin).

In contrast to the Akita basin, the hydrocarbon potential in the Kitakami basin may be handicapped by aspects of the geologic setting such as low heat flow and an oxygen-rich sedimentary environment. However, this basin has a high sand percentage and an abundance of sediments (over 4,000 m thick); moreover, geologic structures are large. Undoubtedly this basin had the same history during the Neogene as the Hidaka oil field, Hokkaido Island. In addition, an exploratory well recently drilled had gas shows at many depths. The hydrocarbon type is considered to be gas rather than oil because of no oil seeps in the nearby onshore.