Oil occurence in basins may generally be related to the relief of basins bottom and to the sedimentary environments.
To clarify the process of the oil accumulation, the study of paleography is a important method and a basic information for the oil exploration.
The writer studied on the stratigraphy of the Neogene strata developed in the surrounding area of the Dewa hilly lands, which include the southern part of the Akita and Yamagata, the eastern part of Miyagi prefectures, and introduced several time-stratigraphic units and its formations. The strgtigraphical relationships of each province are discussed, and inter-provincial correlation are shown in Table 1.
On the study of the isopach map, down ward sinking and upward movement area of the sedimentary basin are clarified. But isopach map alone will not give a trusty aspect of a depositional basin. The lithofacies and the biofacies are equally important method for the restoration of basin depth and sediment-tary environments. Combinations of the lithofacies, biofacies and isopach, make the aspect of a depositional basin clear. Accordingly, on the basis of the ecological study of the fossil foraminifera, the six biofacies are recognized, namely (1) littoral zone biofacies, (2) inner neritic zone biofacies, (3) outer neritic zone biofacies, (4) bathyal zone biofacies, (5) deep stagnant zone biofacies, (5) deep stagnant zone biofacies, (6) shallow stagnant zone biofacies.
The writer prepared the paleogeographical maps of the Nishikurosawa stage, Onnagawa stage, Funakawa stage, Kitaura stage and Wakimoto stage, making account of the lithofacies, biofacies and isopach, those are shown in Fig. 18-22.
As the result of the study, the Dewa Hilly lands was situated on the subsiding zone during the Onnagawa stage, but in the late Funakawa stage, was slowly uplifted from south to north.
Oil occurence is closely related to the paleogeographical aspects. Submarine ridges in the deeper stagnant sea are one of the most favorable places for the oil accumuration. The Amarume and Fukura oil fields may be an example of this relationship.
The Innai-Katsurazaka and Horiuchi oil fields are situated in the hinge belt, where the sand-shale ratio is bitween 1/8 to 1.