Rumoi oil field is one of the complicated structure in Hokkaido. Many unsolved problems remain here, especially regarding to the structure and sedimentation. In this paper one interpretation is proposed, namely 1) stratigraphically the each horizon discussed, and 2) "Yudoro Facies" newly proposed, and 3) the facies being decided as one of the delta type affected by turbidity currents, and 4) accumulated in the sinking trough. Occasionaly Air-Photo is applied.
The deposit of Lake Suwa due to special local conditions of sedimentation. The test well for natural gas prospecting digged down at Shibuzaki by Geological Survey of Japan in 1952-'53 and the deepest of it amounted to 371m. There are several considerations to be taken into account in regard to the exploitation of Lake Suwa deposit property. As one of them, we must take notice of the problem of fossil diatoms in the deposit. Microscopical experiments are then made on 28 samples of clayey rocks, and each of which contained fossil diatoms. Diatom is a common microplant, which knows in both marine and fresh water form, and is widely distributed in Japan. In the deposit of Lake Suwa, 597 species which was found are almost all fresh water and existing forms. The figures of the table were checked by the estimation of species on each material. The deposit represents a succession of fresh water sediments consisting of clay, tufaceous sand, sandy clay and tuff, and should be regarded as a deposit of the Quaternary Period. The position of the well is situated in a recent deposit near the southern shore of the lake, and represents a horizon stratigraphically uppermost the sediments of Lake Suwa.