Fossa Magna is a remarkable tectonic zone which divides the Japanese main island (Honshu) into two parts, those are northeastern Japan and southwestern Japan. This tectonic zone runs from Niigata Prefecture through Nagano Pref. to Shizuoka Pref. and was introduced by E. Naumann in 1885.
In the northern part of the Fossa Magna, the Niigata Tertiary sedimantary basin is in contact with the Nagano basin. Strong tectonic movement and Quaternary volcanoes make it very difficult to directly combine these two basins.
The writers tried to compile these basins from the view point of petroleum geology and to give some considerations about the possibility of petroleum in the Nagano district.
At first, they made a stratigraphic correlation of the Nagano and the Nishikubiki (southwestern margin of the Niigata basin) districts. Afterward, they considered sedimentary conditions and sedimentary enviroments using foraminiferal data and isopach maps, then estimated the existence of "Nishikubiki up-lift" (provisional name) to the northwest of this area. They supposed that this uplift stands opposite the "Nagano Central up-lift" which is located along the southeast of this area. With the progress of intermittent rising of these up-lifts during the Neogene age, the sedimentary basin between the Nishikubiki and Nagano Central up-lifts became deeper and narrower. A vast amount of sand and mud deposited in the basin. Some of these sediments show flysh facies.
The writers considered, as well, about the occurrence of many oil-gas showings and oil-gas fields of the Nagano district in connection with the geologic development of this area. It is important, above all, that the Aoki and the Bessho formations (lower-middle Miocene) are valuable as the source rook and that some oil showings were found in the "Green Tuff" (lower miocene pyroclastic rocks).

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The Tsuchizaki-oki Oil Field is located in the Japan Sea, approximately two kilomters west of the town of Akita-shi. The discovery well was completed at II zone, 550 meters depth, in 1959. The purpose of this study was to determine the production history of the Tsuchizaki-oki Oil Field. In order to resolve this problem, it was necessary to assume the reservoir pressure decline curves. Although early in the development period of the field bottom hole pressure serveys were made, the measurement of bottom hole pressure did not carry out since the reservoir pressure was depleted. k_g/k₀ curves were calculated from the field data. The results obtained from this study are as follows:
(1) The drainage mechanism of II zone might be a solution gas drive.
(2) The oil in place in the central area of II zone might be about 250, 000kl.
(3) The relationship between the reservoir pressure and the recovery is shown in Fig. 8. Curve J in Fig. 6 was used in this calculation. The pressure history was considered to be represented by curve J.

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The purpose of this study is to reveal any relationships between various crude oil properties and geology in the Kubiki (Katamachi) gas and oil field, Japan. Taking the reservoir conditions into account, we found out tendencies that some properties of crude oils vary with the geologic conditions in the field. The Kubiki gas and oil field is one of the largest field in Japan and includes several producing structures (Figure 1). The general stratigraphic column is shown in Figure 3. Many reservoirs are intercalated in the dark grey mudstone of the late Miocene Teradomari Formation. Amoung these structures and reservoirs the Ie reservoir in the Katamachi structure is producing much oil. Thus, our study was carried out mainly on the Ie oils and Ie reservoir there. The Katamachi structure is a gentle dome cut by many minor faults (Figure 2). The Ie reservoir is more or less tuffaceous or shaly sand and remarkably changes in thickness (Figure 4).
We sampled crude oils from 73 producing wells (Table 1) and measured specific gravity and absolute viscosity. Furthermore, we conducted fractional distillation and made a gas-chromatographic analysis of lighter cut of the crude oils (lower than 105°C). On the other hand we collected various reservoir data including initial water-oil ratio.
Through the above-mentioned work, we have come to the following conclusion.
1) The initial boiling point of these crude oils is relatively high and the amount of lighter fraction is comparatively small (Figure 9). As for fraction of 250-275°C the crude oils are generally of naphthenic base.
2) Specific gravity of crude oils from a reservoir in a small area vary very much (Figure 5). Taking the initial water-oil ratio (Figure 6) and the thickness of the reservoir into account, it is suggested that wells producing heavier crude oil are generally situated at the places where the sand reservoir is thin and initial water-oil ratio is large and vice versa, almost regardless of the setting of the wells in the Katamachi structure. This tendency is quite remarkable in the block east of d-d' fault. This is probably because in the portion where the reservoir is thin the reservoir is shalier and contains much of water. But this tendency is not clearly recognized in the block west of d-d' fault. Consequently we propose schematic models of the Ie reservoir in the Katamachi structure (Figure 7).
3) Absolute viscosity has a strong correlation with specific gravity (Figure 8), however, the above tendency (described in 2)) cannot be ascrived to the variation of viscosity of the crude oils. Relation of viscosity vs. specific gravity of the Katamachi crude is quite different from that of Khafji field, Middle East. This fact can be explained from the distillation histograms (Figure 9).
4) Relative amount of lighter hydrocarbons shows a considerable variation (Table 2). However, the Kubiki crude has much of cycloparaffins and branched paraffins, and thermodynamical equilibrium is not found out. These facts suggest that the Kubiki crude is young and less-evolved oil.

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