1) The region lies on the nothernmost part of Nagano prefecture and on the southern end of Tomikura anticline. 2) The region have 2 oil and 12 gas seepages, 3 of gas seepages are not inflamable because of predominant of CO2 component. 3) The Tertiary formations are mostly hiden under overlying volcanic detritus of Quaternary volcanisms. 4) The anticline axis is presumed to cross the Nojiri-ko, therefore the structure borings are desirable on the line across this presumed anticline structure.
In this gas region, Tertiary sedimentary rocks are covered by Quaternary volcanic ash. Natural gas of the region can be classified into the following three groups. I. CO2-(N2) group II. CH4-(N2) group III. CH4-CO2-(N2) group CO2-(N2) group occures in the western district, where the thickness of volcanic ash is large. And the writers think that the gas of this group originated probably from volcanic activity. On the contrary, CH4-(N2) group is recognized in the district, where Tertiary sedimentary rocks or the old lake deposits are exposed. Except the Alluvial gas, the vertical destirbution of the natural gas quality is as follows. Shallower part........................CO2 rich gas (I group) ................................................................ Deeper part.......................CH4 rich gas (III group)
Chemical properties (pH, HCO3-, Cl-, NH4+) of interstitial vater, specific gravity and water content of cores were determined. Fig. 1 shows the data of core analysis, and from this figure the writer expects the existence of natural gas with saline water at the depth of 240m.
Oshamambe natural gas field had been surveyed geochemically and geologically by the Geological Survey of Japan in 1953 and 1954. The writers performed the core analysis and lift test of natural gas well, Oshamambe R-1, which was drilled under planning of Oshamambe town-office, having 600m of depth and being located about 200m north from Oshamambe station. As a casing pipe, gas-pipes 4 inches in diameter are used about 500m of length. The gas-pipe was perforated between 350-390m, 410-550m in depth, and non-casing bore hole in adepth of 551-613m. The production data obtained under natural flowing are as follows: gas volume water volume water temperature 350-400m3/d 710m3/d 49.0°C And the data obtained by the lift test are as follows gas volume water volume water temperature 850-900m3/d 1, 150-1, 200m3/d 50.0°C The geothermal gradient of this place is almost twice in ordinary region. As a conclusion, the writers suppose that the promising gas-reservoir beds in the Kuromatsunai formation of Pliocence age exist probably in the deeper part than 600m in depth.