Mineral assemblage of the argillaceous rocks of the Miocene Takashio-Tamada formations in southwest of Yaita City, Tochigi, Japan, is identified as the detrital montmorillonite and authigenic mordenite-clinoptilolite-cristobalite zone. This mineral assemblage indicates that the argillaceous rocks of these formations in the studied area have been subjected to temperatures between 56 and 69°C in the geologic past and consequently the paleogeothermal gradient is estimated to be approximately 2.2°C/100m and the stratigraphic deficiency after the deposition of Tamada formation is calculated to be about 1800m.
Hydrocarbons/organic carbon ratio indicates that the hydrocarbons in the argillaceous rocks of the area have not matured yet. Also, the quantity of organic matter is decidedly low.
It is expected that the primary migration of carrier water of oil in the Miocene of the area would occur when the burial depth of the sediments reached between about 1100 and 2700m. The possibility of the presence of oil pools may be poor because the generation of petroleum hydrocarbons during the stage of primary migration is considered to have been low. Natural gas pools, however, may be expected to exist in the surrounding area.
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