The author has studied on the timing of primary hydrocarbon migration in the main oil and gas fields of the Northeast Japan, mainly from the geohistorical aspect, and also speculated its relation-ship with hydrocarbon generation through geochemical and coal petrographical study.
As a conclusion, the principal timing of primary migration is at the early stage of hydrocarbonization of kerogen, that is to say, with in the mature range of 0.5 to 0.8% in vitrinite reflectance (
R0), and also, hydrocarbons generated at the late stage should have been unable to accumulate in significantly large quantities.
In the Japanese oil bearing Neogene systems, biginning depths of oil formation are generally from 1500 to 3000m, and their corresponding subsurface temperatures are from 348 to 388K (from 75 to 115°C), respectively. This variation of temperatures necessary for hydrocarbon generation is related to that of effective heating time of Miocene and Pliocene sediments.
Most of oil and gas fields in Japan are formed during very young geologic ages. Therefore, as controlling factors for the formation of important oil and gas accumulations, deep burial with thick sediments and presence of deep and large synclines adjacent to anticlines or higher geothermal gradient more than 3.5°C/100m in case of thin sediments are required. These necessary conditions are fully satisfied in the main oil and gas fields in the Northeast Japan.
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