2020 Volume 85 Issue 6 Pages 299-308
The coal-bearing formations of the Eocene Ishikari Group, Hokkaido, Japan, are supposed to be the petroleum source rock strata for the Yufutsu Field, Hokkaido, Japan. Geochemical and sedimentological analyses were performed for the Ishikari Group to clarify the relationships between petroleum source rock potentials and sedimentary facies and depositional sequences.
In the case of the Ishikari Group, coals have higher source rock potentials than coaly shales and shales. Back marshes in meandering fluvial systems that develop in the coastal plains near the sea are most suitable for the formation of coal source rocks. Coal beds rarely form in the upper transgressive systems tracts （TST） and lower highstand systems tracts （HST） due to marine incursions during the maximum flooding periods in the third order depositional sequences.
Thus, coal beds mainly develop in the middle to upper HST and lower to middle TST. Source rock potentials are higher in the TST than in the HST.
For the formation of the coal-bearing layers, balances between the relative sea-level change rates and peat accumulation rates are important for keeping the fluvial environment near the sea for a long time. Such sedimentary environment is likely to occur at two times in a sequence cycle, one is around a sequence boundary and the other is around a maximum flooding surface, in which the accumulation mode is prone to be aggradation. Among them, coals with high oil source rock potential tend to be formed in the transgressive periods. The raised bog model can be applied to the formation of the peat layers with high source rock potential. Higher contributions of herbaceous plants over woody plants and better preservation of organic matter in the raised bogs compared to the low-lying bogs should be the key factors for the formation of oil-generating coals
