Paleogene Oil Belt-Source of Hokkaido Oils

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Oils in Paleogene sedimentary basin, Hokkaido, North Japan are characterized by high wax content, low sulfur concentration, and abundant terrestrial biomarkers, showing that they are mainly derived from terrestrial organic matter. Eocene coals and/or coaly shales are considered as the possible source rocks. These coals are rich in vitrinite, which generally comprises more than 85% of total macerals. Eocene vitrinite coals in Hokkaido have abnormanlly high hydrogen index (300 to 600 mgHC/gTOC) compared to conventional Type III kerogen. Geochemical analyzes for various coals and coaly shales were carried out to clarify the origin of Eocene oil-prone terrestrial source rocks in high latitude areas.Eocene coals, coaly shales, and shales of non-marine Yubari, Bibai, and Ikushunbetsu Formations are obtained from three outcrops exposed in western Yubari Mountains, Hokkaido. Sedimentary structures indicate that most of the samples were formed under floodplain or backswamp depositional environm … More ent. Their TOC concentrations range from 0.2 to 60%. CPI of n-alkane and 20S/ (20S+20R) of C_<29> regular steranes are in the range of 1.5 to 1.8 and 0.20 to 0.25, respectively, showing that all the samples are immature for hydrocarbon generation. HI increases with increasing TOC and decreases with increasing T_<max> Coals from low energy backswamp facies have the highest HI,the lowest T_<max>, and the lowest generation activation energy distribution, where the pre-exponential factor is assumed to be the same. The samples with high HI and high TOC are generally characterized by high abundance of phenols in pyrolysates of py-GC,high pristane/phytane ratio more than 10, and high abundance of C_<31> homohopane, which is generally the most abundant among all triterpanes.C_<31> homohopane abundance is positively correlated to pristane/phytane ratio. High abundance of homohopanes can be explained by their formation due to oxidation and decarboxylation of bacteriopolyhopanols during diagenesis. High C_<31> homohopane abundance and high pristane/phytane ratio of oil-prone terrestrial source rocks show their highly oxic depositional environment. On the other hand, oil-prone source rocks tend to be rich in oleanane and pimaranes (norpimaranes, norisopimaranes, and isopimaranes) , and poor in tetracyclic diterpanes such as phyllocladanes, suggesting much angiosperm contribution. Eocene is one of the typical epochs of global warming. Plant fossils and stable oxygen isotope ratio of foraminifera show that Hokkaido, in the Eocene, was in the subtropical region. The subtropical climate is convenient for vegetation of waxy flowering plants.Hopanoids such as trisnorhopanes, norhopanes, hopanes, and homohopanes are the major triterpanes of oil-prone source rocks, and they often occupy more than 10% of total alkanes. Whilst, they are very poor in steranes, showing few contribution of algal organic matter. Abundant hopanoids mainly derived from micro-organisms strongly suggest a marked bacterial activity in the organic-rich sediment layr. This is also supported by high abundance of anteiso-alkanes with odd carbon number preference. Biodegradation often plays an important role in modifying initial organic composition. Much contribution of waxy flowering plants and their biodegradation to enrich aliphatic structures can be responsible for the formation of oil-prone terrestrial source rocks. Less