Paleogene Oil Belt-Source of Hokkaido Oils

日本北部北海道古近系沉积盆地的原油具有高蜡、低硫和丰富的陆源生物标志物的特征，表明其主要来源于陆源有机质。始新世煤和/或含煤页岩被认为是可能的烃源岩。这些煤富含镜质组，其通常占总显微组分的85%以上。与常规III型干酪根相比，北海道始新世镜质组煤具有异常高的氢指数（300 ~ 600 mgHC/gTOC）。为阐明高纬度地区始新世陆相生油岩的成因，对北海道夕张山西部3个露头的始新世煤、煤质页岩以及非海相夕张组、Bibai组和Ikushunbetsu组的页岩进行了地球化学分析。沉积构造表明，大部分样品形成于洪泛平原或沼泽沉积环境之下 ...更多信息 它们的TOC浓度范围为0.2%至60%。正构烷烃的CPI值为1.5 ~ 1.8，C_（20）S/（20 S +20 R）<29>值为0.20 ~ 0.25，均为未成熟烃。HI随TOC的增加而增加，随T_增加而减小<max>。低能沼泽相煤具有最高的HI、最低的T_<max>和最低的生成活化能分布，其中指前因子假定为相同的。高HI、高TOC样品热解产物中酚类含量高，植烷/姥鲛烷比值大于10，C_<31>高藿烷含量高，且C_<31>高藿烷含量与植烷/姥鲛烷比值呈正相关。高丰度的均藿烷可以解释为它们的形成，由于成岩作用过程中的细菌多酚的氧化和脱羧。陆相生油岩高C_<31>高藿烷丰度和高姥鲛烷/植烷比值反映了其高氧沉积环境。另一方面，油倾向的烃源岩往往富含齐墩果烷和海松烷（降海松烷、降异海松烷和异海松烷），而四环二萜烷如叶枝烷贫乏，表明被子植物的贡献较大。始新世是全球变暖的典型时期之一。植物化石和有孔虫稳定氧同位素比值表明，北海道在始新世时处于亚热带地区。亚热带气候有利于蜡质开花植物的生长，三降藿烷、降藿烷、藿烷和高藿烷等类藿烷是生油岩中主要的三萜烷，占总烷烃的10%以上。同时，它们的甾烷含量很低，表明藻类有机质的贡献很小。丰富的hopanoids主要来源于微生物强烈建议一个显着的细菌活动的有机质丰富的沉积层。这也得到了具有奇数碳数偏好的反异构烷烃的高丰度的支持。生物降解往往在改变初始有机组成方面起着重要作用。蜡质开花植物及其生物降解作用对富脂烃结构的贡献是陆相生油岩形成的重要原因。少

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