Paleogene Oil Belt-Source of Hokkaido Oils

日本北部北海道古近系沉积盆地原油含蜡量高、硫含量低、陆相生物标志物丰富，表明原油主要来源于陆源有机质。始新世煤和/或煤质页岩被认为是可能的源岩。这些煤中含有丰富的镜质组，镜质组一般占总显微组分的85%以上。北海道始新世镜质组煤与常规的III型干酪根相比，具有异常高的氢指数(300~600mgHC/gTOC)。为了阐明高纬度地区始新世含油陆相烃源岩的成因，对不同类型的煤和煤质页岩进行了地球化学分析。始新世煤、煤质页岩和陆相Yubari组、Bibai组和Ikushunbetsu组的页岩是从北海道尤巴里山西部露头的3个露头中获得的。沉积构造表明，大部分样品形成于…的洪泛平原或后滩沉积环境。更重要的是。它们的总有机碳浓度从0.2%到60%不等。正构烷烃和C&lt；29&gt；正构烷烃的20S/(20S+20R)CPI分别在1.5~1.8和0.20~0.25之间，均为未成熟生烃。低能沼泽相煤具有最高的HI值、最低的Tlt；max&Gt；和最低的生成活化能分布，指数前因子相同。高HI和高TOC的样品一般具有高丰度的Py-GC热解产物，高的伯烷/植烷比大于10，C&lt；31&gt；高杂环戊烷丰度较高，通常是所有三萜中最丰富的，C&lt；31&gt；高杂环戊烷丰度与伯斯坦/植烷比值正相关。在成岩作用过程中，细菌多聚己烷醇发生氧化和脱羧化作用，从而形成了高并戊烷，从而解释了其高丰度。含油陆相烃源岩具有较高的C&lt；31&gt；高正庚烷丰度和高的伯烷/植烷比值，反映了其高氧化的沉积环境。另一方面，油性烃源岩往往富含齐墩果烷和海松烷(降冰片烷、降冰片烷和异海松烷)，而贫四环二萜类化合物，如叶状枝烷，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。植物化石和稳定的有孔虫氧同位素比值表明，北海道始新世处于亚热带地区。亚热带气候有利于蜡质开花植物的生长，含油烃源岩的主要三萜类化合物如三降冰片烷烃、降冰片石烷、石榴石烷和高石蜡石烷类化合物往往占烷烃总量的10%以上。同时，它们的甾烷含量很低，对藻类有机质的贡献很小。主要来源于微生物的丰富的类胡萝卜素强烈表明富含有机物质的沉积物中具有明显的细菌活动。这也得到了具有奇数碳数偏好的正构烷烃的高丰度的支持。生物降解往往在改变初始有机成分方面起着重要作用。蜡质开花植物的大量贡献及其对脂肪结构的生物降解作用可能是陆相生油岩形成的主要原因。较少

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