デグラデナイトに富む北海道始新世石炭・炭質泥岩の石油地球化学的特徴

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Eocene coals and coaly mudstones of Yubari, Bibai, and Ikushunbetsu Formations are obtained from three outcrops exposed in western Yubari Mountains, Hokkaido. Microscopic observations under reflected light show that the samples consist of vitrinite group macerals and degradinite as major macerals. Thin degradinite bands are embedded in vitrinite group macerals, and are characterized by non-continuous laminations, which fluoresce visible light under ultraviolet light. Scanning electron microscopy revealed that degradinite is much porous compared to vitrinite group macerals. Bacteria-like textures are often observed in degradinite pores. Vitrinite reflectance and 20S/(20S+20R) of C_<29> regular steranes are in the ranges of 0.44 to 0.59 Ro% and 0.20 to 0.35, respectively, showing that almost all the samples are immature for hydrocarbon generation. However, T_<max> is variable, ranging from 425 to 445℃. Hydrogen index (HI) increases with increasing TOC and decreases with increasing T_<max>. The samples characterized by high HI are generally rich in degradinite. Degradinite and vitrinite group macerals isolated from the sample were analyzed by pyrolysis gas chromatography and open system pyrolysis. Phenols, which are typical pyrolysis products of lignin, are major compounds of pyrolysates of vitrinite group macerals. Pyrolysates of degradinite are rich in aliphatic compounds and poor in phenols, being similar to those of plant cuticles. HI of degradinite is estimated to be more than 800mgHC/gTOC, showing strongly oil-prone characteristics like Type I kerogen. Degradinite is characterized by lower T_<max> compared to vitrinite group macerals. Considering the micro-textures and pyrolysates of degradinite, biodegradation and selective preservation of resistant biomacromolecular substances such as plant cuticles can be an essential process for the formation of degradinite.
日本有機地球化学会の論文
1999-06-30

デグラデナイトに富む北海道始新世石炭・炭質泥岩の石油地球化学的特徴

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