[SRD33-P12] Geological and geochemical studies on Miocene diatomaceous sedimentary rocks at Akita and Aomori in Japan; role of sulfur in maturation of organic matter and petroleum formation
Keywords:Biomarker, Organic sulfur, Petroleum
The role of sulfur in maturation of organic matter and petroleum formation is controversy. Petroleum is mostly formed through the partial decomposition of kerogen in response to thermal stress during diagenetic stage . For example, kerogen contain significant amounts of sulfur (8–14 wt %) at Miocene Monterey oil field in California. The Monterey oil field generated many of the early products of OM such as low gravity oils , suggesting the possibility that the existence of sulfur in OM could accelerate petroleum formation. This phenomenon has been explained by the early cleavage of C–S and S–S bonds, which are relatively weaker than C–C bond, in the sulfur-rich kerogen in the previous studies [2, 4]. However, previous investigators also questioned for oil generation model promoted by presence of sulfur.
Therefore, in order to constrain the role of sulfur in petroleum formation, EOM and kerogen in sulfur-rich Miocene sedimentary rocks were analyzed in the present study. Geological surveys were conducted in Takanosu (Akita) and Ajigasawa (Aomori) areas. Examined rocks in both areas are equivalent in age to Onnagawa Formation. We found petroleum-like matter on the outcrop during the geological survey. Kerogen was isolated by the HCl/HF acid treatment from the collected samples. The sulfur concentrations of kerogen samples were 2 to 9 wt %, after pyrite separation, indicating sulfur-rich organic matter are certainly present in examined samples.
The Soxhlet extraction of EOM was performed on collected samples. The EOM were analyzed for biomarker compounds and sulfur isotopic composition. Organic sulfur compounds such as highly branched isoprenoid (HBI) thiophenes and alkylthiophenes were detected by gas chromatography / mass spectrometry analysis. Based on C31 hopanes 22S/(22S + 22R) ratios and C29 steranes 20S/(20S + 20R) ratios, maturation grade of organic matter in these rocks was too low to generate petroleum. This is enigmatic to explain the oil generation in this area.
Most likely oil generation was happening in distal areas from the sample collections, through various sulfur-organic compounds with help of heat energy addition by local volcanisms. These results would suggest that the presence of sulfur in organic matter accelerates decomposition of OM, and petroleum generation even in a younger sediments.
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