Source rock lithology prediction based on oil diacholestane abundance in the siliceous-clastic Akita sedimentary basin, Japan

In an effort to predict the source rock lithology from oil biomarkers in the Akita Basin, the relationship of diacholestane abundance to clay mineral content, thermal maturation, and the oxic/anoxic depositional conditions of representative mudstone samples has been investigated. The relative diacholestane abundance of samples with similar maturity levels increases with increasing clay mineral content. The oxic/anoxic depositional conditions of the samples, estimated by C35-homohopane index, are less influential for diacholestane formation in the case of the Yabase oil field samples. A crossplot of the diacholestane abundance versus ethylcholestanes 20S/(20S+20R) and clay mineral content was applied to evaluate the source rock lithology of the Yabase oils. The source of this oil is consequently predicted to be the clay-poor siliceous rocks of the Miocene Onnagawa Formation. Although this study did not achieve quantitative clay content prediction, the results demonstrate that this approach is useful for the characterization of source rock lithology, especially for moderately mature oils.