Biomarker distributions in oils from the Akita and Niigata Basins, Japan

Oils from the Akita and Niigata Basins, Japan, are characterized by the predominance of C15C17 alkanes, pristane/phytane ratio < 2, abundant C27 regular steranes and the presence of C26 steranes. These are consistent with the marine depositional environment of the source rocks. However, geochemical parameters for the Niigata Basin petroleums show higher terrestrial[ input, and are characterized by enhanced concentrations of oleanane, tetracyclic terpane and C29 steranes. Abundances of diasteranes in oils are controlled both by maturation and lithology of source rocks. Comparatively low concentrations of diasteranes in the Akita oils can be attributed to the low abundance of clay minerals in their siliceous source rocks. Oils from the Akita and Niigata Basins can be well discriminated on the basis of geochemical indicators using factor analysis. Evaluation of maturity levels of oils from the Akita Basin shows that oil expulsion occurred at lower maturity levels in siliceous shales than in argillites in the Niigata Basin.