Organic geochemical relationship between pooled oils and distribution of their source rocks in the Niigata basin, Japan.

The paleoenvironment and the distribution of source rocks in the Niigata oil and gas fields was detailed (Sato et al., 1995, this volume).We describe the organic geochemical characteristics of oils produced and probable source rocks, and oil-to-oil and oil-to-source rock correlation using aliphatic biomarkers.Petroleum potential evaluation based on the S1+S2 values on Rock Eval pyrolysis revealed that the argillaceous rocks in the Lower Teradomari LT2 and LT4 zones correspond to possible or potential source rocks (Hirai and Miyamoto, 1992).Triterpane and sterane compositions of the oils and bitumens extracted from argillaceous rocks were compared with each other.The results of the triterpane (m/z 191) and sterane (m/z 217) fragmentograms showed that these oils are matured, and two types of oils can be distinguished (Hirai et al., 1992), and their corresponding source rocks are suggested.(1) The Niitsu type oils produced mainly in the eastern oil and/or gas fields with low oleanane/norhopane ratios (lower than 1.0) and relatively low C29/C27 sterane ratios, suggesting that they were generated from the effective source rocks in the Lower Teradomari LT4 zone with larger contribution of marine organic matter.The Shintainai, Shiunji, Aga-Oki, Higashi-Niigata, Matsuzaki, Minami-Aga, Minami-Niitsu, Kuwayama, Yoshida and Mitsuke fields are classified in this type.(2) The Kubiki type oils are produced mainly in the western oil and/or gas fields with high oleanane/norhopane ratios (higher than 1.0) and relatively high C29/C27 sterane ratios, suggesting that they were generated from the effective source rocks in the Lower Teradomari LT2 zone with larger contribution of terrestrial organic matter.The Fujikawa, Minami-Nagaoka, Tamugiyama, Amaze, Nishiyama, Katamachi, Kuroi, Meiji and Gohtsu fields are classified in this type.