八橋油田における油の根源岩と生成・移動・集積

Abstract

The authors studied on the generation, migration and accumulation of oils in the Yabase Oil Field from the geochemical and geological view points.The Yabase oils reveal a wide range of gravity varying between 19°and 39°API. Heavier oils are considered to be biodegraded because they are relatively abundant in asphaltenes and relatively lack in gasoline range hydrocarbons and normal alkanes. All oils containing a relatively high quantities of alkanes are considered as paraffinic oils except for biodegraded oils. They have similar alkane distributions ranging from n-C10 to n-C35 with fair quantities of n-alkanes above n-C25 and pristane/phytane ratios below 2. It indicates that the oils have been generated from a similar source including predominantly sapropelic material deposited in an anoxic environment at low to middle maturity. On the basis of gas chromatography/mass spectrometry (GC/MS) analysis, the oils have very similar sterane and triterpane compositions. It suggests that they have been generated from very similar marine source at low to middle maturity. Carbon isotope data confirm the conclusions from GC/MS analysis.Effective oil source rocks are probably argillaceous rocks of the Onnagawa and the Nishikurosawa formations at depths greater than 2, 000m in this area. The Yabase oils have not generated from the Funakawa and the Onnagawa formations around the reservoirs, because they are at immature stage.Correlation between oils and several candidates of source rocks indicates that the Yabase oils have been derived from the argillacecous rocks of the Onnagawa and the Nishikurosawa formations placed in early to middle mature zone.These source rocks have entered the oil window in the latest Pliocene (Sasaoka age). Most of the oils have migrated and accumlated in the Quaternary to Recent.