Abstract
The Yabase field is the largest oil field in Japan. It has produced approximately 34 million barrels of oil. Oil to source-rock correlation and organic-geochemical characterization of the Onnagawa and Funakawa Formations were attempted using triterpanes and steranes in the northern part of the field.
Based on the ratios of C29-norhopane/C30-hopane, C27-diasteranes/C27-steranes, and 20S/(20S+20R) of C29-steranes, produced oils from the Kitaakita and Sotoasahikawa fields can be correlated to the Onnagawa Formation and the lower part of the Funakawa Formation in the Horikawa AK-1 well, and to the lower part of the Onnagawa Formation in the Iiokayama-1 and Tofuiwa-1 wells. The oils from the Nigorikawa and Asahikawa fields can be correlated to the lowermost part of the Onnagawa Formation in the Tofuiwa-1 well.
The westward dilution of biogenic silica with clay minerals is also suggested in the Onnagawa and Funakawa Formations. This characteristic can be attributed to the difference in depositional environment within the Akita basin.
