ABSTRACT: Geochemical Characterization of Oils, Gases and Source Rocks in the Yurihara and Ayukawa Fields, Akita, Japan

Waseda, Amane, Hideki Nishita, Yoshiteru Kajiwara, Hirotsugu Iwano, and Keietsu Kato , Japan Petroleum Exploration Co., Ltd, Chiba, Japan

The Yurihara and Ayukawa fields are located in the Akita Basin, Northeast Japan. The former field is 4 km south of the latter. Oils and gases are mainly produced from silicious shales and dolerites in the Middle to Late Miocene Onnagawa Formation, and acidic tuffs and basalts in the Early to Middle Miocene Nishikurosawa Formation. The compositions of the oils, condensates and gases vary with each reservoir. Although source rocks are generally believed to be siliceous shales of the Onnagawa Formation, some part of shales of the Nishikurosawa Formation also have good source potential.

To clarify the main factors controlling the compositions of hydrocarbons in both fields, we investigated geochemical data using statistical factor analyses. Oil-source rock correlation study was also performed. Geochemical analyses including biomarkers and carbon isotopes were performed for 36 oils and condensates, and 35 gases. To group the oils, eight parameters were selected for the factor analyses. Source-related seven parameters were used for the oil-source rock correlation.

All oils and condensates are correlated to the source rocks of the Onnagawa siliceous shales and not to the Nishikurosawa shales. The main factors controlling the compositions of the oils and gases are the maturity and biodegradation. The oils reserved in the Onnagawa Formation are more biodegraded and less mature than the oils in the Nishikurosawa Formation. The condensates reserved in the Onnagawa Formation are interpreted to be generated by the evaporative fractionation process.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia