石狩～日高堆積盆における石油システム

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The petroleum system of the Ishikari-Hidaka basin was investigated using the geochemical data and basin modeling of the Yufutsu oil/gas field. The Yufutsu field is located on the Tomakomai ridge, a north-south trending basement high at the center of the Ishikari-Hidaka basin. In the Yufutsu field, gas and pale yellow-orange waxy condensate oil are produced from so called "Deep Reservoir", the Cretaceous granitic rocks and the conglomerate member of the Eocene Ishikari Group. In addition, bitumen is observed on cores of Deep Reservoir. Further, black oils are found in the "Shallow Reservoirs", the upper Eocene coal bearing member of the Ishikari Group, the upper Oligocene Minami-naganuma Formation and the Miocene Takinoue Formation. Oils from the Shallow and Deep reservoirs are correlated to the coal bearing member of the Ishikari Group, from biomarker and carbon isotope data. However, there is subtle compositional difference between oils in the Shallow and Deep Reservoirs. From higher sterane C29-20S/(20S+20R), Ts/(Ts+Tm), Diahopane/C30-hopane, Oleanane/C30hopane ratios, oil from the Deep reservoir is highly matured. On the other hand, oils from the Shallow reservoirs are of moderate maturity. From basin modeling at the main kitchen to the east of the Yufutsu field, the time of peak oil generation was 10-15 Ma. The distribution of oils in the Shallow and Deep Reservoirs are explained as follows : 1) At earlier stage, moderately matured oil was generated in the main kitchen, and some oil migrated into the Deep Reservoir. 2) At later stage, with increasing burial, gas generation and cracking of retained bitumen began, and the gas and condensate migrated into the Deep Reservoir. As a result, de-asphaltation occurred. At the same time, coal and coaly shale around the Yufutsu field became mature, and black oil migrated into sandstone in the Ishikari Group. For oils in the Minami-naganuma and the Takinoue Formations, the timing of accumulation is not clear.