2018 Volume 83 Issue 2 Pages 169-180
The factor analysis of biomarker composition on the marine oils from the Akita and Yamagata Basins, Japan, was conducted to clarify their source rock characteristics. Three major factors controlling source rock facies are interpreted to be lithologies （Factor 1: clastic vs. siliceous/calcareous）, organic origins （Factor 2: variable contributions of land plants） and oxic/anoxic depositional conditions （Factor 3）. The Akita and Yamagata oils are differentiated into six types by the score plot of the Factor 1 and 2. The areal distribution of the oil types revealed regional variation of source rock facies, such as the higher clastic input in the Yamagata basin than in the Akita basin, higher marine organic input in the west anticline series than the east series in the Yabase oil field area. The Factor 1 and 2 are mainly represented by oleanane/norhopane ratio and the relative abundance of C29 sterane among C27-29 steranes, respectively. Based on these two biomarker parameters, the source rock lithologies in the Sea of Japan side of the Tohoku District are interpreted to change from siliceous/calcareous in the Akita area to clastic in the southwest Niigata area. The source rocks of the oils from the Sagara area in the Pacific side of the main island are evaluated to have higher land plant contributions than those from the Sea of Japan side. The oils distributed in the coal-bearing basins extending from the central Hokkaido to the offshore Pacific side of the Tohoku District contain much higher amount of C29 sterane, suggesting their source rocks are coals or coaly shales.
