Geochemical characteristics of terrigenous- and marine-sourced oils in Hokkaido, Japan

Abstract

Crude oils in Hokkaido, Japan, are divided into two genetic groups based on bulk properties, carbon-isotope compositions and biomarker distributions. One group is characterized by high carbon-isotope canonical variables ( Sofer, 1984), high wax and low sulfur content, high Pr/Ph ratios, low C 27/(C 27+C 29) sterane ratios, relatively low Ts/(Ts+Tm) ratios, and the absence of tricyclic terpanes and C 35 homohopanes. These characteristics suggest that the oils were generated mainly from terrigenous organic matter in source rocks deposited under highly oxic conditions. The other group is characterized by low canonical variables, low wax and high sulfur content, low Pr/Ph ratios, high C 27/(C 27+C 29) sterane ratios, relatively high Ts/(Ts+Tm) ratios, and the presence of tricyclic terpanes and C 35 homohopanes, properties that suggest that they were generated from marine shales. Based on the biomarker distributions, the terrigenous oils correlate to Paleogene coals and coaly shales, whereas the marine oils correlate to Miocene marine shales. The atomic H/C ratios of the coals indicate that they have oil-generating potential. The geographic distribution of the terrigenous and marine source rocks, and their thermal maturity control the distribution of the oils in Hokkaido. The geographic distribution of the Paleogene coal-bearing formations corresponds to the distribution of the terrigenous oils in central Hokkaido. The marine oils are restricted either to areas of high heat flow related to volcanic activity, or to the area with thick Neogene and Quaternary sediments, where the Miocene source rocks are buried deep enough for oil generation.