ABSTRACT

Stable carbon isotope study was carried out on coexisting bitumen and kerogen in sediments of the Shinjo basin. The δ¹³C values of bitumen and kerogen change from −26 to −22‰ and from −25 to −21‰, respectively, from the top to the bottom sediments of an about 1, 600m thick profile. The difference in δ¹³C between bitumen and kerogen at each horizon is approximately constant (average, −0.89‰), even down to the horizones where oil generated. This result shows that bitumen was liberated from kerogen, but no specific isotopic fractionation was involved in the oil-generating zone in comparison to non oil-generating zone. It also shows that oil components at each horizon have been little contaminated by those from adjacent horizones above and below.