Stable carbon isotopes of organic matter in Neogene sediments of the Shinjo basin

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.