ABSTRACT

Carbon isotopic ratios (¹³C/¹²C) of methane, ethane and propane were analyzed for natural gases from several oil and gas fields in Niigata and Akita in Japan. A strong correlation between δ¹³C(C₂H₆) and δ¹³C(C₃H₈) was observed. This relation is well explained by a kinetic theory proposed by Chung et al. (1988). It propounds that carbon isotopic ratios of the gases are mainly decided by kinetics. on the other hand, only a weak correlation was observed between δ¹³C(CH₄) and δ¹³C(C₂H₆). The weak correlation is inferred to be due to mixing of biogenic gas that has low δ¹³C(CH₄). Further, δ¹³C of source kerogen of these gases can be calculated from δ¹³C(C₂H₆) and δ¹³C(C₃H₈). It is inferred that δ¹³C of source kerogen of the gases in Akita and Niigata are close to each other.