Carbon isotopic ratios of methane, ethane and propane in natural gases from Niigata and Akita in Japan: Factors affecting the parameters

Carbon isotopic ratios (C-13/C-12) of methane, ethane and propane were anal yzed for natural gases from several oil and gas fields in Niigata and Akita in Japan. A strong correlation between delta(13)C(C2H6) and delta(13)C(C3H8) was obse rved. 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 m ainly decided by kinetics, on the other hand, only a weak correlation was o bserved between delta(13)C(CH4) and delta(13)C(C2H6). The weak correlation is inferred to be due to mixing of biogenic gas that has low delta(13)C(CH4 ). Further, delta(13)C of source kerogen of these gases can be calculated f rom delta(13)C(C2H6) and delta(13)C(C3H8). It is inferred that delta(13)C o f source kerogen of the gases in Akita and Niigata are close to each other.