Pentane and hexane isomers in natural gases from oil and gas fields in Akita, Niigata and Hokkaido, Japan: Determination factor in their isomer ratios

Hydrocarbons are the main components in natural gases. Although the molecular distribution has been often used to study the origin of hydrocarbon, the occurrence of pentane and hexane isomers has not been discussed for natural gases. This study first reports controlling factors of pentane and hexane isomer ratios of natural gases from Akita, Niigata and Hokkaido in Japan. For the samples from Akita and Niigata, pentane and hexane isomer ratios such as neopentane/isopentane and 2,2-dimethylbutane/2,3-dimethylbutane show positive correlations with δ13C (ethane), suggesting that the ratios are affected by the generation temperature. Relation between kerogen type and isobutane/n-butane ratio is also discussed. By the comparison between Akita and Niigata gases that originated from type II kerogen and Hokkaido gases that originated from type III kerogen, the gases from type III kerogen show a higher isobutane/n-butane ratio compared with those originated from type II kerogen. Although carbon isotopic ratios of ethane and propane are good parameters for the gas generation temperature, this study indicates that combinations among pentane isomer ratios and hexane isomer ratios also provide good parameters for the gas generation temperature.