Isotopic criteria for predicting the phase composition of hydrocarbons in Riphean and Vendian deposits of the Lena–Tunguska petroleum province

Abstract The isotopic criteria used to predict the hydrocarbon phase composition were justified by a comparison of regional trends in the δ13C values for hydrocarbon liquids and gases from Riphean and Vendian–Cambrian deposits of the Lena–Tunguska petroleum province. The δ13C ratios for liquid hydrocarbons and gases, which are defined by kinetic isotope effects due to kerogen cracking during petroleum generation, were used as a criterion for establishing a genetic relationship between different hydrocarbon phases. The study has revealed that oil-versus gas-prone areas exhibit significant differences in the δ13C values and the reverse isotope distributions in the liquids and gases. In the oil-prone areas, methane becomes progressively enriched in 12C compared to oils and condensates, which is the normal trend of carbon isotopic compositions of liquid hydrocarbons and gases generated by organic matter of the same type and same maturity level. In the gas-prone areas, the δ13C values for methane either overlap with those of liquids or methane may show a general trend of enrichment in 13C relative to liquids. This relationship suggests that gases were probably generated and migrated later than oils, which may provide explanation for predominant gas accumulation in the study areas. The results of the isotopic studies and analysis of the geologic history of the region allowed the recognition of hydrocarbon kitchen areas that were potentially expelling late-stage gas to mostly gas-prone areas. The proposed isotopic method, i.e., a comparison of variations in carbon isotopic compositions of hydrocarbon liquids and gases, allowed us to establish a genetic relationship and discriminate between different hydrocarbon phases on the basis of the δ13C ratios. This can provide additional information about the timing of liquid hydrocarbons and gases generation and help to establish the likely locations of hydrocarbon kitchen areas and the history of hydrocarbon accumulations, thus improving the reliability of predictions for the hydrocarbon resource potential in the study areas and the phase composition of accumulated hydrocarbons.