Implications of carbon isotope fractionation of natural gas, pyrobitumen, and source rock on Sinian gas reservoirs evolution, central Sichuan Basin

Hydrocarbon reservoirs in ancient and deeply buried formations typically exhibit complex evolutionary histories. After experiencing high temperatures, pressures, and multiple types of secondary modifications, the geological information carried by hydrocarbons is superimposed and difficult to interpret. Methane carbon isotopes (δ¹³C) in the Sinian natural gas in the central Sichuan Basin are heavier than those of the reservoir solid pyrobitumen (SB) and are considered an ‘anomalous’ fractionation. Therefore, based on the abundant δ¹³C data of the source rock, SB, and natural gas, this study aimed to interpret the geological significance recorded by the ‘anomalous’ fractionation combined with the analysis of geological elements and the evolution process of the gas reservoir. The results showed that the combined contributions of multiple source rocks lead to differences in the original δ¹³C of crude oil in paleo-oil reservoirs. Among them, the slope of the paleo-uplift was closer to the Deyang–Anyue rift trough, where the δ¹³C of the main Cambrian source rocks was negatively biased, making the δ¹³C of the paleo-oil reservoirs more negative. During the late thermal evolution, deasphalting was caused by gas produced from oil cracking under high temperatures and pressures. In contrast, the adsorption of clay minerals and gas intrusion due to kerogen degradation in the source rocks had little effect on deasphalting. The δ¹³C values of the bulk SB precipitated by deasphalting were light and similar to those of the contemporaneous oil. However, the cracked gas with substantial negative δ¹³C produced in the early phase completely escaped as the reservoir pressure increased; the traps concentrated only the late cracked gas, which was isotopically heavier than all the SB produced in the different stages. This study provides new insights into the evolution of isotopic fractionation in ancient oil and gas systems involving oil cracking and phase transformation.