Carbon isotopes of asphaltene-occluded hydrocarbons: A proxy for bitumen vein source identification in northwest Sichuan Basin, China

The Lower Cambrian bitumen veins in northwestern Sichuan Basin originated from Precambrian paleo-oil reservoirs, yet the severe alterations of bitumen and high maturity of potential source rocks challenge conventional oil-source correlation. In this study, we focus on analyzing the stability of carbon isotopic compositions in asphaltene-occluded hydrocarbons to reveal their preservation of original organic signatures. Asphaltenes from the solid bitumen were investigated through gold tube closed-system thermal simulation experiments (300–400 ℃). Free saturated hydrocarbons in raw samples exhibit ¹³C-enrichment (δ¹³C = −31.3‰) due to biodegradation, while thermal simulation experiments reveal progressive ¹³C-depletion of free saturated hydrocarbons (Δδ¹³C = −4.7‰) caused by kinetic isotope effects during thermal cracking. In contrast, asphaltene-occluded saturated hydrocarbons maintain exceptional δ¹³C stability (−28.5‰ ± 0.2‰) across the same stages, demonstrating complete protection by macromolecular shielding, and adsorbed hydrocarbons display intermediate behavior with moderate δ¹³C shifts, indicating partial shielding at asphaltene surfaces. Biomarker distributions (C₂₇-dominant steranes, n-alkane bimodality, and n-alk-(1)-enes) in occluded hydrocarbons probably preserve original signatures unaffected by secondary alterations. The combined δ¹³C signature (within the −26‰ to −31‰ range characteristic of shallow-water Doushantuo Formation) and biomarker evidence (consistent with Precambrian eukaryotic algal contribution) in occluded hydrocarbons conclusively identify the Ediacaran Doushantuo Formation as the bitumen source. These results demonstrate that asphaltene nanoaggregates effectively shield occluded components from secondary alterations and thermal maturation to a certain extent, providing reliable proxies for Precambrian oil-source correlation in altered reservoirs.