Geochemical Heterogeneity of Fracture-Filling Vein Pyrobitumen in Tectonically Reworked Reservoirs of the Western Jiangnan–Xuefeng Uplift, South China

Vein-type pyrobitumen, widely developed along the western margin of the Jiangnan–Xuefeng Uplift, preserves important records of structural modification and post-accumulation alteration within tectonically reworked reservoirs. In this study, Rock-Eval pyrolysis, molecular geochemistry, Raman spectroscopy, X-ray diffraction (XRD), trace and rare earth element (REE) geochemistry, and fluid inclusion analyses were integrated to investigate the origin of geochemical heterogeneity and the evolution of residual hydrocarbons. The analyzed samples are uniformly overmature, with high T_max values (527–594 °C), extremely low S₁ and HI values, and poorly ordered turbostratic carbon structures composed of defect-rich aromatic domains. Despite the comparable thermal maturity, marked variations occur in n-alkane distributions, NSO fractions, asphaltene abundance, and aromatic hydrocarbon compositions. These differences are not systematically related to maturity parameters but instead appear to reflect selective retention and localized redistribution of pyrobituminous material during structural reworking. Late Indosinian–Early Yanshanian compression generated NE-trending shear fractures together with NW-oriented extensional faults, forming a fault–fracture network that served as the principal pathways for fluid circulation and localized emplacement of pyrobituminous material. Trace-element and REE signatures, together with fluid inclusion microthermometry, suggest localized fluid–rock interaction and possible hydrothermal overprinting during pyrobitumen emplacement. The combined results suggest that tectonically driven mobilization, selective compositional fractionation, and late-stage fluid–rock interaction collectively contributed to the observed compositional heterogeneity of the vein-type pyrobitumen. This study provides additional constraints on hydrocarbon redistribution and fluid evolution in structurally reworked, highly evolved petroleum reservoirs.