Origin and Source Correlation of Permian Hydrocarbons in the Kekeya Area, Southwestern Tarim Basin, China

The Southwestern Depression of the Tarim Basin has considerable petroleum exploration potential, and several hydrocarbon accumulations have been discovered in Permian reservoirs of the Kekeya area. However, the sources of Permian crude oils and the genetic types of natural gases remain debated because multiple potential source rocks and multi-source hydrocarbon charging occur in this area. This study integrates GC-MS analyses of crude oils and source-rock extracts with biomarker parameters, bulk and fraction carbon-isotope data, compound-specific carbon-isotope data for n-alkanes, and natural-gas composition and carbon-isotope data to characterize Permian hydrocarbons and constrain oil sources and gas genesis. Methylphenanthrene index (MPI) estimates indicate equivalent vitrinite reflectance (Rc) values of 0.61–1.00% for Permian oils. For natural gases, methane-isotope-derived Rc estimates are dependent on the genetic-type equation applied; sensitivity calculations using the oil-type and coal-derived equations yield ranges of 0.59–1.81% and 0.37–1.14%, respectively. GC-MS results show that Permian oils contain a complete C19–C24 tricyclic terpane series, characterized by a C19TT < C20TT < C21TT distribution pattern with C21TT as the dominant peak, together with abundant C24TeT, C30 rearranged hopane, and relatively high gammacerane contents. These features resemble those of Permian source rocks but differ markedly from Carboniferous source rocks (C19TT > C20TT > C21TT). The C27–C28–C29 regular steranes display an asymmetric shallow V-shaped pattern. Saturated hydrocarbon biomarker parameters (Pr/nC17, Ph/nC18, Ga, and C30H) and aromatic fluorene-series compositions (SF-OF-F) further indicate similar hydrocarbon precursors and strong geochemical affinity between Permian oils and the Pusige Formation source rocks in terms of organic input and depositional environment. Bulk oil δ13C values (−31.2‰ to −26.8‰) and compound-specific δ13C values of n-alkanes (−41.4‰ to −30.7‰) indicate mixed marine/lacustrine algal and terrestrial higher-plant contributions, supporting the Pusige Formation as the main oil source and distinguishing it from Carboniferous source rocks. Gas compositions and carbon isotopes show that gases in the Permian reservoirs are predominantly thermogenic and mainly represent primary kerogen-cracking gas, although some samples record higher maturity, mixed origins, and carbon isotope reversal. These results improve the understanding of hydrocarbon sources and gas genesis in complex petroleum systems of the Southwestern Depression and provide geochemical constraints for exploration in similar foreland basins.