Gas inclusion carbon isotopes and calcite U–Pb geochronology constrain multistage gas charging and vertical redistribution in the Wuqia thrust belt, SW Tarim Basin

Foreland thrust belts commonly host vertically stacked gas accumulations in multiple stratigraphic intervals, yet the charging chronology, source relationships, and three-dimensional accumulation processes of deep and shallow gases remain difficult to constrain precisely. The Wuqia structural belt in the southwestern Tarim Basin contains deep Carboniferous–Permian carbonate gas reservoirs and shallow Cretaceous Kezilesu Group clastic gas reservoirs, providing a representative setting for investigating multistage gas charging and three-dimensional accumulation. This study integrates petrographic observations, fluid-inclusion petrography, calcite U–Pb dating, bulk-crushing carbon-isotope analysis of gas inclusions, and geochemical data from present-day natural gases to reconstruct the charging stages and redistribution history of deep and shallow gas systems. Calcite veins, bitumen–pyrite associations, and fluorite occur in the deep carbonate reservoirs and, together with H 2 S in natural gas and thiadiamondoids in condensate, indicate limited thermochemical sulfate reduction (TSR) of early liquid hydrocarbons. Calcite veins hosting primary gas inclusions in wells AT101 and AS1 yield U–Pb ages of 35 ± 16 Ma and 33.5 ± 8.6 Ma, respectively, providing direct constraints on early deep-gas charging. The younger AS1 calcite ages of 17.3 ± 5.1 Ma and 4.51 ± 0.34 Ma record later calcite precipitation and fluid activity rather than independent gas-charging events, with the latter providing a maximum age for secondary gas-inclusion trapping. Calcite U–Pb ages, combined with fluid-inclusion microthermometry and burial–thermal histories, support a multistage deep-gas charging history. In the shallow Cretaceous reservoirs, coeval aqueous inclusions in detrital quartz have homogenization temperatures mainly of 80°C–150 °C, and matching these temperatures with burial–thermal histories suggests late gas charging during the Miocene–Pliocene interval. Deep gas inclusions have δ 13 C 1 values of −43.8‰ to −31.4‰, markedly more negative than those of present-day reservoir gases (−31.3‰ to −26.2‰), suggesting that the inclusions record relatively lower-maturity early gas, whereas the present reservoirs contain late highly mature to overmature gas. Geochemical comparisons show consistent source affinities between deep gas from Well AS1 and shallow gas from the Akemomu gas field. The earlier charging of deep reservoirs, later charging of shallow reservoirs, shared source characteristics, and vertical transmissibility of thrust faults collectively suggest that part of the shallow gas was derived from late vertical redistribution and reaccumulation of pre-existing deep gas. The Wuqia structural belt records limited early charging of light oil or condensate with local TSR modification, multistage deep gas charging beginning in the Oligocene, and late Miocene upward redistribution of part of the deep gas into shallow Cretaceous reservoirs, ultimately forming a vertically stacked, multilevel gas accumulation system.