Characteristics of Fluid Inclusions and Analysis of Hydrocarbon Charging History in the Permian–Triassic Reservoir of the Shawan Sag, Junggar Basin

Abstract: The Permian–Triassic in the Shawan Sag, Junggar Basin, serves as a key reserve replacement area of the basin; however, its hydrocarbon charging history is complex due to multi-source and multi-stage charging. This study integrates inclusion analysis (including petrographic analysis, fluorescence spectroscopy, microthermometry, and gas-liquid ratio measurements) with source rock thermal evolution to systematically reconstruct the hydrocarbon charging history of the Permian–Triassic reservoirs in the Shawan Sag. Results demonstrate that two stages of hydrocarbon inclusions were developed in quartz grains and calcite cements of the Triassic Karamay Formation, with the fluorescence spectral maximum-intensity wavelength (λ_max) of 473.69–500.33 nm and 436.67–470.51 nm, respectively, and homogenization temperature (T_h) peaks of the coexisting aqueous inclusions at 90–100 ℃ and 120–130 ℃. These correspond to hydrocarbon charging events in the Early–Middle Jurassic and Early–Late Cretaceous. The hydrocarbon inclusions formed during the Cretaceous charging have a gas-liquid ratio (F_v) of approximately 4.43%–6.67%, and the paleo-pressure coefficient during accumulation, which is calculated via inclusion paleo-pressure analysis is about 1.25–1.40, indicating an overpressured environment. Three stages of hydrocarbon inclusions were identified in quartz grains, siliceous cements, and calcite cements of the Permian Upper Wuerhe Formation, with λ_max of 477.33–496.88 nm, 458.68–473.33 nm, and 436.67–455.38 nm, respectively. The T_h peaks of the coexisting aqueous inclusions are respectively 80–90 ℃, 110–120 ℃, and 140–150 ℃, corresponding to Late Triassic–Early Jurassic, Early Cretaceous, and Paleocene–Eocenecharging events, respectively. The hydrocarbon inclusions formed during the Early Cretaceous charging have a F_v of 3.08%–4.45% and the calculated reservoir pressure coefficient is 1.35–1.60. Hydrocarbon inclusions formed since the Paleogene have a F_v of 6.32% and a reservoir pressure coefficient of 1.74, indicating a strongly overpressured environment. Integrated analysis reveals three phases of hydrocarbon charging in the study area: ① Late Triassic–Jurassic: The source rocks of the Lower Wuerhe Formation entered the hydrocarbon generation window. Early hydrocarbons migrated along active faults and were initially trapped, forming inclusions within quartz grains. ② Cretaceous–early Paleogene: Source rocks reached the oil generation peak, the formation overpressure reactivated faults, enabling large-scale hydrocarbon charging, with hydrocarbon inclusions mainly trapped within quartz and calcite cementation. ③ Paleogene–present: Source rocks have entered the high- to over-mature stage, intense overpressure reactivated the faults again, high mature oil and natural gas charged into the reservoirs, with hydrocarbon inclusions predominantly trapped in calcite cements.

Keywords: Junggar Basin; Shawan Sag; Permian–Triassic; fluid inclusions; paleo-pressure; hydrocarbon charging history