Tracing hydrocarbon accumulation and adjustment in a thrust belt using fluid inclusions: A case study of the Cretaceous system in the Kela-2 Gas Field, Kuqa Depression

The complexity of hydrocarbon exploration in foreland thrust belts results from multiphase hydrocarbon charging, multistage thrusting, and complex pressure evolution. Based on fluid inclusion analysis, this study constrains the multiphase hydrocarbon charging history in Cretaceous strata in the Kela-2 gas field, Kuqa Depression. Paleopressure reconstruction was conducted using PVTsim modeling. By combining compositional data from fluid inclusions of varying origins and timing, the dynamic adjustment process of hydrocarbon has been systematically elucidated. Results show that both the Cretaceous Bashijiqike Formation(K₁bs) and Cretaceous Yageliemu Formation(K₁y) dry gas reservoirs in the Kela-2 gas field experienced two phases of hydrocarbon charging, consisting of blue-fluorescent oil and natural gas. The hydrocarbon accumulation process in the Kela-2 gas field can be divided into four stages. During 23 to 15 Ma, blue-fluorescent oil charged into the K₁y reservoir. The ratio of the number of quartz grains with oil inclusions therein to the total number of quartz grains exceeds 5%, indicating the presence of a paleo-oil accumulation. During 15 to 5.3 Ma, thrust faulting triggered the upward adjustment of crude oil from the K₁y to the K₁bs reservoirs. Overpressure was absent in the reservoirs at this stage. The oil inclusions from both intervals exhibit similar geochemical characteristics, with MPI derived equivalent vitrinite reflectance (Ro) values of approximately 0.8, suggesting a common-source adjustment. During 5.3 to 2.5 Ma, intensified fault activity drove the charging of high-maturity coal-derived gas. The methane δ¹³C of gas inclusions in K₁y is -29‰, lighter than that of its present-day gas reservoir (-25.7‰). Meanwhile, the current methane δ¹³C in K₁bs is -28.3‰, which is lighter than that in the current K₁y reservoir. It indicates sustained late-stage charging of high-maturity gas, with preferential migration into the K₁y reservoir. A massive gas charging and oil displacement, triggering rapid pressurization of the reservoir to a maximum pressure coefficient of 2.0. From 2.5 Ma to the present, sustained fault activity and persistent gas charging led to the formation of a dry gas reservoir (dryness coefficient >0.99) and the continued development of overpressure. Integrated fluid inclusion analysis demonstrates a distinct temporal coupling among fault reactivation, hydrocarbon charging, and overpressure evolution, which collectively governed the final accumulation and preservation of the Kela-2 gas field.