Multistage hydrocarbon remigration is common in foreland-basin thrust belts, but its timing and pathways remain poorly constrained. We investigated the Lower Cretaceous Yageliemu and Bashijiqike reservoirs in the Kela-2 gas field, Kuqa foreland basin. Molecular geochemical analyses of reservoir extracts and oil inclusions, combined with fluid-inclusion observations, were used to reconstruct the hydrocarbon charging and remigration history. Reservoir extracts and oil inclusions from both reservoirs show consistent molecular signatures, indicating a common origin from the Triassic Huangshanjie lacustrine source rocks. Fluid-inclusion thermometry combined with burial-history reconstruction indicates that lacustrine oil charged the deeper K₁y reservoir earlier than the shallower K₁bs reservoir, supporting a later phase of upward cross-stratal remigration associated with progressive fault reactivation. From approximately 5.3 Ma onward, large-scale coal-derived gas charging increasingly modified the earlier oil accumulations, and after ca. 2.5 Ma of sustained gas charging, the fully developed Kela-2 anticlinal trap became the dominant accumulation process.

The Dibei area in the northern Kuqa foreland basin is an important target for hydrocarbon exploration in deep tight sandstone reservoirs. The anticline and slope belts in Dibei area record markedly different hydrocarbon properties and accumulation histories, and the origin of this contrast remains unclear. We integrated oil and gas geochemistry, fluid inclusion petrography and microthermometry, and burial and thermal history modeling to reconstruct hydrocarbon charging in the Jurassic Ahe Formation. The integrated geochemical data are consistent with qualitative contributions from Triassic lacustrine and Jurassic coal-measure source rocks to the reservoir oils, although their relative contributions cannot be reliably quantified. Molecular and carbon-isotope data indicate a predominantly coal-derived origin for the natural gas, with the Yangxia Formation considered the most plausible principal source. The anticline belt records two episodes of oil charging at approximately 21 and 15 Ma, represented by oil inclusions with yellow-green and blue fluorescence, respectively, followed by gas charging at approximately 6 Ma. In contrast, the slope belt preserves only the later oil charge at approximately 15 Ma and subsequent gas charging at approximately 9 Ma. These differences attributed to the timing of source rock maturation, fault activity, and effective trap development. Mature source rocks and early trap formation provided favorable conditions for early oil accumulation in the anticline belt. Subsequent fault activity enhanced connectivity between source rocks and reservoirs, enabling later oil charging in both belts. Gas invasion and renewed fault activity then modified and redistributed the earlier oil accumulations. These findings provide a representative example for understanding differential hydrocarbon accumulation in tight sandstone reservoirs of foreland basins.

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.