Cross-Stratal Hydrocarbon Vertical Redistribution in Foreland Thrust Belts: A Fluid Inclusion Molecular Geochemical Perspective

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.