Mechanism of differential hydrocarbon accumulation in multi-stage channels of the H5 gas reservoir in the N gas field, Xihu Sag

【Objective】 The Xihu Sag is one of the most important petroliferous sags in the East China Sea Basin, and the Huagang Formation is the main exploration and development interval in this sag. Recent exploration and development of large-scale reversed anticlines in the Xihu Sag have revealed significant differences in hydrocarbon accumulation among multi-stage channels within the same gas field. The characteristics of natural gas charging, migration pathways, and accumulation mechanisms remain unclear, which restricts further exploration and development deployment in the deep-sag area.【Methods】This paper takes the two-stage channels of the H5 main producing interval in the N gas field as the research object. Based on reservoir characteristics, natural gas geochemical analysis, and systematic comparison of multiple geochemical tracing indicators (δ¹³C₁, ⁴⁰Ar/³⁶Ar, ³He/⁴He, light/heavy hydrocarbon ratio, saturated/aromatic ratio, adamantane A/D and 1MA/2MA, etc.), the accumulation conditions and processes of the two-stage channels are analyzed.【Results】The results show that: (1) Natural gas in both channels is coal-type gas, sharing the same single source and one-stage filling characteristics, indicating that gas source and filling stage are not the primary factors controlling differential accumulation. (2) Regional and inter-well geochemical tracers consistently indicate that natural gas migrated vertically along source-connected faults into the H5 layer and then laterally from west to east along the channel sand bodies. (3) Affected by differences in reservoir physical properties and fault blocking, the variation rates of all tracers in the early Channel 1 are significantly larger than those in the late Channel 2, reflecting stronger migration-induced fractionation and lower charging efficiency in Channel 1.【Conclusion】Accordingly, a hydrocarbon accumulation model characterized by “one-stage same source, vertical drainage, lateral migration along channels, and differential enrichment” is established. This model indicates that late-stage high-quality channels are favorable zones for natural gas enrichment in the deep-sag area, and low-lying traps formed by fault barriers also have exploration potential. The research results provide a theoretical basis for studying differential accumulation in multi-stage channels in the deep-sag area of the Xihu Sag.