Key accumulation conditions and accumulation model of ultra-shallow natural gas in Lingshui 36-A area, Qiongdongnan Basin

To reveal the core mechanisms controlling the large-scale accumulation of ultra-shallow natural gas in the ultra-deep water Lingshui 36-A area of the Qiongdongnan Basin, to solve the scientific problems that are difficult to explain by traditional accumulation theory, and to provide theoretical support for the exploration and development of similar gas fields, this study takes the LS36-A ultra-shallow gas field as the research object. Based on the regional geological background, the key accumulation conditions and accumulation model were systematically investigated by adopting an integrated multidisciplinary method, including geochemical analysis, microscopic observation of reservoir characteristics, fluid inclusion homogenization measurements, seismic interpretation, and burial history simulation using PetroMod software. The results showed that the shallow gas in the gas field was of mixed origin, mainly thermogenic gas generated from the source rocks of the Yacheng Formation in the Lingshui Sag. The reservoir was composed of submarine fan sandstone of the Quaternary Ledong Formation, exhibiting high-quality reservoir properties such as high porosity and high permeability. The caprock was composed of gas hydrate-bearing strata, deep-sea mud, and mass transport deposits, which were spatially superimposed to form a composite sealing system. Conduction channels included long-distance lateral migration channels and short-distance vertical migration channels, with lateral migration contributing more significantly to gas accumulation. Analysis of key accumulation conditions showed that conduction was the prerequisite for ultra-shallow natural gas accumulation, and preservation was the fundamental factor determining the location and scale of accumulation. Through fluid inclusion homogenization temperature measurements and burial history restoration, it was determined that the gas field experienced ultra-late-stage dynamic charging since 2 Ma. A dynamic accumulation model dominated by long-distance lateral migration superimposed by short-distance vertical migration was established. It was clarified that the LS36-A gas field was a large quasi-steady secondary ultra-shallow gas reservoir exhibiting unique oil and gas migration characteristics. This study clarifies the gas source supply, migration pathways, and accumulation process of the gas field, providing an important reference for ultra-deep water and ultra-shallow oil and gas exploration worldwide.