Enrichment mechanisms and accumulation model of ultra-deep water and ultra-shallow gas: A case study of Lingshui 36-1 gas field in Qiongdongnan Basin, South China Sea

Based on petroleum exploration and new progress of oil and gas geology study in the Qiongdongnan Basin, combined with seismic, logging, drilling, core, sidewall coring, geochemistry data, a systematic study is conducted on the source, reservoir-cap conditions, trap types, migration and accumulation characteristics, enrichment mechanisms, and reservoir formation models of ultra-deep water and ultra-shallow natural gas, taking the Lingshui 36-1 gas field as an example. (1) The genetic types of the ultra-deep water and ultra-shallow natural gas in the Qiongdongnan Basin include thermogenic gas and biogenic gas, and dominated by thermogenic gas. (2) The reservoirs are mainly composed of the Quaternary deep-water submarine fan sandstone. (3) The types of cap rocks include deep-sea mudstone, mass transport deposits mudstone, and hydrate-bearing formations. (4) The types of traps are mainly lithological, and also include structural- lithological traps. (5) The migration channels include vertical transport channels such as faults, gas chimneys, fracture zones, and lateral transport layers such as large sand bodies and unconformity surfaces, forming a single or composite transport framework. A new natural gas accumulation model is proposed for ultra-deep water and ultra-shallow layers, that is, dual source hydrocarbon supply, gas chimney and submarine fan composite migration, deep-sea mudstone-mass transport deposits mudstone-hydrate-bearing strata ternary sealing, late dynamic accumulation, and large-scale enrichment at ridges. The new understanding obtained from the research has reference and enlightening significance for the next step of deepwater and ultra-shallow layers, as well as oil and gas exploration in related fields or regions.