Quaternary deep-water sedimentary characteristics and their relationship with the gas hydrate accumulations in the Qiongdongnan Basin, Northwest South China Sea

Recent geological investigations and offshore drillings have revealed the great prospects of the gas hydrate resources in the Quaternary sediments of the Qiongdongnan Basin (QDNB). Pore-filling gas hydrates were first discovered in the sand layers during the 2019 gas hydrate drilling expedition (GMGS6) in the QDNB. Logging data, 2D/3D seismic data, cores, and grain size analysis were used to study the sedimentary facies and their relationships with the gas hydrates. It was found that the Quaternary sediments are hemipelagic deposits, mass transport deposits (MTDs), and channel-levee facies. The MTDs are characterized by vertical superposition and transverse migration. Both fracture-filling (in vein, massive, and nodular occurrences) and pore-filling (disseminated) gas hydrates were identified. The fracture-filling gas hydrates mainly occur in the muds. The pore-filling gas hydrates mainly occur in the sand layers of the channel-levee facies, which can provide reservoir spaces for the gas hydrates. It is speculated that the MTDs could be a favorable seal for the accumulation of gas hydrates. Multi-stage gas charging may have caused the multi-layer distribution of the gas hydrates. Under the premise of sufficient gas sources, the combination of channel-levee facies and overlying MTDs is conducive to the enrichment and preservation of pore-filling gas hydrates, which could be an exploration target. This study provides a reference for gas hydrate prediction and exploration in the QDNB and other prospecting areas with similar geological settings around the world.