Mechanism whereby Sediment Properties Control Gas Hydrate Accumulation in Fine-Grained Sediments

Gas hydrates are mostly concentrated in silt–clay sediments, and the sediment properties (including the particle size and mineral composition) play an important role in controlling gas hydrate accumulation. The northern part of the South China Sea (SCS) is one of the most representative areas with fine-grained gas hydrate reservoirs. In this study, the particle size and mineral composition of 597 sediment samples from gas hydrate reservoirs and adjacent layers at seven sites in the Dongsha, Shenhu, and Qiongdongnan areas, northern SCS, were analyzed, and the sediment properties of the fine-grained gas hydrate reservoirs were determined. Specific surface area and irreducible water saturation analyses were conducted to reveal the controlling significance of the sediment properties on the accumulation of gas hydrates in the fine-grained sediments. The results show that the mineral composition has a controlling effect on the gas hydrate reservoirs and nongas hydrate layers. Due to the adsorption and constraint of illite and smectite mixed layers (I/S) on the gas and fluid and the complex microstructure of I/S, the gas hydrate reservoirs have low I/S and high felsic mineral contents. The sediment particle size has a significant controlling effect on the gas hydrate saturation and morphology in the fine-grained reservoirs. Dispersed low saturation gas hydrates are mainly formed via pore filling, and they appear in the gas hydrate reservoirs dominated by >16 μm sediment particles. Visible high saturation gas hydrates are mainly formed via particle displacement, and they appear in the gas hydrate reservoirs dominated by <8 μm sediment particles. The results of this study are helpful for further revealing the law of gas hydrate enrichment in fine-grained sediments and have important significance for gas hydrate exploration and development and its environmental effects.