Distribution and characteristics of natural gas hydrates in the Shenhu Sea Area, South China Sea

Structure type, gas composition and cage occupancy are important gas hydrate characteristics which play an important role in calculating gas hydrate stability and estimating the amount of global hydrate-bound methane. China conducted the Fourth gas hydrate drilling campaign (GMGS4) in the Shenhu Sea Area (SSA) of the South China Sea in 2016, during which comprehensive Logging-While-Drilling (LWD) data and natural gas hydrate samples were obtained. The LWD data shows high electrical resistivity and high sound velocity at various depth, indicating the occurrence of gas hydrate in this area. Coexistence of structure I (sI) and structure II (sII) gas hydrates in the South China Sea was confirmed for the first time by using Raman spectra analysis. The methane cage occupancies of large cages and small cages in sI hydrate are 0.95 and 0.96, respectively. The hydration number of sI hydrate is 6.03. Gas composition analysis shows that methane (>98.7%) is the dominate gas, with small amount of heavier hydrocarbon gases (ethane: 0.006%-0.976%, propane: 0.033%-0.216%, isobutane: 0.004%-0.047%) and other non-hydrocarbon gases (carbon dioxide: <0.008%). Gas composition (C1/(C2+C3): 84-2333) and isotope composition (δ13C: −50.9‰∼-46.46‰, δD: −177.69‰∼-148.90‰) indicate hydrate-bound gas is dominated by thermogenic source, mixed with certain amount of biogenic source. Existence of sII hydrate potentially deepen the base of sI gas hydrate stability zone in the marine sediment. Coexistence of sI and sII gas hydrates increases the complexity of the gas-water-hydrate system which need to be considered when studying the gas hydrate distribution, estimating the amount of global natural hydrate-bound gases and modeling the gas production.