Chemical and structural characteristics of gas hydrates from the Haima cold seeps in the Qiongdongnan Basin of the South China Sea

Abstract

Crystalline structure, cage occupancy, and gas composition are important gas hydrate characteristics, which can be used to calculate the gas hydrate’s stability and to estimate the energy potential. In May and October of 2015, a Guangzhou Marine Geological Survey team investigated two sites in the Haima cold seeps in the Qiongdongnan Basin, northern South China Sea. They recovered massive gas hydrate samples by conventional gravity coring. After the Shenhu Sea area and Dongsha Sea area, the Qiongdongnan Basin became a new area on the northern slope of the South China Sea where gas hydrates have been found. In order to reveal the structural and geochemical characteristics of the natural gas hydrates, samples were analyzed by micro-Raman spectroscopy and X-ray diffraction under ambient pressure and low-temperature conditions. The results indicate that the gas hydrate samples from the Haima cold seeps are typical structure I hydrates with a hydration number of 6.12–6.19. In addition, trace amounts of H2S trapped in the hydrate were identified based on its characteristic vibrational signature. The gas composition and δ13C-CH4 of the hydrate-bound gas samples were analyzed for gas-source correlation. All of the gas samples are dominated by methane with small amounts of ethane and propane and had relatively light δ13C-CH4, indicating that all of the hydrate-bound gases are mixtures of biogenic and thermogenic gas. The thermogenic gas is inferred to be mainly derived from the coal layers of the Late Miocene-Pliocene Yacheng Formation in the northern Lingshui sag.