Chemical and structural characteristics of gas hydrates from the Haima cold seeps in the Qiongdongnan Basin of the South China Sea. (15th September 2019)

• Record Type: Journal Article
• Authors: Fang, Yunxin; Wei, Jiangong; Lu, Hailong; Liang, Jinqiang; Lu, Jing'an; Fu, Juan; Cao, Ju
• Is Part Of: Journal of Asian earth sciences. Volume 182(2019)
• Publication Date: 2019-09-15
• Subjects: Gas hydrates -- Cold seep -- Gas source -- Raman spectroscopy -- Qiongdongnan Basin; Earth sciences -- Asia -- Periodicals; Sciences de la terre -- Asie -- Périodiques; Earth sciences; Asia; Periodicals; 555.05
• DOI: 10.1016/j.jseaes.2019.103924
• Languages: English
• ISSNs: 1367-9120

Abstract

Graphic abstract: The Qiongdongnan Basin became a new area in the South China Sea where gas hydrates were discovered. The data shows that gas hydrate from the Haima cold seeps are structure I hydrates. For the first time that H2 S is discovered to be trapped in the gas hydrate in the South China Sea. The hydrate-bound gas are dominated by light δ 13 C methane with small amounts of ethane and propane, indicating a mixture of biogenic and thermogenic gas. Raman characteristics of the gas hydrates from the Haima cold seep. (HM1=Haima-1; HM2=Haima-2). Highlights: Gas hydrates were sampled from the Haima cold seep in the South China Sea. H2S is discovered in the gas hydrates from the South China Sea for the first time. Hydrate-bound gases at the Haima cold seep are mixed by biogenic and thermogenic gas. 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 H2 S trapped in the hydrate were identified based on its characteristic vibrational signature. The gas composition and δ 13 C-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 δ 13 C-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.