Progress of Geochemical Study on Gas Hydrae Formation in Northern South China Sea
According to the release of sub-project Geochemical Study on Gas Hydrate Formation of Northern South China (No.2009CB219506), a part of project Research on the Enrichment Characteristics and Production Knowledge Base of gas Hydrae in the South China Sea (National Basic Research Program, 973, No. 2009CB219500) at Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, the scientists have made some new progress of the geochemical study on gas hydrate formation in the northern South China Sea。
Firstly, complicated authigenic minerals were identified by XRD and SEM in the study sediment samples from Qiongdongnan Basin, northern South China Sea, such as miscellaneous carbonates, sulphates and frambiodal pyrite, which consistently indicate the gas hydrate and methane seep to occur in the area. The assemblage and fabric features are similar to that being found in cold-seep sediments, which were thought to be related with microorganisms fueled by dissolved methane. Results of chemical compositions analysis in pore water of the sediment samples show that the concentrations of SO42-, Ca2+, Mg2+ and Sr2+ decrease clearly, and the ratios of Mg2+/Ca2+ and Sr2+/Ca2+ increase sharply as the depth increased. These geochemical characteristics are similar to chemical compositions abnormalities in pore water of the shallow sediments where the gas hydrate occurs in the world. Those results strongly indicate there should be gas hydrates or deep water oil (gas) reservoirs underneath.
Secondly, the geochemical characteristics of shallow sediments for HD319, HD196A and GC10 from the gas hydrate-bearing northeastern continental slope, the South China Sea were studied. Complicated authigenic minerals were identified by XRD and SEM in the study sediment samples, such as miscellaneous carbonates, sulphates and pyrite, which consistently indicate the gas hydrate and methane seep to occur in the area. Results of chemical compositions analysis in pore water of the sediment samples show that the concentrations of SO42-, Ca2+, Mg2+ and Sr2+ decrease clearly, the concentrations of CH4 and H2S increase sharply, and the ratios of Mg2+/Ca2+ and Sr2+/Ca2+ increase sharply as the depth increases. These geochemical characteristics are similar to chemical compositions abnormalities in pore water of the shallow sediments where the gas hydrate occurs in the world. There are abundant of gas sources in the research area based on the results of free hydrocarbon in the headspace gas and the distribution characteristics of chemical compositions in the core sediments, specially the concentrations of SO42-, H2S and CH4 changed sharply with the depth. Furthermore, those results strongly indicate there should be gas hydrates or deep water oil (gas) reservoirs underneath， and the sulphate-methane interface (SMI) should be shallow, about 8m below the seafloor in the area.
Thirdly, the mineralogical features and C-O isotope composition of the Authigenic carbonate build-up of sites TVG-1 and TVG-11 from Jiulong methane reef in the South China Sea were studied. XRD results confirmed that the samples were composed of miscellaneous carbonates, i.e., aragonite, Mg-calcite and dolomite, and detrital minerals were less abundant. In one of the samples, TVG-1, aragonite was replaced by calcite and the pseudomorph of the former was observed. This suggests that this sample was formed earlier than the other so that phase transformation of aragonite to calcite has occurred. SEM observation revealed that micron-scale acicular aragonite was usually associated with bio-filaments which may be extracelular polymeric substances produced by microorganisms. These observations confirm that carbonate precipitation was related to heterotrophic microbial communities fueled by hydrocarbon seepage with presently only weak activity. Stable isotope composition of the carbonates is characterized by strong 13C depletion, with a d13C value -46.22 and -52.88 ‰ PDB for TVG-1 and TVG-11 respectively, indicating that the carbon source is most probably related to anaerobic oxidation of methane (AOM). Another distinct feature of the carbonate is its enrichment of d18O, +3.19 and +4.07‰ for TVG-1 and TVG-11 respectively. 13C depletion and 18O enrichment along with the micro-fabric textural evidence, suggest that the Jiulong carbonate build-ups might have been precipitated from fluids originated from gas-hydrate decomposition, which indicated that this area is rather optimistic for gas hydrate resources.The above results were published on ACTA OCEANOLOGICAL SINICA, JOURNAL OF TROPICAL OCEANOGRAPHY and MARINE GEOLOGY & QUATERNARY GEOLOGY in 2009.