Early Diagenesis Records And Geochemical Characteristics Of Gas Hydrate In The South China Sea

Gas hydrate is becoming one of the most exciting research fields in the world within three decades as its great gas resources. According to the known gas hydrate reserves, the South China Sea (SCS) has the most favourable settings for gas hydrate to occur around offshore of the South China Sea. Based on the geological background and conditions for gas hydrate formation, such as high sedimentation rate, thick sediments, distinctive geological structure in these area, it has indicated that Taixinan Basin, Pearl River Mouth Basin, Xisha Trough, Qiongdongnan Basin and Dongsha Area in the South China Sea are very probable for gas hydrate reserves.In this paper, the marine core sediments of site T1 in Qiongdongnan Basin, site T2 in Jianfengbei Basin, sites CG10, HD196A and HD319 in Taixinan Basin, and other sediments of sites TVG-1 and TVG-11 in Jiulong Methane Reef were collected. We have researched the early diagenesis and the geochemistry characteristics systematically of the gas hydrate in the South China Sea by analyzing the authigenic mineral compositions, chemical compositions in pore water, content of organic carbon, sedimentation rate, methane concentrations in the headspace of sediments, hydrocarbon compounds and so on. Finally, in the light of some indicators inferred from the above methods and some results collected from the former research, some prospective gas hydrates are briefly pointed out in the northern slope of South China Sea, especially in Taixinan Basin and Qiongdongnan Basin. The main achievements are giver as follows:1. Complicated authigenic minerals were identified by XRD and SEM, such as miscellaneous carbonates, sulphates and frambiodal pyrite in the sediment samples from Qiongdongnan Basin, Taixinan Basin and Jianfengbei Basin. These authigenic minerals consistently indicate the gas hydrate to occur in the area. The assemblage and fabric characters of carbonates are similar to what being found in cold-seep sediments, which is thought to be related with microorganisms fueled by dissolved methane, especially found in Jiulong Carbonate Reef. 2. The cold-seep evidences have been found in TVG-1 and TVG-11 samples from the authigenic minerals assemblage and microstructure tectonics in Jiulong Methane Reef. Their high value of CH₄/Ba²⁺ values are very similar to the cold-seeps in Gulf of Mexico and Black Sea. The microstructure in the sediments of Methane Reef shows that carbonate deposition is related with microorganisms. The sample TVG-1 was formed earlier than the TVG-11, because aragonite has been found in TVG-11, and aragonite had transformed into calcite in TVG-1.3. Results of chemical compositions analysis in pore water of sediment samples from Qiongdongnan Basin and Taixinan Basin, show that the concentrations of SO₄^2- decrease sharply in pore water, and the depths of SMI (Sulfate-methane interface) are shallow. The concentrations of Ca²⁺,Mg²⁺,Sr²⁺ decrease clearly, and the ratios of Mg²⁺/Ca²⁺,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.4. Abundant organic carbon content in Taixinan Basin sediments, special in the site HD196A, which is over 1% in most of its sediments. That indicates that there are enough gas resources to form gas hydrate. The deposition rate in the Taixinan Basin is fast, ranges from 16.7 to 20.8 cm/ka since the Late Pleistocene. Methane concentration in the headspace of sediments increases sharply in the bottom of core samplem, which indicates that there is positively a hydrocarbon reserviors underneath the sampling site. Therefore, Taixinan Basin, special the site HD196A, is a positive area for gas hydrate formation.5. The main hydrocarbon compounds in the core sediments of HD196A from Taixinan Basin are: n-alkanes, isoprenoid, branched alkanes, polycyclic aromatic hydrocarbons (PAHs), etc. Based on the distribution curve of n-alkanes, range of carbon chains, the position of their main peaks and the value of CPI and OEP, it is believed that organic matter in the sediments was a mixture of terrestrial plants, marine phytoplankton, algae and bacteria, but dominated by terrestrial higher plants. Squalene was identified in sample, which show that biological methane synthesizing and bacteria function were occurrence in this area. The ubiquity of phenanthrene series, chrysene series compounds, perylene and retene compounds in the sediment indicate that PAHs came from terrestrial higher plants. Abundance of perylene designates fast accumulation of terrestrial organic matter and reductive condition. The methylation and methyl reset of PAHs indicate that the organisms in the sediment were higher therm-evolvement and were matured in this area. Relative concentration of S₈ increases sharply from the middle to the bottom of the core sample, and the abnormality of SO₄^2- concentrations in pore water and methane concentrations in the headspace of sediments, which all imply that the bottom of the core is close to the sulfate-methane interface (SMI). These evidences all indicate that the researched area is rather optimistic for oil, gas and/or gas hydrate resources.6. In summary, because of thick sediments, abundance organic matters, the gas resources for gas hydrate, temperature and pressure conditions, entrapped basin and so on, Jianfengbei Basin, Qiongdongnan Basin and Taixinan Basin in the South China Sea all have the most favourable conditions for gas hydrate to occur. Overall Taixinan Basin has the best chance for gas hydrate formation, and then Qiongdongnan Basin. Gas hydrate is a very complicated reservoir system, and there is much investigation and research on gas hydrate need to be carried out.