【摘要】 天然气水合物是近年来国际上发现的一种新型能源,大量赋存在海底沉积物中。西沙海槽位于南海北部陆坡区,周边有多个大型深水油气田区。对该区地形地貌、地质构造和沉积条件分析以及地球物理BSR分布表明,西沙海槽是我国海洋天然气水合物资源勘查的一个有利远景区。文章主要研究了位于西沙海槽最大BSR区内的XS-01站位沉积物孔隙水的地球化学特征,发现该站位孔隙水阴阳离子浓度和微量元素组成特征变化显示出可能与天然气水合物有关的明显地球化学异常,与国际上己发现有天然气水合物地区的异常相类似。因此,认为该站位是西沙海槽区最有利的天然气水合物赋存区,值得进一步的勘查工作。更多还原

【Abstract】 Gas hydrate is a new source of energy found in recent years mostly from marine sediments.In recent years,we have conducted gas hydrate exploration in the South China Sea.The Xisha Trough which is located in the northern margin of the South China Sea and adjacent to several large oil and gas fields is one of the promising target areas for gas hydrate.The Xisha Trough extends for 420km long with water depths of 1500m in the west part and 3400m in the east part of the trough and has thick sediments deposited with an organic matter content of 0.41%～1.02%.Previous studies on topographical features,geological P-T conditions,structure geology,sedimentary geology and geophysical bottom simulating reflectors（BSR） in the Xisha Trough suggest that this area is a favorable place for the formation and accumulation of gas hydrate.In this paper,we presented geochemical analyses for the sediment and pore water from a piston core at Site XS-01 in the Xisha Trough.Seven pore water samples have been analyzed for their anion(Cl^-,SO^2-₄,Br^-,I^-) contents,cation（Na,K,Ca,Mg） contents and trace element（Li,B,Sr,Ba,Rb,Mn） contents.Eight sediment samples have been analyzed for stable carbon and oxygen isotopic compositions.A number of geochemical anomalies such as anions(e.g.,Cl^-,SO^2-₄),cations（e.g.,Ca,Mg） and trace elements（e.g.,Sr,Ba,B） were found in this study.For example,the concentrations of Cl^- and SO^2-₄ in pore water show a decrease trend with depth.The estimated sulfate-methane interface（SMI） is very shallow of only 18m,which is quite similar to the SMI value of 23m in the ODP 164 Leg 997 at Blake Ridge.The pore water concentrations of Ca,Mg,and Sr also decrease with depth,but concentration of Ba,and Mg/Ca and Sr/Ca ratios increase with depth.These geochemical anomalies are quite similar to those found in gas hydrate locations in the world such as the Blake Ridge and may have related to the formation and dissociation of gas hydrates.The salt exclusion effect during the gas hydrate formation will cause an increase in major ion concentrations in the pore waters that diffused upward such as Cl.The Anaerobic Methane Oxidation（AMO） may lead to change of SO^2-₄ and other cations such as Ca,Mg,Sr and Ba in pore water.It is suggested that low δ¹³C value of authigenic carbonates is an efficient indicator for gas hydrate occurrence.However,the bulk sediment samples we analyzed all show normal δ¹³C values similar to biogenic marine carbonates,and this may either suggest that no gas hydrate-related authigenic carbonates exist or their amounts are too low to be detectable using this bulk analytical methods.In conclusion,we suggest that the XS-01 Site in the Xisha Trough of northern margin of the South China Sea is a potential area for further gas hydrate exploration.更多还原