南海北部神狐东南海域沉积物孔隙水地球化学特征及其对天然气水合物的指示

PORE WATER GEOCHEMISTRY IN SHALLOW SEDIMENTS FROM SOUTHEASTERN SHENHU AREA OF NORTHERN SOUTH CHINA SEA AND THEIR IMPLICATIONS FOR GAS HYDRATE OCCURRENCE

SH-CL13、SH-CL16与SH-CL17站位位于南海北部神狐东南海域BSR发育区内。地球化学分析结果显示,SH-CL16与SH-CL17柱状样孔隙水中的氯离子(Cl^-)浓度及氢同位素(δD)值分别随深度明显降低和升高,指示下伏沉积物可能发育水合物。3个站位的浅表层沉积物甲烷通量很低,甲烷通量的大小控制了SMI的深浅和硫酸盐通量。孔隙水SO_4^(2-)浓度变化趋势及δ^(13)C_(DIC)值表明,在浅表层沉积物中硫酸盐消耗均由有机质硫酸盐还原作用(OSR)所控制,甲烷缺氧氧化作用(AOM)发生在较深的层位。综合地球化学和地球物理研究成果,3个站位位于水合物有利发育区内,由此推测神狐东南海域可能发育扩散型水合物,具有良好的水合物勘探前景。

Proper conditions occur in the southeastern Shenhu area of northern South China Sea for gas hydrate accumulation.The geophysical signature of gas hydrate,i.e.the bottom simulating reflector（BSR）,has been discovered.Site SH-CL13,SH-CL16 and SH-CL17 are within the area of BSR.Clconcentrations andδD values of pore water samples from gravity-piston cores of SH-CL16 and SHCL17 significantly decrease and increase with depth,respectively,which suggests gas hydrate accumulation underneath.The contents of methane in headspace gas samples from the three sites vary between10.0 and 37.7μL/kg,indicating very low methane fluxes in shallow sediments.The depths of sulfatemethane interface（SMI）are between 28.5 and 39.5 m,and sulfate（or methane）fluxes change between 3.6and 5.9mmol·m^（-2）·a^（-1）.As we know,the depth of SMI and sulfate fluxes are constrained by methane fluxes.The trend of SO_4^（2-） variations and δ^（13）C_（DIC） values of pore water indicates that consumption of sulfate is dominated by organoclastic sulfate reduction（OSR）in shallow sediments and anaerobic oxidation of methane（AOM）occurs in relatively deep layers.In summary,the three sites are within the potential areas of gas hydrate occurrence.