南海神狐海域有孔虫与高饱和度水合物的储存关系

Relations between Biogenic Component (Foraminifera) and Highly Saturated Gas Hydrates Distribution from Shenhu Area,Northern South China Sea

通过对神狐海域沉积物组分与水合物成藏关系的研究,得到SH7B孔含水合物层（155~177m）有孔虫丰度以及有孔虫壳体微结构与水合物饱和度的关系.结果表明,有孔虫丰度与水合物饱和度有良好的对应关系,有孔虫丰度高,水合物饱和度也高;反之亦然.有孔虫丰度与水合物饱和度二者的相关系数为0.72,说明有孔虫与水合物的分布和富集有关.扫描电镜研究表明,有孔虫成岩作用不明显,有孔虫为有效孔隙,有孔虫独特的壳体结构增加了沉积物的孔隙空间,有利于水合物的储存和富集.大部分有孔虫壳体大小相当于砂粒级,它的存在一方面增加沉积物粗组分砂的含量,另一方面增加沉积物的孔隙度.沉积物中生物组分——有孔虫,是南海神狐海域水合物富集的重要因素之一.

Based on studies of sedimentary features in the sediment interval containing high-saturation gas hydrate（HS-GHZ）in the Shenhu area,this paper presents results of Shenhu gas hydrates drilling site SH7B,at which HS-GHZ is observed at two major sediment intervals：between 155and 166m,and between 172and 177m.Our results suggest that the fine-grained sediments are foraminifera-contained clayey silt and silt;with very few siliceous microfossils.Two sediment intervals with high abundance of foraminifera coincide with two intervals of HS-GHZs,with high correlation coefficient up to 72%.Scanning electron microscope analysis on microstructure of sediments shows that existing of abundant sand-sized foramineral shells makes obvious contribution to the increase of sand fraction in these sediment intervals and shell-room space of foraminifers（intraframe pore space）are significantly larger than pore space between silt-sized grains（inter-particle pore space）.Only a few numbers of foraminiferal shell-rooms are filled with fine silt-or clay-sized grains or pyrites,suggesting slight influence of post-sedimentary diagenesis on foraminifera shells.In addition,shell-room spaces of foraminifers are generally in round-form with smooth pore-walls,which surely increases volume-containing capability in comparison with irregular and same diameter-sized inter-particle pore space.These evidences suggested that abundant foraminifers do not only increase in porosity of fine-grained sediments,but also provide high volume-containing capability of pore spaces for accumulating of gas hydrate in those two HSGHZs at site SH7B.Furthermore,HS-GHZs are also well correlated to abundant foraminiferal intervals at two adjacent gas hydrates obtained sites,e.g.sites SH2Band SH3B,in the Shenhu area.All of these results lead to the assumption and explanation that a large portion of gas hydrates should be stored in shell-room spaces of abundant foraminifers in fine-grained sediment layers in Shenhu area,which are different from the results obtained by ODP gas hydrate drillings;and existence of abundant foraminifers is an important sedimentary factor for controlling accumulation and storage of high saturation of gas hydrates in Shenhu area.