Relations between Biogenic Component (Foraminifera) and Highly Saturated Gas Hydrates Distribution from Shenhu Area, Northern South China Sea
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摘要: 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(155~177 m)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.Abstract: 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 155 and 166 m, and between 172 and 177 m. 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 (intra-frame 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 HS-GHZs at site SH7B. Furthermore, HS-GHZs are also well correlated to abundant foraminiferal intervals at two adjacent gas hydrates obtained sites, e.g.sites SH2B and 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.
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Key words:
- foraminifera /
- saturation /
- gas hydrates /
- sediments /
- South China Sea
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图 1 南海北部神狐水合物钻探区构造位置(a)和钻孔位置(b)(据匡增桂和郭依群，2011修改)
Fig. 1. Geography and physiognomy of the gas hydrate sampling (a) and drilling locations (b) in Shenhu area, the northern South China Sea
图 2 SH7B孔含水合物层沉积物组分与水合物饱和度的关系(符号▲表示扫描电镜观察沉积物结构层位)
Fig. 2. The relationship between the biogenic components of sediments and the saturations of gas hydrates of core SH7B
图 3 SH7B孔含水合物层有孔虫丰度、碳酸钙含量与水合物饱和度相关性分析
Fig. 3. Plots of hydrate saturation vs. foraminifera abundance (a) and hydrate saturation vs. content of CaCO3 (b)
图 4 水合物饱和度高的样品1(a)与水合物饱和度低的样品2(b)中的有孔虫数量及分布(扫描电镜照片)
Fig. 4. Foraminifera abundance in high hydrate saturation sample (a) and low hydrate saturation sample (b), showing a different abundance (SEM images)
图 5 有孔虫含量不同的含水合物样品在水中的分解现象
a.岩心沉积物样品1(有孔虫丰度112个/克)；b.岩心沉积物样品1在水中未见明显水合物分解气体柱，说明水合物饱和度低；c.岩心沉积物样品2(有孔虫丰度525个/克)，有孔虫颗粒肉眼可见；d.岩心沉积物样品2在水中剧烈分解，形成明显的气体柱，说明水合物饱和度高
Fig. 5. Gas hydrate dissociation from sediments with different foraminifera contents
图 6 SH7B孔含水合物层16 835~16 840 cm有孔虫显微结构(有孔虫大部分房室未被充填)
Fig. 6. Foraminifera microstructure at the interval between 16 835-16 840 cm in core SH7B
图 7 SH7B孔含水合物层15 924~15 929 cm有孔虫房室充填物(a)及其成分分析谱图(b)
Fig. 7. Foraminifera microstructure at the interval between 15 924-15 929 cm in core SH7B, the rooms were filled (a) and the energy dispersive X-ray spectrometry of the fillings (b)
图 8 SH7B孔含水合物层沉积物主要颗粒组成和孔隙类型(原位薄片观察)
Fig. 8. SEM image of gas hydrate-bearing sediments in core SH7B
图 9 SH7B孔15 924~15 929 cm层段水合物(有孔虫房室内白色物)产状示意图
Fig. 9. Gas hydrate occurrence at the interval between 15 924-15 929 cm in the sediments of core SH7B
表 1 SH7B孔含水合物层样品有孔虫丰度与水合物饱和度的相关信息(扫描电镜)
Table 1. The relationship between gas hydrate saturation and foraminifera abundance in core SH7B
样品号 样品深度(cm) 有孔虫丰度(个/单位面积) 水合物饱和度(%) 1 15 924~15 929 26 33 2 16 835~16 840 8 4 3 17 352~17 357 17 13 
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