BIT INDEX FOR SOURCE AND INPUT IDENTIFICATION OF THE GAS-HYDRATE LAYER AT SH7B STATION IN SHENHU AREA, SOUTH CHINA SEA
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摘要: 实际的天然气水合物明显不同于冷泉碳酸盐岩样品。南海已有诸多冷泉碳酸盐岩的研究,针对具有水合物实际产出的神狐海域沉积物样品开展了GDGTs分析测试研究,试图为其成因、来源、AOM作用、生源等提供信息。研究结果表明:SH7B站位96.5~225.2 m的岩心样品中,传统指标总有机碳的δ13C值和C/N值对南海神狐海域天然气水合物赋存段的沉积物之生源输入指示效果不明显;GDGTs主要以类异戊二烯GDGTs为主,水合物赋存段(155~177 m)较非水合物赋存段,支链GDGTs相对百分含量明显增多,类异戊二烯GDGTs相对百分含量明显减少。BIT值进一步明确说明:水合物赋存段比非水合物赋存段,沉积物的陆源有机质输入明显增多。前人研究表明~6 Ma左右南海北部接受了较强的河流碎屑和季风碎屑输入,此次事件造成水合物赋存段(6.0~6.74 Ma)陆源有机质输入增多,结果导致沉积物颗粒粒度增大,孔隙空间增多。较粗颗粒物的大量注入可以改善储层物性,形成上细下粗的有利储集体系,从而为神狐海域天然气水合物提供良好储集空间。Abstract: Gas hydrate is very different from cold seep carbonate. There have been a lot of researches on cold seep carbonate in the South China Sea. This paper is devoted to the GDGTs of the coring samples from the Shenhu area and its implication for the origin, biogenetic source and AOM of gas hydrates. The results show that traditional indicators, such as TOC, δ13C value and C/N ratio, are not effective to the analysis the source of the cores from the SH7B station. The GDGTs of core samples from the interval of 96.5-225.2 m of SH7B station is mainly dominated by isoprenoid GDGTs. Relative content of branched GDGTs of sediments in gas-hydrate-bearing layer from 155-177 m increases more significantly than that in the layer without gas hydrate, while the relative percentage of isoprenoid GDGTs of the samples in gas-hydrate layer reduces significantly. BIT values further indicate that the sediments of gas-hydrate-bearing layer receive more terrestrial organic matter input than those of the non-hydrate layer. Previous studies documented that the northern South China Sea received a great amount of river deposits and monsoon input about 6Ma and led to the increase in terrestrial organic matter input in the sediments of hydrate bearing layer (6.0~6.74 Ma), as well as the increase in grain size and pore space. The input of coarser deposits may improve the reservoir property and form a favorable reservoir system in a coarsening upwards order, and thus provide a better reservoir for gas-hydrates accumulation in the Shenhu area.
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Key words:
- Shenhu area /
- BIT index /
- source /
- gas hydrate /
- terrigenous clastic input
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