BIT INDEX FOR SOURCE AND INPUT IDENTIFICATION OF THE GAS-HYDRATE LAYER AT SH7B STATION IN SHENHU AREA, SOUTH CHINA SEA

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.