A Comparative Study On Geochemical Characteristics Of The Sediments Containing Natural Gas Hydrates Of Different Genesis In The South China Sea

Natural gas hydrate(NGH)is a new type of clean energy.There are many areas in the South China Sea,such as Qiongdongnan,Shenhu,Dongsha and Southwestern Taiwan,has been proved to have great potential of natural gas hydrate resources.Gas sources of NGH can be divided into thermogenic gas,biogenic gas and mixed gas,and all of these gas have been found in the South China Sea.There are also some differences in the process of NGH accumulation and consumption,which may lead to differences in the geochemical characteristics of hydrate deposits.At present,there are few studies on the geochemical characteristics of the deposits of gas hydrate,especially for the deposits of gas hydrate of different genesis,there is a lack of comparative analysis on their geochemical characteristics,therefore,it is urgent to strengthen the research in this field,so as to fully reveal the accumulation and consumption process of NGH deposits of different genesis.In this paper,four NGH stations in Qiongdongnan,Shenhu and Dongsha sea areas are selected to analyze the characteristics of decomposed gas components and stable carbon isotopes of some NGH stations,as well as the geochemical characteristics of NGH deposits at all four NGH stations.Combined with various biomarkers and related geochemical indexes,the differences in geochemical characteristics of the sediments caused by gas hydrates of different genesis are discussed.The main points are as follows:1.The organic matter input source of four stations is not the same.The TOC/TN and n-alkanes index of sediments show that W03 and W09 from Qiongdongnan and Dongsha 16 station all show mixed organic matter from sea and land.But the mother source of organic matter at Shenhu W01 station is not a typical natural source,it may have been influenced by unconventional organic carbon inputs.2.The results of gas composition and stable carbon isotopes of methane from NGH decomposition gas at W03 station in Qiongdongnan indicate that the gas source is mixed-origin gas dominated by biogas.The station has high TOC,low TS,and high porosity in some of the sample layers,which provide good conditions for biogas generation.The L/H values of n-alkanes and?n-Alk/C16 indicate that the sediments at this station are close to normal Marine sediments.High intensity UCM bulges(C12-C19 range)were found in the saturated hydrocarbons of the NGH enrichment deposits,indicating the existence of traces of microbial modification.It is speculated that the sediments at this station are suitable for the activities of methanogens,resulting in the generation of a large amount of biogas and the formation of NGH enrichment areas.3.The results of gas composition and stable carbon isotopes of methane from NGH decomposition gas at W09 station in Qiongdongnan indicate that the gas source is mixed gas with thermogenic gas as the main source.The relatively high TS and the low correlation between TOC and TS may be due to the higher methane flux in the sediments at the site.The L/H values of n-alkanes and a series of ratios of pristane(Pr)to phytane(PH)in the sediments of the NGH enrichment layer at this station indicate that there is a certain low carbon number oil-source hydrocarbon mixing in some sample layers.At the same time,the variation of n C₁₇/Pr value indicates that there is a certain microbial degradation activity in the oil-source hydrocarbon mixing horizon.There were high intensity UCM bulges in the saturated hydrocarbon chromatogram of the gas hydrate enrichment layer at the station,which was also the evidence of microbial activity.4.The composition of decomposed gas and its related gases and the stable carbon isotopes of methane at Shenhu W01 station transition from biogenic gas to thermogenic gas with the increase of sediment depth,it is indicated that thermogenic gas source exists in the deep of the station.This station also has high TS and low TOC-TS correlation,which is presumed to be related to the high natural gas flux in the sediments.The L/H values,?n-Al K/C₁₆and carbon dominance index(CPI)of the sediments in some strata at W01 station indicate that the sediments at this station may have been contaminated by deep oil-source hydrocarbon.At the same time,by calculating the Pr/Ph,Pr and Ph of the station and comparing them with hopane compounds,it was found that there was oil-source hydrocarbon mixing in the station,in addition,compared with W09 station,the source hydrocarbon of the mixed contamination was closer to the complete petroleum component.In addition,there was a strong UCM bulge in the saturated hydrocarbon chromatographic map of the station.Through comparison,it was confirmed that the UCM caused by oil-source hydrocarbon mixing and the UCM caused by microbial degradation activities existed.5.The gas composition of gas hydrates and their decomposed gases at Dongsha16 station,as well as the co-location of methane carbon,indicate that they are typical microbial gas.The elemental characteristics of sediments from this station are close to those of normal Marine sediments,and their organic geochemical indexes are in the normal range compared with those of normal sediments,and there is no obvious oil-source hydrocarbon mixing sign.