Geochemical Characteristics Of Sediment From The Makran Accretionary Prism And Their Implications On Hydrate Accumulation

In order to further understand the implications of sediment geochemical anomaly characteristics for gas hydrate identification,and improve the accuracy of gas hydrate identification in the Makran accretionary prism sea area,Pakistan,this paper,based on the project of "Exploration of natural gas hydrate around the Indian Ocean" undertaken by Qingdao Institute of Marine Geology,analyzes the content of major and trace elements and pore water ions in sediment samples collected from two sites in Pakistan sea area(S2 and S3 stations are located in BSR development area of Makran accretionary prism sea area).Combining with previous research results,we can speculate the source and environment of sedimentary materials,and discuss the relationship between abnormal indication characteristics of geochemical element content of pore water and natural gas hydrates.The results show that the trace elements are mainly Sr and Ba,and the contents of Cr,Ni and Zn are medium.The sediments are thought to be composed not only of authigenic marine sediments from pelagic and semi-pelagic,but also of Indus River sediments and the basalts and ultramafic rocks of Makran-Bela ophiolite complexes and rocks of the Murray ridge.The sediment is rich in organic matter,TOC content is more than 1%.The provenance background of the Makran accretionary prism mainly belongs to the island arc and the active continental margin.The chemical index of alteration(CIA)in the study area is 35.59?57.65,with an average of 48.52,which belongs to the initial weathering stage(CIA<70).The index of compositional variability(ICV)is 1.49?2.60,with an average of 1.87,indicating that the sediment has not undergone recycling process.Indicators such as V/Cr,Ni/Co and V/(V+Ni)indicate that the column sediments at the two sites are in an oxidation?sub-oxidation environment,but the V/Cr ratio tends to increase with the depth at 3.0m below the seafloor.Compared with other areas where hydrates have been found,the sediments under 3.0m below the seafloor have the trend of gradual reduction.The porous hydrate has been collected near S3 site,which shows that the deep gas source in the study area is abundant,and the methane flux from the decomposition of natural gas hydrate is high.The content of anions and cations,hydrogen and oxygen isotopes,dissolved methane,dissolved inorganic carbon(DIC)and carbon isotopes in pore water of sediment at S2 and S3 sites were measured and analyzed,the results show that the methane anaerobic oxidation reaction(AOM)exists at the S2 and S3 sites,and the SMI(sulfate-methane interface)depth is 4.6m and 4m below the seafloor,respectively.Under 210 cm,the development of AOM at S2 site is indicated by the diagram of ?(DIC+Ca2++Mg2+)-?(SO42-)and the diagram of SO42-and I-relationship.According to the variation trend of SO42-,Ba2+,DIC,?13Cv-PDB contents and alkalinity with depth,it is believed that the whole column core sediment at the S3 site has a strong AOM effect,a larger methane flux and better prospects for gas hydrate exploration.The ??113Cv-PDB content of dissolved inorganic carbon in two sites shows that the hydrocarbon gas in S2 site is mainly biogenic,and the hydrocarbon gas in S3 site is mainly thermogenic.Combined with the geological and structure and other aspects,it is considered that there are good gas hydrate accumulation conditions in the Makran accretionary prism sea area.