Geochemical Characteristics Of Carbonate Crusts And Sediments Carbonate Minerals In The Gas Hydrates Potential Area And Their Indicative Significance In The Northern South China Sea

As a new unconventional green energy,natural gas hydrate has attracted much attention,and more than 97%of the global gas hydrate is found in continental margin marine sediments.When the temperature and pressure of gas hydrate reservoir change,hydrate will decompose and release methane.The vertical migration of methane is associated with the anaerobic oxidation of methane by sulfate(SR-AOM)in the sediment pore water,resulting in the formation of authigenic minerals such as carbonates and sulfides,and affecting their morphology and structure.Methane fluid will form cold seep carbonate with different occurrence and mineral assemblages.As the major carbon reservoir in the global carbon cycle,carbonate rocks are also the stable forms of methane carbon in the submarine seep,so the mineralogical and geochemical characteristics of authigenic minerals in the submarine crust and sediments can effectively indicate methane seep events.To sum up,the study of morphology,mineralogical and geochemical characteristics for carbonate in gas hydrate potential area,not only helps us to understand how the anaerobic oxidation of methane to affect the carbonate form structure,mineral composition and geochemical characteristics;but also to explore genetic relationships between carbonate minerals,carbonate crust.That research can also help us to identify the deep gas hydrate decomposition,and for exploration and mining of the submarine gas hydrates in Northern South China Sea.The purpose of this paper is to explore the relationship between the methane leakage activity in the potential nature gas gydrate zone and the morphological structure,mineral assemblages and geochemical characteristics of carbonate rocks.Samples of surface crust particles from MWT-4 station in the mud-volcano area of the pearl river estuary basin and sediment columnar samples from A27 and SH1 stations in the shenhu potential gas hydrate area were selected as research objects.Carbonate mineral morphology observation,microscopic section identification,XRD analysis,mineral step extraction,sediment element analysis and carbonate carbon and oxygen isotope analysis were carried out.Mainly gained the following understanding:(1)The TOC content of the sediments below the depth of 240 cmbsf at A27 and SH1 in the Shenhu Sea Area increased with the C/N>8,indicating that the organic matter at this depth was significantly controlled by terrestrial source.However,as the depth gets shallower,the C/N value decreases.Affected by topography and water depth,the C/N of SH1 is less affected by terrigenous.(2)The correlation analysis of sediment elements of two sites shows that the Mg and Ca elements are greatly affeted by terrigenous,and even negatively correlated with the land-source indicator elements Al and Ti.Combined with the characteristics of Mg/Ca values in sediments and the results of our research about the material source of sites,the sediment Mg mainly comes from clay minerals and carbonate minerals,while Ca mainly comes from Marine calcareous organisms.(3)Tube and microporous structures of carbonate crust in the surface sediments in the Northern South China Sea are effective channels for deep fluid migration.Under the influence of fluid composition,the main mineral composition of carbonate crust is high magnesium calcite,and the molar percentage of MgCO3 is up to 13.08%.However,due to the influence of hydrocarbon leakage from Liuhua oilfield near the site,the ?13C of crust shows a positive value.(4)The carbonate minerals in the Northern South China Sea mainly include calcite,aragonite,dolomite,etc.A few strata contain some ankerite,siderite and magnesite.Affected by the anaerobic oxidation of methane in the sediments,the relative contents of aragonite and dolomite increased,accompanied by the decrease of calcite MgCO3 content and the negative value of total carbonate ?13C in the sediments.And the autogenic carbonate minerals,associated with pyrite and biocarbonate shells,are in the form of white fibrous and needle-like cementation.(5)Synthesize the analysis results of carbonate mineral assemblages and pore water geochemical characteristics,the upper boundary of the sulfate methane transition zone(SMTZ)at SH1 is located at the depth of 400 cmbsf,while the upper boundary of SMTZ at A27 is located at the depth of 340 cmbsf.Furthermore,due to the weakening of SR-AOM in SMTZ,the growth of Weak combination state of inorganic carbon and the dolomite content in the sediments slowed down at the bottom of station A27.(6)The variation trend of Mg/Ca value with depth can indicate the dolomite precipitation related to methane leakage in the sediments.In addition,the increase of relative content of carbonate minerals in the sediments,along with the decrease of calcite MgCO3 content and the negative value of carbonate ?13C,can indicate the influence of methane anaerobic oxidation(SR-AOM).Therefore,the peak of Mg/Ca,aragonite and dolomite contents in the sediment sulfate methane transition zone(SMTZ),accompanied by the low value of calcite MgCO3 content and the negative value of carbonate ?13C,may indicate the decomposition of the underlying gas hydrate and further indicate the underlying natural gas hydrate reservoir.