A Numerical Study On The Control Condition Of Authigenic Mineral Formation Induced By The Methane Leakage At The Sea-Bed

Anaerobic oxidation of methane(AOM) is an important biogeochemical process in the seabed anaerobic sediments. AOM significantly regulates the balance of global methane budget, which play an important role in the global carbon and sulfur cycle. At the same time, the biogeochemical characteristics associated with AOM have also become the mark for the exploration of natural gas hydrate. Recent studies mainly concentrate on the controls of AOM in the view of biological and bioenergy, however, there is little comprehensive study considering diagenesis, mineral dissolution/precipitation and solute transport to realizethe formation of authigenic mineral under the methane seepage at the sea-bed.With supporting by the program Formation of Authigenic Minerals related to Natural Gas Hydrate. Southwest Taiwan Basinin the northern slope of the South China Sea was selected as the study area. Based on the physical and geochemical data the control conditions of the formation of the authigenic minerals underthe methane seepage at the seabed are discussed using TOUGHREACT.According to the actual physical parameters, varies scenariosare used to realize the temporal and spatial variation of the sulfate methane interface and authigenic under different methane leakage rate and hydrogeochemical conditions. The results shows that the formation of the authigenic mineralsis significantly impacted by the methane leakage rate. The greater methane leakage rate, the shallower sulfate methane interface. The AOM reaction is fiercer at the interface, calcite precipitation amount is large, and the precipitation of calcite at the interface is mainly controlled by AOM. When the methane leakage rates 20 times higher than the basic scheme(7.60×10-2 mmol cm-2a-1), aragonite starts to precipitate. When the methane leakage rate larger than this value, the critical time of aragonite precipitation is becoming earlier. Shallower precipitation position shallower, larger the precipitation amount is obtained. Methane leakage rate is the main control factor for formation of aragonite. Sulfate concentration significantly affects the precipitation of ankerite. The lower concentration of sulfate, the greater amount of ankerite precipitation. When the concentration of sulfate decreased by a factor of 3 times, the precipitation amount of ankerite increases to more than 3 orders of magnitude. The calcium concentration is mainly affect the precipitation of calcite and magnesite, when increasing the calcium concentration to 5 times, the precipitation of calcite is 0.16% higher than that of the base-case. By decreasing the Calcium concentration by a factor of 5 times, magnesite precipitation ratio increasing by about 5 orders of magnitude. The low calcium environment is conducive to magnesite precipitation. The influence of temperature on authigenic mineral mainly in the forms of the reaction rate and equilibrium constant. By raising temperature, the dissolved amount of oligoclase, k-feldspar, smectite-Na, smectite-Ca, chlorite increase, however, the precipitation amount of calcite, magnesite and pyrite increase. The higher temperature, the higher precipitation amount of magnesite and ankerite.In summary, from the comprehensive view of biological function, diagenesis and solute transport to study the formation of authigenic minerals in the background of methane seepage. The formation of varies carbonates and sulfides under methane seepage indicate different sedimentary environments. Therefore, the mineral assemblage is specific for certain methane seepage and combined with the mineral assemblage in sediment can better indicate the possible existence of gas hydrate at the bottom in the actual work.