Abiotic Marine Hydrate Stability In Serpentinization Area

Natural gas hydrate is a white ice crystal solid formed by natural gas molecules dominated by methane at low temperature and high pressure,which is mainly distributed in high latitude permafrost zone and continental margin sea area.Hydrate is an energy source with large reserves,clean combustion and high energy density,so 164 m³ of methane and 0.8 m³ of water can be released per cubic meter of hydrate.It is estimated that the natural gas contained in the global hydrate is about 0.2-120×10¹⁵m³.At the same time,since natural hydrates are mainly methane gas,when the environment changes,it will release a large amount of strong greenhouse gases such as methane.Moreover,this process will also have a crucial impact on global climate and environmental changes.Therefore,the investigation and study of natural gas hydrate has not only attracted scientists from various countries,but also is one of the hottest international studies in recent decades.The hydrate has been fully investigated and studied internationally.Moreover,it is necessary to determine the temperature and pressure region where gas hydrate can stably exist during the research process.In the seabed sediment layer,the region where the gas hydrate can stable exist under certain thermodynamic conditions is the low temperature and high pressure region where the gas hydrate stable zone is mainly distributed in the seabed with water depth greater than 500 m and under the seabed.In addition,the deepest position of the gas hydrate stable zone is the bottom boundary of the gas hydrate stable zone,which is also the bottom of the stable formationt of gas hydrate.Therefore,the determination of the bottom of the gas hydrate stable zone has always been an important part of the gas hydrate investigation.As a specific area under the seabed,the mineralization and distribution of seabed gas hydrate in the stable zone have a vital influence,and its depth determines the reserves of hydrate.The factors affecting the stability zone generally include geothermal gradient,water depth,seabed temperature,pore fluid,salinity and gas composition,etc.Moreover,the deepest part of the stable zone is the bottom boundary of the stable zone.However,due to the complex practical conditions,the depth of the bottom boundary of the stable zone is not very accurate,so accurate calculation of the bottom boundary of the stable zone is the premise for carrying out research on natural gas hydrate resources.Natural gas hydrate can only be formed under the conditions of low temperature,high pressure and sufficient methane supply.In nature,natural gas hydrates are mainly distributed in permafrost and continental margin waters.For example,research shows that evidence of hydrate existence or samples of hydrate have been found in the northern slope of Alaska,the Mackenzie Delta of Canada,the outer continental margin of North America's east coast,the Gulf of Mexico,the edge of the African Atlantic,the waters off the North California-Oregon coast,the Philippine Sea,and the eastern part of the South China Sea trough.Therefore,the ocean environment has been neglected by people,because it is believed that the ocean environment is not an ideal sea area for the formation of natural gas hydrates.Although the ocean environment generally meets thermodynamic conditions such as temperature and pressure for hydrate formation,the ocean environment lacks natural gas sources.With the deepening of deep-sea investigation and research,the interaction between oceanic basic and ultrabasic rocks and water can generate serpentinization to generate methane and other hydrocarbon gases,which provide sufficient gas supply for hydrate formation on the seabed in the oceanic region.Serpentinization usually occurs in geological tectonic environments such as slowly expanding mid-ocean ridges,subduction zones and continental ophiolite systems.In order to evaluate the thermodynamic conditions for the formation of oceanic serpentinized inorganic methane hydrate and the distribution characteristics of hydrate stability zones,this thesis selects three serpentinized zones with different geological environments.These three zones of action are respectively Mariana Trench forearc in the subduction zone,the Fram Strait in the North Atlantic of the ultra-slow spreading ridge and the Lost City of the slow spreading ridge.Based on measured in-situ temperature,water depth,geothermal gradient and combined with methane hydration-water-free gas three-phase equilibrium temperature and pressure conditions,this thesis also calculates the bottom of stable zone for methane hydrate formation in seabed environment of ocean region where serpentinization is developed in three different geological tectonic environments,and evaluates its hydrate development potential.And the research shows that methane hydrate may be developed in the serpentine mud volcano Mariana Trench forearc and the ultra-slow spreading ridge of the Fram Strait in the North Atlantic Ocean,and the possibility of methane hydrate formation near the Lost City vent in the Atlantic Ocean is small.However,with further research,it is shown that extensive serpentinization of oceanic crust not only generates methane,but also generates a large amount of hydrogen.Therefore,hydrogen should be considered when calculating the bottom of hydrate stable zone in the serpentinization zone for that the existence of hydrogen will affect the thermodynamic conditions for hydrate formation and the formation characteristics in serpentine mud volcanic sediments.According to the drilling data of IODP 366 conducted by serpentine mud volcano in front of the Mariana Trench forearc,the thermodynamic model of methane-hydrogen hydrate is applied to calculate the distribution characteristic depth of methane-hydrogen hydrate stable zone with different hydrogen content in the Mariana Trench.The calculation results show that the conditions for methane-hydrogen hydrate formation at stations 1493 and 1497 of three serpentine mud volcanoes in the Mariana front forearc are extremely unlikely.Moreover,1491,1492,1496 and 1498 may have methane-hydrogen hydrate development temperature and pressure and gas composition,and the lower the bottom boundary of methane-hydrogen hydrate stability zone will be with the increase of hydrogen ratio.