Geochemical Characteristics And Numerical Modeling Of Pore Water In The Gas Hydrate-bearing Zone Of The Northern South China Sea

It is a hot topic to study the theory of oil and gas accumulation by means of hydrogeological and isotope hydro-geochemical theories.China's natural gas reserves are abundant.However the cost of conventional gas extraction is high.So it is necessary to carry out unconventional exploration of oil and gas resources.Gas hydrate is a highly efficient clean new energy,attracting the attention of various countries in recent years.Marine sediment is the main reservoir of gas hydrate.It exists only in low temperature and high pressure conditions.One of the important research fields is methane,as well as its gas source.Methane is one of the products due to decomposition of organic matter,and its solubility in porewater of marine sediments can be obtained thermodynamically.According to the methane-water phase diagram,when the dissolved methane exceeds its solubility,solid hydrate will accumulate in so-called hydrate stability zone.In order to further explore the controlling factors of natural gas hydrates and clarify the indices for hydrate exploration,this dissertation aims to improve gas hydrate theories on the basis of previous studies,focusing on the geochemical characteristics of sedimentary porewater in hydrate-bearing areas and introducing hydrological methods(i.e.solute transport models and numerical simulation)to research on reactive transport processes of chemical components in porewater.The model developed in this dissertation consists principally of two parts:fluid transport and component reactions.The fluid transport mainly involves the processes of advection and diffusion,which are explained in detail respectively.The component reactions are mostly related to early diagenesis that a series of microbial geochemical processes take place,including organic matter decomposition with different terminal electron acceptors,which can be quantified by chemical kinetics.The general reactive transport equation is deduced and its parameter sensitivity is also analyzed.Certainly in practical applications,the general model needs to be revised in order to provide a reliable foundation for the exploration and evaluation of gas hydrate resources.When used in hydrate formation in drilling areas,the concept of saturation is necessary and porewater salinity is represented by chloride concentration.When used for porewater reactive processes in shallow sediments,it is necessary to model dissolved inorganic carbon and its stable carbon isotope,tracing methane of different gas sources.Finally,the measured data(including deep drilling cores and/or gravity piston columns)of the continental slope of the northern part of the South China Sea exemplify the application of our l-D reactive transport model,with which we explore the mineralization characteristics of the natural gas hydrate in the South China Sea in a quantitative manner.For the Shenhu drilling area,the advection velocity of porewater during the Holocene was estimated by the molar ratio of bromine to iodine,and a non-steady state multiphase fluid transport model was constructed.We found that the hydrate accumulation in the study area,i.e.Site SH7,was in decline since the Pleistocene.For the Dongsha prospecting area,our stable isotope reactive transport model shows that Site CL11 is more developed than Site CL7 in fluid flow rate and the biological activity.For a comparison of the continental slopes of the South China Sea,our model revealed that,for the same geological and hydrogeological settings,the quantitative contrast between the rate of methane production and external deep fluids affects the gas source of hydrates.Due to the openness of fluid,which causes endmember mixing,coupled with delayed response of porewater,the deep reaction may not be sufficient to,through fluid advection,affect shallow sediments.