Experimental Simulations For Elements Geochemical Behavior Of Marine Gas Hydrates

Gas hydrate is one of the most important potential energy resources, distributing mainly in marine sediments beside continental slopes and continental perennial frost zones. On the other hand, owing to its instability and high percents of methane contents, gas hydrates is a factor inducing geological hazards and global climate changes.To understand more about gas hydrates, it is necessary to strengthen basic studies, especially in experimental simulations. Advantage of studying the environments and mechanisms of gas hydrates through laboratory modeling not only include its low costs and low risk, but also include its high theoretic importance. At present, elements geochemical data about gas hydrates are from sediments containing gas hydrates and related pore fluids. Due to the instability of gas hydrates, sampling processes are often unable to guarantee the actual situations of the gas hydrates strata. As a whole, studies on elements geochemistry during gas hydrates forming processes are relatively inadequate. Purposes of this study are to illustrate the changes in ionic concentrations and characteristics of hydrogen and oxygen isotopic fractionation during gas hydrates formation through experimental simulation.To achieve the goal, an experimental flow is designed; method of determining the element concentrations instantly through the gas hydrates forming processes is proposed. With the experimental data, factors affecting the experimental results, including temperature, pressure, vibration, filter type and sediment sizes, are in turn discussed. At the end, an optimal experimental condition is proposed.Preliminary studies of ionic concentration changes in marine water-methane system and sediment-marine water-methane system showed that, concentrations for many ions increased during gas hydrates forming period, due to the salt exclusion process. Among them, several ions (including Na~+,Sr~2+ and Cl~-)concentrations variation correlated well with the salt exclusion process. Their anomalies in porewaters might be important indicators of gas hydrates. Studies of the sediment-marine water-methane system also showed that, medium sediment size is propitious to the genesis of gas hydrates.In the marine water-methane system, concentration changes of chlorine ion, hydrogen and oxygen isotope composition were determined. Fractionation factors of H and O isotope showed that, the heavier isotopes have an aptitude to concentrate in gas hydrates. Considering the ubiquitous isotope anomalies in the sediment pore-waters rich in gas hydrates, we think that salt exclusion during gas hydrates formation is the cause for these anomalies.