Kinetic Study Of Hydrogen Isotopic Fractionation During The Generation Of Natural Gas And Its Application

Two kinds of simulation experiments were designed, one is gas generation experiment with samples of source rock, crude oil and model compounds with different functional groups under anhydrous condition, the other is gas generation from n-C₉18)H₃₈ cracking under different aqueous medium conditions. Based on the two kinds of experiments, the mechanism and influential factors of hydrogen isotopic fractionation during the generation of natural gas were investigated, and the participation mechanism and behaviors of aqueous medium were especially investigated. Hydrogen isotopic fractionation model based on chemical kinetics was established, calibrated and then extrapolated to geological settings.During the generation of natural gas, the hydrogen isotopic composition has a tendence of becoming heavier in general, and becoming lighter in the low or high temperature periods, it aslo has a normal sequence distribution in accordance with geological settings. These features are similar to those of carbon isotopic fractionation. The fact that hydrogen isotopic composition becoming lighter in the low or high temperature periods is mainly caused by the heterogeneity of the parent material and the complexity of gas generation mechanism, which can lead to a reverse of the hydrogen isotopic sequence to some extent. During the high temperature period, the cracking of heavy hydrocarbon gas (C_2-5) has an obvious effect on the becoming lighter of its hydrogen isotopic composition.The n-C₁₈H₃₈ cracking experiments with different aqueous amounts suggest that, the plentiful formation water can increase the generation quantity of natural gas (hydrocarbon gas, H₂ and CO₂), especially during the high maturity stage. As the hydrogen and oxygen sources, the formation water can provide hydrogen and oxygen free radicals which have contribution to the pyrolysis reaction, at the same time more H₂ will be produced through the combination of hydrogen free radical. With the gas quantity increasing, the formation water will prolong the mature process of organic matter. In addition, the hydrogen isotopic composition of the hydrocarbon gas generated during hydrous experiment is obvious lighter, especially for that of ethane, compared with that of anhydrous experiment. The difference of hydrogen isotopic composition curves is more obvious than that of yield curves under different aqueous amounts, which indicates as an geological index the hydrogen isotopic index of natural gas is sensitive, and also suggests that the impact of hydrogen from formation water during the gas generation process will be more reflected through the hydrogen isotope composition of natural gas. Compared with water amount, the quality of aqueous medium has less effect on the yields, components and hydrogen isotopic composition.Besides the effects of the hydrogen isotopic composition and thermal maturation of parent matter, the sediment enviroment (aqueous mediums enviroment) has an important influence on the hydrogen isotopic composition of natural gas. As the formation water widely exists in geological situation, the effect of the formation wate must be taken into account when the hydrogen isotopic fractionation during the gas generation process is used to investigate the gas migration and accumualtion history.The free radical reaction is the main mechanism of hydrocarbon generation from organic matter. The participation of the formation water goes on through the hydrolysis disproportionation of organic matter during the evolvement process of organic matter. On the one hand, the water-derived hydrogen (hydrogen free radical) can be combinated with the hydrocarbon (alkene and alkane) free radical generated during the thermal maturation process of organic matter. On the other hand, organic matter can be oxidated by formation water following the hydrocarbon-alcohol-ketone-carboxylic acid-CO₂ sequence.The consistency of carbon and hydrogen isotopic fractionation mechanisms determines that hydrogen isotopic fractionation can be described by using the carbon isotopic fractionation model. The geological extrapolation results show that a more exact and optimistic conclusion of natural gas resource assessment can be drawn if we consider the effect of formation water, and more abundant and reliable geological information can be obtained through the combination applications of carbon and hydrogen isotopic indexes.