Kinetics Study Of Carbon Isotope Fractionation During The Course Of Kukersite And Olylite Cracking Gas

Since natural gas mainly includes few hydrocarbon of low molecular weight, its important cause information of formation can be gained by isotope. Stable isotope measure of hydrocarbons of C1~C5 can provide very important fingerprint information, which can be used to evaluate the property and thermal maturity of the potential hydrocarbon source rock, and can be used to judge the migration path of the natural gas and study whether the natural gas was from poly sources and the history of the gas gathering and gas dissipating and so on.We can know from the experiment that when the oil cracks into natural gas, the evolvement law of alkane gas carbon isotope is coherent with that of kerogen's cracking into natural gas, that is, 1) with the increase of maturity, carbon isotope ratio of alkane gas increases; 2) with the increase of carbon number in the alkane molecule, δ13C of alkane gas increases. In addition, from the relationship chart of volume percent and temperature, we can see that methane decreases in its first period and then increases in its last period, while the development tendencies of ethane, propane and butane are reverse, which shows that with the increase of the temperature (evolvement degree), ethane, propane and butane crack secondly.There exists the regular tendency in relationship of the methane isotope evolvement and the maturity of source rock, which has been observed by many geochemists. These geochemists build the static relationship model of methane isotope and kerogen vitrinite reflectivity, which takes very important effect for the study of natural gas. In the late ten years, with the development of thermal simulation experiment, especially with the study of hydrocarbon generation kinetics, many kinetics models about methane isotope evolution have been built, including Berner model, Rooney model, Lorant model, Cramer model and Tang model and so on, which all have their own merits and drawbacks. This paper based on hydrocarbon generation kinetics and thermal cracking experiment, according to the data fitting result of kukersite and olylite thermal cracking gas, we gain their kinetic parameters of alkane gas, and then build the isotope fractionation kinetic model based on the relative theories and gain the kinetic parameters. Compared with Berner model, this model has the following merits: 1) from the simulation result we know that this model not only simulates the tendency of carbon isotope evolvement in the middle temperature part, but also simulates the complex change of low temperature and high temperature; 2) this model considers the influence of temperature to isotope fractionation effect; 3) this model divides the generated alkane gas into the gas which includes 13C and the gas which includes I2C, and then gains their kinetic parameters respectively.