Study Of Carbon Isotopes Fractionation During The Process Of Nature Gas Generation

Nature gas is a kind of mixture in which light hydrocarbons are dominant, it is very difficult to acquire some important origin information because macromolecule fingerprint fossil is absent. Presence of isotope effect fetches up this regret. Important factors affecting carbon isotope value of nature gas are parent material type and maturity. Under the effect of "structure effect", carbon isotope value of coaliferous gas generated from humus is greater than that of petroliferous gas generated from sapropelic substance if they have same maturity. Popularly, carbon isotope value of nature gas from certain parent material becomes greater and greater as its maturity increases. Different functional groups in generating gas parent material have distinct isotope composition and ?Ea between 12C-12C and 13C-12C. The models describing nature gas carbon isotope distillation become more and more perfect from early static models to models based on Rayleigh expression and kinetic models. Static models can not dynamically describe nature gas evolvement course and can not forecast all kinds of complicated variety in the process of natural gas generation; and models based on Rayleigh expression incarnate carbon isotope distillation using a constant distillation gene; but kinetic models realize isotope kinetic domino offect is a function of temperature and time. But models of Cramer1 and Cramer2 have bugs: they consider different generating gas functional groups corresponding with parallel reactions have same carbon isotope constitute, this is not reasonable. In Cramer1 model it is boldly supposed that ?Ea between 12C-12C and 13C-12C of different functional groups is equal which breaches quantum chemistry theory. Cramer3 model casts off these flaws, it calculates isotope kinetic effect using actual genertated quantity of 12C1 and 13C1. Models'fitting effect is different as they are founded on distinct bases: models based on Rayleigh expression and Cramer1 model can only simulate the part in which carbon isotope value become great when temperature increases, for abnormal varieties in parties of low temperature and high temperature they fail. Cramer2 model can simulate carbon isotope value curve preferably, but fluctuate is unavoidable. Only Cramer3 model can simulate it completely, so it is the most perfect. Genesis type and maturity of nature gas can be confirmed using carbon isotope value by static models. Using kinetic models and combining basin's thermal history and embedding history we can study gas reservoir form history.