Hydrocarbon Generation Mechanism And Carbon Isotope Fractionation Kinetics Of Crude Oil Cracking

In recent years,based on the theory of organic hydrocarbon generation and thermal simulation experiments,the research on the characteristics and evaluation of crude oil cracking gas,the degree of cracking and the pyrolysis kinetics have made good progress.At present,the analysis of the carbon isotope of pyrolysis gas and the study of hydrocarbon generation kinetics lacks the unified theory of the two to further explore the causes and accumulation of gas reservoirs.In this paper,crude oil samples from Tarim and Zhongyuan areas were selected for autoclave continuous thermal simulation experiments.The mechanism of hydrocarbon cracking,the main factors of cracking and the kinetics of carbon isotope fractionation in pyrolysis experiments were discussed and studied.Firstly,based on the results of characterization analysis,the mechanism of cracking hydrocarbon generation in crude oil in two regions was explored,and the differences in hydrocarbon generation mechanism were compared and discussed.Among them,the content of saturated hydrocarbons in Fuyang crude oil was higher,and the content of aromatic compounds and heteroatom compounds was higher.Relatively few,the inhibitory effect on the pyrolysis reaction is relatively weak,so the cracking reaction occurs at a relatively low temperature,and the higher molecular saturated hydrocarbon content of the C₂₀ or higher of the Puyang crude oil leads to a relatively prolonged process of deep pyrolysis.It also led to the difference in the inflection point of the two crude oils.Secondly,the analysis results of carbon isotope of pyrolysis gas show that the distribution of?¹³C of C_2-5 hydrocarbons in crude oil cracking gas in Tarim region is-40.5‰?-10.5‰,and the distribution of?¹³C of methane is-53.3‰?-27.4‰.The distribution of?¹³C of C_2-5hydrocarbons in crude oil cracking gas products in Puyang area is-36.3‰?-11.4‰,and the distribution of?¹³C of methane is-44.8‰?-33.6‰.With the increase of pyrolysis temperature,the evolution law of carbon isotope conforms to the temperature-order effect.?¹³C has different degrees of lightening and then becoming heavy,and the temperature inflection point is different.Due to some differences in hydrocarbon generation mechanism,the initial total carbon isotope difference and geological environment and formation process have affected the carbon isotope variation of cracked gas during crude oil cracking.Finally,the isotope fractionation kinetics model of Cramer3 and Tang was established,and the kinetic parameters of crude oil isotope fractionation were calculated.The difference of pyrolysis mechanism of crude oil was also proved to some extent.At the same time,according to the isotope fractionation kinetic model,the calculated values and the carbon isotope experimental values are well fitted.The results provide a comparative quantitative measure for the difference in methane carbon isotope fractionation of crude oil cracking gas in different regions.The research results also provide an important theoretical basis for the formation,enrichment and evaluation of large gas fields in the Tarim and Central Plains,and the study of hydrocarbon accumulation mechanisms.