A Kinetic Model of Stable Carbon Isotope Ratios in Gaseous Hydrocarbons Generated from Thermal Cracking of ntetracosane and Its Geological Significance

Key words: 
• ntetracosane /
• crude oil cracking gas /
• heavy hydrocarbon gas /
• carbon isotopic kinetics

Abstract: Based on quantitative modeling and experimental data on ntetracosane cracking to generate gaseous hydrocarbons in a confined system, we calculated the generation kinetics and carbon isotope kinetics of methane, ethane and propane and determined the main controlling factors influencing the variation of δ13C2 and δ13C3. The carbon isotope of gaseous hydrocarbons from nC24 cracking to gas are comparable to those from crude oil reported recently, and they can be used to investigate the geochemical characteristics of crude oil cracking to gas under geological conditions. The geological model of nC24 cracking suggests it is stable at 150～160℃ and its cracking temperature ranges 180℃ to 200℃, consistent with geological models of crude oil cracking currently reported. With increasing degree of thermal stress, the changes in δ13C2 and δ13C3 are larger than that of δ13C1. It is shown that isotope accumulation controlled by the recharging history of a reservoir has a considerable impact on the carbon isotope distribution of natural gas. Compared to that of cumulative gas, the carbon isotope ratio of partly accumulation gas is heavier and influences the fractionation of the δ13C2 and δ13C3 curves to a greater extent. The geological model of nC24 cracking to gas has been used to interpret the origin of carbon isotopic variation of natural gas in some reservoirs in the Tarim basin.