A kinetic model of stable carbon isotope ratios in gaseous hydrocarbons generated from thermal cracking of n-tetracosane and its application to the Tarim Basin

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 δ13C2 and δ13C3. The carbon isotope of gaseous hydrocarbons from n-C24 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-C24 cracking suggests it is stable at 150 – 160 °C and its cracking temperature ranges from 180 to 200 °C, consistent with geological models of crude oil cracking currently reported. With increasing degree of thermal stress, the changes in δ13C2 and δ13C3 are larger than that of δ13C1. 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 δ13C2 and δ13C3 curves to a greater extent. The geological model of n-C24 cracking to gas has been used to interpret the origin of carbon isotopic variation of natural gas in some reservoirs in the Tarim Basin.