Abstract The purpose of this paper is to present a new kinetic model for the generation of hydrocarbon gas, linking isotopic fractionation and molecular compositions. Pyrolysis experiments were performed with a Type II kerogen in a confined system under isothermal and anhydrous conditions. A mathematical formalism is applied to a compositional kinetic scheme using the pyrolysis data. The experimental and numerical simulations show that the δ 13C of the hydrocarbon gas species increase and diverge in value at high maturity, and that the C 2–C 5 become more enriched in 13C than the initial kerogen. Such an experimental isotopic evolution is not observed in most geological cases, where the δ 13C of the thermogenic gas hydrocarbons tend to converge when the maturity increases. From a comparison between experimental isotopic data from closed and open systems, we propose that the two different trends—divergence vs. convergence—may be explained by taking into account the residence time of the gas in the source, for a given generation rate. Indeed, the residence time appears to be a strongly controlling factor for the isotopic and molecular genetic signatures ( δ 13C and dryness) of the thermogenic hydrocarbons. This assumption is tested by comparing modeling results with experiments and natural data using a diagram showing the difference in δ 13C of ethane and propane as a function of the C 2/C 3 ratio. Results show that the evolution trends observed in such a diagram obey a logic depending on both the maturity and the expulsion rate of hydrocarbons.