The thermal decomposition of sedimentary organic matter, or kerogen, within the metagenesis zone (T > 160 degreesC) leads to the formation of large amounts of late gas, mainly composed by methane. With the purpose of understanding and quantifying the mechanisms of late methane generation, artificial maturation experiments were performed in closed system on natural samples of type II and type III mature kerogens (R-0 > 1.3%, H/C < 0.65). For each experiment, mass and atomic (C, H, and 0) balances were obtained by recovering, fractionating and quantifying the entire pyrolysis effluents. These data were interpreted by means of a kinetic schema based on three bulk reactions: one corresponding to the decomposition of short chain alkylated polyaromatics and diarenylalkanes that constitute partly mature kerogens, another one related to demethylation processes, a last one based on the autohydrogenation of the remaining pyrolysis residue. Kinetic parameters derived from these experiments confirmed that late methane is generated in sedimentary basins after the conditions of peak oil generation, between 160 degreesC and up to 240 degreesC. However, differences between type II and type III kerogens were observed for the generation of methane, revealing structural disparities between these two samples. This study showed also that the formation of methane was significantly affected by the experimental conditions. Compared to closed-system experiments, the amount of methane recovered in open-system experiments was 2 to 3 times lower. Moreover, the rate of methane generation in open system was not accounted for by the kinetic model calibrated on closed-system data. These discrepancies might be related to the modes of oxygen and hydrogen consumption during the early pyrolysis stages of these mature kerogen. Thus, it is essential to consider carefully the type of experimental data used in gas generation kinetics, since they might lead to diverging predictions for geological conditions.