Thermal analysis has been used to determine the impact of heating on the decomposition reaction of two Moroccan oil shales between ambient temperature and 500°C. During pyrolysis of raw oil shale, the residual organic matter (residual carbon) obtained for both shales depends on the heating rate (5 to 40°C min-1). Three stages characterize the overall process: the concentration of carbonaceous residue decreases with increase of heating rate, become stable around 12°C min-1 and continue to decrease at higher heating rates. Activation energies were determined using the Coats-Redfern method. Results show a change in the reaction mechanism at around 350°C. Below this temperature, the activation energy was 41.3 kJ mol-1 for the decomposition of Timahdit, and 40.5 kJ mol-1 for Tarfaya shale. Above this temperature the respective values are 64.3 and 61.3 kJ mol-1. The reactivity of Timahdit and Tarfaya oil shale residual carbon prepared at 12°C min-1 was subject to a dynamic air atmosphere to determine their thermal behaviour. Residual carbon obtained from Tarfaya oil shale is shown to be more reactive than that obtained from Timahdit oil shale.