A new kinetic model accounting for the generation of “early” CO2 from kerogen within the Boom Clay Formation was developed and calibrated using a large set of experimental pyrolysis data. The model also was constrained by the present day temperature of the clay where no CO2 generation is assumed. The kinetic scheme involved three competitive reactions characterized by relatively low activation energies, i.e., Ea <35 kcal/mol (146.5 kJ/mol) with an associated gas potential of ca. 49 mg of CO2 per gram of organic carbon. Focusing on the fastest reaction specifically, this study showed that the earliest stage of CO2 formation is associated with the thermal decomposition of moieties containing activated carboxylic acid functions present in the thermolabile fraction of the kerogen. Considering the temperature elevation within the Boom Clay Formation that would be imposed by high-level nuclear waste, the kinetic model predicts that (i) CO2 release would start about 1 year after disposal, and might reach the limits of the clay layer after about 100 years and (ii) for each meter length of gallery, the absolute cumulative mass of CO2 generated in a radius of 50 m around the gallery might reach ca. 3 tons.