The main purpose of the present study is to evaluate the nature and amount of gaseous compounds that would be generated from the Boom Clay kerogen due to the foreseen thermal stress associated with the geological disposal of high activity nuclear waste. To this end, pyrolysis experiments were carried out on this low maturity, O-rich kerogen with focus on mild conditions, including Rock–Eval and closed pyrolyses using a wide range of temperature/time conditions. The residual kerogen recovered after the closed pyrolyses was re-examined by Rock–Eval and elemental analyses and the components of the gas fractions were identified and quantified by gas chromatography. These experiments showed substantial production of CO2 (corresponding to ca. 1/5 of the total O content of the kerogen) under mild thermal stress. The kinetic parameters (frequency factor and distribution of activation energy) of this early production of CO2 were determined and used to simulate the possible consequences for the deep disposal of highly radioactive waste. Extrapolation to thermal stresses, corresponding to 80 and 100 °C over 1 ka, indicated that this production of CO2 might influence the geochemistry and perhaps therefore the effectiveness of the geological barrier. For example, unless diffusion out of the heated zone counterbalances the effect of CO2 generation, significant acidification and large changes in bicarbonate concentration may take place, in the interstitial water of the clay, at a time scale of only tens to a few hundred years.