Experiments were conducted on bulk Boom Clay and Boom Clay kerogen concentrate in order to study bitumen formation during air oxidation. Samples were collected in the HADES underground facility of Mol (Belgium) during excavation of the Praclay gallery. Rock–Eval and elemental analyses show that the organic matter is immature and primarily of type III. Air oxidation experiments were carried out during 62, 114 and 139 days at 130 °C. These experimental conditions were used to simulate natural oxidation occurring during the excavation and operation of an underground disposal facility. Molecular and spectroscopic characterizations show that during air oxidation, the kerogen is enriched in oxygenated aromatics while it releases an aliphatic rich bitumen. The clay minerals, in their turn, catalyze the air oxidation of the released bitumen constituents (e.g. n-alkanes). The consequence is that air oxidation of Boom Clay leads to the release of oxygen functionalized organic matter likely to be mobilized by water.