CO2 geosequestration [carbon capture and storage (CCS)] and enhanced oil recovery (EOR) by CO2 injection in hydrocarbon-depleted reservoirs could limit the CO2 atmospheric accumulation. In the case of CO2 capture by oxy-combustion, the main annex gas associated with CO2 is O-2. O-2 that remains in the flue gas for injection can induce the oxidation of the hydrocarbons contained in the reservoirs. The effect of O-2 must be studied in terms of benefit and/or risk for CCS or EOR. To investigate the mechanism of hydrocarbon oxidation, it is essential to analyze the distributions of the formed oxygenated compounds. That is why experiments have been performed with a model compound (n-octane) in a closed reactor under high pressure at different temperatures and with different oxygen concentrations. The product distribution suggests two pathways of n-alkane oxidation, with (i) the preservation of the aliphatic chain length of the initial n-alkane, which generates oxygenated products with the same number of carbon, and (ii) the breakdown processes of the initial n-alkane, which generates low-molecular-weight oxygenated products. The new understanding of the mechanism of n-alkane oxidation could be incorporated into the detailed kinetic model of our previous study, which is specific to the reservoir conditions.