Erroneous coal maturity assessment caused by low temperature oxidation.
Résumé
Previous work carried out on different outcrop coals from the French Massif Central revealed abnormally high Tmax values, which initially were attributed to high temperature oxidising fluids. This increase in Tmax, which was exclusively observed for medium to low volatile bituminous coals (Rr≥1.5%), was accompanied by a very clear exponential-like decrease of the hydrogen index (HI) together with an increase in the oxygen index (OI). With the aim of checking the possibility that this rise in Tmax could simply be caused by weathering, a few mature coal samples (Rr>1.4%) were experimentally oxidised at rather low temperatures (110, 125 and 150 °C) and for durations of up to 6.5 month. Then the oxidation residues were subjected to Rock–Eval 6 pyrolysis and to vitrinite reflectance measurements. These experiments effectively reproduce the geochemical changes displayed by the naturally altered coals, thus supporting the hypothesis that the previously studied natural alteration process could have been ultimately caused by weathering. The observed artificial oxidation path could be divided into two stages: (i) a strong decrease in HI and an increase in OI and (ii) a constant HI value and a slow rise in OI. The increase in Tmax seems related to the transition between the first and the second stages. This behaviour of HI and OI during artificial oxidation suggests that these two Rock–Eval derived indices can be practically used to detect a moderate and even a low oxidation degree. Finally, if one ignores the constancy of the vitrinite reflectance values all along the oxidation path, the decrease of HI and the concomitant increase of Tmax mimic a thermal evolution pathway.