Erroneous maturity assessment given by Tmax and HI Rock-Eval parameters on highly mature weathered coals

Abstract

Two sets of mature coals have been collected at outcrop on two dipping coal seams in the Alès Carboniferous coalfield (Massif Central, France). Visual examination, petrographic observations, and Rock-Eval pyrolysis studies unequivocally suggest that these coals have been weathered, the intensity of this alteration increasing progressively towards the soil. According to the reflectance (Rr) values of 1.7–1.8%, these coals have reached low volatile bituminous rank. In contrast to the reflectance, which does not exhibit any marked variation along the studied sections, T max, although classically considered as a rank parameter, increases progressively with decreasing distance from the surface. This T max increase is accompanied by a decrease in the hydrogen index (HI) and with an increase in oxygen index (OI). All these changes are undoubtedly consequential to weathering. Thus in the present case, T max must not be considered as a maturity indicator, but as an oxidation parameter like OI and the O/C atomic ratio. This conclusion is also supported by the positive correlation between T max and proportion of oxidised vitrinites (i.e. bearing oxidation marks) provided by maceral analysis. This peculiar behaviour of T max in response to the weathering of mature coals is in total contrast with its invariance during low-mature coal alteration. This finding obliges us to moderate or even to revise the conclusions of a previous work on mature coals sampled at outcrop [Int. J. Coal Geol. 44 (2000) 49] where higher T max than expected from Rr values, were supposed to have been necessarily caused by warm (and oxidizing) fluids.