Discrepancies between different organic maturity indicators in a coal series affected by an abnormal thermal event (Viséan, Niger)

Abstract

Classical petrographical and geochemical approaches have been used to study Viséan coal seams sampled in two areas 50 km apart, in northern Niger. T max-values given by Rock-Eval® pyrolysis and individual molecular ratios determined on hopanes and steranes are in agreement with the burial history of the two sets of samples studied. The difference of maturity of the two sample sets revealed by these various parameters is consistent with their current difference in burial depth (∼ 400 m). Only small changes of these parameters with depth were observed owing to the small depth interval sampled at both sites (15- and 40-m). These variations also appear to be related to normal burial diagenesis. No maturity differences between the two sample sets are revealed by vitrinite reflectance data and atomic H/C ratios which are in the same range at both sites, but display very large variations with depth ( R v ≈ 0.60 −1.40% and (H/C) at ≈ 0.80–0.90 to 0.55). These data indicate rapid entrance into the oil window, this conclusion also being supported by microspectrofluorimetric observations showing oil production and expulsion and by the presence of pyrobitumens containing large devolatilization vacuoles in the deepest samples. The abnormally high maturity gradient could have been caused by a hydrothermal event. The present observations on coals confirm and supplement previous conclusions drawn from the study of organic materials disseminated in ZnPb-sulfide ore-bearing sedimentary rocks. The difference in sensitivity shown by the various maturity indicators is considered very likely to be due to differences in the kinetics of the transformations on which they each depend.