Structural modifications of kerogen during natural evolution as derived from 13C CP/MAS NMR, IR spectroscopy and Rock-Eval pyrolysis of Toarcian shales

Abstract

Four series of solvent-extracted samples of Toarcian shales from northern Germany have been analysed by 13C CP/MAS NMR, IR spectroscopy and Rock-Eval pyrolysis with respect to maturity dependent changes in kerogen structure. The four series correspond to different evolution stages characterized by average reflectance values of 0.48, 0.66, 0.88 and 1.45%. Aliphatic carbon concentrations derived from NMR and IR agree reasonably well within the maturity range of 0.66–1.45%. The obvious discrepancy in the case of immature samples may essentially be attributed to partial quenching of IR aliphatic CH intensities by oxygenated functions. The net increase of NMR aromaticities with increasing values of vitrinite reflectance is accompanied by an enrichment in methyl groups of the residual aliphatic fraction. The ratio of S 2 pyrolysis yields to IR aliphatic carbon concentrations is found to decrease with increasing maturity of the rock samples reflecting an enhanced conversion of aliphatic carbon to fixed aromatic carbon. There is some evidence that a more important fraction of reactive aliphatic carbon is transformed into fixed aromatic carbon during the natural evolution of the organic matter. The continuous decrease of aliphatic carbon content with increasing maturity is parallelled by a decrease of total organic carbon only up to R m = 0.88%. The apparent maturity independence of mean TOC values in the final interval may be due partly to the production of insoluble pyrobitumen by cracking of oil constituents and partly to the aromatization of aliphatic kerogen carbon with concomitant generation of hydrogen.