Origin of compositional differences in organic matter abundance and oil potential of cherty and clayey Cenomanian black levels in the Umbria-Marche basin (Italy)

Rock-Eval pyrolysis of a large set of Cenomanian samples, collected from the black levels (clayey, cherty and mixed) in three sections of the Umbria-Marche basin, showed large differences in organic matter (OM) quantity and quality. The chert samples systematically exhibit much lower TOC contents, markedly lower HI and higher OI. This reflects the extensive oxidative destruction of the initial kerogen that occurred upon the chertification of some clayey sediments. A comparative study, by a combination of microscopic, spectroscopic and pyrolytic methods, was performed on kerogens of the chert and clay layers of a representative mixed level. The various fractions of the initial kerogen underwent differential destruction or alteration during chertification, resulting in (i) relative enrichments of microfossils and woody debris although lignin was altered by demethoxylation and (ii) extensive destruction of the amorphous fraction while it remained predominant. The amorphous fraction retained in the chert kerogen showed large changes in composition related to oxygen incorporation and probably escaped complete destruction owing to oxidative reticulation. The above features account for the pronounced systematic differences in OM abundance and oil potential between the chert and clay layers in the black levels.