Organic carbon composition of marine sediments: Effect of oxygen exposure on oil generation potential

Anaerobic sedimentary conditions have traditionally been linked to the gene ration of the source rocks for petroleum formation. However, the influence of sedimentary redox conditions on the composition of freshly deposited org anic matter (OM) is not clear. We assessed the effect of in situ exposure t ime to oxic conditions on the composition of OM accumulating in different c oastal and deep-sea sediments using solid-state C-13 nuclear magnetic reson ance (NMR). C-13 NMR spectra were resolved into mixtures of mode[ component s to distinguish between alkyl carbon present in protein and nonprotein str uctures. There is an inverse relation between the length of exposure to oxi c conditions and the relative abundance of nonprotein alkyl (alkyl,) carbon , whose concentration is two orders of magnitude higher in coastal sediment s with short exposure times than in deep-sea sediments with long exposure t imes. All alkyl(NP)-rich samples contain a physically separate polymethylen e component similar in composition to algaenans and kerogens in type I oil shales. The duration of exposure to oxic conditions appears to directly inf luence the quality and oil generation potential of OM in marine shales.