First indications of the formation of kerogen amorphous fractions by selective preservation. Role of non-hydrolysable macromolecular constituents of Eubacterial cell walls

Two marine kerogens supposed to contain an important bacterial component were analysed and their spectroscopic and pyrolytic features compared with those of the amorphous non-hydrolysable macromolecular materials (bacterans) isolated from extent bacteria. Examination by Transmissio Electron Microscopy revealed that Dontrien (Lower Toarcian Shales, Paris Basin) is essentially composed of amorphous organic matter and OSB3 (Marl Slate Formation, N.E. England) is exclusively amorphous. The formation upon kerogen pyrolysis of series of products (3-methylalkanes, dimethylalkanes, ethylesters and alcohols) characteristic of bacteran pyrolysates is consistent with the contribution of bacterans to the formation of these two kerogens. The nature and distribution of the acids of the pyrolysates indicate that bacterial biomass was implicated in OSB3 and Dontrien kerogen formation via two pathways: bacteran selective preservation and incorporation of partly degraded lipids. Comparison between Dontrien and OSB3 indicates a weaker contribution of the selective preservation pathway to OSB3 kerogen whereas bacteria lipid incorporation may have been more significant in OSB3 kerogen. The selective preservation pathway was so far considered as restricted to the formation of kerogens having retained some of the morphological features of the source organisms. An important point derived from this study is therefore the ability of this pathway to also contribute to the formation of amorphous kerogens.