The structure of kerogen and related materials. A review of recent progress and future trends

Abstract

Unravelling the chemical constitution of kerogen, asphaltenes, coal and humic substances is the most challenging objective in molecular organic geochemistry. Some compositional constraints are obtained from elemental analysis and from the determination of functional groups or the degree of aromaticity by IR or NMR spectroscopy. Pyrolysis of kerogen or related materials yields small structural units, some of which may be representative of moieties originally present in the macromolecules whereas others may have been formed by secondary reactions. In any case, there is no information on the mode of connection of the various structural units among each other. Chemical degradation of kerogen and related materials so far commonly involved the application of strongly oxidizing reagents (e.g. KMnO 4), but also was done by reductive cleavage (hydrogenolysis). Although a variety of methods has been used over the years, much of the work lacked adequate detailed analysis of the reaction products and/or the reactions were not sufficiently specific to allow a reconstruction of larger structural entities. Nevertheless, a combination of elemental analysis, spectroscopic information and chemical data from pyrolysis, bitumen composition in natural rocks and sometimes chemical degradation were used to develop constitutional models of kerogens or asphaltenes of different origins and different levels of thermal evolution. The models at least were internally consistent with respect to the data set used, and although a fair amount of “chemophantasy” may have been incorporated, they certainly stimulated further work. Recently, specific chemical degradation reactions like ether cleavage with boron trichloride followed by lithium aluminium deuteride reduction of the halides or oxidation with ruthenium tetroxide have shown how certain low-molecular-weight products are linked to kerogen or asphaltene macromolecules. Further reactions of this type will have to be looked for. Any attempt to achieve substantial progress in kerogen structure elucidation will require the concerted efforts of (1) consecutive specific chemical degradation reactions, (2) the detailed quantitative molecular analysis of small degradation products, (3) a study of the degradation residues by spectroscopy and pyrolysis, and (4) the application of refined concepts of the preservation of biological macromolecules (e.g. aliphatic biopolymers).