Chemical composition of the precursor compounds and mechanisms of humic substances formation at the post-sedimentation stage of the organic compounds evolution

Abstract

The publication is a review that presents in a concise form information about the chemical composition of living matter components and the mechanisms of their transformations, the result of which is the geopolymers formation. Among geopolymers, humic substances, including humic and fulvic acids, attract our attention. The relevance of this review lies in the importance of understanding multidirectional reactions, the result of which is the secondary polymerization of organic matter chemically active components that have passed the biodegradation barrier at the stage of sedimentation and diagenetic transformations. Humic substances, in their turn, are precursors of kerogen, therefore, an understanding of reaction mechanisms and their products provides complete information about the conditions of various types of kerogen formation, which are characterized by different ability to produce oil and gas. We paid special attention to polyphenols, which have high chemical activity and the ability to react with increasing molecular weight. In addition to the traditional Maillard reaction, among the condensation mechanisms we considered oxidative crosslinking of phenols, oxidative condensation of polyunsaturated fatty acids, and esterification of fatty acids with phenols. For each mechanism, the conditions for its implementation and probable contribution to the formation of humic substances are briefly considered. Analysis of probable mechanisms of formation of humic substances showed that condensation reactions can occur under geochemical conditions of sedimentation and early diagenesis. At the same time, their speeds are low, and the precursors necessary for the reactions, formed as a result of biological degradation, are contained in very small concentrations. We conclude that kerogen contains two components – primary, which enters its structure without any significant changes, and secondary, which is the result of a series of complex multidirectional reactions.