Kerogen origin, evolution and structure

It is generally accepted that the formation of kerogens is the result of the so-called depolymerization-recondensation pathway (Tissot and Welte, 1984; Durand, 1980), Thus, naturally occurring macromolecular substances such as polysaccharides and proteins are enzymatically depolymerized to oligo- and monomers, which for the most part are mineralized. However, a small part of them are thought to condense with other substances such as low-molecular-weight lipids in a random way (Fig. 1). During diagenesis, the “geopolymers” thus formed continuously undergo chemical transformations by which they become more and more insoluble and resistant. Kerogens are being formed, which—depending on the nature of the original organic matter contributions—can generate various amounts and sorts of oil under thermal stress.

A Review of Macromolecular Organic Compounds That Comprise Living Organisms and Their Role in Kerogen, Coal, and Petroleum Formation

Pyrolysis of immature Torbanite and of the resistant biopolymer (PRB A) isolated from extant alga botryococcus braunii. Mechanism of formation and structure of Torbanite

Extensive knowledge about refractory biomacromolecules has been accumulating for 20 years. The components, characterized by a conspicuous resistance to drastic base and acid laboratory hydrolyses, also exhibit a relatively high resistance to degradation under natural conditions. These refractory biomacromolecules, identified in vascular plants and microalgae, probably play a major role in living organism protection, and they are thought to be important for organic matter sources, composition, and turnover in soils and sediments as well. In addition, some refractory macromolecules are formed in natural environments as the result of condensation/aromatization processes. This review is concerned with six families of refractory macromolecules: lignins, sporopollenins, aliphatic macromolecules (algaenans, cutans, suberans), tannins, black carbon, and proteins. The origin and composition of each family and its contribution to organic matter in soils and sediments are discussed, focusing on recent advances and on questions that are still pending.

A review of some important families of refractory macromolecules: composition, origin, and fate in soils and sediments

A review of our current understanding of resistant biomacromolecules derived from present and past algae and higher plants is presented. Insight in the nature of recent and fossil macromolecules is strongly hampered by the difficulties in obtaining the material in pure and unaltered form. For the extant material, avoiding artificial condensation and structural alteration as a result of chemical isolation and purification of biomacromolecules requires constant attention. To date, several types of sporopollenin seem to occur. One type is characterised by oxygenated aromatic building blocks, in particular p-coumaric acid and ferrulic acid. The other type is thought to consist predominantly of an aliphatic biopolymer. In this review it is concluded that extant sporopollenin consists of the aromatic type, whereas the aliphatic component of fossil sporopollenin is due to early-diagenetic oxidative polymerization of unsaturated lipids. The cuticles of most higher plants contain the aliphatic biopolyester cutin. Additionally, cuticles of drought-adapted, mostly CAM plants, seem to contain the non-hydrolysable aliphatic biopolymer cutan. Only a very few algae are able to biosynthesize resistant, (fossilisable) cell walls: some Chlorophyta, Eustigmatophyta and Prasinophyta produce the aliphatic biopolymer algaenan. Some Dinophyta are also capable of producing algaenan cell walls. Additionally, some taxa produce highly resistant cyst-walls with a high proportion of aromatic moieties. For the morphologically well-preserved fossil material, contamination by organic particles other than the target taxon is hard to eliminate and can contribute to either the aliphatic or aromatic signal. Furthermore, post-mortem migration of aliphatic moieties into, and their condensation onto the macromolecule might occur, e.g. by oxidative polymerization. These phenomena hamper the evaluation of the aliphatic signature of fossil plant material and may for example explain the preservation of initially cutin-based cuticles or non-algaenan containing algae. The extent to which migration and in situ formation of aromatic moieties plays a role in modifying resistant algal macromolecules, notably under elevated temperature and/or pressure conditions, still remains an open question.

/pdf/biomacromolecules-of-algae-and-plants-and-their-fossil-2t3gg3cuui.pdf

Metzger S. Peracca

Papers

The resistant polymer of the walls of the hydrocarbon-rich alga Botryococcus braunii☆