Kerogen is a solid sedimentary, insoluble, organic material found in the upper crust of the Earth in dispersed form (see Durand 1980). Its properties and spontaneous evolution in time are best understood by representing each sample in the 2-D van Krevelen diagram (Fig. 1), by a point (P) whose abscissa and ordinate are respectively the ratio of O to C concentrations, and of H to C concentrations (all in number of atoms). The shallowest samples lie in the upper right side of this diagram; they are mainly composed of long aliphatic carbon chains. As time goes by, the samples are buried deeper and deeper, and their composition changes in an orderly manner: first, in the "diagenesis" phase, mostly oxygen is expelled in the form of CO, CO2 and H2O and the representative point shifts nearly horizontally to the left. Then, in "catagenesis", it rolls down towards the origin as mostly H is expelled in the form of methane, a process which breaks the aliphatic chains and allows aromatic rings to form and coalesce in clusters, which are thermodynamically more stable (see left side of figure).
This clear relationship between composition and structure explains why the location in the van Krevelen diagram roughly defines the properties of the material and, in particular, its IR spectrum. The initial properties of kerogen depend on its geographical location, so the samples are roughly classified as of lake (I), marine (II) and continental (III) origins, in order of decreasing initial H content. In Fig. 1, the corresponding evolutionary tracks lie in three roughly horizontal strips which, at low O/C values, coalesce like branches of a tree into one nearly vertical stem extending towards the origin (see, for instance, Durand 1980, p. 129). In this diagram, which was originally proposed for coals (van Krevelen 1993), the latter are scattered, according to mining depth (i.e. evolutionary stage), nearly along the same bent strip as kerogens of type III. This is illustrated graphically in Durand (1980) and Papoular et al. (1996), Fig. 4, and is not duplicated here, in Fig. 1, for the sake of clarity. The main difference between kerogens and coals is that the latter are found in the form of bulk rocks and the former in dispersed form (sand-like).
The important point for present purposes is that hundreds of kerogen samples have been analyzed thoroughly, using all available analytical techniques: thermal, thermogravimetric, IR , Raman and NMR spectroscopy, etc. To each sample corresponds a point in the van Krevelen diagram, a complete chemical composition and, in many cases, an IR absorbance spectrum with integrated absorbances given for the main bands (example in Fig. 2). This made it possible to find correlations between bands and assign each band to a chemical functional group.
Moreover, the natural evolution (and the attendant displacement along the corresponding strip in the van Krevelen diagram) could be mimicked by annealing "young" samples (from shallow mines) up to a few hundred degrees Celcius. This shows that mild heating accelerates evolution in the natural (spontaneous) direction, an important hint to the evolutionary processes in space.
All these efforts culminated in the building up of a chemical representation of the structure of kerogens as a function of their type and evolutionary state, i.e. their location in the diagram of Fig. 1. The work of Behar & Vandenbroucke (1986) is a good example of this development. Inspired by theoretical models of coal, they found that a disordered array of at least 1500 atoms of C, H, O, N and S in different configurations are necessary to describe the properties of a given kerogen. It is not possible to adequately describe the observed spontaneous and continuous changes in properties in terms of a limited number of small, specific, molecules. In order to fit the measurements on different samples, one has rather to build a random array made of a dominant carbon skeleton with functional groups of heteroatoms (H, O, N, S) attached randomly to it, then statistically tailor the number of different bondings (C-C, C=C, etc.), functional groups (C-H, C=O, etc.) and aromatic or polyaromatic rings, etc. The variety of environments of the functional groups ensures that their characteristic vibrations will blend into bands of the right width. The relative intensities of the latter will change according to the concentrations of the corresponding functional groups. This scheme accounts for the continuous set of IR spectra observed along the evolutionary track.
Behar & Vandenbroucke give numbers for the structural parameters enumerated above and for 8 representative evolutionary stages. They also give sketches of the corresponding structures, an example of which will be shown and discussed below (Fig. 4).
Copyright ESO 2001