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Kerogens and Coals
Kerogen and Coal structures
Kerogen is defined as the fraction of sedimentary organic matter that is insoluble in good solvents like toluene. A large range of possible compositions can be found, depending on the origin of the organic matter (algae, bacteria, higher plants) , the conditions of preservation (anoxicity, pH) and thermal maturity. The latter refers to the progressive loss of labile functional groups , then the breaking of alkyl chains, as a result of pyrolysis reactions in the natural environment (heating during burial) or in oil shale pyrolysis.
The kerogen model units presented in this web page (175 to 260 carbon atoms) have been built in the MedeA®environment from the sole consideration of the elemental analysis and functional group analysis (Ungerer, P., J. Collell and M. Yiannourakou, Molecular modeling of the volumetric and thermodynamic properties of kerogen : influence of organic type and maturity. Energy & Fuels, 2014).
In this work it was shown for the first time that quantitative predictions of standard thermodynamic properties could be obtained from quantum mechanics and volumetric properties from molecular dynamics. The heat capacity in the ideal gas state is predicted to increase as a function of temperature as obtained from quantum mechanics at the semi-empirical level on an isolated kerogen unit (MOPAC -PM7). Standard heats of formation can be predicted as well. Kerogen density trends with thermal maturity and organic type are found in good agreement with experimental data, but this requires simulation of several model units in a simulation box with periodic boundary conditions.
In the following list of kerogen model units, the roman number (I, II, III) refers to the organic type in the sense of petroleum geochemistry, and the letter (A, B, C, D) refers to successive maturity stages. H/C and O/C are atomic ratios.
kerogen I-A: kerogen of lacustrine (algal) origin
- immature with respect to hydrocarbon generation ( H/C = 1.53 ; O/C = 0.05).
- Excellent potential for oil generation.
kerogen II-A: organic matter deposited in anoxic marine shales
- immature with respect to hydrocarbon generation ( H/C = 1.17 ; O/C = 0.10).
- Good potential for oil generation.
kerogen II-B: organic matter deposited in anoxic marine shales
- beginning of oil generation ( H/C = 1.12 ; O/C = 0.06).
- Still fair potential for oil generation.
kerogen II-C: organic matter deposited in anoxic marine shales
- middle/end of oil generation ( H/C = 0.90 ; O/C = 0.05).
- Most of the potential for oil generation is exhausted, significant potential for gas generation.
kerogen II-D: organic matter deposited in anoxic marine shales
- overmature with respect to hydrocarbon generation ( H/C = 0.58 ; O/C = 0.05).
- No potential for oil generation, limited potential for gas generation.
kerogen III-A: organic matter from higher plants deposited in post-jurassic coals and related shales
- having lost much of the oxygen-bearing function but still immature with respect to hydrocarbon generation ( H/C = 0.88 ; O/C = 0.12).
- Good potential for oil generation.
Note that the simulation of the total organic matter (including generated molecules like hydrocarbons, gas and water) requires simulation of several kerogen units and associated molecules in a large simulation box with periodic boundary conditions ( Collell, J., P. Ungerer, G. Galliero, M. Yiannourakou, F. Montel, M. Pujol, Molecular simulation of bulk organic matter in type II shales in the Middle of the oil formation zone. Energy & Fuels, 2014)
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