Laboratory experimentation on whole-rock Rock-Eval pyrolysis has shown that the characterization of organic matter through the use of a modified van Krevelen-type diagram, where the hydrogen and oxygen indices are substituted for the H/C and O/C ratios, is questionable. The hydrogen and oxygen indices are strongly affected by the matrix mineralogy and by organic enrichment.

Others have assumed that the S₃ peak on the Rock-Eval represents CO₂ liberated solely from the organic matter because the pyrolysis temperature of 390°C, at which trapping of CO₂ ceases, is significantly below the decomposition temperatures for the principal matrix minerals. However, Rock-Eval pyrolysis of pure mineral specimens with decomposition temperatures in excess of 800°C has produced CO₂. Published data suggest that this yield of inorganic CO₂ may be partly a consequence of crystallographic imperfections. Because the oxygen index is calculated relative to the organic carbon content, the leaner the rocks the greater the error will be resulting from interference of inorganic CO₂.

The hydrocarbon yield (S₂) was found also to be dependent on the mineral matrix. Each kerogen type produced greater hydrocarbon yields when associated with a carbonate matrix as compared to an argillaceous matrix. These differences were found to be greater in leaner rocks. In addition, the hydrocarbon yield did not appear to increase proportionately with increasing organic carbon content. Hydrocarbon yields relative to organic carbon content were found to be greater in richer rocks than in leaner ones. Thus, the hydrogen index for a given rock appears to depend not only on the type of organic matter, but also on concentration of the organic matter and the character of the mineral matrix.

It appears, therefore, that although there are advantages to the van Krevelen diagram for tracing evolution pathways as organic matter matures, the complications discussed above require that an alternative method be developed for evaluating pyrolysis data.

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