Using natural maturation series to evaluate the utility of parallel reaction kinetics models: an investigation of Toarcian shales and Carboniferous coals, Germany

Abstract

Open-system pyrolysis is routinely performed on immature samples in order to determine the kinetic parameters of petroleum generation at both bulk and molecular levels. This study tested such predictions for Type II and Type III organic matter by making calibrations with both artificial and natural maturity sequences of Toarcian shales (Posidonia shale; 0.48–1.44% R r) and Carboniferous vitrains (0.74–2.81% R r). Both natural series showed little or no compositional variability attributable to kerogen type. Artificially matured samples were prepared by non-isothermal heating (0.7 K/min) of the least mature samples up to end temperatures between 375 and 470°C under either closed- or open-system conditions. Measured generation rate versus temperature curves were analyzed assuming a distributed system of activation energies and a single frequency factor in each case. The resulting kinetic parameters were then used to assess bulk petroleum formation rates for geological heating conditions. In the case of all artificially matured samples, measured and predicted bulk petroleum formation rate vs. temperature curves for each maturation stage remain within the original envelope defined by the least mature sample, despite an upward shift of T max temperatures. This confirms that the reactions taking place during both the pyrolysis measurements and simulated maturation processes are the same, involving mainly homolytic cracking. A similar pattern of measured and predicted rate curves is reproduced by the natural maturation sequence of the Toarcian shales in the maturity range of 0.53 to 1.44% R r suggesting that petroleum generation within these natural systems also results from cracking reactions, and therefore that petroleum generation over geological time can be reliably extrapolated from open-system pyrolysis of the appropriate immature sample. By contrast, significant deviations are observed for the natural coal series, with measured and predicted rate curves extending beyond the immature envelopes. In accordance with the pronounced increase of frequency factors and of protonated aromatic carbon concentrations, this behavior is attributed to solid state aromatization reactions which compete with product generation during natural coalification, but which are not reproducible to the same extent by experimental heating. It is concluded that petroleum generation from vitrinitic coals over geological time cannot be reliably extrapolated from open-system pyrolysis of low rank samples.