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

Open-system pyrolysis is routinely performed on immature samples in order t o 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 shares (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 degrees C under either closed- or open-system conditions. Measured generation rate Ve rsus temperature curves were analyzed assuming a distributed system of acti vation energies and a single frequency factor in each case. The resulting k inetic parameters were then used to assess bulk petroleum formation rates f or geological heating conditions. In the case of all artificially matured s amples, measured and predicted bulk petroleum formation rate vs. temperatur e curves for each maturation stage remain within the original envelope defi ned by the least mature sample, despite an upward shift of T-max temperatur es. This confirms that the reactions taking place during both the pyrolysis measurements and simulated maturation processes are the same, involving ma inly homolytic cracking. A similar pattern of measured and predicted rate c urves is reproduced by the natural maturation sequence of the Toarcian shal es in the maturity range of 0.53 to 1.44% R-r suggesting that petroleum gen eration within these natural systems also results from cracking, reactions, and therefore that petroleum generation over geological time can be reliab ly extrapolated from open-system pyrolysis of the appropriate immature samp le. By contrast, significant deviations are observed for the natural coal s eries, with measured and predicted rate curves extending beyond the immatur e envelopes. In accordance with the pronounced increase of frequency factor s and of protonated;aromatic carbon concentrations, this behavior is attrib uted to solid state aromatization reactions which compete with product gene ration during natural coalification. but which are not reproducible to the same extent by experimental heating. It is concluded that petroleum generat ion from vitrinitic coals over geological time cannot be reliably extrapola ted from open-system pyrolysis of low rank samples. (C) 1998 Published by E lsevier Science Ltd. All rights reserved.