Maturity-related variation in the bulk-transformation kinetics of a suite of compositionally related New Zealand coals

Bulk HI transformation kinetics were determined by open-system pyrolysis for a suite of compositionally related Late Cretaceous to Paleocene coals from Tara-1 well, Great South Basin, New Zealand. These coals span a maturity range encompassing the onset of significant oil generation and expulsion. Kinetic parameters were also obtained for one of the immature samples following artificial maturation by hydrous pyrolysis. Kinetic models were based on three modes of optimisation of kinetic parameters: discrete-E-act distribution with single variable A value; discrete-E-act distribution with fixed A (10(14) s(-1)); and Gaussian-E-act distribution with single variable A value. HI transformation-rate vs. temperature envelopes for the naturally matured coals exhibited an abrupt jump to higher temperature near Rank(S-t) 12.5, apparently coinciding with the onset of significant paraffinic oil expulsion, for all optimisation modes. The artificially matured sample does not predict this shift in transformation envelopes, which suggests that certain of the reactions that occur under natural maturation do not occur during laboratory pyrolysis. The structural rearrangements occurring up to the expulsion threshold, some or all of which are responsible for the increase in HI up to Rank(S-r) similar to 11, are not manifested in changes in HI transformation-rate envelopes. However, the presence of adsorbed bitumen may mask the influence of structural rearrangements during optimisation of the kinetic data, which could explain the behaviour of the transformation-rate envelopes in the Rank(S-r) 11.0- 12.5 range obtained from the discrete-E-act free-A optimisation. This behaviour involves a regression in transformation-rate envelopes to lower temperatures as the amount of retained bitumen increases prior to expulsion. The almost perfect nesting of transformation-rate envelopes subsequent to oil expulsion suggests either that any further structural rearrangement does not produce a significant change in the A and Ea,, distributions of hydrocarbon-generating bonds, or that the expulsion drastically reduces the degree of further rearrangements involving hydrocarbon-generating moieties within the kerogen. It follows that kinetic parameters for immature coals should not be considered ideal for modelling the whole phase of hydrocarbon generation. Nevertheless, the kinetic parameters for the least mature Tara coal (Rank(S-r) similar to 8.5) reproduced the normalised HI trend for the entire suite of coals with reasonable accuracy over the maturity range considered (Rank(S-r) similar to 8.5 - 15.0), only slightly underestimating conversion at the lower maturity levels and overestimating it slightly at the higher levels. (C) 2002 Elsevier Science Ltd. All rights reserved.