Compositional information for kinetic modelling and petroleum type prediction

Abstract

The process of petroleum generation can be described in terms of timing and quantity by kinetic modelling. Compositional changes which occur during maturation are usually not taken into account in these models, despite the fact that they play an important role in defining physical behaviour of the petroleum under changing subsurface conditions. With this in mind, both open and closed system pyrolysis methods were applied to a set of immature Posidonia Shale samples from the Lower Saxony Basin (NW Germany) for studying both qualitative and quantitative aspects of the petroleum generation process. With the open system non-isothermal pyrolysis method, discrete distributions of activation energies in the range of 47–53 kcal/mol and one single frequency factor were determined. A maturity sequence was established by pyrolysing aliquots of immature Posidonia Shale samples under closed system conditions for various times and temperatures. The compositional changes and the kinetics of formation of single compounds and groups in the C 1–C 35 range were then studied quantitatively using gas chromatography. The sequence was characterised by a progressive increase in both gas/oil ratio (GOR) and paraffinicity. There was only minor variation of the chain length distribution of n-alkanes, indicating a limited pool of precursors in the Posidonia Shale kerogen. Different kinds of single reaction models were used to determine the kinetic parameters for n-alkane generation. A satisfactory fit to the experimental data could not be obtained with first-order kinetics. A good fit could be made using a kinetic model of autocatalytic reactions.