PRACTICAL CRITERIA FOR CLASSIFYING KEROGENS: SOME OBSERVATIONS FROM PYROLYSIS-GAS CHROMATOGRAPHY

Abstract

Two pyrolysis-gas chromatography (Py-GC) techniques were used to gain a succinct perspective of kerogen composition and structure which, by comparison with (13C NMR data, appears to be representative of the kerogen structure in toto. One-step Py-GC was used to determine the chain length distribution of selected compounds in kerogen pyrolysates and, by correlation with petroleum composition, to construct a scheme for evaluating kerogen quality. This scheme indicates that, inter alia, hydrogen-deficiency is not unequivocally synonymous with inferior oil potential, and waxy crude oil can be generated from both aquatic and terrigenous organic matter. Sequential two-step Py-GC allowed kerogen composition to be further defined in terms of how petroleum precursors are distributed throughout the kerogen macro-structure (homogeneously or heterogeneously) and the ease with which these precursors are released during pyrolysis (maturation characteristics). Routinely defining kerogen types according to not only genetic potential (from Rock-Eval) but also the aforementioned criteria is therefore a step towards unifying kerogen geochemistry and petroleum geochemistry. Maturation characteristics play a crucial role in defining the nature of the oil generation window, particularly as regards assessing the relative roles played by (1) precursor depletion and (2) product cracking, in defining the upper maturity limit of oil generation.