Mechanisms of hydrocarbon formation during flash pyrolysis of kerogen

Abstract

Geomacromolecules, which account for by far the greatest part of organic matter in sediments, are, due to their physical properties and complex structures, not easily accessible for common spectroscopic methods to eludicate their structure. Therefore, thermal and chemical degradation techniques are often applied to obtain structural information on these substances. Flash pyrolysis (Py) in combination with gas chromatography (GC) and mass spectrometry (MS) has proven to be a useful method of characterizing geomacromolecules (e.g. kerogen) at a molecular level. Upon flash pyrolysis of kerogen mixtures of nalkanes, n-alk- I-enes, saturated and aromatic (poly)cyclic hydrocarbons are commonly generated. However, the mechanisms by which these compounds are formed are not completely known. This is surprising since a full understanding of the thermal degradation processes is essential for interpretation of the compound distributions with respect to the macromolecular structure. Therefore, we have flash pyrolysed silicon-bound hydrocarbons, which serve as model compounds for hydrocarbon moieties in kerogen. Although these experiments gave some insight into the thermal degradation of hydrocarbon moieties. the major pyrolysis products were formed by processes involving the 0-Si and Si-C bonds and are thus not really representative of hydrocarbon moieties in kerogen. In this paper we report the C₀-C₄ alkylbenzene distributions in flash pyrolysates of two kerogens and flash pyrolysis experiments with new model compounds (sodium salts of fatty acids) aimed at the interpretation of the alkylbenzene distributions of the kerogen pyrolysates. CONCLUSION Alkylbenzene distributions in kerogen pyrolysates have a potential to give information on biological contributions and palaeoenvironmental conditions. However, much more work aimed at the understanding of the thermal processes responsible for their formation has to be performed before we can fully decode the information contained in pyrolysis product distributions. Experiments with model compounds are essential in this respect but our preliminary investigations are rather confusing and warrant further investigation.