Laboratory Experiments to Determine the Origin of High Molecular Weight Hydrocarbons (>C40) in Geological Samples

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Biomarkers are compounds in geologic samples whose basic carbon skeleton resembles that of their precursors which occur in source materials such as algae, bacteria and higher plants. For the most part the biomarkers discovered to date have been in the C10-C40 range. Limitations in both chromatography and mass spectrometry in the past, have prevented the successful investigation of biomarkers in the region above C40. In the past two years we have taken advantage in the development of high temperature capillary columns and tandem mass spectrometers to enter into the frontier area of biomarker geochemistry in the range of C40-C100. Extracts from a variety of samples have been investigated by tandem mass spectrometry in combination with gas chromatography or the direct insertion probe. Tandem mass spectrometry has been used in combination with micropyrolysis techniques to investigate the high molecular weight fragments produced by pyrolytic degradation of kerogens and asphaltenes. This work has clearly demonstrated the presence of complex mixtures of high molecular weight components in a wide variety of samples. These compounds are dominated by n-alkanes and alkylclohexanes extending to at least C100. In addition to analytical limitations, another reason why these components have not been investigated as thoroughly as components below C40 is that in many cases their low volatility precludes their presence in crude oils collected at the well head for example. Furthermore the low volatility of these components also means it is possible that they may not necessarily occur in the saturate fraction containing compounds below C40. The present goal is to investigate how these molecular weight compounds are formed. In some preliminary experiments we have shown that artificial maturation of fatty acids will lead to polymerization of these compounds to form high molecular weight alkanes, cyclohexanes and complex mixtures of alkylated naphthalenes. It is proposed to expand these experiments and perform them with a variety of lipids, as well as naturally occurring organic material such as higher plant material and algae, in the presence and absence of catalysts, such as bentonite, and at a variety of temperatures. Preliminary experiments have also been conducted in the presence of elemental sulphur and it has been shown that sulphur is incorporated into many of these reaction products. Although similar maturation experiments have been performed previously, the earlier experiments were typically designed to examine thermal degradation products and not polymerization products. The work described herein will provide us with information on the way in which these high molecular components may be produced and conditions under which these compounds will be formed.