Oligomerization of fatty acids as a possible source for high molecular weight hydrocarbons and sulphur-containing compounds in sediments

Abstract

In an attempt to understand some of the possible mechanisms for the formation of high molecular weight hydrocarbons present in oils and source rocks, artificial maturation experiments have been performed with naturally-occurring fatty acids (C 16-C 26) both in the presence and absence of the clay mineral bentonite and elemental sulphur. The heating experiments, performed at 300°C for 168 h, resulted in the formation of complex mixtures of high molecular weight hydrocarbons (>C 40) and organic sulphur compounds, comprising mainly di- and trimerization products of the original fatty acids. In the presence of bentonite, and absence of elemental sulphur, relatively high proportions of di- and trimerization products were obtained with the total amounts of saturate hydrocarbons recovered being 30–40% of the weight of the original fatty acid. The presence of elemental sulphur in the reaction mixtures leads to preferential decarboxylation and little if any oligomerization. Furthermore the presence of elemental sulphur leads to the formation of series of 2,5-dialkylthiophenes and 2,4-di- n-alkylbenzo(b)thiophenes. The high molecular weight compounds produced in these experiments were analysed by high temperature gas chromatography (HTGC), direct insertion probe-mass spectrometry (DIP-MIS) and, in selected cases, gas chromatography-mass spectrometry (GC-MS). In this paper, the results of these investigations are discussed in terms of the possible origin of the high molecular weight hydrocarbons and organic sulphur compounds recently isolated from different geological materials.