Waxy oils from lacustrine source rocks
by Paul Farrimond
Many lacustrine-sourced oils of different ages and from different parts of the world are observed to be waxy, containing high proportions of long-chain n-alkanes, typically showing a slight predominance of odd carbon number chain lengths (Fig.1).
Petroleum geochemists tend to consider long-chain (C23 to C35) n-alkanes as being typical of higher plants (from wax coatings of leaves), and often use them as potential indicators of land plant contribution to the source rock of an oil. However, in these high-wax lacustrine-sourced oils, they are derived from oil-prone algal biomass.
Figure 1: Whole oil gas chromatogram of a lacustrine-sourced waxy oil (North Falkland Basin; Farrimond et al., 2015); Pr = pristane; Ph = phytane; n-alkanes labeled by carbon number.
Algaenan is a highly aliphatic biopolymer found in various microalgae, being particularly common in lacustrine green algae (e.g. Tetraedron, Scenedesmus, Chlorella & Botryococcus); it is not often found in marine organisms (Kodner et al., 2009). Occurring in the cell walls, algaenan contains long hydrocarbon chains joined by ether linkages, forming a recalcitrant structure that is resistant to degradation allowing selective preservation in kerogen (Tegelaar et al., 1989).
Other common geochemical characteristics of these lacustrine waxy oils include pristane/phytane ratio usually >1 and high hopane/sterane ratios – both characteristics that are also associated with high land plant input. Their bulk carbon isotopic composition can vary widely from very light to very heavy values depending on the particular source depositional environment and organic matter inputs.
Lacustrine source rocks typically contain Type I kerogens, and these have previously been shown to be often characterized by a narrow range of activation energies leading to hydrocarbon generation over a narrow maturity range (e.g. Dessort et al., 1997; Wang Min et al., 2011) – in other words, the kerogen falls apart relatively rapidly. This feature is a direct consequence of the composition of the algaenan biopolymer which can comprise a high proportion of the kerogen of such source rocks. An understanding of kerogen kinetics is essential for predicting hydrocarbon generation in basin modelling, and if actual source rock data are lacking, then oil composition can provide information on the likely source; in such cases it is essential to distinguish between long-chain n-alkanes from lacustrine algae and those from higher plants, as kerogens with these different organic matter types have very different hydrocarbon generation characteristics.
References:
Dessort D., Connan J., Derenne S. & Largeau C. (1997). Comparative studies of the kinetic parameters of various algaenans and kerogens via open-system pyrolysis. Organic Geochemistry 26, 705-720.
Farrimond P., Green A. & Williams L. (2015). Petroleum Geochemistry of the Sea Lion Field, North Falkland Basin. Petroleum Geoscience 21, 125-135.
Kodner R.B., Summons R.E. & Knoll A.H. (2009). Phylogenetic investigation of the aliphatic, non-hydrolyzable biopolymer algaenan, with a focus on green algae. Organic Geochemistry 40, 854-862.
Tegelaar E.W., de Leeuw J.W., Derenne S. & Largeau C. (1989). A reappraisal of kerogen formation. Geochimica et Cosmochimica Acta 53, 3103-3106.
Wang Min, Lu Shuangfang, Xue Haitao, Wang Weiming, Liu Min, Dong Qi & Wang Guihua (2011). Hydrocarbon generation kinetic characteristics from different types of organic matter. Acta Geologica Sinica (English Edition) 85, 702-711.