Phytane sources

Of particnlar significance in the study of petroleum weathering are the biomarker molecules (e.g., pristane, phytane, the hopanes and steranes). Historically, the biomarkers have been employed as crude oil signatures in prospecting and characterization. More recently, such molecules have also been employed in the environmental field, both for the determination of pollutant source and estimation of the degree of weathering. 

The importance of these depositional environments makes it desirable that studies concerned with the reconstruction of palaeoenvironments from sediments or source rocks of oils also establish molecular parameters for palaeohypersalinity. Recently, ten Haven et al. (7-9) have summarized a number of "organic geochemical phenomena" related to hypersaline depositional environments. In addition to previously known parameters, such as an even-over-odd carbon number predominance of n-alkanes and a low pristane/ phytane ratio (<0.5), several new parameters were suggested. These parameters, however, are mainly based on empirical relations. 

Branched and cyclic hydrocarbons have also been used as paleomarkers, principally in lake sediments, to interpret past changes in organic matter sources. In particular, isoprenoid hydrocarbons such as pristane (C19 isoprenoid hydrocarbon) and phytane (C20 isoprenoid hydrocarbon) (both derived from phytol) have been used as indicators of herbivorous grazing and methanogenesis, respectively. 

Liquid hydrocarbons derived from coal have a composition that is somewhat distinctive, and that distinguishes them from oils derived from algal-dominated type I and II source rocks. For example, petroleum derived from coal tends to have high pristane/phytane ratios a ratio >4 is... 

In Refsum s disease, an autosomal recessive disorder, the defect is probably in the a-hydroxylation of phytanic acid. Phytanic acid is a 20-carbon, branched-chain fatty acid derived from the plant alcohol phytol, which is present as an ester in chlorophyll. Thus, its origin in the body is from dietary sources. The oxidation of phytanic acid is shown in Figure 18-6. The clinical characteristics of Refsum s disease include peripheral neuropathy and ataxia, retinitis pigmentosa, and abnormalities of skin and bones. Significant improvement has been observed when patients are kept on low-phytanic acid diets for prolonged periods (e.g., diets that exclude dairy and ruminant fat). 

Evidence for the biological origin of reduced carbon in sedimentary rocks comes from two sources. One is the existence of chemofossils, that is, characteristic remnants of biologically important compounds more resistant to chemical degradation than others. Notable examples are the isoprenoids pristane (2,4,6,10-tetramethyl pentadecane) and phytane (2,4,6,10-tetramethyl hexadecane), which arise from the decay of chlorophyll (Eglin-ton and Calvin, 1967 Didyk et al., 1978 McKirdy and Hahn, 1982 Hahn,... 

Isoprenoids. While both pristane and phytane (as well as other isoprenoids) are quite common in ancient sedimentary rocks (28,29), only pristane has been found in uncontaminated recent sediments. Blumer and Snyder (30) have suggested that organisms rich in pristane, such as copepods, are a major source of sedimentary pristane. 

The ratio of the isoprenoids pristane (Ci9) and phytane (C2o). When these compounds are resolved from the normal C17 and Cig peaks with which they are closely associated in the chromatogram, their ratio (if near one) is diagnostic for petroleum, or (if pristane > > phytane) for biogenic sources. Pristane is common in plankton (cope-pods) and various marine animals phytane has only rarely been reported except as a component of petroleum (30). 

In Table I all the Ci8 phytane ratios are between 0.8 and 2.3, as would be expected for a petroleum source. Pristane iphytane ratios are (with one exception) substantially greater than one, indicating a planktonic source as well. 

FIGURE 27.8 GC traces of the saturated fractions of the weathered source oil (top), the moderately weathered sample 1 (middle), and the severely weathered sample 2 (bottom) from the Baffin Island Oil Spill project conducted at Baffin Island in the Canadian Arctic from 1980 to 1983, illustrating the effects of over 15 years field weathering on chemical composition changes and the weathering trend of the spilled oil. IS, pr, and ph represent the internal standard, pristane, and phytane, respectively. 

Hughes, W. B., Holba, A. G. Dzou, L. I. P. 1995. The ratios of dibenzothiophene to phenathrene and pristane to phytane as indicators of deposi-tional environment and lithology of petroleum source rocks. Geochimica et Cosmochimica Acta, 59, 3581-3598. 

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