Relationship between Ratio of Pristane to Phytane, Crude Oil Composition and Geological Environment in Australia

Abstract

ACYCLIC isoprenoid hydrocarbons have been identified in sediments^1–4, coal⁵, and petroleum^6–11. Although isoprenoid hydrocarbons up to C₂₅ have been recognized¹¹, pristane (C₁₉) and phytane (C₂₀) are usually the most important in terms of concentration. Both are considered to be products of the diagenesis of the phytyl side chain of chlorophyll⁵, although other sources are possible^12–14. In coals⁵, pristane is formed by decarboxylation of phytanic acid, and phytane by dehydration and hydrogenation of phytol. Differences in the ratio of pristane to phytane may reflect variations in the degree of oxidation during the early stages of chlorophyll degradation⁵. Thus the formation of phytanic acid, the precursor of pristane, should occur to a greater extent on land during the initial, aerobic stages of plant decay than in an aquatic environment where totally anaerobic decomposition is more likely. High ratios of pristane to phytane, which are found in bituminous coals⁵ and certain Australian oils⁵, may reflect source material of terrestrial origin. The ratios of phytanic acid to phytol (plus dihydrophytol) in Dead Sea sediments were 4.7 and 5.5 in two oxidizing environments and 1.1 and 3.4 in two reducing environments¹⁴. Clearly the formation of phytanic acid is favoured in an oxidizing environment. But the precise mode of formation of phytanic acid in the natural environment is not fully understood because reduotion of the double bond as well as oxidation of the alcohol part of the molecule is necessary to form phytanic acid from phytol.