Insights on the origin of pristane and phytane in sediments and oils from laboratory heating experiments

Abstract

We investigated a formation pathway of pristane (Pr) and phytane (Ph) in crude oils and ancient sediments from the phytyl side chain of chlorophyll and degradation of Pr and Ph during thermal maturation by laboratory heating experiments of pure dihydrophytol (DHP), Pr and Ph, followed by measurement of carbon isotopic compositions. In the first heating experiment, dihydrophytol (DHP) was pyrolyzed in a closed system at 320–350°C for 1-12 h. DHP yields large amounts of Pr and prist-1-ene with lesser amounts of Ph and phytenes. The Pr/Ph ratio which was obtained after hydrogenation of prist-1-ene and phytenes increases from 0.7 to 21 with increasing thermal degradation of DHP. We conclude that DHP originating from chlorophyll pigments can be an important precursor of Pr under geological conditions. Pr and Ph produced from DHP are slightly depleted in 13C compared to the original DHP and δ13C of Pr is similar to that of Ph. This result supports the idea of a common source for both Pr and Ph, primarily chlorophyll pigments. In the second heating experiment which was intended to evaluate the role of thermal stress on the fate of Pr and Ph in petroleum, Pr and Ph were heated at 380 or 420°C under vacuum for 5–800 h, and the shift in their carbon isotopic composition was studied as a function of thermal decomposition. The decomposition of Pr and Ph follows apparently first-order kinetics. Pr/Ph ratio does not change significantly during thermal decomposition of 90% of the initial amount of these isoprenoids. The fractionation factors for both Pr and Ph are practically identical: α = 0.9994 ± 0.0002 (n = 23). The effect of temperature on the fractionation factor is negligible. Although Pr and Ph are enriched in 13C with increasing thermal decomposition, the magnitude of the enrichment is small and 90% of Pr and Ph must be thermally decomposed to cause a 1.4 ± 0.4increase in δ13C values. The results also indicate that Pr/Ph ratio does not change significantly during thermal decomposition. Therefore, we conclude that small changes in Pr/Ph ratios and in δ13C of Pr and Ph with increasing thermal maturity may be a notable feature of occurrence of thermocracking of Pr and Ph in crude oils.