User:GuannanDong/PhytaneSandbox

Phytane is a type of isoprenoid alkane that is mostly formed from dehydroxylation of phytol. Phytol is part of the chlorophyll molecule. Similarly, pristane is also mainly formed from phytol but has one less carbon. Pristane and phytane are common constituents in petroleum and have been used as a biomarker for indicating depositional conditions and correlating oil and its source rock (from where oil formed). In environmental studies, pristane and phytane are target compounds for investigating oil spill.

Chemistry
Phytane is a clear and colorless liquid at room temperature. It is a head-to-tail linked regular isoprenoid, a non-polar compound.

Phytane has many structural isomers with chemical formula C20H42, 366319 to be exact. Among them, crocetane is a tail-to-tail linked isoprenoid and often co-elutes with phytane on GC/MS due to the structural similarity.

The substituent of phytane is phytanyl. Phytanyl groups are frequently found in archaeal membrane lipids of methanogen and halophilic archaea, for example, in archaeol.

Phytene is the singly unsaturated version of phytane. Phytene is also found as the functional group phytyl in many organic molecules of biological importance such as chlorophyll, tocopherol (Vitamin E) and phylloquinone (Vitamin K1). Phytene's corresponding alcohol is phytol.

Geranylgeranene is the fully unsaturated form of phytane. The corresponding substituent is geranylgeranyl.

Preservation
In suitable environment, biomolecules like chlorophyll can be converted and preserved in recognizable form as biomarker. Conversion during diagenesis often causes the chemical loss of functional groups like double bonds and hydroxyl groups. Early studies suggested that pristane and phytane are diagenetic products of phytol under different redox conditions. Pristane can be formed in oxidizing conditions by phytol oxidation to phytenic acid, which may then undergo decarboxylation to pristene, before finally being reduced to pristane. In contrast, phytane is likely from reduction and dehydration of phytol (via dihydrophytol or phytene) under relatively anoxic conditions.

However, different biotic and abiotic processes may play a significant role during the diagenesis of chlorophyll and phytol, and the exact reactions are more complicated and not well-correlated to redox conditions.

Pristane/Phytane ratio
Pristane/Phytane, or Pr/Ph, is an indicator for redox conditions in the depositional environment. This index is based on the assumption that pristane is formed from phytol by an oxidative pathway, while phytane is generated through various reductive pathways. In non-biodegraded crude oil, Pr/Ph less than 0.8 indicates saline to hypersaline conditions associated with evaporite and carbonate deposition; whereas organic-lean terrigenous, fluvial, and deltaic sediments under oxic to suboxic conditions usually generate crude oil with Pr/Ph above 3. Pr/Ph is commonly applied because pristane and phytane are measured easily using gas chromatography.

However, the index should be used very cautiously: pristane and phytane may not result from degradation of the same precursor (chlorophyll phytyl chain and ether lipids of archaea for both pristane and phytane, α-tocopherols and MMTCs only for pristane). Also, pristane, but not phytane, can be produced in reducing environments by clay-catalysed degradation of phytol and subsequent reduction. Additionally, during catagenesis, Pr/Ph tends to increase. This variation may be due to preferential release of sulfur-bound phytols from source rocks during early maturation.

Pristane/nC17 and phytane/nC18 ratios
Pristane/n-heptadecane (or Pr/nC17) and phytane/n-octadecane (or Ph/C18) are sometimes used in petroleum correlation studies. Oils from rocks deposited under open-ocean conditions showed Pr/nC17 < 0.5, while those from inland peat swamp had ratios greater than 1.

The ratios should be used with caution for several reasons. Both Pr/nC17 and Ph/nC18 decrease with thermal maturity of petroleum because isprenoids are less thermal stable than linear ones. However, biodegradation increases these ratios because aerobic bacteria generally attack linear alkanes before the isprenoids. Therefore, biodegraded oil is similar to low-maturity non-degraded oil in the sense of low n-alkane relative to pristane and phytane.

Biodegradation scale
Pristane and phytane are more resistant to biodegradation than n-alkanes, but less so than steranes and hopanes. The presence or depletion of pristane and phytane can be used to determine the level of biodegradation.

Carbon isotopes
Carbon isotope composition of pristane and phytane generally reflects the kinetic isotope fractionation in photosynthesis. For example, δ13C (PDB) of pristane and phytane in gilsonites from the Uinta Basin, Utah is -33 to -34‰, suggesting an origin from photic zone organism. The isotopic ratios match those in the proposed Mahogany Ledge oil shale source rocks.

Carbon isotope compositions of pristane and phytane in crude oil can help to constrain their source. Pristane and phytane from a common precursor should have δ13C values differing by no more than 0.3 ‰.

Hydrogen isotopes
Hydrogen isotope composition of phytol in marine phytoplankton and algae is highly depleted, with δD (VSMOW) from -360 to -280‰. Thermal maturation preferentially releases light isotopes and pristane and phytane becomes progressively heavier with maturation.

Case study: limitation of Pr/Ph as a redox indicator
Inferences from Pr/Ph on the redox potential of the source sediments should always be supported by other geochemical and geological data, such as sulfur content or the C35 homohopane index. The Baghewala-1 oil from India has low Pr/Ph (0.9), high sulfur (1.2 wt.%) and high C35 homohopane index (12), consistent with anoxia during deposition of the source rock.

However, drawing conclusion on the oxicity of depositional environment from Pr/Ph alone can be misleading because salinity effect seems to control the Pr/Ph in hypersaline environments. The decrease in Pr/Ph during deposition of the Permian Kupferschiefer sequence in Germany, which is in coincidence with an increase in trimethylated 2-methyl-2-(4,8,12-trimethyltridecyl)chromans, aromatic compounds believed to be markers of salinity.