Phytane is the Diterpenoid alkane formed when phytol, a chemical substituent of chlorophyll, loses its Hydroxy group group.
Pristane and phytane are common constituents in petroleum and have been used as proxies for depositional redox conditions, as well as for correlating oil and its source rock (i.e. elucidating where oil formed). In environmental studies, pristane and phytane are target compounds for investigating .
Chemistry
Phytane is a non-polar
organic compound that is a clear and colorless liquid at room temperature. It is a linked regular
Terpenoid with chemical formula C
20H
42.
Phytane has many structural isomers. Among them, crocetane is a linked Terpenoid and often co-elutes with phytane during gas chromatography (GC) due to its structural similarity.
Phytane also has many Stereoisomerism because of its three stereo carbons, C-6, C-10 and C-14. Whereas pristane has two stereo carbons, C-6 and C-10. Direct measurement of these isomers has not been reported using gas chromatography.
Sources
The major source of phytane and pristane is thought to be
chlorophyll.
Chlorophyll is one of the most important
photosynthesis in plants,
algae, and
cyanobacteria, and is the most abundant
tetrapyrrole in the biosphere.
Hydrolysis of chlorophyll
a,
b,
d, and
f during diagenesis in marine sediments, or during
invertebrate feeding
releases
phytol, which is then converted to phytane or pristane.
Another possible source of phytane and pristane is
Ether lipid. Laboratory studies show that thermal maturation of methanogenic archaea generates pristane and phytane from
Archaeol (archaeols).
In addition, pristane can be derived from
Preservation
In suitable environments, biomolecules like chlorophyll can be transformed and preserved in recognizable forms as biomarkers. Conversion during diagenesis often causes the chemical loss of functional groups like
and
Hydroxy group.
Studies suggested that pristane and phytane are formed via diagenesis of phytol under different
Redox.
Pristane can be formed in oxic (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, various biotic and abiotic processes may control the diagenesis of chlorophyll and phytol, and the exact reactions are more complicated and not strictly-correlated to redox conditions.
In thermally immature sediments, pristane and phytane has a configuration dominated by 6R,10S stereochemistry (equivalent to 6S, 10R), which is inherited from C-7 and C-11 in phytol. During thermal maturation, isomerization at C-6 and C-10 leads to a mixture of 6R, 10S, 6S, 10S, and 6R, 10R.
Geochemical parameters
Pristane/Phytane ratio
Pristane/phytane (Pr/Ph) is the ratio of abundances of pristane and phytane. It is a proxy for
Redox in the depositional environments. The Pr/Ph 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-
Biodegradation crude oil, Pr/Ph less than 0.8 indicates saline to
Hypersaline lake conditions associated with evaporite and carbonate deposition, whereas organic-lean terrigenous, fluvial,
and
River delta sediments under oxic to suboxic conditions usually generate
Petroleum 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 with caution, as pristane and phytane may not result from degradation of the same precursor (see *Source*). Also, pristane, but not phytane, can be produced in reducing environments by clay-catalysed degradation of phytol and subsequent reduction.
Pristane/nC17 and phytane/nC18 ratios
Pristane/n-heptadecane (Pr/nC
17) and phytane/n-octadecane (Ph/C
18) are sometimes used to correlate oil and its
source rock (i.e. to elucidate where oil formed). Oils from rocks deposited under open-ocean conditions showed Pr/nC
17< 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/nC17and Ph/nC18 decrease with thermal maturity of petroleum because Terpenoid are less thermally stable than linear alkanes. In contrast, biodegradation increases these ratios because aerobic bacteria generally attack linear alkanes before the isoprenoids. Therefore, biodegraded oil is similar to low-maturity non-degraded oil in the sense of exhibiting low abundance of n-alkanes relative to pristane and phytane.
Biodegradation scale
Pristane and phytane are more resistant to
biodegradation than n-alkanes, but less so than
and
. The substantial depletion and complete elimination of pristane and phytane correspond to a Biomarker Biodegradation Scale of 3 and 4, respectively.
Compound specific isotope analyses
Carbon isotopes
The carbon isotopic composition of pristane and phytane generally reflects the kinetic isotope fractionation that occurs during photosynthesis. For example, δ
13C(PDB) of phytane in marine sediments and oils has been used to reconstruct ancient atmospheric CO
2levels, which affects the carbon isotopic fractionation associated with photosynthesis, over the past 500 million years.
In this study,
partial pressure of CO
2 reached more than 1000 ppm at maxima compared to 410 ppm today.
Carbon isotope compositions of pristane and phytane in crude oil can also 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 starts out as highly depleted, with δD (VSMOW) ranging from -360 to -280‰.
Thermal maturation preferentially releases light isotopes, causing and pristane and phytane to become progressively heavier with maturation.
Case study: limitation of Pr/Ph as a redox indicator
Inferences from Pr/Ph on the redox potential of source sediments should always be supported by other geochemical and geological data, such as
Sulfur or the C
35 homohopane index (i.e. the abundance of C
35 homohopane relative to that of C
31-C
35 homohopanes). For example, the Baghewala-1 oil from India has low Pr/Ph (0.9), high sulfur (1.2 wt.%) and high C35 homohopane index, which are consistent with anoxia during deposition of the source rock.
However, drawing conclusion on the oxic state of depositional environments only from Pr/Ph ratio can be misleading because salinity often controls the Pr/Ph in Hypersaline lake environments. In another example, the decrease in Pr/Ph during deposition of the sequence in Germany is in coincidence with an increase in trimethylated 2-methyl-2-(4,8,12-trimethyltridecyl)chromans, an aromatic compound believed to be markers of salinity.
See also
