Pristane phytane ratio

A number of selected molecular parameters obtained from analysis of immature crude oils and sediment extracts are evaluated as indicators of palaeosalinity. The nature of these parameters is discussed taking into account the role of intermolecular and intramolecular incorporation of sulfur into specific functionalized lipids. Specific distribution patterns of methylated chromans and C20 isoprenoid thiophenes and the relative abundance of gammacerane are excellent indicators for palaeosalinity, whilst other parameters such as 14< (H),17a(H)/140(H),170(H) -sterane ratios, the pristane/phytane ratio, the even-over-odd carbon number predominance of n-alkanes and the relative abundance of C35 hopanes and/or hopenes may indicate palaeohypersalinity but are affected by environmental factors other than hypersalinity and by diagenesis. 

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. 

A more critical evaluation of the above mentioned ratios and phenomena reveals the usefulness of the various palaeosalinity indicators. Distribution patterns of methylated chromans and the relative abundance of gammacerane are not influenced by sulfur incorporation reactions and may directly reflect species distributions in the palaeoenvironment. To some extent this holds for 14a(H),17a(H)/140(H),170(H)-steraneratios as well, although incorporation of sulfur may influence this ratio and original A7/A5-sterol ratios do not always correlate with hypersaline environments. The isoprenoid thiophene ratio is highly useful as a palaeosalinity indicator since the distribution of the C20 isoprenoid thiophenes directly reflects the distribution of their substrates. The other parameters (pristane/phytane ratio, odd-over-even carbon number predominance of n-alkanes, relative abundance of C35 hopanes and/or hopenes) should be used with caution because they obviously depend on the quenching by sulfur of specific lipids, a process which is not restricted to hypersaline environments. 

The pristane/phytane ratio (Figure 3d) also shows a trend which is apparently influenced by the three facies types. This ratio is thought to reflect changes in anoxicity and/or salinity (30-32). 

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... 

Bitumens, were separated by chromatography, urea clathration and 5A molecular sieve occlusion before and after analyses of many of the aliphatic sub-fractions by GC and gas chromatography-mass spectrometry (GC-MS). Experimental details are noted in a previous publication (16) in which the distribution of cyclic alkanes in two lacustrine deposits of Devonian (N.E. Scotland) and Permian (Autun, France) age, (the D and C series samples) were discussed, Chromatographic separation into aliphatic, aromatic and polar compounds of the bitumens extracted from the shales gave the results shown in Table VI. Carbon Preference Indices and pristane/phytane ratios were measured in this work space limitations precluded... 

Most of the alphatic fractions were separated into normal and branched alkane fractions after the removal of unsaturated hydrocarbon. Most of these subfractions were analysed by GO to give fingerprint chromatograms. These analyses were used to provide data that allowed carbon preference indices, pristane/phytane ratios etc. to be measured. Also, the relative amounts of (17 H, 21/ H 17/3 H, 21 oCH and 17 H, 21(3 H) hopanes, steranes, 4-methyl steranes, carotenoid and other related compounds were assessed from gas chromatograms and reconstructed ion chromatograms. A large number of Kovats Retention Indices (0V-101) were measured and tabulated for these compounds also (10). 

Dunn 1987), thus closely matching the values (—28.2 2.7, = 3) found by Elvsborg et al. (1985). The pristane/phytane ratio of extracts from this formation fall in the range of typical type II marine shales, e.g. 0.6-1.60 (Elvsborg et al. 1985 Cohen Dunn 1987) and with a uniform sterane distribution centring around C27 = 34%, Cjg = 33%, C29 = 33% (Cohen Dunn 1987). 

On the whole, all the oils appear to be chemically alike, which is suggestive that they are generated from the same or congeneric source rocks. The distributions of saturated hydrocarbons (Cio) and the pristane/phytane ratio do not feature conspicuous distinctions. However, an analysis of the gasoline C -Cy fraction has revealed certain distinctions among the oils. 

The Devonian oils in reservoirs D,-Tj of the Illizi Basin are characterised by a 5 C with a rather narrow spread of -28.10 to -29.85% and their pristane/phytane ratio of... 

Field Well Depth (m) Age Pristane/ phytane ratio Oil (0) or bituminoid (B)... 

Table 5.3. Average of pristane/phytane ratio for oils from Oued el-Mya fields...

Field Well Age No. of samples analyzed Pristane/phytane ratio ... 

Gas Chromatography. Whereas N.M.R. effectively senses the f mctional groups present in a mixture, differences between retention times enable G.C. to identify individual members (provided they are moderately volatile) of a homologous series such as the n-alkanes. Positive identification requires calibration by co-injection with authentic samples (here n-alkanes) or coupling of M.S. with, preferably, a capillary G.C. column. Although G.C. stick diagrams of n-alkane distributions have been used for comparing extracts from fossil fuels, we advocate acyclic-isoprenoid-hydrocarbon distributions, and particularly the pristane/phytane ratio, as more reliable indicators of fossil fuel maturation 6). 

G.C.-M.S, analysis. The n-alkane distribution is distinctive, with a marked preference of odd over even carbon number (carbon preference index, 1.9, as in low-rank coals) in the range above C25 but with a smooth, nearly Gaussian, distribution for the lower range (peaked at Cie or C17) also, the pristane/phytane ratio is high (U.5), as in sediments of marine origin. 

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