140 -sterane 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 second sterane ratio, also used extensively for many years, is the 14a(H),17a(H)/(14a(H), 17a(H)-b 14 8(H),17 8(H)), or j3l3/(aa + jBjB) ratio. Initially, it was proposed that this ratio changed as a result of the aa-isomers being converted to the 8 8-isomers with increasing maturity (Mackenzie et al., 1982 Seifert and Moldowan, 1986). It was also observed in certain samples, particularly those from hypersaline evaporitic environments, that very immature samples contained relatively high concentrations... 

M., de LeeuwJ.W., Sinninghe DamsteJ.S. (1998)The effect of clay minerals on diasterane/sterane ratios. Geochim. Cosmochim.Acta 62, 2923—9. 

C28R/C29R, sterane ratio, 24-methyl-5a,14a,17a(H)-cholestane (20R)/24-ethyl-5a,14a,17a(H)-cholestane (20R). This ratio of 20R sterane isomers has been adapted from discussions in Grantham and Wakefield (1988) and Waples and Machihara (1991), in which suites of steranes were considered rather than two individual epimers. 

The single sample (00-67) of a natural crude-oil seep, located offshore from Anacapa Island (Fig. 1), has geochemical parameters that closely match the Group B samples (Table 2). For example, the BI value is 1.4, sterane ratio is 1.3, hopane-sterane ratio is 0.70, and the PAH-RJ value is 26. The presence of trisnorhopane in this sample is a characteristic of both Groups A and B. The value of -23.4%o matches samples in Groups A, B, and C. 

The terpane and sterane distributions for the six oil samples are remarkably similar and only a few selected ratios will be discussed (Table 2). Grantham and Wakefield (1988) observed that the C28/C29 sterane ratios for crude oils generated from marine source rocks with no significant terrigenous organic matter input are less than 0.5 for Lower Paleozoic and older oils, 0.4-0.7 for Upper Paleozoic to Lower Jurassic oils, and greater than about... 

Fig. 4. C28/C29 sterane ratios for crude oils generated from marine source rocks (stippled area) generally increase from the Paleozoic to the present due to increasing phytoplankton diversity (Grantham and Wakefield, 1988). The six Algerian oil samples have C28/C29 sterane ratios in the range 0.38-0.44 (solid bar), consistent with Paleozoic source rock. G, O, S, D, C, P, Tr, J, K, T, Cz—Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary, and Cenozoic, respectively.

Sterane hydrocarbons present in fossil fuels are additional usefiil molecular marker indicators for petroleum impact. Their homolog distributions (C27 versus Cjg) are useful for determining the reservoir origin of petroleum. The epimerization ratio at C-20 of the Cjg sterane (ratio nos. 16 and 17) is characteristic of petroleums, confirming such residues in the SWMs and their leachates. 

Luellen, D.A. and Shea, D. 2003, Semipermeable membrane devices accumulate conserved ratios of sterane and hopane petroleum biomarkers. Chemosphere S3 705-713. 

M ile not all of the maturity parameters will be discussed in detail, it is useful to make a few comments concerning those that are most commonly used. Ratios based on changes in sterane... 

Carbonates and Low diasterane (67)/sterane (66) ratios van Kaam-Peters et al. 

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). 

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