N-Alkane abundances

Closer examination of n-alkane abundances among ozokerites support this possibility. All three Galician samples, for example, show an abun-... 

Vaporization. Maxima in n-alkane abundance ranging from C5 to Ci 1 in most crude oils probably result from the vaporization of normally gaseous constituents, primarily through reduction in pressure. The position of the maximum should depend primarily on the gas-to-oil ratio, greater ratios giving higher maxima. 

Inertness. The final favored range of n-alkane abundance at C51-C59 may represent vestiges of an infinite series that survived by reason of inertness through, say, insolubility. On occasion, n-alkanes above C8o were observed, and homologs as high as C100 may be present in minute amounts. 

Analytical data for the other classes of organic compounds show normal fatty acids to be more than an order of magnitude greater than n-alkanes in modem sediments, typically 26 /Agrams/gram and 1.5 /Agram/ gram, respectively for ancient sediments, the abundances were found to... 

Ozokerites in any case provided a logical material for extending previous work on the n-alkanes in crude oils (12). There the abundance of n-alkanes was found to be a significant parameter in the geochemistry. 

In certain ranges of some crude oils, odd-numbered n-alkanes predominate appreciably over the even-numbered homologs. Especially striking are odd predominances from Cn to C19 that accompany declines in abundance from Ci6 to Ci8 and from Ci8 to C2o (i2). Odd-even predominances were also observed in ozokerites, but not as prominently and at higher carbon numbers they will be presented and discussed in detail elsewhere. 

The saturated hydrocarbon distributions of the marl samples are dominated by long-chain n-alkanes of higher land plant origin (21) with a strong odd-over-even carbon number predominance. Hopanoid hydrocarbons are the next most abundant constituents, but other hydrocarbons particularly abundant in the laminite samples described hereafter are also clearly recognizable. 

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. 

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. 

In summary, Raney Ni desulfurisation of the polar fraction of the Northern Apennines Marl further supports the presence of (poly)sulfide-linked phytanyl, docosanyl and cholestanyl moieties (some of them with additional intramolecular sulfur linkages) in the resin fraction as proposed from the pyrolysis experiments. In addition, a number of other structural units are revealed e.g. pentakishomohopane, carotenoids, n-alkanes and isoprenoid alkanes. The reason why these structural units are not revealed by the pyrolysis experiments may be (i) their much lower relative abundance (e.g.9 other n-alkanes and isoprenoid alkanes), (ii) their attachment in the macromolecules by more than one (poly)sulfide linkage, which make their release from the macromolecule by flash pyrolysis unlikely ([Pg.522]

Oil from Well TJ-210 is put in a class by itself and is different from the other classes. It is produced from the same sands of the L-5 reservoir as Class 4 oils, although a short distance away. It is depleted in n-alkanes in the Cg-C13 range, but shows an abundant distribution from C1A-C35. Pristane and phytane are both prominent. It does not appear to be unduly affected by water washing as it contains an appreciable amount of benzene and toluene. The distribution of components in the CA-C7 range shows a larger proportion of naphthenic hydrocarbons compared to the rest of the oils. Its gross chemical composition would otherwise suggest a Class 2A-type oil. The oil contains many more saturates and n-alkanes than any other of the Miocene-reservoir crudes analyzed. 

Finally, even when HC composition and cuticular transpiration are correlated, causation cannot be assumed. For example, higher cuticular water-loss rates in the desert ant, Pogonomyrmex barbatus, are correlated with a decrease in abundance of an n-alkane and an increase in a methylalkane (Figure 6.2 Johnson and Gibbs, 2004). This is exactly what one would expect if lipid melting points affect cuticular permeability, but this increase is also accompanied by a change in mating status. Mated, de-alate queens that have founded... 

When using different n-alkanes for precipitation, distribution of total nitrogen is about the same in all cases (see Table III) nearly half of the total nitrogen is concentrated in the resin fraction while Type II concentration in asphaltenes increases when going from C5 to C7. This indicates that compounds with Type II elements are more abundant in higher molecular weight species and that C5 precipitation dilutes them with other types of molecules. 

The CPI25-35 for plant wax n-alkanes is in the range of 5-10, meaning on average that each odd-numbered hydrocarbon is 5-10 times more abundant than the even-numbered molecules with one more or one... 

The fragment ion m/z 141 occurs abundantly in the mass spectra of alkylnaphthalenes and as a weak ion in those of n-alkanes, and this fact has been used to monitor aspects of increasing aromaticity in a variety of kerogens (21, 37). These changes are well represented in the present work thus in... 

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