Sterane distributions

Sterane distributions (m/z 217) for three samples representing the three facies are similar (Figure 4). 

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). 

The sterane distribution of DST and inclusions samples from Halten Vest, Smorbukk and Smorbukk Sor (Fig. 26) suggests that both inclusions and DSTs have been sourced from type II marine source rocks, i.e. the Spekk Formation. There is no statistically relevant difference between the biomarker parameters extracted from inclusions in 6506/12-1, 6506/11-1, 6506/12-4 and Smorbukk Sor. This conclusively shows that inclusions formed in these... 

Fig. 26. The sterane distribution for fluid inclusions from Halten Vest is similar to the variation observed for the inclusions and present DSTs from the Smorbukk and Smorbukk Sor petroleum systems. This suggests strongly that their SR facies origin is the same, i.e. the same type II marine Spekk Formation The trend along the C27-C29 axis is representing phase fractionation/evaporative fractionation plus maturity effects (cf. Karlsen et al. 1995).

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... 

Boeda et al. (1996) identified bitumen on a flint scraper and a Levallois flake, discovered in Mousterian levels (about 40 000 BP) at the site of Umm el Tlel in Syria. The occurrence of polyaromatic hydrocarbons such as fluoranthene, pyrene, phenanthrenes and chrysenes suggested that the raw bitumen had been subjected to high temperature. The distribution of the sterane and terpane biomarkers in the bitumen did not correspond to the well-known bitumen occurrences in these areas. In other studies of bitumen associated with a wide variety of artefacts of later date, especially from the 6th Millennium BC onwards, molecular and isotopic methods have proved successful in recognizing different sources of bitumen enabling trade routes to be determined through time (Connan et al., 1992 Connan and Deschesne, 1996 Connan, 1999 Harrell and Lewan, 2002). 

The distribution of thiophenes in the microbial mat and in the paleosoil of mangrove are noticeably different. In the microbial mat, isoprenoid, branched, midchain and n-alkylthiophenes occur mainly between C14 and C21 and are dominated by the C20 thiophenic isoprenoids. In the paleosoil of mangrove, if some of the isoprenoids and branched thiophenes are present, the sulfur compounds are dominated by the C28-C30 n-alkylthiophenes and by a presumed C30 n-alkylthiolane thiol, and in the extract by a C29 sterane-thiol. These differences are likely to be a reflection of the type of molecules in which sulfur might have been incorporated. 

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. 

Moldowan et al (5) investigated a sediment core, spanning a depth range of 5m, of Lower Toarcian shales from W. Germany specifically at a transition zone from a rather oxidized, shallow-marine, marly sediment to an organic matter-rich, black shale. Variations in distributions of isoprenoid hydrocarbons, steranes and monoaromatic steroids were observed and were related to variations in oxidation/reduction conditions during and shortly after sedimentation. 

The data described above can be used to predict the location of better source rocks in vertically drained basins, especially in deltaic-type environments with relatively young source rocks. With long-distance vertical migration, some of the biomarker parameters may become skewed. A number of factors must first be considered before applying this approach first, some of the parameters vary with maturity second, C30 steranes are not present in lacustrine samples and so the approach will not work in that situation and finally, it will not work where the oils were deposited prior to land plant evolution, since no vitrinite was present at that time. Oils from mixed source rocks also complicate the issue. The ability to predict source rock properties on the basis of biomarker distributions in cmde oils is a very interesting concept, since most exploration efforts try to discover oil and not source rocks. 

Grantham P. J. and Wakefield L. L. (1988) Variations in the sterane carbon number distributions of marine source rock derived crude oils through geological time. Org. Geochem. 12, 61-73. 

Fig. 5. 27 Variations over geological time in sterane C-number distributions in oils from marine source rocks (age refers to inferred source after Grantham Wakefield 1988).

Sometimes the presence (or even the absence) of a range of biomarkers can be diagnostic. For example, despite the reservations about sterane C-number distributions noted in Section 5.4.2, dominant C29 steranes... 

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