Alkanes from rock

Molecular fossils have been successfully identified in younger Precambrian rocks and linked to certain classes of biological source material. In organic analyses of ancient sediments the cleaned, pulverized rocks are treated with organic solvents to extract a soluble fraction containing the less complex and more easily identifiable compounds. However, this fraction is more subject to contamination since it is not locked within the rock matrix. Normal alkanes have been identified in extracts of the 3 billion year old Fig Tree Shale. These alkanes have a probable biological origin in cellular lipids. The odd and even-numbered alkanes are evenly distributed, a characteristic of alkanes from ancient rocks. It is uncertain, however, whether these compounds were present at the time of deposition or derived from a later source [24]. 

Levy et al. [392] briefly investigated alternative fluids for on-line SFE-capil-lary GC with C02, N20, and SF6 for the extraction of PAHs and alkanes from solid waste, sediment, and shale rock. They initially compared the extraction efficiency of pure fluids and then some fluid mixtures. They found that 20% SF6 in C02 was more effective at 375 bar, and 50 °C for 30 min than each pure fluid for removing both PAHs and alkanes. 

Geochemists and the oil industry routinely screen rock samples for bio-markers that indicate the age and source of hydrocarbon constituents. The conventional extraction takes 48 hours and requires extract clean-up by thin-layer chromatography to separate the alkanes from aromatic hydrocarbons and heteroatom containing species (16). Here the selectivity of SFE in extracting only the alkanes from a mature source rock... 

Figure 9. Chromatogram of shale oil from Rock Springs No. 9, a true in situ experiment. The alkene/alkane ratios are very low (coking) and the naphthalene content are very high (combustion and associated cracking). The naphthalene/methylnaphthalene ratios are high compared with the...

The band resulting from the methylene rocking vibration (p CH2), in which all of the methylene groups rock in phase, appears near 720 cm-1 for straight-chain alkanes of seven or more carbon atoms. This band may appear as a doublet in the spectra of solid samples. In the lower members of the w-alkane series, the band appears at somewhat higher frequencies. 

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. 

Figure 3. Partial FID chromatograms (/z-Cg to n-C9 region) from flash pyrolysis of Monterey Fm. kerogens, Santa Barbara basin. Open and closed circles indicate w-alk-l-enes and /z-alkanes respectively. Depth of burial, Rock-Eval pyrolysis Tmax, Hydrogen Index (HI), and the thiophene ration (TR) are shown.

The molecular distribution and compound-specific carbon-isotopic composition of hydrocarbons can be used to qualify and quantify their sources and pathways in the environment. Molecular source apportionment borrows from molecular methods that were developed and applied extensively for fundamental oil biomarker studies, oil-oil and oil source rock correlation analysis. Additionally, petroleum refinement produces well-defined mass and volatility ranges that are used as indicators of specific petroleum product sources in the environment. Compound-specific carbon-isotopic measurement is a more recent addition to the arsenal of methods for hydrocarbon source apportionment. Carbon isotopic discrimination of i-alkanes, biomarkers, and PAHs has shown that the technique is highly complementary to molecular apportionment methods. 

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