Abstract
On the basis of the distribution and abundance of normal and isoprenoid alkanes, the examined oils were classified in four groups. The first group of samples (group i) includes the oils A and B, with relatively large amounts of n-alkanes (n-C18 > Phy, and n-C17 > Pr). The oils C and D represent group ii, with the contents of n-alkanes (n-C18 Pr) in two maxim at n-C17 and n-C25. In the oils E and F, group iii, lower molecular n-alkanes are the most abundant in the samples, n-C15 as the major compound and phytane > pristine. The last group iv, oils G and H, show the n-alkanes detectable at trace and the isoprenoids pristane and phytane as the major compounds in the oils. This n-alkanes distribution indicates that biodegradation has not occurred to samples from A to F, contrary to the samples G and H that are biodegraded. The tryciclic and triterpenoids are depicted in the key ion plots m/z 191, and show mainly the 17α (H),21β (H)-hopane series ranging from C27 to C35. The crude oil samples A and G have relatively high tricyclic terpane indices, suggesting marine source. Gammacerane was present in all the studied samples, but in measurable amounts only in the G oil. The distribution of tetracyclic steranes studied on GC/MS by monitoring the ion m/z=217, showed that the most abundant tetracyclic components in A oil are the C27 steranes. A and G crude oils exhibit abundance of diasteranes. The distribution of aromatic compounds were studied on GC/MS by monitoring the characteristic ions to methylbenzene (DMB, TMB, TeMB and PMB); methylnaphthalenes (DMN, TMN e TeMN); methyl phenanthrene (MP, DMP and TMP) and tricyclic aromatic steranes. Aromatic hydrocarbons, especially, phenanthrene and naphthalenes derivatives are resistant to biodegradation. The percentage of phenanthrene in the tricyclic aromatic fraction is proportional to oil maturity. By this parameter the crude oils A, C, D and G show the same maturation. By monitoring the ion m/z 231 and 245 the G, C and E oils show distributions of C26 to C28 and C28 to C30 respectively to tricyclic aromatic sterane, confirming the oil maturation.
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