42 
A Generally, the determination of very small peaks, such as the 25-norhopane peak 
in Figures 32 and 33, should be avoided, if their role is unclear. The determination 
of very small peaks is not only associated with a bigger analytical error. There is 
also the high risk that the wrong peaks are chosen from the chromatograms: some 
crude oils may show such complex hopane-patterns that even the identification of the major 
peaks of the hopane-series may become difficult (Figure 34). The determination of 25- 
norhopane in these cases, for example, would require structure determination of a small peak 
among clusters of overlapping peaks of similar compounds, which is beyond the scope of Oil 
Spill Identification and simply not practicable in acute cases. 
Fig. 34: Examples of more complex hopane-clusters (Angola Quinguila crude, left, Leadon 
crude, North Sea, right). 
The broader platform area of the northern North Sea, including Statfjord, Gullfacs, Brent, 
Oseberg, Troll etc., seems to be especially characterized by relatively high concentrations of 
C28-bisnorhopane, a compound, which is not a member of the regular hopane series. Its 
formation from a C30 or C35 biological precursor would have required cleavage of two 
carbon-carbon bonds in the side chain, which is very improbable. 28-bisnorhopane thus can 
be regarded as a specific source parameter. Beside in North Sea crudes it is found only in 
selective crude oils in other areas of the world. 
There is roughly a general decline of the C28-bisnorhopane concentrations to the south, up to 
the Ekofisk-area and the Danish platforms (Figure 35). But this trend is interrupted by the 
platforms of the Flotta catchment area (Piper, Claymore etc.), where the highest 
concentrations of C28-bisnorhopane are found (up to about 50% in relation to hopane).