37 
Fig. 30: GCs of four Venezuelan crude oils showing the general variation from n-paraffin 
(upper left) to naphthene based crudes (lower right). 
2.5.1.2 GC-MS analysis 
Biomarkers and their relations generally represent source and maturity of crude oils. While 
source parameters are derived from the specific biomass from which an oil has been formed, 
maturity parameters represent chemical changes occurring during oil maturation. 
Some of them can be highly characteristic, i.e. specific of a particular production area, or even 
of an oil field. But others may have only a very low meaning, when an oil spill sample is 
compared with samples from suspected sources. 
The relation of the C31+ enantiomers in the hopane series, for example, indicates maturity of 
organic matter in samples of potential source rocks. With increasing maturity this value 
reaches about 60:40 although it may slightly vary depending on the carbon number of the 
homologues being measured. The value of the second pair, expressed as 100*32abS/32abR 
(%32abS), of all 260 samples of the collection of crude oils from all over the world has found 
to be 58 with a standard deviation of only 2.2% (Figure 31).