Molecular structures of sedimentary 8,14-secohopanes inferred from their gas chromatographic retention behaviour

The branched and cyclic alkane fractions from a biodegraded crude oil and the solvent extract of a mature sediment were analysed for extended 17α-hopanes/moretanes and extended 8,14-secohopanes using gas chromatography-mass spectrometry techniques. The biodegraded crude oil contained abundant 8,14-secohopanes in the form of pseudohomologous series with carbon numbers ranging from 31 to 35. Comparison of the chromatograms of the extended 8,14-secohopanes with those of the extended 17α-hopanes and moretanes from the sediment extract revealed some interesting similarities. Firstly, the C31 moretanes (22S, 22R) were not resolved but the corresponding C31 17α-hopanes were well resolved. In the case of the 8,14-secohopanes three of the six groups of C31 compounds were either poorly resolved into doublets or appeared as single peaks while the other three compound groups were well-resolved into doublets. This suggests that one group of 8,14-secohopanes behave in a similar manner to the moretanes and another group have a behaviour similar to that of the 17α-hopanes. Another feature of the distribution of extended moretanes, namely the greater relative abundance of the second eluting isomer of each pair (22S and 22R diasteromers), was also observed in the “moretane-like” 8,14-secohopanes. These similarities in GC behaviour and in the relative abundances of isomers has been interpreted as evidence for “hopane-type” and “moretane-type” extended 8,14-secohopanes.