C₂₆ and C₂₈-C₃₄ 28-norhopanes in sediments and petroleum

A complete series of 28-norhopanes (C₂₆ and C₂₈-C₃₄) has been detected in oil samples and rock extracts from West Greenland and the North Sea. Only the C₂₈ members of the series (28,30-bisnorhopanes) and the related 25,28,30-trisnorhopanes have been described in the literature. 28,30-Bisnorhopanes are often the only 28-norhopanes in oils and sediments, and their isotopic composition can be different from that of the regular hopanes, suggesting that 28,30-bisnorhopanes have a different origin. In other cases, 28-norhopanes and regular hopanes have a similar distribution of homologues, and there is no isotopic evidence for a different origin of the two series. When the complete series of 28-norhopanes is present, it is usually accompanied by high concentrations of the corresponding demethylated aromatic 8,14-secohopanes. The 28-norhopanes seem to be less resistant to biodegradation than regular hopanes, and there is a preferential demethylation of the low-molecular-weight 28-norhopanes. C₂₅ and C₂₇-C₃₁ 25,28-bisnorhopanes have been identified in biodegraded oils. The C₂₈-C₃₄ 28-norhopanes are best studied using the m/z 355 mass chromatogram, since this fragment is comparatively intense and the interference from other compounds is usually low. The 17β(H),21α(H)/(17α(H),21β(H) + 17β(H),21α(H)) ratios of the C₂₉ and C₃₀ 28-norhopanes can be used as maturity parameters. In immature samples, a large proportion of the 28-norhopanes (especially C₂₈ and C₃₀) occurs in the bitumen. However, hydrous pyrolysis experiments have shown that 28-norhopanes are also part of the kerogen.