By Paul Farrimond
These aromatic biomarkers are unfortunately rarely used in petroleum geochemistry. They have the five-ring structure of the hopanes but with an aromatic ring fused to the E-ring in place of the normal side chain. They occur as a series from C32 to C35 and are present in most oils and source rocks, although typically in low abundance, being found in the m/z 191 mass chromatogram.
When a compound class separation is performed in the laboratory, the benzohopanes are normally present in the aromatic hydrocarbon fraction; however, they can also be found in the saturated hydrocarbon fraction as they are not strongly aromatic. This difficulty in separation is probably one of the reasons that they have not been regularly used in geochemical studies. If you find four unknown peaks eluting in the later part of the m/z 191 hopane mass chromatogram (after C33), they may be benzohopanes.
They are diagenetic products of the polyfunctionalised hopanoids present in bacteria and are usually more abundant in carbonate environments (Hussler et al., 1984). A related series of hexacyclic hopanes have been found in evaporite and hypersaline carbonate environments (Connan & Dessort, 1987) which together with the benzohopanes offer significant potential as environmental markers in oils.
References:
Connan, J. & Dessort, D. (1987). Novel family of hexacyclic hopanoid alkanes (C32-C35) occurring in sediments and oils from anoxic paleoenvironments. Organic Geochemistry 11, 103-113.
Hussler, G., Connan, J. & Albrecht, P. (1984). Novel families of tetra- and hexacyclic aromatic hopanoids predominant in carbonate rocks and crude oils. Organic Geochemistry 6, 39-49.
