2-Methylhopane - by Paul Farrimond
Hopanes are widely employed in petroleum geochemistry and have a near-ubiquitous occurrence in oils and source rocks, being derived
from bacteria. 2-Methylhopanes have an additional methyl group at the C-2 position and are less abundant than hopanes, but offer useful biomarker potential, being derived from a more limited range of bacterial types, and particularly associated with cyanobacteria in shallow marine environments.
2-Methylhopanes occur as a series (C28-C36) with variable length of the side chain at C-21, and as both 22S and 22R isomers for the C32+ homologues. Whilst immature sediments contain 17b(H),21b(H) isomers, these are absent in oils and mature source rocks where the 17a(H),21b(H) isomers dominate. 2-Methyhopanes are identified and quantified from the m/z 205 mass chromatogram, but often only the most abundant members (usually C31) of the series are reported in analytical reports. Nevertheless, useful parameters can be calculated using these compounds and used in oil-oil and oil-source rock correlations (Farrimond et al., 2004).
Farrimond P., Talbot H.M., Watson D.F., Schulz L.K. & Wilhelms A. (2005). Methylhopanoids: Molecular indicators of ancient bacteria and a petroleum correlation tool. Geochimica et Cosmochimica Acta68, 3873-3882.