Changes

==Biodegradation and hopane distribution==

==Biodegradation and hopane distribution==

[[file:oiloil-and-oilsource-rock-correlations_fig8-41.png|300px|thumb|{{figure number|2}}Regular hopane and moretane series in a nondegraded oil (top), as shown in the m/z 191 mass chromatogram, compared to the series of demethylated hopanes and moretanes in a heavily biodegraded oil (bottom), revealed in the m/z 177 mass chromatogram. From Volkman et al.;<ref name=ch08r57>Volkman, J., K., Alexander, R., Kagi, R., I., Woodhouse, G., W., 1983, Demethylated hopanes in crude oils and their applications in petroleum geochemistry: Geochimica et Cosmochimica Acta, vol. 47, p. 785–794, DOI: 10.1016/0016-7037(83)90112-6.</ref> reprinted with permission from Elsevier.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-41.png|300px|thumb|{{figure number|2}}Regular hopane and moretane series in a nondegraded oil (top), as shown in the m/z 191 mass chromatogram, compared to the series of demethylated hopanes and moretanes in a heavily biodegraded oil (bottom), revealed in the m/z 177 mass chromatogram. From Volkman et al.;<ref name=ch08r57>Volkman, J. K., R. Alexander, R. I. Kagi, G. W. Woodhouse, 1983, Demethylated hopanes in crude oils and their applications in petroleum geochemistry: Geochimica et Cosmochimica Acta, vol. 47, p. 785–794, DOI: 10.1016/0016-7037(83)90112-6.</ref> reprinted with permission from Elsevier.]]

Although hopane distributions are well known to change during extreme [http://www.oiltracers.com/services/exploration-geochemistry/oil-biodegradation.aspx biodegradation], the causes for these changes are controversial and poorly understood. At very high levels of biodegradation, hopanes and moretanes disappear. In their place appear series of demethylated hopanes and moretanes (25-norhopanes and 25-normoretanes). Although workers originally believed the regular hopanes and moretanes were converted to their demethylated forms by bacterial removal of a single methyl group, that explanation has been disputed. Some workers today believe that the hopanes and moretanes simply disappear, and their disappearance merely reveals pre-existing series of less abundant demethylated species that could not be seen in the presence of regular hopanes and moretanes.

Although hopane distributions are well known to change during extreme [http://www.oiltracers.com/services/exploration-geochemistry/oil-biodegradation.aspx biodegradation], the causes for these changes are controversial and poorly understood. At very high levels of biodegradation, hopanes and moretanes disappear. In their place appear series of demethylated hopanes and moretanes (25-norhopanes and 25-normoretanes). Although workers originally believed the regular hopanes and moretanes were converted to their demethylated forms by bacterial removal of a single methyl group, that explanation has been disputed. Some workers today believe that the hopanes and moretanes simply disappear, and their disappearance merely reveals pre-existing series of less abundant demethylated species that could not be seen in the presence of regular hopanes and moretanes.