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Line 19: − Biodegradation, where severe, can also cause major changes in sterane and triterpane distributions. The ααα-20R steranes (regular steranes with the 20R configuration) are lost selectively during the early stages of severe biodegradation, followed by loss of all ααα steranes. The figure below illustrates this trend. It shows the m/z 217.2 mass chromatograms of three oils from central Myanmar in successive stages of biodegradation, ranging from not degraded (top) to extremely degraded (bottom). The severely degraded oil has lost almost all of its regular steranes, with greater loss of 20R than 20S. Gas chromatograms of these three oils are shown in Figure 8-20. − + + + + + + − The figure below shows an example of the 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.+ − [[file:oiloil-and-oilsource-rock-correlations_fig8-41.png|thumb|{{figure number|8-41}}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., 10., 1016/0016-7037(83)90112-6</ref> reprinted with permission from Elsevier.]]+ − ==Another hopane distribution example==+ − The figure below, which shows m/z 191 mass chromatograms of two genetically related oils from Papua New Guinea, gives another example of a major difference in hopane distribution. This difference could erroneously be considered genetic but is actually an unusual result of severe biodegradation. The top oil, recovered from a drill-stem test and not biodegraded, contains a full suite of triterpanes. The bottom seep oil, in contrast, is heavily biodegraded (gravity 30 hopane and homohopanes. The C<sub>29</sub> hopane is either unaffected or only slightly reduced in concentration. T<sub>m</sub>, T<sub>s</sub>, moretanes, and C<sub>z</sub> (indicated with *) also appear unaffected at this level of biodegradation. − + − Internal standards can appear in GC/MS data as well as in gas chromatograms. The figure below shows m/z 191 (top) and 217 (bottom) mass chromatograms for a seep oil from Papua New Guinea. The three peaks in the top chromatogram to the left of T<sub>s</sub> come from the internal standard, which was supposed to be a single compound but is actually a mixture (probably of various isomers of the same compound). The internal standard was unlabelled on this mass chromatogram and, as such, poses a serious risk even for experienced interpreters since the internal standard peaks might be thought to represent indigenous compounds. − + + +
Gas chromatography/mass spectrometry (GC/MS): limitations (view source)
Revision as of 15:15, 12 February 2014
, 15:15, 12 February 2014no edit summary
==Biodegradation and sterane distributions==
==Biodegradation and sterane distributions==
[[file:oiloil-and-oilsource-rock-correlations_fig8-40.png|thumb|{{figure number|8-40}}From Curiale;<ref name=ch08r12>Curiale, J., A., 1994, Correlation of oils and [[source rocks]]—a conceptual and historical perspective, in Magoon, L., B., Dow, W., G., eds., The [[Petroleum system]]—From Source to Trap: AAPG Memoir 60, p. 251–260.</ref> reprinted with permission from AAPG.]]
[[file:oiloil-and-oilsource-rock-correlations_fig8-40.png|thumb|{{figure number|1}}From Curiale;<ref name=ch08r12>Curiale, J., A., 1994, Correlation of oils and [[source rocks]]—a conceptual and historical perspective, in Magoon, L., B., Dow, W., G., eds., The [[Petroleum system]]—From Source to Trap: AAPG Memoir 60, p. 251–260.</ref> reprinted with permission from AAPG.]]
Biodegradation, where severe, can also cause major changes in sterane and triterpane distributions. The ααα-20R steranes (regular steranes with the 20R configuration) are lost selectively during the early stages of severe biodegradation, followed by loss of all ααα steranes. [[:file:oiloil-and-oilsource-rock-correlations_fig8-40.png|Figure 1]] illustrates this trend. It shows the m/z 217.2 mass chromatograms of three oils from central Myanmar in successive stages of biodegradation, ranging from not degraded (top) to extremely degraded (bottom). The severely degraded oil has lost almost all of its regular steranes, with greater loss of 20R than 20S. Gas chromatograms of these three oils are shown in Figure 8-20.
==Biodegradation and hopane distribution==
==Biodegradation and hopane distribution==
[[file:oiloil-and-oilsource-rock-correlations_fig8-41.png|thumb|{{figure number|2}}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., 10., 1016/0016-7037(83)90112-6</ref> reprinted with permission from Elsevier.]]
Although hopane distributions are well known to change during extreme 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 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.
[[:file:oiloil-and-oilsource-rock-correlations_fig8-41.png|Figure 2]] shows an example of the 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.
==Another hopane distribution example==
[[file:oiloil-and-oilsource-rock-correlations_fig8-42.png|thumb|{{figure number|3}}See text for explanation.]]
[[file:oiloil-and-oilsource-rock-correlations_fig8-42.png|thumb|{{figure number|8-42}}See text for explanation.]]
[[:file:oiloil-and-oilsource-rock-correlations_fig8-42.png|Figure 3]], which shows m/z 191 mass chromatograms of two genetically related oils from Papua New Guinea, gives another example of a major difference in hopane distribution. This difference could erroneously be considered genetic but is actually an unusual result of severe biodegradation. The top oil, recovered from a drill-stem test and not biodegraded, contains a full suite of triterpanes. The bottom seep oil, in contrast, is heavily biodegraded (gravity 30 hopane and homohopanes. The C<sub>29</sub> hopane is either unaffected or only slightly reduced in concentration. T<sub>m</sub>, T<sub>s</sub>, moretanes, and C<sub>z</sub> (indicated with *) also appear unaffected at this level of biodegradation.
==Internal standards==
==Internal standards==
[[file:oiloil-and-oilsource-rock-correlations_fig8-43.png|thumb|{{figure number|8-43}}See text for explanation.]]
[[file:oiloil-and-oilsource-rock-correlations_fig8-43.png|thumb|{{figure number|4}}See text for explanation.]]
Internal standards can appear in GC/MS data as well as in gas chromatograms. [[:file:oiloil-and-oilsource-rock-correlations_fig8-43.png|Figure 4]] shows m/z 191 (top) and 217 (bottom) mass chromatograms for a seep oil from Papua New Guinea. The three peaks in the top chromatogram to the left of T<sub>s</sub> come from the internal standard, which was supposed to be a single compound but is actually a mixture (probably of various isomers of the same compound). The internal standard was unlabelled on this mass chromatogram and, as such, poses a serious risk even for experienced interpreters since the internal standard peaks might be thought to represent indigenous compounds.
==For additional information==
==For additional information==