Changes

==Effects of maturation and migration==

==Effects of maturation and migration==

Sterane and triterpane concentrations decrease greatly during oil cracking. Therefore, thermal condensates (light oils) do not normally contain large amounts of steranes and triterpanes. Consequently, steranes and triterpanes are usually of little value in high-maturity oils and condensates. Furthermore, some condensates that do contain steranes and triterpanes may have picked up all or most of them during [[migration]] by extraction from the rocks through which they passed (see Waples and Machihara<ref name=ch08r59>Waples, D. W., and T. Machihara, 1991, Biomarkers for geologists: Tulsa, AAPG, 91 p.</ref> for examples and references). Finally, because triterpane and particularly sterane distributions change greatly during maturation, correlations of those biomarkers between an immature [[source rock]] and an oil can be very difficult. In such cases a strictly numerical approach, working with peak ratios and relative proportions, can be better than a visual one, since the eye can be unduly influenced by maturity-related differences instead of genetic characteristics.

[[Sterane]] and [[triterpane]] concentrations decrease greatly during oil [[cracking]]. Therefore, thermal condensates (light oils) do not normally contain large amounts of steranes and triterpanes. Consequently, steranes and triterpanes are usually of little value in high-[[maturity]] oils and condensates. Furthermore, some condensates that do contain steranes and triterpanes may have picked up all or most of them during [[migration]] by extraction from the rocks through which they passed (see Waples and Machihara<ref name=ch08r59>Waples, D. W., and T. Machihara, 1991, Biomarkers for geologists: Tulsa, AAPG, 91 p.</ref> for examples and references). Finally, because triterpane and particularly sterane distributions change greatly during [[maturation]], correlations of those biomarkers between an immature [[source rock]] and an oil can be very difficult. In such cases a strictly numerical approach, working with peak ratios and relative proportions, can be better than a visual one, since the eye can be unduly influenced by maturity-related differences instead of genetic characteristics.

==Biodegradation and sterane distributions==

==Biodegradation and sterane distributions==

[[file:oiloil-and-oilsource-rock-correlations_fig8-40.png|300px|thumb|{{figure number|1}}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. From Curiale<ref name=ch08r12>Curiale, J. A., 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0250/0251.htm Correlation of oils and source rocks—a conceptual and historical perspective], in L. B. Magoon, and W. G. Dow, eds., The [[Petroleum system]]—From Source to Trap: [http://store.aapg.org/detail.aspx?id=1022 AAPG Memoir 60], p. 251–260.</ref> reprinted with permission from AAPG.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-40.png|300px|thumb|{{figure number|1}}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. From Curiale<ref name=ch08r12>Curiale, J. A., 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0250/0251.htm Correlation of oils and source rocks—a conceptual and historical perspective], in L. B. Magoon, and W. G. Dow, eds., The [[Petroleum system]]—From Source to Trap: [http://store.aapg.org/detail.aspx?id=1022 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]], 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.

[[file:oiloil-and-oilsource-rock-correlations_fig8-42.png|300px|thumb|{{figure number|3}}m/z 191 mass chromatograms of two genetically related oils from Papua New Guinea.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-42.png|300px|thumb|{{figure number|3}}m/z 191 mass chromatograms of two genetically related oils from Papua New Guinea.]]

[[: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 C₂₉ hopane is either unaffected or only slightly reduced in concentration. T_m, T_s, moretanes, and C_z (indicated with *) also appear unaffected at this level of biodegradation.

[[: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 C₂₉ hopane is either unaffected or only slightly reduced in concentration. T_m, T_s, moretanes, and C_z (indicated with *) also appear unaffected at this level of biodegradation.

==Internal standards==

==Internal standards==

[[Category:Critical elements of the petroleum system]]  

[[Category:Critical elements of the petroleum system]]  

[[Category:Oil–oil and oil–source rock correlations]]

[[Category:Oil–oil and oil–source rock correlations]]