Changes

  | part    = Critical elements of the petroleum system

  | part    = Critical elements of the petroleum system

  | chapter = Oil–oil and oil–source rock correlations

  | chapter = Oil–oil and oil–source rock correlations

  | frompg  = 8-1

  | frompg  = 8-18

  | topg    = 8-71

  | topg    = 8-29

  | author  = Douglas W. Waples, Joseph A. Curiale

  | author  = Douglas W. Waples, Joseph A. Curiale

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch08/ch08.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch08/ch08.htm

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

[[File:GCMS closed.jpg|thumb|400px|GCMS closed|Gas chromatography mass spectrometer. Photo by [https://commons.wikimedia.org/wiki/User:Polimerek Polimerek].]]

[[File:GCMS_closed.jpg|thumb|400px|GCMS closed|Gas chromatography mass spectrometer. Photo by [https://commons.wikimedia.org/wiki/User:Polimerek Polimerek].]]

In [[gas chromatography]] the distributions of [[Wikipedia:Alkane#Linear_alkanes|''n''-alkanes]] and [[Wikipedia:Terpenoid|isoprenoids]] are usually most useful. Of particular significance for correlations are the wax content (C₂₃₊), the [[Wikipedia:Carbon Preference Index|carbon preference index (CPI)]]<ref name=ch08r3>Bray, E., E., Evans, E., D., 1961, Distribution of n-paraffins as a clue to recognition of source beds; ''Geochimica et Cosmochimica Acta'', vol. 22, p. 2–15., 10., 1016/0016-7037(61)90069-2</ref> and the [[pristane-phytane ratio]] (Pr/Ph). The following figures show gas chromatograms of the [[Wikipedia:Saturate, aromatic, resin and asphaltene|saturate fractions]] of several oils, illustrating the differences that can be observed in these compound classes as a result of differences in the source material.

In [[gas chromatography]] the distributions of [[Wikipedia:Alkane#Linear_alkanes|''n''-alkanes]] and [[Wikipedia:Terpenoid|isoprenoids]] are usually most useful. Of particular significance for correlations are the wax content (C₂₃₊), the [[Wikipedia:Carbon Preference Index|carbon preference index (CPI)]]<ref name=ch08r3>Bray, E. E., E. D. Evans, 1961, Distribution of n-paraffins as a clue to recognition of source beds; ''Geochimica et Cosmochimica Acta'', vol. 22, p. 2–15., 10., 1016/0016-7037(61)90069-2</ref> and the [[pristane-phytane ratio]] (Pr/Ph). The following figures show gas chromatograms of the [[Wikipedia:Saturate, aromatic, resin and asphaltene|saturate fractions]] of several oils, illustrating the differences that can be observed in these compound classes as a result of differences in the source material.

{{Limit TOC|2}}

{{Limit TOC|2}}

==Examples==

==Examples==

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file:oiloil-and-oilsource-rock-correlations_fig8-5.png|{{figure number|1}}Gas chromatogram of a high-wax oil of terrestrial origin with an odd-carbon preference in the wax region and a high pristane-phytane ratio typical of coaly or certain nearshore aquatic environments. From Robinson;<ref name=ch08r46>Robinson, K., M., 1987, An overview of [[source rock]]s and oils in Indonesia: Proceedings, Indonesian Petroleum Association 16th Annual Convention, p. 97–122.</ref> reprinted with permission from the Indonesian Petroleum Association.

file:oiloil-and-oilsource-rock-correlations_fig8-5.png|{{figure number|1}}Gas chromatogram of a high-wax oil of terrestrial origin with an odd-carbon preference in the wax region and a high pristane-phytane ratio typical of coaly or certain nearshore aquatic environments. From Robinson;<ref name=ch08r46>Robinson, K. M., 1987, An overview of [[source rock]]s and oils in Indonesia: Proceedings, Indonesian Petroleum Association 16th Annual Convention, p. 97–122.</ref> reprinted with permission from the Indonesian Petroleum Association.

file:oiloil-and-oilsource-rock-correlations_fig8-6.png|{{figure number|2}}Gas chromatogram of an extremely waxy oil from the Paradox basin, Utah. Copyright: Unocal.

file:oiloil-and-oilsource-rock-correlations_fig8-6.png|{{figure number|2}}Gas chromatogram of an extremely waxy oil from the Paradox basin, Utah. Copyright: Unocal.

file:oiloil-and-oilsource-rock-correlations_fig8-7.png|{{figure number|3}}Gas chromatogram of a low-wax oil derived from typical marine algae. Copyright: Unocal.

file:oiloil-and-oilsource-rock-correlations_fig8-7.png|{{figure number|3}}Gas chromatogram of a low-wax oil derived from typical marine algae. Copyright: Unocal.

===Example: Oil derived from ''G. prisca''===

===Example: Oil derived from ''G. prisca''===

[[:file:oiloil-and-oilsource-rock-correlations_fig8-8.png|Figure 4]] is a gas chromatogram of saturates from an [[Ordovician]]-sourced oil from the Illinois Basin. It shows a low-wax oil derived primarily from the primitive organism ''Gloeocapsamorpha prisca''. Samples derived from ''G. prisca'' show strong odd-carbon preferences up to ''n''-C₁₉, and have very low concentrations of both pristane and phytane.<ref name=ch08r44>Reed, J., D., Illich, H., A., Horsfield, B., 1986, Biochemical evolutionary significance of Ordovician oils and their sources: Organic Geochemistry, vol. 10, p. 347–358., 10., 1016/0146-6380(86)90035-5</ref><ref name=ch08r28>Longman, M., W., Palmer, S., E., 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0008/0900/0938.htm Organic geochemistry of mid-continent middle and late Ordovician oils]: AAPG Bulletin, vol. 71, p. 938–950.</ref><ref name=ch08r18>Hatch, J., R., Risatti, J., B., King, J., D., 1990, [http://archives.datapages.com/data/specpubs/basinar3/data/a134/a134/0001/0400/0403.htm Geochemistry of Illinois basin oils and hydrocarbon source rocks], in Leighton, M., W., Kolata, D., R., Oltz, D., F., Eidel, J., J., eds., Interior cratonic basins: [http://store.aapg.org/detail.aspx?id=1182 AAPG Memoir 51], p. 403–423.</ref><ref name=ch08r17>Guthrie, J., M., Pratt, L., M., 1995, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0079/0011/1600/1631.htm Geochemical character and origin of oils in Ordovician reservoir rock, Illinois and Indiana]: AAPG Bulletin, vol. 79, p. 1631–1649.</ref>

[[:file:oiloil-and-oilsource-rock-correlations_fig8-8.png|Figure 4]] is a gas chromatogram of saturates from an [[Ordovician]]-sourced oil from the Illinois Basin. It shows a low-wax oil derived primarily from the primitive organism ''Gloeocapsamorpha prisca''. Samples derived from ''G. prisca'' show strong odd-carbon preferences up to ''n''-C₁₉, and have very low concentrations of both pristane and phytane.<ref name=ch08r44>Reed, J. D., H. A. Illich, B. Horsfield, 1986, Biochemical evolutionary significance of Ordovician oils and their sources: Organic Geochemistry, vol. 10, p. 347–358., 10., 1016/0146-6380(86)90035-5</ref><ref name=ch08r28>Longman, M. W., S. E. Palmer, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0008/0900/0938.htm Organic geochemistry of mid-continent middle and late Ordovician oils]: AAPG Bulletin, vol. 71, p. 938–950.</ref><ref name=ch08r18>Hatch, J. R., J. B. Risatti, and J. D. King, 1990, [http://archives.datapages.com/data/specpubs/basinar3/data/a134/a134/0001/0400/0403.htm Geochemistry of Illinois basin oils and hydrocarbon source rocks], in M. W. Leighton, D. R. Kolata, D. F. Oltz, and J. J. Eidel, eds., Interior cratonic basins: [http://store.aapg.org/detail.aspx?id=1182 AAPG Memoir 51], p. 403–423.</ref><ref name=ch08r17>Guthrie, J. M., and L. M. Pratt, 1995, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0079/0011/1600/1631.htm Geochemical character and origin of oils in Ordovician reservoir rock, Illinois and Indiana]: AAPG Bulletin, vol. 79, p. 1631–1649.</ref>

==G. prisca==

==G. prisca==

==Bisnorhopane==

==Bisnorhopane==

[[file:oiloil-and-oilsource-rock-correlations_fig8-11.png|300px|left|thumb|{{figure number|7}}Gas chromatograms of two extracts of immature source rock candidates in southern California (Monterey Formation, Santa Maria basin). From Curiale et al.;<ref name=ch08r13>Curiale, J., A., Cameron, D., Davis, D., V., 1985, Biological marker distribution and significance in oils and rocks of the Monterey Formation, California: Geochimica et Cosmochimica Acta, vol. 49, p. 271–288., 10., 1016/0016-7037(85)90210-8</ref> reprinted with permission from Elsevier.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-11.png|300px|left|thumb|{{figure number|7}}Gas chromatograms of two extracts of immature source rock candidates in southern California (Monterey Formation, Santa Maria basin). From Curiale et al.;<ref name=ch08r13>Curiale, J. A., D. Cameron, and D. V. Davis, 1985, Biological marker distribution and significance in oils and rocks of the Monterey Formation, California: Geochimica et Cosmochimica Acta, vol. 49, p. 271–288., 10., 1016/0016-7037(85)90210-8</ref> reprinted with permission from Elsevier.]]

An unusual biomarker is 28, 30-bisnorhopane, which can be extremely abundant in those few samples where it is present. [[:file:oiloil-and-oilsource-rock-correlations_fig8-11.png|Figure 7]] shows gas chromatograms of two extracts of immature [[source rock]] candidates in southern California ([[Monterey Formation]], [[Santa Maria Basin]]). They contain bisnorhopane as well as a series of monoaromatic steroid hydrocarbons. These two biomarker types, together with the low [[pristane-phytane ratio]]s shown here, are characteristic of Monterey oils. These distinctive characteristics provide a fairly confident correlation to Monterey oils, despite the very low [[Maturation|maturity]] of the source rocks.

An unusual biomarker is 28, 30-bisnorhopane, which can be extremely abundant in those few samples where it is present. [[:file:oiloil-and-oilsource-rock-correlations_fig8-11.png|Figure 7]] shows gas chromatograms of two extracts of immature [[source rock]] candidates in southern California ([[Monterey Formation]], [[Santa Maria Basin]]). They contain bisnorhopane as well as a series of monoaromatic steroid hydrocarbons. These two biomarker types, together with the low [[pristane-phytane ratio]]s shown here, are characteristic of Monterey oils. These distinctive characteristics provide a fairly confident correlation to Monterey oils, despite the very low [[Maturation|maturity]] of the source rocks.

Other minor components present in gas chromatograms can be used for correlations, even when compound identities are not known. However, one should always treat unidentified compounds with caution, since they may represent contaminants, or their concentrations may be affected by [[Maturation|maturity]] or alteration effects.

Other minor components present in gas chromatograms can be used for correlations, even when compound identities are not known. However, one should always treat unidentified compounds with caution, since they may represent contaminants, or their concentrations may be affected by [[Maturation|maturity]] or alteration effects.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-12.png|Figure 8]] shows gas chromatograms of two oils from Texas and Oklahoma. Full-scale chromatograms on the left show the oils are not [http://www.oiltracers.com/services/exploration-geochemistry/oil-biodegradation.aspx degraded], a condition necessary for this type of correlation. The shaded areas indicate the regions expanded on the right. Based on the many similarities in the expanded-scale chromatograms and on other geochemical similarities, it was concluded that these two oils were derived from the same lower Paleozoic source facies.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-12.png|Figure 8]] shows gas chromatograms of two oils from Texas and Oklahoma. Full-scale chromatograms on the left show the oils are not [[biodegradation|degraded]], a condition necessary for this type of correlation. The shaded areas indicate the regions expanded on the right. Based on the many similarities in the expanded-scale chromatograms and on other geochemical similarities, it was concluded that these two oils were derived from the same lower Paleozoic source facies.

==Star diagrams==

==Star diagrams==

Star diagrams (polar plots) are sometimes used to display the relative amounts of a series of peaks in a chromatogram. See, for example, Kaufman et al.<ref name=ch08r23>Kaufman, R., L., Ahmad, A., S., Hempkins, W., B., 1987, A new technique for the analysis of commingled oils and its application to production allocation calculations, in Proceedings of the 16th Annual Convention of the Indonesian Petroleum Association: p. 247–268.</ref>

Star diagrams (polar plots) are sometimes used to display the relative amounts of a series of peaks in a chromatogram. See, for example, Kaufman et al.<ref name=ch08r23>Kaufman, R. L., A. S. Ahmad, and W. B. Hempkins, 1987, A new technique for the analysis of commingled oils and its application to production allocation calculations, in Proceedings of the 16th Annual Convention of the Indonesian Petroleum Association: p. 247–268.</ref>

==Limitations due to maturity differences==

==Limitations due to maturity differences==

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file:oiloil-and-oilsource-rock-correlations_fig8-13.png|{{figure number|9}}Gas chromatograms of two oils from Wyoming. From Peters and Moldowan;<ref name=ch08r39>Peters, K., E., Moldowan, J., M., 1993, The Biomarker Guide—Interpreting [[Molecular fossils]] in Petroleum and Ancient Sediments: Englewood Cliffs, New Jersey, Prentice-Hall, 363 p.</ref> reprinted with permission from Prentice-Hall.

file:oiloil-and-oilsource-rock-correlations_fig8-13.png|{{figure number|9}}Gas chromatograms of two oils from Wyoming. From Peters and Moldowan;<ref name=ch08r39>Peters, K., E., and J. M. Moldowan, 1993, The Biomarker Guide—Interpreting [[Molecular fossils]] in Petroleum and Ancient Sediments: Englewood Cliffs, New Jersey, Prentice-Hall, 363 p.</ref> reprinted with permission from Prentice-Hall.

file:oiloil-and-oilsource-rock-correlations_fig8-14.png|{{figure number|10}}Gas chromatograms of saturated hydrocarbons from an immature extract of coaly organic matter (top) and an oil with a fairly high wax content believed to have been sourced from a similar facies (bottom). Copyright: Unocal.

file:oiloil-and-oilsource-rock-correlations_fig8-14.png|{{figure number|10}}Gas chromatograms of saturated hydrocarbons from an immature extract of coaly organic matter (top) and an oil with a fairly high wax content believed to have been sourced from a similar facies (bottom). Copyright: Unocal.

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</gallery>

==Limitations due to evaporative loss==

==Limitations due to evaporative loss==

Evaporative loss can affect the appearance of gas chromatograms. When the solvent is removed too fast during recovery of source rock extracts, selective loss of some of the C₁₅₊ components can occur. This loss not only alters the appearance of the gas chromatograms but may also change compound ratios (for example, the [[pristane-phytane ratio]] through selective loss of pristane). In some instances, [http://www.gazprominfo.com/terms/topping-gas/ gas stripping] in the reservoir can selectively remove lighter components, leaving an oil or residue that looks lightly [http://www.oiltracers.com/services/exploration-geochemistry/oil-biodegradation.aspx biodegraded] or, in some instances, waxy.

Evaporative loss can affect the appearance of gas chromatograms. When the solvent is removed too fast during recovery of source rock extracts, selective loss of some of the C₁₅₊ components can occur. This loss not only alters the appearance of the gas chromatograms but may also change compound ratios (for example, the [[pristane-phytane ratio]] through selective loss of pristane). In some instances, [http://www.gazprominfo.com/terms/topping-gas/ gas stripping] in the reservoir can selectively remove lighter components, leaving an oil or residue that looks lightly [[biodegradation|biodegraded]] or, in some instances, waxy.

==Example of loss of C₁₅₊ fraction==

==Example of loss of C₁₅₊ fraction==

==Limitations due to internal standards==

==Limitations due to internal standards==

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file:oiloil-and-oilsource-rock-correlations_fig8-17.png|{{figure number|13}}Gas chromatograms of the saturated hydrocarbons from two seep oils from Papua New Guinea.

file:Figure8-17.png|{{figure number|13}}Gas chromatograms of the saturated hydrocarbons from two seep oils from Papua New Guinea.

file:oiloil-and-oilsource-rock-correlations_fig8-18.png|{{figure number|14}}Gas chromatogram of the saturated hydrocarbons from the Lufa seep oil, another seep oil from Papua New Guinea. Copyright: Murray et al.;<ref name=Murray>Murray, A. P., R. E. Summons, J. Bradshaw, and B. Pawih, 1993, Cenozoic Ooil in Paua New Guinea--evidence from geochemical analysis of two newly discovered seeps, in G. J. Carman and Z. Carman, eds., Petroleum Exploration and Development in Papua New Guinea: Proceedings of the Second PNG Petroleum Convention, Australian Geological Survey, p. 489-498</ref> courtesy Australian Geological Survey Organization.

file:oiloil-and-oilsource-rock-correlations_fig8-18.png|{{figure number|14}}Gas chromatogram of the saturated hydrocarbons from the Lufa seep oil, another seep oil from Papua New Guinea. Copyright: Murray et al.;<ref name=Murray>Murray, A. P., R. E. Summons, J. Bradshaw, and B. Pawih, 1993, Cenozoic Ooil in Paua New Guinea--evidence from geochemical analysis of two newly discovered seeps, in G. J. Carman and Z. Carman, eds., Petroleum Exploration and Development in Papua New Guinea: Proceedings of the Second PNG Petroleum Convention, Australian Geological Survey, p. 489-498</ref> courtesy Australian Geological Survey Organization.

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===Examples of problems with internal standards===

===Examples of problems with internal standards===

[[:file:oiloil-and-oilsource-rock-correlations_fig8-17.png|Figure 13]] shows gas chromatograms of the saturated hydrocarbons from two seep oils from Papua New Guinea. Both samples contain an internal standard that [http://www.thefreedictionary.com/elution elutes] just before ''n''-C₂₁. Because the left oil is [http://www.oiltracers.com/services/exploration-geochemistry/oil-biodegradation.aspx biodegraded], the detector sensitivity was set very high to record the trace amounts of remaining [[Wikipedia:Alkane#Linear_alkanes|''n''-alkanes]]. Consequently, the internal standard peak is very tall. In contrast, the right oil (undegraded) shows a much smaller internal standard peak because the detector sensitivity was lower.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-17.png|Figure 13]] shows gas chromatograms of the saturated hydrocarbons from two seep oils from Papua New Guinea. Both samples contain an internal standard that [http://www.thefreedictionary.com/elution elutes] just before ''n''-C₂₁. Because the left oil is [[biodegradation|biodegraded]], the detector sensitivity was set very high to record the trace amounts of remaining [[Wikipedia:Alkane#Linear_alkanes|''n''-alkanes]]. Consequently, the internal standard peak is very tall. In contrast, the right oil (undegraded) shows a much smaller internal standard peak because the detector sensitivity was lower.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-18.png|Figure 14]] shows a gas chromatogram of the saturated hydrocarbons from the Lufa seep oil, another seep oil from Papua New Guinea. Because this work was carried out by a different laboratory than the analyses in the previous figure, the internal standard used here is different. This standard elutes just before ''n''-C₂₂. The pristane–''n''-C₁₇ ratio suggests that this oil is different from those in [[:file:oiloil-and-oilsource-rock-correlations_fig8-17.png|Figure 13]], a fact confirmed by other geochemical evidence.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-18.png|Figure 14]] shows a gas chromatogram of the saturated hydrocarbons from the Lufa seep oil, another seep oil from Papua New Guinea. Because this work was carried out by a different laboratory than the analyses in the previous figure, the internal standard used here is different. This standard elutes just before ''n''-C₂₂. The pristane–''n''-C₁₇ ratio suggests that this oil is different from those in [[:file:oiloil-and-oilsource-rock-correlations_fig8-17.png|Figure 13]], a fact confirmed by other geochemical evidence.

==Biodegradation limitations==

==Biodegradation limitations==

[[file:oiloil-and-oilsource-rock-correlations_fig8-20.png|400px|thumb|{{figure number|16}}(left) Gas chromatograms of three oils from a common source in central Myanmar. A waxy oil (top) is sequentially converted into an extremely biodegraded remnant oil (bottom). (right) Whole-oil gas chromatograms of three oils from the offshore Mackenzie Delta, northwestern Canada, showing a modest decrease in the pristane–phytane ratio, apparently as the result of minor biodegradation. From Curiale et al.;<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 Magoon, L., B., Dow, W., G., eds., The [[Petroleum system]]—From Source to Trap: [http://store.aapg.org/detail.aspx?id=1022 AAPG Memoir 60], p. 251–260.</ref> and Curiale<ref name=ch08r9>Curiale, J., A., 1991, The petroleum geochemistry of Canadian Beaufort Tertiary “non-marine” oils: Chemical Geology, vol. 93, p. 21–45., 10., 1016/0009-2541(91)90062-V</ref> reprinted with permission from Elsevier.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-20.png|400px|thumb|{{figure number|16}}(left) Gas chromatograms of three oils from a common source in central Myanmar. A waxy oil (top) is sequentially converted into an extremely biodegraded remnant oil (bottom). (right) Whole-oil gas chromatograms of three oils from the offshore Mackenzie Delta, northwestern Canada, showing a modest decrease in the pristane–phytane ratio, apparently as the result of minor biodegradation. From Curiale et al.;<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> and Curiale<ref name=ch08r9>Curiale, J. A., 1991, The petroleum geochemistry of Canadian Beaufort Tertiary “non-marine” oils: Chemical Geology, vol. 93, p. 21–45., 10., 1016/0009-2541(91)90062-V</ref> reprinted with permission from Elsevier.]]

Biodegradation can severely alter gas chromatograms. In the earliest stages of biodegradation, ''n''-alkanes are removed selectively, leading to significant loss of information.

[[Biodegradation]] can severely alter gas chromatograms. In the earliest stages of biodegradation, [[Wikipedia:Alkane#Linear_alkanes|''n''-alkanes]] are removed selectively, leading to significant loss of information.

===Examples of biodegradation===

===Examples of biodegradation===

[[:file:oiloil-and-oilsource-rock-correlations_fig8-20.png|Figure 16]] (left) shows gas chromatograms of three oils from a common source in central Myanmar. A waxy oil (top) is sequentially converted into an extremely biodegraded remnant oil (bottom). The API gravity of the oil decreases and sulfur content increases substantially with increasing biodegradation.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-20.png|Figure 16]] (left) shows gas chromatograms of three oils from a common source in central Myanmar. A waxy oil (top) is sequentially converted into an extremely biodegraded remnant oil (bottom). The [[API gravity]] of the oil decreases and sulfur content increases substantially with increasing biodegradation.

Pristane–phytane ratios may also be affected slightly at low stages of biodegradation. At moderate levels these compounds are removed completely (left figure). The right figure below shows whole-oil gas chromatograms of three oils from the offshore Mackenzie Delta, northwestern Canada, showing a modest decrease in the pristane–phytane ratio, apparently as the result of minor biodegradation.

[[Pristane-phytane ratio]]s may also be affected slightly at low stages of biodegradation. At moderate levels these compounds are removed completely (left figure). The right figure below shows whole-oil gas chromatograms of three oils from the offshore [[Mackenzie Delta]], northwestern Canada, showing a modest decrease in the pristane-phytane ratio, apparently as the result of minor biodegradation.

==See also==

==See also==

[[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]]