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==Applications==

In gas chromatography the distributions of ''n''-alkanes and isoprenoids are usually most useful. Of particular significance for correlations are the wax content (C₂₃₊), the 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 (Pr/Ph) ratio. The following figures show gas chromatograms of the 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 ''n''-alkanes and isoprenoids are usually most useful. Of particular significance for correlations are the wax content (C₂₃₊), the 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 (Pr/Ph) ratio. The following figures show gas chromatograms of the 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}}

 

==Example: oil of terrestrial origin==

===Example: oil of terrestrial origin===

The figure below is a 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. Significant input of terrigenous organic matter is indicated by a bimodal ''n''-alkane distribution (a second mode in the wax region, from ''n''-C₂₃ to ''n''-C₃₁), a pristane-phytane ratio greater than 2.0, and a strong odd-carbon ''n''-alkane dominance from ''n''-C₂₅ to ''n''-C₃₁. These features are characteristic of deltaic or lacustrine-sourced oils (in this case, from Indonesia).

The figure below is a 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. Significant input of terrigenous organic matter is indicated by a bimodal ''n''-alkane distribution (a second mode in the wax region, from ''n''-C₂₃ to ''n''-C₃₁), a pristane-phytane ratio greater than 2.0, and a strong odd-carbon ''n''-alkane dominance from ''n''-C₂₅ to ''n''-C₃₁. These features are characteristic of deltaic or lacustrine-sourced oils (in this case, from Indonesia).

[[file:oiloil-and-oilsource-rock-correlations_fig8-5.png|thumb|{{figure number|8-5}}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|thumb|{{figure number|8-5}}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.]]

==Example: oil of marine origin==

===Example: oil of marine origin===

The figure below is a gas chromatogram of an extremely waxy oil from the Paradox basin, Utah. It was sourced from a marine anoxic evaporitic carbonate. Here the waxes, which are not derived from terrestrial plants, show an even-carbon preference, and the pristane-phytane ratio is very low (

The figure below is a gas chromatogram of an extremely waxy oil from the Paradox basin, Utah. It was sourced from a marine anoxic evaporitic carbonate. Here the waxes, which are not derived from terrestrial plants, show an even-carbon preference, and the pristane-phytane ratio is very low (

[[file:oiloil-and-oilsource-rock-correlations_fig8-6.png|thumb|{{figure number|8-6}}. Copyright: Unocal.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-6.png|thumb|{{figure number|8-6}}. Copyright: Unocal.]]

==Example: oil of marine algal origin==

===Example: oil of marine algal origin===

The figure below shows a gas chromatogram of a low-wax oil derived from typical marine algae. Pristane-phytane ratios for such oils tend to be slightly above 1.0. This Alaskan oil was derived from a source rock containing predominantly marine algal organic matter.

The figure below shows a gas chromatogram of a low-wax oil derived from typical marine algae. Pristane-phytane ratios for such oils tend to be slightly above 1.0. This Alaskan oil was derived from a source rock containing predominantly marine algal organic matter.

[[file:oiloil-and-oilsource-rock-correlations_fig8-7.png|thumb|{{figure number|8-7}}. Copyright: Unocal.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-7.png|thumb|{{figure number|8-7}}. Copyright: Unocal.]]

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

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

The figure below 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, 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, 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: AAPG Memoir 51, p. 403–423.</ref><ref name=ch08r17>Guthrie, J., M., Pratt, L., M., 1995, Geochemical character and origin of oils in Ordovician reservoir rock, Illinois and Indiana: AAPG Bulletin, vol. 79, p. 1631–1649.</ref>

The figure below 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, 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, 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: AAPG Memoir 51, p. 403–423.</ref><ref name=ch08r17>Guthrie, J., M., Pratt, L., M., 1995, 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_fig8-11.png|thumb|{{figure number|8-11}}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|thumb|{{figure number|8-11}}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.]]

===Other minor components===

==Other minor components==

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 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 maturity or alteration effects.

[[file:oiloil-and-oilsource-rock-correlations_fig8-12.png|thumb|{{figure number|8-12}}From Curiale;<ref name=ch08r10>Curiale, J., A., 1992, Petroleum geochemistry of Texas and Oklahoma oils from the Marathon/Ouachita foldbelt: Chemical Geology, vol. 98, p. 151–173., 10., 1016/0009-2541(92)90096-N</ref> reprinted with permission from Elsevier.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-12.png|thumb|{{figure number|8-12}}From Curiale;<ref name=ch08r10>Curiale, J., A., 1992, Petroleum geochemistry of Texas and Oklahoma oils from the Marathon/Ouachita foldbelt: Chemical Geology, vol. 98, p. 151–173., 10., 1016/0009-2541(92)90096-N</ref> reprinted with permission from Elsevier.]]

===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., 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>

===Limitations due to maturity differences===

==Limitations due to maturity differences==

Maturity-related differences are often observed when comparing oils with each other and with source rock extracts. High-wax oils are affected most strongly by maturity. Maturity differences involving source rocks can present particular difficulties when source rock samples are extremely immature.

Maturity-related differences are often observed when comparing oils with each other and with source rock extracts. High-wax oils are affected most strongly by maturity. Maturity differences involving source rocks can present particular difficulties when source rock samples are extremely immature.

For these reasons, and also because of the possibility of fortuitous similarities between unrelated samples, gas chromatography seldom provides definitive positive correlations. It may, however, provide fairly definitive negative correlations. When used in conjunction with other correlation parameters, it can often be valuable for positive correlations as well, as demonstrated in the following two examples.

For these reasons, and also because of the possibility of fortuitous similarities between unrelated samples, gas chromatography seldom provides definitive positive correlations. It may, however, provide fairly definitive negative correlations. When used in conjunction with other correlation parameters, it can often be valuable for positive correlations as well, as demonstrated in the following two examples.

===Maturity difference example===

==Maturity difference example===

The figure below shows gas chromatograms of two oils from Wyoming. Both were sourced from the Permian Phosphoria Formation, but are reservoired in different fields. The bimodal distribution of ''n''-alkanes in the top oil is consistent with a lower level of maturity than that of the unimodal oil at the bottom. Comparison of these oils using gas chromatography for the purpose of oil–oil correlation must be done with caution because of the maturity differences.

The figure below shows gas chromatograms of two oils from Wyoming. Both were sourced from the Permian Phosphoria Formation, but are reservoired in different fields. The bimodal distribution of ''n''-alkanes in the top oil is consistent with a lower level of maturity than that of the unimodal oil at the bottom. Comparison of these oils using gas chromatography for the purpose of oil–oil correlation must be done with caution because of the maturity differences.

[[file:oiloil-and-oilsource-rock-correlations_fig8-14.png|thumb|{{figure number|8-14}}. Copyright: Unocal.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-14.png|thumb|{{figure number|8-14}}. Copyright: Unocal.]]

===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, gas stripping in the reservoir can selectively remove lighter components, leaving an oil or residue that looks lightly 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, gas stripping in the reservoir can selectively remove lighter components, leaving an oil or residue that looks lightly biodegraded or, in some instances, waxy.

===Example of loss of c₁₅₊ fraction===

==Example of loss of C₁₅₊ fraction==

The figure below shows a gas chromatogram of saturated hydrocarbons in a source rock extract from which the extracting solvent was evaporated too vigorously, leading to loss of some of the C₁₅₊ fraction. This problem can be recognized by looking at the ''n''-C₁₇ and ''n''-C₁₈ peaks. Normally they are almost the same size, but in this case the ''n''-C₁₇ peak is much smaller. Pristane has also been depleted relative to phytane, leading to an erroneous pristane–phytane ratio unless some correction is made.

The figure below shows a gas chromatogram of saturated hydrocarbons in a source rock extract from which the extracting solvent was evaporated too vigorously, leading to loss of some of the C₁₅₊ fraction. This problem can be recognized by looking at the ''n''-C₁₇ and ''n''-C₁₈ peaks. Normally they are almost the same size, but in this case the ''n''-C₁₇ peak is much smaller. Pristane has also been depleted relative to phytane, leading to an erroneous pristane–phytane ratio unless some correction is made.

[[file:oiloil-and-oilsource-rock-correlations_fig8-16.png|thumb|{{figure number|8-16}}. Copyright: Unocal.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-16.png|thumb|{{figure number|8-16}}. Copyright: Unocal.]]

===Limitations due to internal standards===

==Limitations due to internal standards==

An internal standard is commonly coinjected with the sample to aid in quantification during gas chromatography. If this internal standard is not clearly labeled on the chromatogram, it may be confused with an indigenous compound and erroneously used in correlation.

An internal standard is commonly coinjected with the sample to aid in quantification during gas chromatography. If this internal standard is not clearly labeled on the chromatogram, it may be confused with an indigenous compound and erroneously used in correlation.

[[file:oiloil-and-oilsource-rock-correlations_fig8-18.png|thumb|{{figure number|8-18}}. Copyright: Murray et al. (1993); courtesy Australian Geological Survey Organization.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-18.png|thumb|{{figure number|8-18}}. Copyright: Murray et al. (1993); courtesy Australian Geological Survey Organization.]]

===Limitations due to contaminated samples===

==Limitations due to contaminated samples==

In other cases, particularly in rock extracts, the sample may be contaminated. Plasticizers, for example, are common but are usually easy to recognize. The figure below shows a gas chromatogram of saturated hydrocarbons from a source rock sample contaminated with bits of plastic. The largest peak is dioctylphthalate, derived from contact with the plastic.

In other cases, particularly in rock extracts, the sample may be contaminated. Plasticizers, for example, are common but are usually easy to recognize. The figure below shows a gas chromatogram of saturated hydrocarbons from a source rock sample contaminated with bits of plastic. The largest peak is dioctylphthalate, derived from contact with the plastic.

[[file:oiloil-and-oilsource-rock-correlations_fig8-19.png|thumb|{{figure number|8-19}}See text for explanation.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-19.png|thumb|{{figure number|8-19}}See text for explanation.]]

===Biodegradation limitations===

==Biodegradation limitations==

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, ''n''-alkanes are removed selectively, leading to significant loss of information.