Changes

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==

==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==

===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.

[[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.]]

[[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.]]

[http://www.oiltracers.com/services/exploration-geochemistry/oil-biodegradation.aspx 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.

[[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===