Changes

==Determining data reliability==

==Determining data reliability==

In any correlation study, we must decide which data are most reliable for answering the questions about genetic relationships and which data have been affected by postgenetic transformations, such as expulsion, migration, [[biodegradation]], water washing, and thermal cracking. In addition, we must resolve other complicating issues, such as differences in maturity of the samples being compared, facies variations in source rocks, possible mixing of oils from different sources in a single reservoir, and the intrinsic differences between oils and source rock bitumens. These topics are discussed in more detail in Curiale.<ref name=ch08r11>Curiale, J. A., 1993, Oil to source rock correlation. Concepts and case studies, in M. H. Engel, and S. A. Macko, eds., Organic Geochemistry: New York, Plenum Press, p. 473–490.</ref><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>

In any correlation study, we must decide which data are most reliable for answering the questions about genetic relationships and which data have been affected by postgenetic transformations, such as expulsion, migration, [[biodegradation]], water washing, and thermal [[cracking]]. In addition, we must resolve other complicating issues, such as differences in maturity of the samples being compared, facies variations in source rocks, possible mixing of oils from different sources in a single reservoir, and the intrinsic differences between oils and source rock bitumens. These topics are discussed in more detail in Curiale.<ref name=ch08r11>Curiale, J. A., 1993, Oil to source rock correlation. Concepts and case studies, in M. H. Engel, and S. A. Macko, eds., Organic Geochemistry: New York, Plenum Press, p. 473–490.</ref><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>

==Positive correlations==

==Positive correlations==

| Analyze as many samples as possible || A large number of samples provides background information on natural variation that could otherwise be confused with genetic differences.

| Analyze as many samples as possible || A large number of samples provides background information on natural variation that could otherwise be confused with genetic differences.

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| Use samples that have suffered as little postgenetic transformation as possible || Postgenetic transformations—especially [[biodegradation]], cracking, and [http://www.gazprominfo.com/terms/topping-gas/ gas stripping] of oils—can make positive correlations very difficult. Also, all oil–source rock correlations are complicated by the fact that significant compositional changes occur as the oil is expelled from the source rock and migrates.

| Use samples that have suffered as little postgenetic transformation as possible || Postgenetic transformations—especially [[biodegradation]], [[cracking]], and [http://www.gazprominfo.com/terms/topping-gas/ gas stripping] of oils—can make positive correlations very difficult. Also, all oil–source rock correlations are complicated by the fact that significant compositional changes occur as the oil is expelled from the source rock and migrates.

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| Use source rock samples that are mature, not post mature || Since many source rock samples are immature, a comparison with mature oils can be difficult—not only because physical appearances and molecular distributions are different, but also because extracts from immature rocks are sometimes genetically unrelated and compositionally dissimilar to mature extracts and oils.<ref name=ch08r26>Kohnen, M. E. L., S. Schouten, J. S. Sinninghe Damste, J. W. de Leeuw, D. Merrit, and J. M. Hayes, 1992, The combined application of organic sulphur and isotope geochemistry to assess multiple sources of palaeobiochemicals with identical carbon skeletons: Organic Geochemistry, vol. 19, p. 403–419, DOI: 10.1016/0146-6380(92)90008-L.</ref>

| Use source rock samples that are mature, not post mature || Since many source rock samples are immature, a comparison with mature oils can be difficult—not only because physical appearances and molecular distributions are different, but also because extracts from immature rocks are sometimes genetically unrelated and compositionally dissimilar to mature extracts and oils.<ref name=ch08r26>Kohnen, M. E. L., S. Schouten, J. S. Sinninghe Damste, J. W. de Leeuw, D. Merrit, and J. M. Hayes, 1992, The combined application of organic sulphur and isotope geochemistry to assess multiple sources of palaeobiochemicals with identical carbon skeletons: Organic Geochemistry, vol. 19, p. 403–419, DOI: 10.1016/0146-6380(92)90008-L.</ref>

==Negative correlations==

==Negative correlations==

If we can find even one major difference between two samples that cannot be explained by either natural variation or postgenetic transformation, the correlation is considered to be negative and we should conclude that the samples are not genetically related. In some cases, the discrepancy may consist of a single major difference (e.g., presence of an important source-related biomarker in one sample and not in the other). In other cases, the discrepancy may represent the sum of numerous minor differences between the samples. It is crucial in evaluating possible negative correlations that postgenetic effects (e.g., cracking, [[biodegradation]]) not be interpreted as genetic characteristics. Moreover, intrinsic differences between immature extracts and mature extracts or oils should not necessarily be interpreted as negative correlations.

If we can find even one major difference between two samples that cannot be explained by either natural variation or postgenetic transformation, the correlation is considered to be negative and we should conclude that the samples are not genetically related. In some cases, the discrepancy may consist of a single major difference (e.g., presence of an important source-related biomarker in one sample and not in the other). In other cases, the discrepancy may represent the sum of numerous minor differences between the samples. It is crucial in evaluating possible negative correlations that postgenetic effects (e.g., [[cracking]], [[biodegradation]]) not be interpreted as genetic characteristics. Moreover, intrinsic differences between immature extracts and mature extracts or oils should not necessarily be interpreted as negative correlations.

==Positive vs. negative correlations==

==Positive vs. negative correlations==