Oil correlation

	Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Critical elements of the petroleum system
Chapter	Oil–oil and oil–source rock correlations
Author	Douglas W. Waples, Joseph A. Curiale
Link	Web page
Store	AAPG Store

What are correlations?

Correlations are comparisons of two or more samples based on the physical and chemical properties of those samples.

Purpose of correlation studies

The purpose of any correlation study is to determine whether a genetic relationship exists among a group of oil samples or between an oil and a proposed source rock. Positive correlations are always useful because they confirm some proposed concept; but negative correlations can be of even greater value in developing new exploration ideas.

Objectives of correlation studies

Objectives of correlation studies include the following:

Classifying oils in genetic families
Establishing oil–source rock relationships
Addressing problems of reservoir continuity

Applications of correlation studies

Exploration applications of correlation studies include the following:

Determining how many effective source rocks exist in a given area
Proposing migration pathways from kitchen areas to new prospects or plays
Providing input data for volumetric calculations
Providing data for development geology

Parameters

These parameters are of three types:

Elemental—the bulk composition of a sample
Isotopic—ratios of one stable isotope to another in a sample
Molecular—the presence and relative or absolute abundance of certain specific molecules in a sample

Molecular parameters are the most important because they provide the most specific data, including data that can sometimes be used for estimating ages of oils or source rocks.

Selection of correlation parameters

As wide a range of data types as possible should be used in correlations. Particular emphasis should be placed on molecular and isotopic parameters because they carry much more information than do elemental parameters.

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.^[1]^[2]

Positive correlations

A positive correlation between two or more samples occurs when all pieces of evidence are compatible with the existence of a genetic relationship among the samples. Since some differences inevitably exist between any two samples, the key to achieving a positive correlation is not to find samples that are identical but rather to find samples in which the differences are explainable by normal transformation processes, maturity differences, intrinsic differences between oils and source rock bitumens, natural variation (e.g., facies), or analytical uncertainty.

Correlation processes

To increase the quality of and your confidence in positive correlations, use the following table.

To increase confidence in your correlation …	Because …
Measure as many different properties as possible	All positive correlations are based on circumstantial evidence. Therefore, measuring more properties means acquiring more evidence.
Use detailed molecular and isotopic comparisons (e.g., GC/MS or compound-specific isotope analyses)	Molecular and isotopic techniques are very sensitive and provide much “fingerprint” detail about sample compositions.
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.
Use samples that have suffered as little postgenetic transformation as possible	Postgenetic transformations—especially biodegradation, cracking, and 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 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.^[3]

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 correlations tend to reinforce and refine existing exploration concepts, whereas negative correlations often refute existing ideas and offer new opportunities for creative thinkers. Both types of correlation have obvious value to explorationists.

A positive correlation is seldom 100% certain. Our confidence level in any positive correlation increases with the amount and quality of the data and samples used in the correlation. Curiale^[2] discusses the problem of inadequate numbers of samples and overreliance on data of questionable validity. Moreover, some samples are quite easy to correlate positively because they share an unusual characteristic (e.g., the presence of an unusual biomarker). Samples that have no unusual or distinguishing features can often be correlated positively with many other samples, and at least some of those apparent correlations are likely to be in error.

A negative correlation, in contrast, can be quite definitive if the differences are large and if they clearly are not the result of postgenetic transformations. Probably the most difficult negative correlations are those based on numerous small differences, each of which in and of itself would not be conclusive. In such cases, we should ascertain that the differences are not related to variations or differences in facies, maturity, or migration.

References

↑ 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.
↑ ^2.0 ^2.1 Curiale, J. A., 1994, 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: AAPG Memoir 60, p. 251–260.
↑ 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.

External links

find literature about
Oil correlation

[ch08r11-1] 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.

[ch08r12-2] 2.0 ^2.1 Curiale, J. A., 1994, 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: AAPG Memoir 60, p. 251–260.

[ch08r26-3] 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.

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