Limitations of oil correlations
| 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 happens to oils and rocks at depth and over time can alter the geochemical parameters measured in correlation studies. Geologists need to be aware of how those changes may limit correlation accuracy. Six main problems limit the utility, accuracy, and confidence of correlations:
- Differing thermal maturities of samples
- Solution-exsolution at different depths
- Biodegradation
- Natural variation among samples
- Multiple sources for oil in a trap
- Lack of distinguishing characteristics
Differing thermal maturities of samples
Oil–oil and oil–source rock correlations are most successful when the samples being compared are at the same or similar thermal maturities. Except in cases of extreme overmaturity of one or more of the samples, these problems can usually be overcome by an experienced interpreter. However, for highly mature samples some information and confidence are inevitably lost. Inexperienced interpreters can easily confuse maturity effects with genetic differences.
Solution-exsolution at different depths
A further limitation in oil–oil correlations exists when migrating gas dissolves part of a crude oil at depth (i.e., at high pressure) and exsolves a portion of its oil components at shallower depths (i.e., at reduced pressure). Because each component of petroleum is soluble to a different degree in natural gas, this solution-exsolution process can radically alter the oil composition and the appearance of gas chromatograms.
Biodegradation
Although biodegradation affects gas chromatograms very strongly, steranes, triterpanes, and isotopes are much less affected. Modern correlation technology can usually overcome differences caused by biodegradation.
Natural variation among samples
Any group of oils will inevitably exhibit variations among the members. This “natural variation” can stem from true differences, sample handling, analytical error, or differences in technique between laboratories. The amount of acceptable or “normal” difference varies greatly, depending on the range of maturities, biodegradation, expulsion and migration (the effects of which are especially large when comparing oils with source rock extracts), and variations in source rock facies. Correlations become increasingly difficult and tenuous as distances between samples increase.
Vertical and lateral rates of change in source rock facies are highly variable but are generally highest in nearshore and nonmarine environments. Katz (in press), for example, documents the extreme variability in source facies in the lacustrine Pematang Formation of central Sumatra and the consequent variability in oil properties. As he notes, correlations in such settings are very difficult and depend critically upon sample coverage of the source facies.
A major part of the art of correlation is deciding whether an observed difference is significant or is better considered to be the result of natural variation. The differences between the various members of a family of oils are often more subtle than the discussions in this chapter suggest. In fact, sometimes the variations within a single family (especially those that cover wide geographic areas or those from nonmarine sources) are as large as the variations between families.
Multiple sources for oil in a trap
Many sedimentary basins contain more than one oil source unit. If two or more source rocks are thermally mature, oils from both may become commingled within a single trap, preventing straightforward oil–source rock correlations. A good example is found in Seifert et al.[1]
Lack of distinguishing characteristics
Some correlation efforts inevitably yield ambiguous results. Some oils are difficult to correlate definitively because they lack distinguishing characteristics. Furthermore, in some basins several source rocks may have very similar geochemical signatures, making it difficult to determine precisely which unit is the actual source for a given oil. In such cases, we often try to acquire additional data in an effort to uncover some unique characteristic that will be the key to a successful correlation.
See also
- Basic principles of oil correlation
- Practical suggestions for oil correlation
- Statistical analysis in oil correlation
References
- ↑ Seifert, W. K., J. M. Moldowan, and R. W. Jones, 1979, Application of biological marker chemistry to petroleum exploration, in Proceedings of the Tenth World Petroleum Congress: London, Heyden & Son, p. 425–440.
External links
| find literature about Limitations of oil correlations |
|
|
|
|