Practical suggestions for 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 |
Use many analytical tools
The importance of a diversified set of analytical tools cannot be overemphasized. We have noted that many published correlation efforts often rely unduly on biomarker analyses, often to the exclusion of elemental data and even stable carbon isotope ratios. This reliance on biomarkers undoubtedly arises because of the widespread and rapid availability of molecular data through gas chromatography/mass spectrometry (GC/MS) analysis and is promoted by the existence of a large database of molecular analyses of crude oils. Nevertheless, excessive reliance on a single analytical technique can lead to erroneous correlation conclusions, especially since many of the biomarkers used in correlations are present only in concentrations of a few parts per million.
Because oils are susceptible to many transformations during expulsion and migration and in the reservoir—including cracking, phase separation, and biodegradation—correlation techniques and parameters must be selected to minimize complications due to postgenetic transformations. In general, oil–source rock correlations are more difficult than oil–oil correlations, largely because we seldom have samples from the effective (mature) source rocks themselves. Problems arise from variations in facies between the basin center and the sampling point as well as from potentially large maturity differences. Condensates can be difficult to correlate because they have lost much of their valuable information during cracking, phase separation, or both.
Integrate geochemistry with geology
It has been all too common to base correlations entirely or mainly on geochemical considerations without proper regard for the geological content. While it is true that geochemical and geological considerations should be separated in the initial stages of a correlation study to ensure the geochemical conclusions are truly independent, the final story should fully integrate the geochemical model within the geologic framework. Although geochemical data can often give new insights, all geochemical conclusions must ultimately be consistent with the geologic facts. Failure to properly integrate geologic and geochemical data can lead to geochemically based correlations that are difficult or impossible to justify geologically.
One of the best examples of such a discrepancy is in an early study of oils of the Michigan basin[1] in which Devonian oils above the thick regional Silurian salt were correlated positively with Ordovician oils below the salt, without regard for the geologic problems involved in the proposed migration. Subsequent studies using more sensitive technology and larger numbers of samples show that the positive correlation was in fact fortuitous[2][3][4][5] and that migration through the salt had not occurred.
See also
- Basic principles of oil correlation
- Limitations of oil correlations
- Statistical analysis in oil correlation
References
- ↑ Vogler, E. A., P. A. Meyers, and W. A. Moore, 1981, Comparison of Michigan basin crude oils: Geochimica et Cosmochimica Acta, vol. 45, p. 2287–2293., 10., 1016/0016-7037(81)90078-8
- ↑ Illich, H. A., and P. L. Grizzle, 1983, Comment on “Comparison of Michigan basin crude oils” by Vogler: Geochimica et Cosmochimica Acta, vol. 47, p. 1151–1155., 10., 1016/0016-7037(83)90245-4
- ↑ Pruitt, J., 1983, Comment on “Comparison of Michigan basin crude oils” by Vogler: Geochimica et Cosmochimica Acta, vol. 47, p. 1157–1159., 10., 1016/0016-7037(83)90246-6
- ↑ Waples, D. W., 1985, Geochemistry in Petroleum Exploration: Boston, IHRDC, 232 p.
- ↑ Rullkötter, J., P. A. Meyers, R. G. Schaefer, and K. W. Dunham, 1986, Oil generation in the Michigan basin: a biological marker and carbon isotope approach, in D. Leythaeuser, and J. Rullkötter, eds., Advances in Organic Geochemistry 1985: Oxford, Pergamon, p. 359–376.
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