Changes

  | part    = Critical elements of the petroleum system

  | part    = Critical elements of the petroleum system

  | chapter = Oil–oil and oil–source rock correlations

  | chapter = Oil–oil and oil–source rock correlations

  | frompg  = 8-1

  | frompg  = 8-55

  | topg    = 8-71

  | topg    = 8-57

  | author  = Douglas W. Waples, Joseph A. Curiale

  | author  = Douglas W. Waples, Joseph A. Curiale

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch08/ch08.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch08/ch08.htm

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

Commercial oil reserves were discovered in the Norwegian sector of the North Sea more than twenty years ago. [[Oil–oil and oil–[[source rock]] correlation]]s are now reasonably well known and constrained for this region. Early efforts at correlating oils in this area were documented by Phillips Petroleum Co. workers<ref name=ch08r20>Hughes, W., B., Holba, A., G., Miller, D., E., Richardson, J., S., 1985, Geochemistry of greater Ekofisk crude oils, in Thomas, B., M., eds., Petroleum geochemistry in exploration of the Norwegian Shelf: London, Graham & Trotman, p. 75–92.</ref> following examination of 30 oils from eight fields. In addition to performing conventional compound-fraction separations and GC and [[GC/MS]] analyses, those workers also examined sulfur compound distributions, sulfur and nitrogen contents, and carbon isotope ratios. Their integrated study provides a good example of a modern oil–oil correlation.

Commercial oil reserves were discovered in the Norwegian sector of the [[North Sea]] more than twenty years ago. [[Oil–oil and oil–source rock correlation]]s are now reasonably well known and constrained for this region. Early efforts at correlating oils in this area were documented by Phillips Petroleum Co. workers<ref name=ch08r20>Hughes, W. B., A. G. Holba, D. E. Miller, and J. S. Richardson, 1985, Geochemistry of greater Ekofisk crude oils, in Thomas, B., M., eds., Petroleum geochemistry in exploration of the Norwegian Shelf: London, Graham & Trotman, p. 75–92.</ref> following examination of 30 oils from eight fields. In addition to performing conventional compound-fraction separations and [[Gas chromatography|GC]] and GC/MS analyses, those workers also examined sulfur compound distributions, sulfur and nitrogen contents, and [[Wikipedia:Isotope geochemistry|carbon isotope ratios]]. Their integrated study provides a good example of a modern oil–oil correlation.

==Initial evidence suggests several oil families==

==Initial evidence suggests several oil families==

The figure shows data for a set of oils from eight fields in the Norwegian sector of the North Sea. The left figure is concentration of sulfur in oil plotted against API gravity, showing a trend of decreasing sulfur content with increasing gravity. Note that the Ekofisk and Eldfisk field oils show only small ranges in sulfur content and gravity. The right figure shows ααα-20S/(20S+20R) ratios plotted for each of the eight fields. This wide range extends from a decidedly marginally mature oil (in Hod field) to oils in which this parameter has reached equilibrium.

[[file:oiloil-and-oilsource-rock-correlations_fig8-46.png|thumb|300px|{{figure number|1}}Data for a set of oils from eight fields in the Norwegian sector of the North Sea. From Hughes et al.<ref name=ch08r20 />); reprinted with permission from Graham and Trotman.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-46.png|thumb|{{figure number|8-46}}From Hughes et al.<ref name=ch08r20 />); reprinted with permission from Graham and Trotman.]]

The eight fields are in the Central Graben of the North Sea, where the oils occur in Cretaceous and Jurassic reservoirs at 2800–3500 m true vertical depth. The source is presumed to be Upper Jurassic mudstones, based on a correlation of Ekofisk oil to Kimmeridgian [[source rocks]].<ref name=ch08r55>van den Bark, E., and O. D. Thomas, 1980, [http://archives.datapages.com/data/specpubs/fieldst2/data/a012/a012/0001/0150/0195.htm Ekofisk: first of the giant oil fields in western Europe], in M. T. Halbouty, ed., Giant Oil and Gas Fields of the Decade 1968–1978: AAPG Memoir 30, p. 195–224.</ref> Initial examination of bulk geochemical parameters in the 30 oils indicated wide compositional variation, as illustrated in the percent sulfur vs. [[API gravity]] plot below. Similar variability exists in the nitrogen concentrations and in the distribution of saturated hydrocarbons, aromatic hydrocarbons, NSO, and asphaltene fractions (not shown). Using these data alone, we might conclude that several source-distinctive families exist.

 

[[:file:oiloil-and-oilsource-rock-correlations_fig8-46.png|Figure 1]] shows data for a set of oils from eight fields in the Norwegian sector of the North Sea. The left figure is concentration of sulfur in oil plotted against API gravity, showing a trend of decreasing sulfur content with increasing gravity. Note that the Ekofisk and Eldfisk field oils show only small ranges in sulfur content and gravity. The right figure shows ααα-20S/(20S+20R) ratios plotted for each of the eight fields. This wide range extends from a decidedly marginally mature oil (in Hod field) to oils in which this parameter has reached equilibrium.

==Further evidence shows oils are similar==

==Further evidence shows oils are similar==

The ternary diagram (top) shows the distribution of ααα-20R-steranes by carbon number. The oval encloses all of the samples. The small range for most of the samples within the oval suggests that a single organic source facies could be responsible for most of the oils. In the m/z 191 mass chromatograms (bottom) for a typical Ekofisk and Eldfisk oil, note the peak-to-peak similarity of these oils of similar maturity.

[[file:oiloil-and-oilsource-rock-correlations_fig8-47.png|thumb|300px|{{figure number|2}}(top) Ternary diagram showing the distribution of ααα-20R-steranes by carbon number. (bottom) mass chromatograms for a typical Ekofisk and Eldfisk oil. From Hughes et al.<ref name=ch08r20 />); reprinted with permission from Norwegian Geological Society.]]

 

Examination of correlation parameters that are more specifically source distinctive, however, suggests that the variations within the set of oils are actually minor. The distribution of C₂₇-C₂₉ regular steranes shows that most of the oils cluster rather tightly, with approximately equal concentrations of each homolog ([[:file:oiloil-and-oilsource-rock-correlations_fig8-47.png|Figure 2]]). Furthermore, Hughes et al.<ref name=ch08r20 />) report that δ¹³C values for the oils range from –28.7 ‰ to –26.8 ‰, with 28 of the 30 oils falling in the –28.3 ‰ to –27.1 ‰ range. Finally, terpane distributions show that many of the oils are very similar to one another ([[:file:oiloil-and-oilsource-rock-correlations_fig8-47.png|Figure 2]], bottom).

[[file:oiloil-and-oilsource-rock-correlations_fig8-47.png|thumb|{{figure number|8-47}}From Hughes et al.<ref name=ch08r20 />); reprinted with permission from Norwegian Geological Society.]]

The ternary diagram ([[:file:oiloil-and-oilsource-rock-correlations_fig8-47.png|Figure 2]], top) shows the distribution of ααα-20R-steranes by carbon number. The oval encloses all of the samples. The small range for most of the samples within the oval suggests that a single organic source facies could be responsible for most of the oils. In the m/z 191 mass chromatograms ([[:file:oiloil-and-oilsource-rock-correlations_fig8-47.png|Figure 2]], bottom) for a typical Ekofisk and Eldfisk oil, note the peak-to-peak similarity of these oils of similar maturity.

==Conclusions drawn from the correlation study==

==Conclusions drawn from the correlation study==

==See also==

==See also==

* [[Case histories]]

* [[Oil correlation case histories]]

==References==

==References==

[[Category:Critical elements of the petroleum system]]  

[[Category:Critical elements of the petroleum system]]  

[[Category:Oil–oil and oil–source rock correlations]]

[[Category:Oil–oil and oil–source rock correlations]]