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  = 8-58

  | topg    = 8-71

  | topg    = 8-60

  | 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

==Geological setting==

==Geological setting==

[[file:oiloil-and-oilsource-rock-correlations_fig8-48.png|thumb|{{figure number|1}}From Mello et al.;<ref name=ch08r30>Mello, M., R., Gaglianone, P., C., Brassell, S., C., Maxwell, J., R., 1988a, Geochemical and biological marker assessment of depositional environments using Brazilian offshore oils: Marine and Petroleum Geology, vol. 5, p. 205–223., 10., 1016/0264-8172(88)90002-5</ref> reprinted with permission from Marine and Petroleum Geology.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-48.png|thumb|300px|{{figure number|1}}Generalized west–east [[cross section]] from the Brazilian [[continental margin]], with the coastline approximately at the western margin. From Mello et al.;<ref name=ch08r30>Mello, M. R., P. C. Gaglianone, S. C. Brassell, and J. R. Maxwell, 1988a, Geochemical and biological marker assessment of depositional environments using Brazilian offshore oils: Marine and Petroleum Geology, vol. 5, p. 205–223, DOI: 10.1016/0264-8172(88)90002-5.</ref> reprinted with permission from Marine and Petroleum Geology.]]

The offshore margin of Brazil contains oil reserves in Cretaceous and lower Tertiary reservoirs within several large basins. These basins, the product of rifting of Africa from South America in the Early Cretaceous, contain several source units representing depositional settings ranging from lacustrine to deltaic to marine.

The [[Offshore exploration and exploitation|offshore]] margin of Brazil contains oil reserves in Cretaceous and lower [[Tertiary]] reservoirs within several large basins. These basins, the product of rifting of Africa from South America in the Early Cretaceous, contain several source units representing depositional settings ranging from lacustrine to deltaic to marine.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-48.png|Figure 1]] is a generalized west–east cross section from the Brazilian continental margin, with the coastline approximately at the western margin. Note the wide range of depositional settings, corresponding to the wide range of potential [[source rock]]s for the five source-distinctive groups of oils in these marginal basins.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-48.png|Figure 1]] is a generalized west–east cross section from the Brazilian continental margin, with the coastline approximately at the western margin. Note the wide range of depositional settings, corresponding to the wide range of potential [[source rock]]s for the five source-distinctive groups of oils in these marginal basins.

==Parameters measured==

==Parameters measured==

To understand the source families responsible for the oils on the Brazilian margin, Mello et al.<ref name=ch08r30 />) measured numerous elemental, isotopic, and molecular characteristics for about 50 oils; 16 of these were examined using techniques of semiquantitative biomarker analysis. Their study is an excellent example of the importance of using diversified analytical methods. A partial list of the parameters measured includes δ¹³C of the whole oil; sulfur content; vanadium–nickel ratio; compound-class distribution; pristane–phytane ratio; various biomarker ratios involving diasteranes, methylsteranes, tricyclic terpanes, bisnorhopane, and gammacerane; and semiquantitative concentration data for various isoprenoids, β-carotane, selected steranes, hopanes, and oleanane. This set of parameters was then used to classify the oils into five source-distinctive groups. No one or two correlative parameters could establish these groupings—only through the combined use of several elemental, isotopic, and molecular parameters could the correlations be made and depositional settings inferred.

To understand the source families responsible for the oils on the Brazilian margin, Mello et al.<ref name=ch08r30 /> measured numerous elemental, isotopic, and molecular characteristics for about 50 oils; 16 of these were examined using techniques of semiquantitative biomarker analysis. Their study is an excellent example of the importance of using diversified analytical methods. A partial list of the parameters measured includes δ¹³C of the whole oil; sulfur content; vanadium–nickel ratio; compound-class distribution; pristane–phytane ratio; various biomarker ratios involving diasteranes, methylsteranes, tricyclic terpanes, bisnorhopane, and gammacerane; and semiquantitative concentration data for various isoprenoids, β-carotane, selected steranes, hopanes, and oleanane. This set of parameters was then used to classify the oils into five source-distinctive groups. No one or two correlative parameters could establish these groupings—only through the combined use of several elemental, isotopic, and molecular parameters could the correlations be made and depositional settings inferred.

==Group I oils, freshwater lacustrine setting==

==Group I oils, freshwater lacustrine setting==

For illustrative purposes, we discuss here differences in the characteristics of group I and group IV oils.<ref name=ch08r30 /> The group I oils are very low in sulfur (13C values more negative than –28.0 ‰. Concentrations of diasteranes, steranes, 4-methylsteranes, and gammacerane are low; ''n''-propylcholestanes (C₃₀ steranes) are absent. A distinctive odd-carbon ''n''-alkane dominance and a waxy appearance to the [[gas chromatogram]] of these oils is evident. These and other features suggest a freshwater lacustrine depositional setting.

For illustrative purposes, we discuss here differences in the characteristics of group I and group IV oils.<ref name=ch08r30 /> The group I oils are very low in sulfur (13C values more negative than –28.0 ‰. Concentrations of diasteranes, steranes, 4-methylsteranes, and gammacerane are low; ''n''-propylcholestanes (C₃₀ steranes) are absent. A distinctive odd-carbon ''n''-alkane dominance and a waxy appearance to the [[gas chromatogram]] of these oils is evident. These and other features suggest a freshwater lacustrine depositional setting.

The figures below are typical gas chromatograms of saturated hydrocarbons for group I (left) and group IV (right) oils from the Brazilian continental margin. Note the many source-distinctive differences, including pristane–phytane ratio and the dominance of waxy ''n''-alkanes in the group I oil. Supplemental numerical data indicate these differences extend to the sulfur concentration, the vanadium–nickel ratio, and the δ¹³C value.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-49.png|Figure 2]] represents typical gas chromatograms of saturated hydrocarbons for group I (left) and group IV (right) oils from the Brazilian continental margin. Note the many source-distinctive differences, including pristane–phytane ratio and the dominance of waxy ''n''-alkanes in the group I oil. Supplemental numerical data indicate these differences extend to the sulfur concentration, the vanadium–nickel ratio, and the δ¹³C value.

 

[[file:oiloil-and-oilsource-rock-correlations_fig8-49.png|thumb|{{figure number|8-49}}From Mello et al.<ref name=ch08r30 />); reprinted with permission from Marine and Petroleum Geology.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-50.png|300px|thumb|{{figure number|3}}Typical m/z 191 mass chromatograms for group I (top) and group IV (bottom) oils from the Brazilian continental margin, showing the differences in terpane distributions. From Mello et al.<ref name=ch08r30 />; reprinted with permission from Marine and Petroleum Geology.]]

In contrast to group I oil, group IV oils contain significantly higher levels of sulfur, higher vanadium-nickel, less negative δ¹³C ratios, and lower wax contents (see Figure 8-49). Sterane concentrations are high, as are concentrations of ''n''-propylcholestanes. In addition, tricyclic and tetracyclic terpanes are present in relatively higher concentration in group IV oils, and the homohopane distribution is irregular, with high C₃₃.

In contrast to group I oil, group IV oils contain significantly higher levels of sulfur, higher vanadium-nickel, less negative δ¹³C ratios, and lower wax contents (see Figure 8-49). Sterane concentrations are high, as are concentrations of ''n''-propylcholestanes. In addition, tricyclic and tetracyclic terpanes are present in relatively higher concentration in group IV oils, and the homohopane distribution is irregular, with high C₃₃.

The figures below are typical m/z 191 mass chromatograms for group I (top) and group IV (bottom) oils from the Brazilian continental margin, showing the differences in terpane distributions. A marine carbonate depositional setting for the source of group IV oils is reflected in the high abundance of tricyclic and tetracyclic terpanes, in the high C₃₃ homohopane concentration, and in the low wax content.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-50.png|Figure 3]] illustrates typical m/z 191 mass chromatograms for group I (top) and group IV (bottom) oils from the Brazilian continental margin, showing the differences in terpane distributions. A marine carbonate depositional setting for the source of group IV oils is reflected in the high abundance of tricyclic and tetracyclic terpanes, in the high C₃₃ homohopane concentration, and in the low wax content.

 

==Geological conclusions==

==Geological conclusions==

==What this case study illustrates==

==What this case study illustrates==

This case study demonstrates the utility of compound concentration data and emphasizes the use of routine oil–oil correlation data in predicting the depositional setting of oils. Later efforts have confirmed several of these predictions using similar correlation tools on prospective source rocks in the region.<ref name=ch08r31>Mello, M., R., Telnæs, N., Gaglianone, P., C., Chicarelli, M., I., Brassell, S., C., Maxwell, J., R., 1988b, Organic geochemical characterisation of depositional palaeoenvironments of source rocks and oils in Brazilian marginal basins: Organic Geochemistry, vol. 13, p. 31–45., 10., 1016/0146-6380(88)90023-X</ref> The resulting oil–source rock correlations are supported by a similarly wide range of analytical approaches.

This case study demonstrates the utility of compound concentration data and emphasizes the use of routine oil–oil correlation data in predicting the depositional setting of oils. Later efforts have confirmed several of these predictions using similar correlation tools on prospective source rocks in the region.<ref name=ch08r31>Mello, M. R., N. Telnæs, P. C. Gaglianone, M. I. Chicarelli, S. C. Brassell, and J. R. Maxwell, 1988b, Organic geochemical characterisation of depositional palaeoenvironments of source rocks and oils in Brazilian marginal basins: Organic Geochemistry, vol. 13, p. 31–45, DOI: 10.1016/0146-6380(88)90023-X.</ref> The resulting oil–source rock correlations are supported by a similarly wide range of analytical approaches.

==See also==

==See also==

[[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]]