Changes

==Geological setting==

==Geological setting==

[[file:oiloil-and-oilsource-rock-correlations_fig8-53.png|300px|thumb|{{figure number|1}}Stratigraphic section of the West Siberian basin. From Peters et al. (1994); courtesy AAPG.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-53.png|300px|thumb|{{figure number|1}}Stratigraphic section of the West Siberian basin. From Peters et al.;<ref name=ch08r41 /> courtesy AAPG.]]

The West Siberian basin, located in central Russia, is approximately bisected north–south by the Ob River. [[:file:oiloil-and-oilsource-rock-correlations_fig8-53.png|Figure 1]] is a stratigraphic section of the basin. Oils are reservoired throughout the section but are concentrated in the Lower Cretaceous and Upper Jurassic.

The West Siberian basin, located in central Russia, is approximately bisected north–south by the Ob River. [[:file:oiloil-and-oilsource-rock-correlations_fig8-53.png|Figure 1]] is a stratigraphic section of the basin. Oils are reservoired throughout the section but are concentrated in the Lower Cretaceous and Upper Jurassic.

Peters et al.<ref name=ch08r41 /> approached both the oil–oil and oil–source rock correlation aspects together, relying upon molecular and isotopic data accompanied by cluster analysis results. The distributions of regular steranes and monoaromatic steroid hydrocarbons (C₂₇– C₂₉) provided an excellent framework for the correlation. The oils (shown as solid circles in the figure below) form two distinct families according to these criteria: 26 oils fall in a group relatively depleted in the C₂₉ homolog, while six oils form a group relatively enriched in C₂₉. This two-family oil–oil classification is also consistent with whole-oil carbon isotope ratios as well as with other biomarker data.

Peters et al.<ref name=ch08r41 /> approached both the oil–oil and oil–source rock correlation aspects together, relying upon molecular and isotopic data accompanied by cluster analysis results. The distributions of regular steranes and monoaromatic steroid hydrocarbons (C₂₇– C₂₉) provided an excellent framework for the correlation. The oils (shown as solid circles in the figure below) form two distinct families according to these criteria: 26 oils fall in a group relatively depleted in the C₂₉ homolog, while six oils form a group relatively enriched in C₂₉. This two-family oil–oil classification is also consistent with whole-oil carbon isotope ratios as well as with other biomarker data.

[[file:oiloil-and-oilsource-rock-correlations_fig8-54.png|thumb|300px|{{figure number|2}}distribution of the C₂₇–C₂₉ regular steranes (left) and monoaromatic steroid hydrocarbons (right) for oils in the West Siberian basin. From Peters et al. (1994); courtesy AAPG.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-54.png|thumb|300px|{{figure number|2}}distribution of the C₂₇–C₂₉ regular steranes (left) and monoaromatic steroid hydrocarbons (right) for oils in the West Siberian basin. From Peters et al.;<ref name=ch08r41 /> courtesy AAPG.]]

The ternary diagrams in [[:file:oiloil-and-oilsource-rock-correlations_fig8-54.png|Figure 2]] show the distribution of the C₂₇–C₂₉ regular steranes (left) and monoaromatic steroid hydrocarbons (right) for oils in the West Siberian basin. Two oil families can be distinguished on the basis of their relative amounts of the C₂₉ homologs. The bitumens from all the Bazhenov Formation rock samples plot together with a single oil family. Molecular structures are shown for the compounds used in the ternary diagrams.

The ternary diagrams in [[:file:oiloil-and-oilsource-rock-correlations_fig8-54.png|Figure 2]] show the distribution of the C₂₇–C₂₉ regular steranes (left) and monoaromatic steroid hydrocarbons (right) for oils in the West Siberian basin. Two oil families can be distinguished on the basis of their relative amounts of the C₂₉ homologs. The bitumens from all the Bazhenov Formation rock samples plot together with a single oil family. Molecular structures are shown for the compounds used in the ternary diagrams.

Several possible source rocks in the basin show steroid carbon-number distributions similar to those of one of the oil families in [[:file:oiloil-and-oilsource-rock-correlations_fig8-54.png|Figure 2]]. In particular, all the Bazhenov Formation samples (uppermost Jurassic) plot adjacent to the oil family that is relatively lean in the C₂₉ homolog. Other rock samples that also plot with this family in these ternary diagrams do not show valid oil-source correlations using other biomarker and isotope parameters.

Several possible source rocks in the basin show steroid carbon-number distributions similar to those of one of the oil families in [[:file:oiloil-and-oilsource-rock-correlations_fig8-54.png|Figure 2]]. In particular, all the Bazhenov Formation samples (uppermost Jurassic) plot adjacent to the oil family that is relatively lean in the C₂₉ homolog. Other rock samples that also plot with this family in these ternary diagrams do not show valid oil-source correlations using other biomarker and isotope parameters.

[[file:oiloil-and-oilsource-rock-correlations_fig8-55.png|thumb|300px|{{figure number|3}}Distributions of homohopanes by carbon number (C₃₁–C₃₅) for bitumens from the Bazhenov Formation are essentially identical to those for the low-C₂₉-sterane oil family. Modified from Peters et al (1994);{{citation needed}} courtesy AAPG.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-55.png|thumb|300px|{{figure number|3}}Distributions of homohopanes by carbon number (C₃₁–C₃₅) for bitumens from the Bazhenov Formation are essentially identical to those for the low-C₂₉-sterane oil family. Modified from Peters et al;<ref name=ch08r41 /> courtesy AAPG.]]

Further support for a Bazhenov source is provided by the distribution of extended hopanes. As indicated in the figure below, the “Bazhenov” oils have extended hopane distributions virtually identical with the Bazhenov source rock extracts (indicated as bitumens in the figure). This positive correlation is strengthened because the homohopane distributions are somewhat unusual; thus, the similarity is unlikely to be fortuitous.

Further support for a Bazhenov source is provided by the distribution of extended hopanes. As indicated in the figure below, the “Bazhenov” oils have extended hopane distributions virtually identical with the Bazhenov source rock extracts (indicated as bitumens in the figure). This positive correlation is strengthened because the homohopane distributions are somewhat unusual; thus, the similarity is unlikely to be fortuitous.