25th International Meeting on Organic Geochemistry IMOG 2011

P-274
Pr/Ph ratio in the Bazhenov formation rock samples (Western
Siberia)
Ivan Goncharov 1 , Vadim Samoylenko 1 , Nikolay Oblasov 1 , Vladimir Volkov 2
1 JSC «TomskNIPIneft», Tomsk, Russian Federation, 2 SE «SAC RNM named after V.I. Shpilman», Tyumen,
Russian Federation (corresponding author:GoncharovIV@nipineft.tomsk.ru)
Bazhenov formation, the major oil source in
Western Siberia, includes an area of about 1 mil.
km 2 . Rock lithology and organic matter (OM)
content and quality changes throughout the central
regions of Western Siberian to its periphery, which,
in its turn, affects the rock potential generation and
molecular composition of generated hydrocarbons.
Thus, in this case, an important factor is the
regional feature of molecular parameter changes,
defining the source rock organic material, as this
gives more reliable correlation of oils of this genetic
type.
One of the major features determining the rock
organic material quality is the oxidation-reduction
regime during sedimentogenesis and diagenesis.
The most frequently applied molecular parameter to
evaluate the oxidation-reduction conditions of
sedimentation is Pr/Ph ratio.
The zone of minimum parameter value Pr/Ph (0.6-
1.0) for Bazhenov formation rocks is confined to the
central part of the sedimentation basin [1]. Pr/Ph
ratio increases from central zones to the periphery.
However, this parameter value for Bazhenov
formation rocks is not more than 2.0 throughout the
area [2].
The regional behavior distortion of Pr/Ph changes
was firstly detected in the eastern part of Western
Siberia within Pyl-Karaminsk arch (east of Khanty-
Mansi Autonomous Okrug). This region pertains to
the marginal distribution boundary of Bazhenov
formation rocks, where eastward they are
substituted by less bituminous rocks of Mariyanov
formation. According to the regional behavior of
Pr/Ph ratio it should be 1.4-1.7. However, research
results of 40 sample-extracts from Bazhenov
formation cross-section, in 8 wells of Pyl-Karaminsk
arch area indicated that the average value of Pr/Ph
ratio is 0.81-1.20, while for some other samples the
Pr/Ph ratio is 0.63. At the same time, OM of
Bazhenov formation rocks is characteristic of rather
low maturity. Thus, the ratio S/(S+R) St29 for all
investigated samples ranges from 0.09 to 0.20.
It is obvious that the Pr/Ph ratio in this region could
not have such a low value. It is possible that such a
low maturity indicates the fact that rock OM has not
even generated the first hydrocarbon portions.
Therefore, free hydrocarbons in the rock pore
space are transformation products of initial OM
during diagenesis. As stated earlier, in several
cases of rather low maturity, Pr/Ph ratio parameter
could be significantly distorted [3].
Actually, the investigated regional behavior of Pr/Ph
ratio in adjoining areas indicated that this value
parameter increases with OM maturity growth (Fig.)
When the S/(S+R) St29 ratio is 0.25-0.30, then
Pr/Ph ratio in Bazhenov formation rocks reaches
the corresponding value of this region. It is obvious
that at this stage of maturity the amount of
generated hydrocarbons already significantly
exceeds the amount of free hydrocarbons, which
were initially in rock pore space.
Pr/Ph
2.0
1.6
1.2
0.8
Pyl-Karaminsk arch
adjoining areas
0.4
0.00 0.10 0.20 0.30 0.40 0.50 0.60
S/(S+R) St29
Fig. Relationship of Pr/Ph and S/(S+R) St29)
References[1] Goncharov I.V. Geochemistry of
oils in West Siberia. Moscow: Nedra, 1987, 181
p.;[2] Goncharov I.V. et al. Source rocks and oils in
eastern part of West Siberia / Oil industry, 2010,
№8, p.24-28; [3] Volkman J.K. and Maxwell J.R.,
1986. Acyclic isoprenoids as biological markers //
Biological Markers in the Sedimentary Record (R.B.
Johns, ed) / Elsever, New York, p. 1-42.
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