25th International Meeting on Organic Geochemistry IMOG 2011

O-21
Study of the stable isotopic composition of severely
biodegraded oils as petroleum system correlation parameters
Norka Marcano, Steve Larter, Bernhard Mayer
University of Calgary, Calgary, Canada (corresponding author:nimarcan@ucalgary.ca)
Oil-source rock or oil-oil correlation is one of the most
challenging steps in the analysis of complex
petroleum systems. It is generally based on molecular
fingerprints (biomarkers) and whole oil or oil fraction
carbon isotopic composition. When biomarkers are
unaltered by post expulsion processes, they can
constitute a powerful tool for correlation but effects on
biomarker concentrations in mixed oil charge
scenarios and even destruction of biomarkers at
severe biodegradation levels, may drastically affect
the viability of the approach. Biodegradation is one of
the most dominant post accumulation processes in
petroleum reservoirs 1,2 . One of the largest
accumulations of heavily to severely biodegraded oil
are the Alberta oil sands, where biodegradation is by
far the most important process responsible for
variations in the composition and oil properties 3 .
In this study, continuous petroleum columns from the
Alberta oil sands containing progressively
biodegraded oil (from level 5PM to 8PM) are studied
using molecular and isotopic analyses, with the
purpose of evaluating the effect of biodegradation in
the C, N, S and H isotopic compositions of bulk oil
and fractions of different polarities to assess the
viability of these proxies as oil correlation parameters.
The results show that even though the total
hydrocarbon fraction is highly depleted by
biodegradation to different degrees, this process does
not cause observable systematic carbon isotope
changes in either the bulk bitumen or in fractions of
different polarities in single oil columns. � 15 N values in
bulk bitumen also do not show any trend with
biodegradation, as may have been expected from the
observed destruction of carbazoles studied in the
polar fractions of the oils, which change both
concentration and distribution with biodegradation
levels. Organic sulphur compounds in the aromatic
hydrocarbon fraction are also progressively removed
down the oil columns, yet � 34 S values of bulk oil
remain essentially constant. Locally however, lower
� 34 S values suggest secondary incorporation of 32 Senriched
sulfur.
Variations in � 15 N and � 34 S measured on the aromatic
hydrocarbons, resins and asphaltene fractions, in oil
columns with increasing biodegradation levels toward
the bottom, suggest observed nitrogen and sulfur
isotopic compositions of oil fractions, unlike whole oil,
may be affected by the biodegradation process.
Finally, it is observed that S and N isotopic
compositions of bulk bitumen and heavy oils from the
main studied pools in the basin differ consistently
displaying what appears to be a regional trend (Fig.1).
The lack of significant changes of the bulk isotopic
composition of progressively biodegraded oils in
single oil columns, suggests that the observed
regional isotopic variations are source charge related.
We conclude that S and N isotopic compositions can
be used in biodegraded oils as stable source rock
proxy signatures.
� 34 S V-CDT (‰)
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
Peace River
Buffalo Head Hills
Alberta border
Athabasca
Lloydminster
0.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0
� 15 N N2 (‰)
Cold Lake
Fig. 1. Variations of � 34 S vs. � 15 N in oil sands bitumens and
heavy oils of the Alberta Basin.
References
[1] Roadifer, R.E., Exploration for Heavy Crude Oil and
Natural Bitumen 1987, 25, 3-23
[2] Head, I.M., Jones, D.M., Larter, S.R., Nature 2003,
426, 344-352
[3] Larter, S., Adams, J., Gates, I.D., Bennett, B., Huang,
H., Journal of Canadian Petroleum Technology 2008,
47, 52-61
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