25th International Meeting on Organic Geochemistry IMOG 2011

O-16
Improved genetic characterization of Brazilian oils using
combined molecular (biomarkers) and isotope geochemistry
Jarbas Vicente Poley Guzzo, Eugenio Santos Neto, Alexandre de Andrade Ferreira
PETROBRAS, Rio de Janeiro, Brazil (corresponding author:guzzo@petrobras.com.br)
An extensive application of isotope geochemistry
combined with the traditional molecular (biomarkers)
approach over a large set of oils, gases and bitumens
enabled a detailed geochemical characterization of
Brazilian petroleum systems and, at the same time, a
rigorous evaluation of the potential and limitations of
both methods.
Carbon isotope profiles of n-alkanes show remarkably
consistent patterns between n-C13 and n-C33,
distinguishing the three main oil families known in
Brazil according to the depositional environment of
source-rocks (lacustrine, marine-evaporitic and open
marine). Lacustrine oils show a typical negative
sloping curve with a subtle sloping up trend between
n-C27 and n-C33 (asymmetric ―U‖ shape), and a large
amplitude of isotopic values (generally higher than
3‰, Fig. 1). Marine-evaporitic oils show a typical
symmetric ―U‖ shape curve between n-C13 and n-
C33, with n-C22 showing the most negative value, a
medium amplitude of isotopic values (generally about
3‰) and in general the heaviest isotopic profile of nalkanes
within the studied dataset. Open marine oils
show an overall ―flat‖ pattern, with small isotopic
variations from the main trend.
Besides its genetic discrimination efficiency, carbon
isotopic profiles of n-alkanes have also been used to:
� demonstrate oil mixing when hardly detected
or evaluated by biomarkers
� solve questions on possible migrationcontamination
of oils
� directly correlate oils and source-rocks
� identify the origin of high maturity oils with
severely degraded biomarker distribution
� describe sub-types of lacustrine and marine
oils
Several features described in this study were already
noted elsewhere, e.g. the trend toward isotopically
lighter values with increasing n-alkane chain length
(negative slope) of lacustrine oils and bitumens, the
flat character of n-alkane isotopic profile of open
marine oils, and the ubiquitous isotopic enrichment
with increasing thermal maturity (e.g. Murray et al.
1994, Pedentchouk 2004, Samuel et al. 2009).
A number of elements and processes of petroleum
systems identified and described using the biomarker
approach have their characterization reinforced using
isotopes. Even subtle compositional heterogeneities
hardly detected by biomarkers may generate
conspicuous features among isotopic profiles as, for
example, the remarkably depleted hydrogen isotope
profiles of early mature oils, and the late or early
petroleum charges detected by the isotopic
composition of gases.
In a more restricted set of oil samples, the carbon
isotopic composition of individual biomarkers
improved the genetic discrimination due to the large
differences between isotopic composition of specific
biomarkers (tricyclic and pentacyclic terpanes) among
oils of different origins.
The combined biomarker/isotope approach produced
a large empirical database that supported and refined
the interpretation of specific geological features and
processes like secondary migration, source-rock
facies succession, reservoir filling history and
complex petroleum systems.
� 13 C
-24
-26
-28
-30
-32
-34
-36
lacustrine marine-evaporitic open marine
3.4 ‰
nC22
4.7 ‰
nC27
1.2‰
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
n -alkane carbon number
Figure 1: Patterns of � 13 C profiles of individual n-alkanes of
the main Brazilian oil families. Notice some diagnostic
features of each oil family, e.g., amplitude of � 13 C values and
the isotopically lightest n-alkane.
References
Murray A. P., Summons R. E., Boreham C. J., Dowling L. M. 1994.. Org.
Geochem. 22(3-5): 521-542
Pedentchouk, Nikolai 2004. Lacustrine paleoenvironments from stable isotopes
of hydrogen and carbon in lipids. Ph.D. Dissertation, Pennsylvania State
University, State College, Pennsylvania, USA.
Samuel O. J., Cornford C., Jones M., Olabisi A. A., Akande S. O. 2009. Org.
Geochem. 40(4): 461-483
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