25th International Meeting on Organic Geochemistry IMOG 2011

P-276
Effects of biomarker biodegradation on tar sands of the
Pirambóia Formation, Paraná Basin, Brazil
Eliane Soares de Souza, Georgiana Feitosa da Cruz, Hélio Severiano Ribeiro
North Fluminense State University/Petroleum Engineering and Exploration Laboratory (UENF/LENEP),
Macaé, Brazil (corresponding author:eliane@lenep.uenf.br)
Major magmatic events are believed to have been
effective in the generation, migration and
accumulation of significant quantities of oil in Paraná
Basin 1 . The genesis of the tar sands accumulations
has been attributed to the Irati-Pirambóia petroleum
system 2 . The Irati Formation has an organic matterrich
rock mature due to the thermal effect of the Early
Cretaceous basic igneous intrusions, and hence offer
real prospects of large-scale oil generation 1 . The oil
migrated along the diabase dike / sedimentary rock
contact to form the reservoir rocks of eolian
sandstone of the Pirambóia Formation 2 .
The geology, organic facies, maturation and
migration on the organic matter-rich rock focused
primarily on the saturated hydrocarbon fractions (e.g.,
n-alkane distribution, isoprenoid/n-alkane ratios,
biomarker maturity parameters) have been discussed
by a number of authors 1,2,3 . However, biomarker
biodegradation studies of these samples have not
been addressed. In this sense, the aim of this work
was to examine to rank the extent of biodegradation
in tar sands of the Pirambóia Formation with particular
emphasis on determination of variations on hopane
(and homologues) biomarker distributions.
We studied 6 (S1 to S6) tar sands samples from
Pirambóia Formation (FB and GI). The samples were
pulverized extracted with dichloromethane in a
Soxhlet apparatus for 40 hours. The extract was
fractionated by liquid chromatography and analyzed
by GC-FID and GC-MS.
The Pirambóia tar sands evaluated were severely
biodegraded and of the n-alkanes, isoprenoid alkanes
and light aromatics compounds were removed by
waterwashing and/or biodegradation. During the
advanced stages of biodegradation, such as these tar
sands, hopanes and other pentacyclic triterpanes can
be altered, being converted either to 25-norhopane
homologues 4 or to other, as yet uncharacterized,
products.
In this work, the hopane distributions have been
altered to differing degrees with the homologues
above C30 decreasing in concentration prior to the
tricyclic and C24-tetracyclic terpanes. The high
resistance this compounds to biodegradation indicate
that they are well suited to serve as correlation
parameters in heavily altered samples. Da Cruz et al. 5
showed a preferential depletion of R C35 hopane in
relation to tricyclic terpane, under aerobic conditions.
Demethylation at the C25 position is observed for
most hopanes in severely biodegraded samples. The
Figure 1 showed relatioship between C35 hopane
index versus 25-norhopane ratio used to rank the
extent of biodegradation.
C35 Hopane Index
40
35
30
25
20
15
10
5
0
S5
S6
S2
S3
S1
0 5 10 15 20 25
25-Norhopnae Ratio
Tar sand FB
Tar sand GI
Fig. 1. Plot of C35 hopane index (100 x C35/C31-35) and the 25norhopane
ratio [(100 x C30-34 25-norhopanes (m/z 177) / �
C31-35 hopanes (m/z 191) + C30-34 25-norhopanes (m/z 177)].
In the most biodegraded samples (S1 and S5,
Fig.1) the hopanes index decrease showing That C35
is more biodegradable than C31-34 homohopanes. The
same effect was observed on 25-norhopane ratio
showing that in severely biodegraded oils the 25norhopanos
are also consumed during the process.
References
[1] Thomaz-Filho, A., Mizusaki, A. M. P., Antonioli, L. (2008)
Marine and Petroleum Geology 25, 143-151.
[2] Araújo, C. C. de, Yamamoto, J. K., Rostirolla, S. P.,
Madrucci, V., Tankard, A. (2005) Marine and Petroleum
Geology 22, 671-685.
[3] Araújo, L. M., Trigüis, J. A., Cerqueira, J. R., Freitas, L.
C. da S. (2000) in Mello, M. R. and Katz, B. J., Eds.,
Petroleum systems of South Atlantic margins: AAPG Memoir
73, 377-402.
[4] Peters, K.E., Walters, C.C., Moldowan, J.M., 2005. The
Biomarker Guide, second ed. Cambridge University Press,
Cambridge, UK.
[5] da Cruz, G. F., Santos Neto, E. V. & Marsaioli, A. J.
(2008) Organic Geochemistry 39, 1204-1209.
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