25th International Meeting on Organic Geochemistry IMOG 2011

P-060
Biomarkers parameters used to assessment of petroleum
biodegradation under laboratory conditions
Georgiana Feitosa da Cruz 1 , Eugênio Vaz dos Santos Neto 2
1 North Fluminense State University/Petroleum Engineering and Exploration Laboratory (UENF/LENEP),
Macaé, Brazil, 2 Leopoldo Américo Miguez de Mello Center for Research and Development (CENPES), Rio
de Janeiro, Brazil (corresponding author:geofec@gmail.com)
Biodegradation geochemical parameters based on
the sequential removal of biomarkers are commonly
applied to indicate the preference metabolic of
degradation of the aerobic and anaerobic
microorganisms 1 . The reliable biomarker
interpretation about these information are usually
based on multiple biomarker parameters which should
be resistant to secondary processes, such as
biodegradation 2 . In this sense, the aim of this study
was to investigate the biomarker parameters that are
influenced by biodegradation process under
laboratory conditions. For this we use the following
ratios: Carbon Preference Index (CPI), pristane/nC17
and phytane/nC18, pristane/phytane (Pr/Ph), C28 and
C29 tricyclic terpanes (22R + 22S) to C35 17�(H),
21�(H)-homohopane (22R + 22S) (C28TT/C35Ho and
C29TT/C35Ho ), C35 17�,21�(H)-homohopane (22R +
22S) to C30 17�,21�(H)-hopane (C35Ho/C30H), C27
18�(H)-22,29,30-trisnorneohopane (Ts) to C27
17�(H)-22,29,30-trisnorhopane (Tm) (Ts/Tm), C35 to
C31-C35 homohopane (22R + 22S) (Homohopane
Index, HI), C27, C28,C29 ��� (20R + 22S) and ���
(20R + 22S) steranes to C29-C33 17�(H)-hopanes
(20R + 22S) (sterane/hopane) and C30-C34 25norhopanes
to C31-C35 hopanes ( 25-NH ratio).
The samples were collected from the Pampo field in
the southern part of the Campos Basin. They were
generated from the Lagoa Feia Formation source rock
in differ temperature and depth, allowing a variable
level of oil degradation (oil A, 82 ºC; 2405–2588 m
and oil B, 71 ºC; 1988–2222 m).
The results showed that in aerobic conditions
occurred progressive removal of even carbon
numbered n-alkanes, increase of ratio Pr/nC17
Ph/nC18 and decrease in the ratio Pr/Ph. It is
commonly accepted that odd number n-alkane
preference is associated with low oil thermal maturity,
however these experimental data reveal that the
alkane carbon number predominance can change
with the type of biodegradation and should be
interpreted with caution. In addition, there was an
increase of ratios C28 and C29TT/C35Ho, (Fig. 1),
sterane/hopane, 25-NH, revealing a preferential
depletion of R C35 hopane in relation to tricyclic
terpane, of hopane in relation to sterane and
enrichment of 25-NH.
Fig. 1. Plot of C28TT/C35Ho ratio during biodegradation
process.
The homohopane index decrease showing that C35
is more biodegradable than C31-C34 homohopanes.
Unlike what occurred under aerobic conditions,
anaerobic consortium showed greater susceptibility to
biodegradation of odd numbered n-alkanes, decrease
of ratios C28 and C29TT/C35Ho, sterane/hopane, 25-
NH and increased of homohopane index. The
geochemical parameters calculated from mixed
consortium revealed that there was joint action
between aerobic and anaerobic consortia, as
expected.
A comparative biodegradation of terpanes,
hopanes, 25-norhopanes and steranes under aerobic
and anerobic conditions indicated that some of the
controversial biomarker parameters can be easily
explained by taking into consideration the microbiota
and oxygen content.
Acknowledgment
Unicamp, Capes, Finep, Petrobrás and Profa Dra Anita J.
Marsaioli.
[1] da Cruz, G. F., Santos Neto, E. V. & Marsaioli, A. J.
(2008) Organic Geochemistry 39, 1204-1209.
[2] Peters, K.E., Walters, C.C., Moldowan, J.M., 2005. The
Biomarker Guide, second ed. Cambridge University Press,
Cambridge, UK.
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