25th International Meeting on Organic Geochemistry IMOG 2011

P-264
Natural gas geochemistry of the southern offshore Brazilian
Basins
Eugenio V Santos Neto 1 , Jose R Cerqueira 1 , Alain Prinzhofer 2
1 Petrobras R&D Center, Rio de Janeiro, Brazil, 2 French Institute of Petroleum, Rueil-Malmaison, France
In the last decades the petroleum exploration in Brazil
has been developed mostly in offshore basins,
especially those located along its southern littoral.
More recently, drilling activities have targeted
prospects pertaining to increasingly deeper, under
thick salt layers and more complex geological
settings. Among many geological aspects that add
complexity to the prospects one is very conspicuous:
the unusual gas composition of many accumulations.
In order to better understand the origin and evolution
of the gas hydrocarbons and non-hydrocarbons was
collected a representative set of gas samples from
two southern Brazilian offshore basins for an
integrated geochemistry study. The samples were
analyzed by GC for the relative quantification of each
gas fraction and carbon isotopic ratios were
measured in hydrocarbons and CO2. That part of the
work was developed in the Gas Geochemistry
Laboratories of the PETROBRAS R&D Center. Noble
gas quantification and isotopes were analyzed in the
Geochemistry Branch of the French Petroleum
Institute.
Gas hydrocarbons have relatively low and
homogeneous values for C2/C3 versus C2/iC4 ratios
suggesting common sources and comparatively
moderate degree of thermal maturity and no evidence
of biodegradation [1]. The low thermal maturity and
the typical primary cracking (kerogen → oil + gas) are
corroborated by the pattern given by the plot � 13 C2-
� 13 C3 versus C2/C3 [2].
The most abundant non-hydrocarbon is CO2 that can
reach up to 80% in some accumulations. Values of
� 13 CCO2 are between -5‰ and -9‰ typical of mineral
origin. Ratios of R/Ra (R = 3 He/ 4 He of sample, Ra =
3 He/ 4 He of the air � 1.4x10 6 ) of helium within those
basins have shown variable but strong mantle
signature. The abundance of CO2 increases with the
strengthening of mantle signature (Fig. 1).
Micropyrolysis performed in samples of the same
stratigraphic interval has shown values of � 13 C for the
produced CO2 significantly different from those found
in natural CO2, e.g., carbonates: +3‰ < � 13 C < +9‰,
carbonates + kerogen � 13 C � -13‰, and kerogen
� 13 C � -20‰.
Carbon isotopic ratios of methane are within -41‰
and -35‰, and C2-C4 mostly in the range of -32‰ and
-29‰. The relatively small difference between � 13 C of
methane and components of the C2+ fraction suggests
a contribution of dry gas from deeper ―gas kitchens‖.
However, the unusual geologic framework of the
region, with extensive crustal thinning and mantle
exhumation [3], may influence somehow the
conventional petroleum system above.
CO2 CO2 (%) (%)
90
80
70
60
50
40
30
20
10
0
0 1 2 3 4 5 6
R/Ra
Fig. 1 – Abundance of CO2 versus R/Ra. Notice that
the values of the end-members for crust and mantle
are 0.01 and 8.00, respectively.
The hydrocarbon accumulations discovered in the
pre-salt in the southern offshore Brazilian basins have
a gas fraction generated mainly by thermal cracking,
with homogeneous low maturities in the C2+ fraction,
and no biodegradation. Isotopically heavy methane
may be sourced by deeper kitchens. High and
variable proportions of CO2, mainly derived from the
mantle, are associated with these fluids.
References
[1] Prinzhofer, A., Mello, M.R., Freitas, L.C.S. & Takaki, T.
(2000). AAPG Memoir 73, p. 107-119.
[2] Lorant, F., Prinzhofer, A., Behar, F., Huc, A.Y. (1998).
Chemical Geology, vol. 147, p. 249-264.
[3] Zalán, P.V., Severino, M.C., Oliveira, J.A.B., Magnavita,
L.P., Mohriak, W.U., Gontijo, R., Viana, A.R., Szatmari,
P. (2009) 2009 AAPG <strong>International</strong> Conference &
Exhibition, Rio de Janeiro, Abstracts.
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