25th International Meeting on Organic Geochemistry IMOG 2011

P-294
Petroleum generation and expulsion zones in Maturín sub-basin:
Evaluation of paleoenvironmental and lithologic variations of
cretaceous source rocks responsible for oil charges
accumulated in Carabobo Area, Orinoco Oil Belt- Eastern
Venezuelan Basin
Wendy Murillo 1 , Carolina Olivares 2
1 PDVSA-Intevep, Los Teques, Venezuela, 2 PDVSA-Oriente, Puerto la Cruz, Venezuela
(Corresponding author:murillow@pdvsa.com)
The petroleum accumulations in Carabobo area
come from lateral migration to long distances from
the north and northwest and were generated from
Cretaceous source rocks in the north of Maturín
sub-basin. The objective of this contribution is to
investigate the oil migration and charging based on
integration of data from organic geochemistry and
1-D petroleum generation and expulsion basin
modeling in wells which penetrated the oversthrust
slab of the Pirital and Frontal Thrust and
rocks beneath them. This integrated approach
provides new insights about the accumulations of
heavy and extra-heavy oils in the Carabobo area
(Orinoco Oil Belt), Eastern Venezuelan Basin.
In the southern part of the Frontal Thrust,
petroleum is still being generated and expulsed
from one tectonic unit. Generation in this unit (here
referred to as tectonic unit 1) occurred from Late
Cretaceous to Recent, while the expulsion
occurred in several episodes during early
Paleocene, early Miocene and late Miocene. In
contrast, in the thrust zone generation and
expulsion occur from two or more tectonic units,
defined and separated by thrust-faults. Oil
generation from the source rocks in the tectonic
unit 2 occurred mainly from early Eocene and
expulsion from early Miocene. There are three
main zones of petroleum expulsion in Maturín subbasin,
which are defined as: zone of expulsion I
where the petroleum expulsion occurred from early
Eocene to early Miocene, although it could be as
old as early Paleocene. In the zone II the expulsion
occurred during the early Miocene-middle Miocene
times while in the zone III it occurred during the
late Miocene to Recent, mainly below the Frontal
thrust (southern part of the deformation zone).
The crude oils in Carabobo area were generated
and expulsed from the Guayuta Group during the
early and middle Miocene, before the
emplacement of the Frontal thrust. These rocks
have continued the petroleum generation in the
sub-thrust positions during the late Miocene-
Recent. Oils existing in the different members of
the Oficina Formation come from predominantly
marine organic matter and correlate with the
calcareous shale and limestone of the Guayuta
Group. There are mixing of unaltered oils with
highly altered residues of earlier generated oils
and oils that do not show evidences of alteration of
n-paraffins, regular steranes nor homohopanes.
In general, the oils in Carabobo area show high
norhopane/ hopanes >1 combinated with C35/C34
22 S-hopane ratio(1.0-2.5) which are characteristic
of crude oils generated from marine carbonate
rock deposited under anoxic conditions with
restricted water circulation1. Nevertheless, the
non-biodegraded oils in the east and southeastern
part of Carabobo area, which have similar maturity
level (early maturity stage to maximum generation
of petroleum at the moment of expulsion), were
generated from a marine source rock with a more
siliciclastic input based on higher diasterane/
sterane ratio and C29/C30 hopane