25th International Meeting on Organic Geochemistry IMOG 2011

P-121
Source rock characteristics of Cretaceous organic-rich black
shales offshore south-western Africa
Alexander Hartwig, Rolando di Primio
GFZ Helmholtz Centre Potsdam German Research Centre for Geosciences, Potsdam, Germany
(corresponding author:dipri@gfz-potsdam.de)
The source rock potential of Cretaceous organic-rich
whole rock samples from deep sea drilling project
(DSDP) wells offshore south-western Africa was
investigated using bulk and quantitative pyrolysis
techniques. The sample material was taken from
organic-rich intervals of Aptian-, Albian-, and
Turonian-aged core samples from DSDP sight 364
offshore Angola, DSDP well 530A north of the Walvis
Ridge offshore Namibia, and DSDP well 361 offshore
South Africa. The analytical program included TOC,
Rock Eval, bulk pyrolysis GC, bulk kinetics and microscale
sealed vessel pyrolysis (MSSV) experiments.
The results were used to determine differences in the
source rock petroleum type organofacies, petroleum
composition, GOR, and PVT behavior of
hydrocarbons generated from Aptian, Albian, and
Turonian black shales for petroleum system modeling
purposes.
The Aptian organic-rich shales investigated are very
similar throughout the eastern South Atlantic. The
highest source rock potential was identified in
sapropelitic shales in DSDP well 364, containing very
homogenous Type II and organic sulfur-rich Type IIS
kerogen (Fig. 1). Bulk kinetic experiments on these
samples indicate that organic sulfur contents
influence kerogen transformation rates, Type IIS
kerogen being the least stable. TOC contents of up to
31% and HI‘s of 663 mg HC/g TOC were measured.
Average values in all wells are generally in the range
of 6.9 % TOC and an HI of 477 mg HC/g TOC in
black shale intervals with a potential to generate P-N-
A low wax to paraffinic high wax oils, whereas Type III
kerogen with similar TOC values and an average HI
of 53 mg HC/g TOC in intercalated silty sandstones of
DSDP well 361 shows a potential for gas/condensate
generation.
The Albian is a very good mixed Type II/III kerogen
source rock in DSDP well 361. Higher contents of
marine organic matter occur north of the Walvis Ridge
in DSDP wells 364 and 530A. The latter also
containing Type IIS kerogen. The total organic carbon
content can be as high as 37% with HI‘s of 526 mg
HC/g TOC offshore Angola. Average TOC values
reach 5.9% with average HI‘s of 327 mg HC/g TOC.
The kerogen has a potential to generate intermediate
to aromatic P-N-A low wax oils.
South of the Walvis Ridge, the Turonian contains
predominantly a Type III. North of the Walvis Ridge,
the Turonian black shales contain Type II kerogen
with an average TOC of 8.6% and HI of 523 mg HC/g
TOC. These source rocks have the potential to
generate paraffinic and P-N-A high wax oils. The
assessment of the phase properties of the potentially
generated fluids is currently ongoing.
Our results demonstrate the excellent source rock
potential of early Cretaceous organic-rich black
shales, especially of the Aptian-aged source rock, that
has been recognized in a number of the South
Atlantic offshore basins. Further, they emphasize the
effects of changes in ocean water circulation related
to the South Atlantic opening and its influence on
organic matter preservation in Cretaceous-aged
organic-rich sediments along the southwest African
margin.
Fig. 1: Kerogen type according to pyrolysis-GC.
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