25th International Meeting on Organic Geochemistry IMOG 2011

P-086
Migration tracers reveal long-range migration in the summan
exploration area
Khaled Arouri 1 , S K Panda 2 , S A Satti 3 , Y Yang 1
1 EXPEC Advanced Research Center, Saudi Aramco, Dhahran, Saudi Arabia, 2 Research and Development
Center, Saudi Aramco, Dhahran, Saudi Arabia, 3 Area Exploration Department, Saudi Aramco, Dhahran,
Saudi Arabia (corresponding author:khaled.arouri@aramco.com)
Exploration around the Summan Platform, north of
Saudi Arabia, is progressively extending west and
south. The shift of exploration effort away from the
kitchen, which lies to the north of the Summan
Platform, has increased the risk of charge access to
the prospects further away to the south and west. A
migration study — based on the geochemical
characteristics of oils — can mitigate this risk and
help better understand the migration extent from the
Late Jurassic kitchen.
Benzocarbazoles
(benzo[a]carbazole/benzo[c]carbazole; BC) have
been suggested as odometers for oil migration (Larter
et al., 1996). The systematic BC behavior — for Late
Jurassic Arab-A oils from oilfields in the Summan
area — suggests that these accumulations are
charged via long-range migration, rather than multiple
sourcing from different local kitchens. The BC ratio
systematically decreases southwards from Field A to
Field E (Fig. 1), apparently as a response to the
increase in migration distance from a kitchen to the
north. While Field E is believed to be the most distal
oil belonging to this petroleum system, any traps
(whether structural or stratigraphic) — along this
north-south trending long-range migration pathway —
are high-graded, especially those closer to the entry
point north of Field A.
The reduction in BC ratio with increasing migration
distance is accompanied by a decrease in oil maturity,
with early-generated, lower-maturity oil (Field E) being
the most-fractionated, shallowest and furthest away
from the source kitchen. This reduction in ratio is also
associated with a clear API gravity gradient across
the basin, together with a gentle compositional
(molecular and isotopic) gradient, indicative of
increasing contribution by more mature oil with depth
toward the north (Field A).
Long-range migration has probably been facilitated by
a displacement (fill-and-spill) mechanism through
intermediary fields along the migration route.
Interestingly, the southward migration effect
manifested at a regional scale is also evident at a field
scale, where the north of each field contains its last
charge, whereas the south contains its migration
front. Vertical and lateral compositional grading in
some Summan fields is likely due to poor mixing of
several viscous charges from a common carbonate
source. This observation highlights the importance of
interpreting the compositional grading of reservoirs
within the larger context of regional migration and
regional compositional grading.
Reference Cited
Larter, S.R., Bowler, B., Li, M., Chen, M., Brincat, D.,
Bennett, B., Noke, K., Donohoe, P., Simmons, D.,
Kohnen, M., Allan, J., Telnaes, N., Horstad, I., 1996.
Benzocarbazoles as molecular indicators of
secondary oil migration distance. Nature 383, 593–
597.
Depth (ft) below sea level
S.L
-2000
-4000
-6000
-8000
-10,000
-12,000
-14,000
S N
E
15
0.28
a c
most-fractionated
= most-migrated
D
23
0.59
a c
C
26
0.63
a c
~ 200 km
B
32
0.89
decreasing API
decreasing oil maturity
lighter � 13 C updip
similar source signature (carbonate)
a c
decreasing BC ratio
A
33
0.97
a c
least-fractionated
= least-migrated
API
Kitchen
BC ratio
Figure 1. Oilfield compositional gradients as a
function of distance from the kitchen, including:
benzocarbazole (BC) ratio, API gravity, isotope ratios
and maturity versus depth below sea level. The
benzo[a]carbazole (labeled a) is preferentially
removed relative to benzo[c]carbazole (labeled c).
229