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coherently in terms of a common hydrodynamic
model (as in the Kraka field example discussed
below). Possible hydrodynamic aquifer influence
on a static (i.e. passive) petroleum accumulation
must also be considered alongside the concepts
of a dynamic petroleum accumulation (e.g. on-
going migration
3.7.3.1 Kraka Field Example
This small chalk reservoir in the Danish sector of
the North Sea provides an interesting account of
the phenomenon of tilted oil-water contacts and
their interpretation. The subtle nature of the tilt
and the use of multiple data sources to confirm an
initially doubtful interpretation are very informa-
tive. A study of the field by Jørgensen and
Andersen (
1991
) included some initial
observations on a tilted oil-water contact, and a
tentative argument that it was due to tectonic
tilting during the Tertiary. A subsequent study
by Thomasen and Jacobsen (
1994
), give a
detailed description and a more thorough basis
for interpretation a 0.6
dip in both free water
level and oil-water contact (Fig.
3.46
).
Their main observations were:
Repeat Formation Tester (RFT) data from
three wells indicated a free-water level
(interpreted from the change in slope of
water and oil zones) falling by about 70 m
over a 2 km distance (Fig.
3.47
).
Due to the heterogeneous and fractured nature
of the chalk reservoir zone, logs from seven
wells show highly variable saturations
(Fig.
3.48
). These were interpreted by best-
fit capillary pressure saturation functions.
Difficulties in fitting a function assuming a
horizontal free-water level were resolved by
fitting functions to individual wells and then
identifying the implied tilt in free water level.
or
leakage)
or
pressure
transients in the aquifer.
Fig. 3.45
Map of the Cairo Pool oilfield, Arkansas
showing a hydrodynamic offset of an oil accumulation
(After Dahlberg
1995
). Contours are 20 foot intervals;
black dots
¼ wells with oil in the reservoir interval,
open circles
¼ wells with water in the reservoir interval
(Redrawn from Dahlberg
1995
(Fig. 12.5), Springer-
Verlag, New York, with kind permission from Springer
Science and Business Media B.V.)
5600
A-7C well trajectory
5800
GOC
6000
6200
1 km
Fig. 3.46
Cross-section through the Kraka field (From
Thomasen and Jacobsen
1994
) showing interpreted fluid
contacts and horizontal well to exploit down-dip reserves
(Redrawn from Thomasen and Jacobsen
1994
,
#
1994,
Society of Petroleum Engineers Inc., reproduced with
permission of SPE. Further reproduction prohibited with-
out permission)
