Tectonic influence on the Jurassic sedimentary architecture in the northern North Sea with focus on the Brent Group - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 397

Table 2. Stratigraphic thicknesses of the different formations seen in wells 33/12-6 and 34/11-A-15 in Fig. 4: depth
of top formation (TVD = true vertical depth), thicknesses (m Ma −1 ), thickness-expansion rates (E-W incr.) and time span
of formation.

W E

33/12-6 34/11-A-15

Top
TVDm Formation Thickness (m) E-W incr ~ Million years Top TVDm Thickness (m)

Heather 2645 295 (29.5 m per my) 37x 10 3970 8 (0.8 m per my)

Ness-Tarbert 2940 250 (50 m per my) 3.5x 5 3978 72 (14.5 m per my)

Rannoch-Etive 3190 65 (26 m per my) 1.2x 2.5 4050 55 (22 m per my)

Drake 3255 115 (10.5 m per my) 1.9x 11 4105 60 (5.5 m per my)

Cook 3370 100 (33 m per my) 1.3x 3 4165 75 (25 m per my)

Amundsen-Burton 3470 220 (36.6 m per my) 1.2x 6 4240 185 (30.8 m per my)

Statfjord 3690 4240

Million years estimates from Husmo et al . (2003).

biostratigraphic samples were derived from cut-
tings, stratigraphic tops are favoured over bases,
due to the inherent problem of caving.

significant surfaces identified from the interpre-
tation of core samples, electrical logs and biostrati-
graphic information were tied to the seismic vol-
umes using velocity log data.

Seismic data

The three reprocessed 3D seismic surveys
ST11M12, ST11M07 and ST09M01 (Figs 4 and 6)
have been used for reflection seismic interpreta-
tion. They are partly overlapping and have a total
areal extent of ~ 1730 km 2 , with a 585 km 2 final
migration size. In-line direction is east-west with
a line spacing of 12.5 m and cross-line direction is
north-south with a line-spacing of 25 m. The verti-
cal seismic resolution (reflection separation) is
depth-dependent and varies throughout the area
from ~ 25 ms Two-Way-Time to ~ 50 ms TWT at a
depth of 3500 ms TWT. Lateral resolution is
dependent on line-spacing and depth and would
typically be 200 m to 300 m at 3500 ms TWT.
The seismic lines have not been depth-converted
since they are used to show relative lateral thick-
ness variations of the sedimentary strata (see also
Table 2 for thickness-variations of the formations).
However, velocity well data have been utilised for
both time and depth migration of the seismic surveys
and show that the velocity field is generally uni-
form laterally for regional (km-scale) interpretation
or depth conversion purposes. Hence, the thick-
ness variations in time reflect true stratigraphic
thickness variations. Time-to-depth relationships
are derived from well data and show that the
equivalent true vertical depth (TVD) at 3500 ms
TWT is approximately 4000 metres.
Interpretation of the seismic data was based on
correlation with the well data. Chronostratigra-
phically significant and lithostratigraphically

PRE-RIFT TO SYN-RIFT SIGNATURES

Geological cross-section across the
Gullfaks-Kvitebjørn area

Fig. 4 shows a regional geological cross section that
runs from the Kvitebjørn Field via the Valemon
Field and across the southern part of the Gullfaks
Field toward the Statfjord Field (Fig. 1, cross-
section 1). The wells along this correlation have a
complete section of the Jurassic stratal package.
Fig. 4 shows the east-west stratal thickness trends
for the Dunlin, Brent and Viking groups. The Dunlin
Group shows a tendency to thicken westward with
a gentle (~1.3 times, see Table 2) thickness increase.
The Brent Group shows a more pronounced (~2
times) westward increase in thickness towards a
Permo-Triassic master-fault (See Fig. 1) but with
significant internal variations. The Rannoch and
Etive formations have a near tabular thickness dis-
tribution, whereas the Ness and Tarbert formations
together show clearer wedge-shaped geometries
that involves a pronounced (~3.5 times; Table 2)
westward thickness increase. Internally, the sedi-
mentary architecture of the lower Ness-Tarbert
formations show near tabular strata while the upper
part show a gradual angular variation upward in the
form of westward bed expansion. An apparently
isolated and several tens of metres thick sandstone
unit occurs locally at the base of Ness Formation
(see well 34/10-23 in Fig. 4).