Applying accommodation versus sediment supply ratio concepts to stratigraphic analysis and zonation of a fluvial reservoir - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 111

increasing A/S when sedimentation, initially
confined to previously formed incised or fluvial
valley (when A/S dropped below zero), overflows
the valley margins and spreads across a much larger
area. This is a local geomorphological effect - no
sudden change in the rate of A/S increase is neces-
sarily associated with this event (the rate of A/S
increase can remain stable) but sedimentation is
not confined anymore and resulting sandstone
body stacking patterns suddenly change to less
accommodation.

Group, visible in grain-size distributions, sand
contents, organic matter contents and 'wetness'
of overbank deposits. The Statfjord Group has
been described as dominated by braided (Tampen
area) to meandering (Utsira High area) channel
systems with coal-bearing overbank (floodplain)
deposits. South of the Tampen area, on the Utsira
High, fluvial sandstones are finer grained and
the associated floodplain deposits contain
higher proportions of non-pedogenic deposits
(Nystuen & Fält, 1995; Ryseth & Ramm, 1996;
Ryseth, 2001).
In parts of the Tampen area, the Statfjord Group
is divided into three lithostratigraphic formations
(Deegan & Scull, 1977; Lervik, 2006; Fig. 3): The
calcrete bearing red beds of the Raude Formation
(uppermost Triassic to lowermost Jurassic age),
the coal-bearing grey-beds with coarse-grained
sandstones of the Eiriksson Formation (lowermost
Jurassic age) and the finer-grained and partially
heterolithic sandstones and mudstones of the
Nansen Formation (lower Jurassic age; Deegan &
Scull, 1977). This transition is interpreted as a
progressive change towards more humid climatic
conditions and possibly hinterland rejuvenation
(Røe & Steel, 1985; Ryseth, 2001).
The Raude Fm is typified by an upward trend
from fewer, relatively stationary and narrower
channel belts to broader and possibly more
mobile braid-belts. Overall N/G is around 45%.
In the lower part, isolated sandstone bodies are
encapsulated in extensive mudstones with pal-
aeosol horizons. Channel sandstone thicknesses
are quite variable and both multi-storey and pos-
sible single-storey channels occur. The latter are
interpreted as possibly anastomosing trunk
channels on the alluvial plain, while the multi-
storey sandstone bodies represent braid belts
(Nystuen & Fält, 1995; Ryseth & Ramm, 1996;
Ryseth, 2001).
The lower part of the Eiriksson Fm is composed
of highly amalgamated sandstone bodies, formed
in sinuous channels, with good lateral and verti-
cal connectivity and a sandstone N/G fraction of
more than 80%. Most of the upper part of the
Eiriksson Fm is interpreted as being deposited in
a lower coastal plain environment with fluvial-
dominated distributary mouth bars and bay fills
in the Tampen area.
The uppermost part of the Eiriksson Fm and the
Nansen Fm are considered to be influenced by
marine processes and witness the onset of trans-
gression that culminates in the deposition of the

APPLICATION TO THE
STATFJORD GROUP

General setting and characteristics

The largely fluvial Statfjord Group on the Tampen
area of the North Sea (Deegan & Scull, 1977; Røe &
Steel, 1985; Nystuen & Fält, 1995; Ryseth & Ramm,
1996; Ryseth, 2001; Lervik, 2006; Figs 1-3 and
Fig. 8) was deposited in a rift basin that experi-
enced relatively high subsidence rates during
deposition. It was deposited on large subsiding
'mega-blocks' in a structurally confined area set
up by thermally driven post-rift subsidence
(Badley et al ., 1988; Steel & Ryseth, 1990; Steel,
1993; the North Sea Viking Graben, Fig. 8). In a
SW to NE direction from the Utsira High into the
Viking Graben (a distance of approximately 140 km)
the formation changes in thickness from about
200 m to 600 m. This change in formation thick-
ness is not reflected in significant overall changes
in depositional environment (Ryseth, 2001) indi-
cating that large-scale subsidence patterns and
differential compaction generally did not affect
reservoir architecture.
Main sediment source areas as identified from
Sm/Nd studies (Mearns et al ., 1989) were located
north and north-west of the Tampen Spur. Possible
source terrains include the Rona Ridge to the west
of the Shetland Islands which was part of the
larger Lewisian basement of western Scotland.
This source terrain might be reactivated several
times due to early Cimmerian tectonics. Other
workers (Dalland et al ., 1995; Morton et al ., 1996)
included Caledonide foldbelt areas to the north of
Tampen Spur as possible sources for at least parts
of the Statfjord Group, which also makes eastern
sources probable.
There is a clear proximal-distal trend from
North (Tampen Spur) to South (Utsira High)
for contemporaneous deposits of the Statfjord