Geoscience Reference
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Shut-off of coarse sandy supply was relatively
abrupt and replaced by a prolonged period of
pelagic claystone deposition. These claystones
form an extensive unit which separates the
Springar and basal Tang Formation sandstones
(i.e. the 'Våle heteroliths') from the overlying Egga
sandstone unit.
upper part of the Springar sandstones is thin and
vice versa. Other production wells, located in
more central parts of the subbasin, have a more
irregular vertical facies trend, probably with vertical
alternations between the various mid-fan facies
associations. This is interpreted to reflect back-and-
forth shifting of mid-fan depositional sub-environ-
ments (i.e. the channel-to-lobe transition, central
lobe and peripheral lobe) through time (Fig. 13C).
Wells off-axis of the main sand-fairway (e.g. well
6305/1-1 to the north; Figs 4 and 6) shows a rela-
tively thick monotonous mudstone section, devel-
oped in predominantly contouritic mudstone facies.
The upper part of the Egga sandstone unit com-
prises another channelised package in the Ormen
Lange subbasin, representing predominantly chan-
nel-belt, sandy inter-channel (sometimes slump-
deformed) and channel-to-lobe transition facies
associations (Fig. 13D). The change from the sedi-
ment-starved conditions represented by the field-
wide pelagic drape across the lower part of the Egga
sandstone unit to the channelised upper part of the
unit is fairly sharp, suggesting a rapid return of inner
fan environments and in places a fairly marked bas-
inward shift in facies tracts. Seismic and core data
suggest relatively shallow channels with sandy
overbanks and inter-channel areas (Figs 6 and 10).
The channelised unit is accordingly interpreted to
represent a braided inner fan area (cf. Decker
et al
.,
2004), where channels could migrate relatively
freely. However, the position of the channel-belts, as
defined by the vertical stacking of channel-fill facies
and seismic data, remained relatively fixed through
time, their position probably controlled downcur-
rent from the outlet of fixed slope or ramp margin
conduits (Fig. 11). Seismic data suggest that these
channel storeys fed frontal lobe storeys or lobe sto-
rey sets/lobe complexes in the adjacent downflow
subbasin, probably from channels incised via knick-
point erosion (see Pirmez
et al
., 2000) into the front-
ing slope ramps. Within the Ormen Lange subbasin,
the inter-channel areas are dominated by relatively
coarse-grained sediments that probably represent
overbank or crevasse channels and splays. Off-axis
or distal sub-environments (well 6305/1-1; Fig. 6)
are dominated by continuous mudstone deposition
of contouritic origin, with localised interbedded,
thin-bedded turbidites. Penecontemporaneous soft-
sediment deformation formed in response to an
unstable substrate, probably as a combination of
higher local sedimentation rates coupled with basin-
floor tilting. Locally, there is an increase in the
amount of soft-sediment deformation towards the
Egga sandstone unit; reservoir architectures
and stratigraphic development
In the Ormen Lange subbasin, the Egga sandstone
unit predominantly consists of central to inner fan
facies tracts. It forms an overall forestepping unit
with a thin, backstepping uppermost part. The
unit can be split into a lower and an upper part,
potentially representing two individual lobe com-
plexes, respectively, separated by a relatively
thick pelagic mudstone unit that extends across
the entire Ormen Lange subbasin.
The lower part of the Egga sandstone unit forms
a high net-to-gross sandstone package, defined by
a series of normally 3 to 7 (maximum 10) metres-
thick bedsets (Fig. 9), again interpreted to repre-
sent individual lobe storeys (Fig. 12). The bedsets
or lobe storeys are commonly separated by variably
thick, mixed hemipelagic-pelagic mudstones.
Each sandstone motif commonly shows a coarsen-
ing-to-fining and thickening-to-thinning upwards
trend, typically paralleled by an associated
upwards change from distal through successively
more proximal facies, subsequently overlain by
distal or marginal lobe facies tracts. The lobe sto-
rey motifs are commonly arranged so that they
define an overall forestepping trend, suggesting an
overall progradational or outbuilding of the sandy
fan (Fig. 5). Such a simple stratigraphic organisa-
tion of facies tracts is best developed in wells to
the south-west and north-west in the Ormen Lange
subbasin (i.e. those located in more downfan posi-
tions). However, there are marked local departures
from this generalised forestepping stacking pat-
tern. Wells within the central-eastern part of the
Ormen Lange subbasin have a basal, overall back-
stepping trend. This is defined by a vertical transi-
tion from relatively thick basal lobe storey(s),
commonly consisting of coarse-grained channel-
fill and channel-to-lobe transition facies types,
overlain by central or mid-fan lobe storeys of alter-
nating central to peripheral lobe deposits. Only
subordinate channel-to-lobe transition facies types
are associated. The basal coarser-grained lobe sto-
reys are thicker where the underlying, disorganised,
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