Geoscience Reference
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Åre 2 Zone
to be a candidate sequence boundary formed dur-
ing significant lowering of base level (c.SB1;
Table 1 and Fig. 13). The top of the zone shows
fairly abrupt transition into overlying floodplain
fines and brackish-water bay-floor mudstones.
This onset of marginal-marine mud deposition
defines a flooding surface (Fig. 4).
Two sub-zones have been defined within this res-
ervoir zone (Åre 2.1 and 2.2; Figs 4 and 13). The
lowermost, the Åre 2.1 Sub-zone (65 m to 80 m
thick), is composed of fluvial coastal plain depos-
its which show an overall increase in the channel
to floodplain ratio (>40% sand) compared to the
underlying Åre 1 Zone, suggesting improved con-
nectivity of channel belts (e.g. Mackey & Bridge,
1995). Single-storey and multi-storey channel
sandstones of limited lateral extent are character-
istic for the lowermost part of this sub-zone, but a
significant increase in correlateable vertically and
laterally accreting channel deposits is observed
upwards in the stratigraphy (Fig. 13). In particu-
lar, one channel belt is identified in the upper-
most part of this reservoir sub-zone and mapped
in the seismic data through the northernmost part
of the Heidrun Field. Seismic mapping and well-
to-well correlations suggest this cut-and-fill-chan-
nel belt has a lateral extent of at least 2.5 km
perpendicular to the estimated N to S-oriented
depositional direction. The frequency of lake and
peat swamp deposits is somewhat lower within
Åre 2.1 Sub-zone than the Åre 1 Zone, suggesting
floodplains were better drained. This change in
floodplain facies, together with increased connec-
tivity of channel sandstone bodies, might be
attributed to deposition during a phase of high-
stand progradation.
The succeeding Åre 2.2 Sub-zone comprises a
relatively thick interval (typically 10 m to 15 m,
maximum 34 m; Fig. 4) of fluvial channel sand-
stones, with a minor component of crevasse splay
sandstones and floodplain mudstones (Fig. 13).
This fluvial channel belt is assumed to be laterally
extensive over large parts of the field, based upon
well correlations, pressure continuity and pro-
duction data. Evidence from core indicates that
the channel sandstones display consistent deposi-
tional characteristics (in terms of bedform thick-
ness, presence of internal scour surfaces, grain
size and sorting) in most wells. The base of the
Åre 2.2 Sub-zone is erosive and typically charac-
terised by a pebble lag. This erosional surface may
reflect field-wide down-cutting forming an incised
valley filled by braided channel deposits. Deepest
incision is seen in the NNW area of the field and
the sand-dominated Åre 2.2 Sub-zone thins
toward the SE. This thinning may define the mar-
gin of the valley. Based on the field-wide extent of
the channel belt, the base of Åre 2.2 is considered
Åre 3 and 4 Zones: Lower delta plain
and brackish water, interdistributary bay
Reservoir zones Åre 3 and 4 (110 m to 125 m com-
bined thickness; Fig. 4) contain deposits that
reflect an interaction of fluvial and marginal
marine processes. Like the underlying reservoir
zones, these facies associations are interpreted to
have formed in the lower coastal plain and include
peat swamp (FA 1), lacustrine muds (FA 2) and
crevasse splay (FA 4; Fig. 9). However, in these
zones, lower coastal plain deposits are intermixed
with marginal marine deposits, including those of
the interdistributary bay facies associations; tide-
influenced distributary channel (FA 6), bay-floor
muds (FA 7), wave-influenced bay-fill (FA 8) and
bay-margin muds (FA 9; Figs 14, 15 and 16).
The brackish-water interdistributary bay depos-
its are dominated by 2 m to 10 m thick, upward-
coarsening/cleaning units (Figs 14 and 16) grading
from: 1) wave-rippled and pinstriped mudstones
and siltstones (FA 7), to 2) mud-prone to sand-
prone heteroliths showing lenticular bedding
and small-scale to medium-scale hummocky
cross-lamination; and 3) very fine-grained to fine-
grained sandstones displaying medium-scale
cross beds, swaley lamina sets, planar stratifica-
tion and local current ripple cross-lamination
(FA 8) (Fig. 16). Synaeresis cracks are abundant
within these deposits. Bioturbation is weak to
locally high and the trace fossil assemblages are
dominated by
Planolites montanus
,
Skolithos
and
Arenicolites carbonarious
(Fig. 14). Carbonaceous,
muddy deposits of FA 7 are interpreted to have
formed in a low energy bay floor setting swept by
occasional wave action. Dominance of wave-
generated structures within the heterolithic and
sand-rich parts of these bay-fills (FA 8) is consist-
ent with a wave-dominated 'shoreface' develop-
ment (Figs 14 and 15). Low-diversity, stressed
trace fossil assemblages, together with abundant
synaeresis cracks, confirm deposition in a brack-
ish water environment and the vertical thickness
of complete bay-fills suggests depositional water-
depths of less than 10 m.
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