Geoscience Reference
In-Depth Information
The Åre 3 Zone is characterised by rapid lateral
and vertical shifts in coastal plain to marginal-
marine bay facies belts (cf. Machado, 2009) and
the base boundary of the zone is defined by a 1 m
to 3.5 m thick mudstone, containing the first evi-
dence of brackish-water conditions, such as syn-
aeresis cracks and trace fossils (e.g.
Arenicolites
carbonarius
). In the northernmost part of the
field, this marine incursion and associated rise in
water table is associated to alternating brackish
water mudstones and locally developed peat
swamp deposits. The distribution of facies asso-
ciations indicates that the geometry of the palaeo-
coastline was probably quite complex during
deposition of this reservoir zone. Correlation of
facies across the Heidrun Field indicates that the
first, rather poorly developed and mud-prone,
wave-reworked bay-fill deposits developed ini-
tially in the SE, with succeeding subsequent
gradual expansion along a SW to NE oriented
coastline (the Åre 3.1 Sub-zone; Figs 4 and 7B).
There is an overall landward shift of facies belts
upwards within the Åre 3 Zone and compared to
the Åre 3.1 Sub-zone deposits, the bay-fill units
in the uppermost part of the reservoir zone (Åre
3.3 Sub-zone; Fig. 4) are better developed, sandier
and more correlateable. In this part of the stratig-
raphy the bay-fill units can be traced for 2 km to
5 km in a NNE to SSW direction and 1 km to 3 km
in a W to E direction.
The top of the Åre 3 reservoir zone is defined at
the base of a 2 to 5 m thick interval comprising
well-laminated carbonaceous claystone, mud-
stone and muddy siltstone (Fig. 8). Pervasive
calcite cementation is in most cases associated
with the uppermost part of this mudstone interval
(cf. Hammer
et al
., 2010). An upward increase
in bioturbation (
Arenicolites carbonarious
and
Planolites montanus
) within the mudstones and
claystones may suggest increased salinities
and more tolerable living conditions in combina-
tion with reduced sedimentation rate during dep-
osition. The characteristic log expression of this
interval, high GR values and a wide negative sep-
aration on the RHOB/NPHI logs together with
high RHOB values indicating the calcite cementa-
tion, facilitates identification across the field
(Figs 8 and 18). This mudstone interval is the
most consistent and reliable correlation marker
within the Åre Formation and the bay-fill cycle
above it (the Åre 4.1 Sub-zone) is the first of
these units to have prograded over a significant
swathe of the southern Heidrun Field. As such,
this surface is considered a maximum flooding
event (MFS2; Table 1, Figs 4 and 18). Results from
the Heidrun Field megaspore study (in-house)
confirm that the uppermost part of the Åre 3 Zone
and locally lowermost part of the Åre 4 Zone cor-
responds to the first occurrence of mesofossils
from the
Kuqaia quadrata
Zone. This biozone can
be correlated regionally across the Halten Terrace
(Morris
et al
., 2009) and, according to Morris
et al
. (2009), it calibrates with the first occurrence
of abundant
Cerebropollenites thiergartii
(palyno-
logical bioevent) which defines the base of the
Sinemurian stage (Fig. 5; Pedersen
et al
., 1989).
Although, it is difficult to pin the base of the
megaspore biozone precisely to the top of the
lithostratigraphically defined Åre 3 reservoir
zone, the base Sinemurian megaspore assemblage
seems to broadly correspond to the MFS2 mud-
stone interval.
Åre 4 Zone
Although the deposits of the Åre 4 Zone (50 m
to 75 m thick) display a similar composition of
facies as the Åre 3 Zone, it is distinguished by an
increase in brackish water bay-fill relative to
lower coastal plain fluvial/alluvial deposits
(Figs 4 and 18). Wave-reworked bay-fill deposits
form packages of 4 m to 5 m thick, stacked upward-
cleaning heteroliths and clean sandstones. Strong
wave-influence is indicated by the presence of
hummocky and swaley cross-stratification. Facies
correlations indicate that these thicker sandy
units are continuous for at least 6 km to 10 km in
NE to SW directions. Compared with the Åre 3
Zone, the deposits of the Åre 4 reservoir zone
contain larger and better organised bay-fill suc-
cessions. This, combined with less frequent dis-
tributary channel deposits, suggests that larger
interdistributary bay systems had developed,
with increased wave reworking of the delta front
areas (Fig. 7C).
Repeated marine incursions during deposition
of the Åre 4 Zone have produced a number of
candidate flooding surfaces that can be correlated
over wide areas (Fig. 4). Hence, several sub-zones
have been identified: Åre 4.1 to 4.4. These sur-
faces most likely represent autocyclic shifts
within the interdistributary bay system where
relatively low gradients of the coastal plain
combined with shallow water depths may have
enhanced the extent of apparent palaeocoastline
migrations.
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