Updated depositional and stratigraphic model of the Lower Jurassic Åre Formation, Heidrun Field, Norway - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 259

image logs (FMI; Fullbore Formation MicroImager)
are used to derive proximal to distal facies rela-
tionships and depositional trends. Facies distri-
bution maps illustrate depositional scenarios and
representative 'time-slices' of sedimentary environ-
ments for each reservoir zone.
Two biostratigraphic studies validate chron-
ostratigraphic correlation in the Åre Formation.
Pedersen et al . (1989) and an in-house palynos-
tratigraphic study documented both non-marine
acritarchs and non-pollen/spore organic debris
and marine palynomorphs (dinoflagellate cysts,
acritarchs and prasinophycean algae). However,
it has been found that the use of palynological
biozones for chronostratigraphic correlation is
limited due to their strong facies dependency.
A separate megaspore zonation study attempted
to improve the stratigraphic resolution of the low-
ermost, non-marine part of the Åre Formation.
Although results from this study are in keeping
with the regional megaspore biozonation of the
Åre Formation in Mid-Norway (based on Morris
et al ., 2009; Fig. 5), the reservoir zone resolution
within the non-marine succession on Heidrun
was not significantly improved by this megaspore
zonation.
Some sedimentological and stratigraphic con-
cepts established during this study are tested
against dynamic well data (in-house studies). For
example, potential and proven barriers and baffles
for flow and connectivity of sandbodies identified
by petrophysical pressure gradients, well test data
and production history have been compared with
the geological understanding of the different res-
ervoir zones (see 'Industrial application' chapter
for further discussion).

Formation is succeeded by the tide-dominated
Tilje Formation of early Pliensbachian to early
Toarcian age (Fig. 3). The lithostratigraphic
framework of the Mesozoic and Cenozoic
succession offshore mid-Norway is defined by
Dalland et al . (1988). The regional chronostrati-
graphic framework is, to a large degree, based
on palynostatigraphy (Pedersen et al ., 1989) and
in-house, unpublished studies and reports. The
Åre Formation is identified in several fields
offshore mid-Norway and northern Norway
(cf. Dalland et al ., 1988). Although the current
stratigraphic subdivision of the Åre Formation
differs from field to field, the formation has
traditionally been sub-divided into two members:
Åre 1 and Åre 2 (Fig. 5). The boundary between
these members is defined as the top of the young-
est coal-bearing strata (Dalland et al ., 1988; Svela,
2001). This boundary separates the dominantly
coastal plain deposits of the lower Åre Formation
from the marginal-marine dominated deposits of
the upper Åre Formation.
The overall transgressive character of the Åre
Formation reflects gradual tectonic development
of the Halten Terrace Basin during development of
the North Atlantic rift system in the early Jurassic.
This intracratonic post-rift basin, estimated to
be 600 km by 200 km, opened northward to the
Tethys Sea (Fig. 2; Doré, 1991; Kjærefjord, 1999).
The oldest part of the Åre Formation consists of a
succession of fluvial channel sandstones, peat
swamp coals and floodplain mudstones, inter-
preted to correspond to deposition in a low-lying
and wet coastal plain environment (Åre 1 and 2
Zones; Rhaetian to Hettangian; Fig. 4 (Dalland
et al ., 1988). A gradual change into a lower delta
plain setting is confirmed by repeated incursions
of marginal-marine interdistributary bay, marsh
and distributary channel fill facies associations in
the middle part of the Åre Formation (Åre 3 and
4 Zones; Hettangian to Sinemurian; Fig. 4;
Kjærefjord, 1999; Svela, 2001). A progressive tran-
sition into more open marine conditions recorded
in the uppermost Åre Formation (Åre 5, 6 and 7
Zones; Sinemurian to early Pliensbachian time;
Fig. 4) is sustained throughout deposition of the
lowermost Tilje Formation (Kjærefjord, 1999;
Martinius et al ., 2001). This transition is ascribed
to a significant change in the basin geometry related
to the establishment of a marine seaway between the
Tethys Sea in the south and the Boreal Sea in
the north (Kjærefjord, 1999; Martinius et al ., 2001).

GEOLOGICAL SETTING

The Åre Formation, of the Early Jurassic Båt
Group, is approximately 670 m thick and com-
prises coastal plain to delta plain sandstones,
mudstones and coals of Rhaetian to early
Pliensbachian age (Figs 3 and 4). Few wells extend
through the base of the Åre succession into the
underlying alluvial Triassic age 'grey beds'. The
Rhaetian age of base Åre is confirmed by both
the megaspore Bankis porites pingus (in-house
study; Fig. 5) and palynological biostratigraphy
Ricciisporites tuberculatus / Limbosporites lund-
bladii events (Pedersen et al ., 1989). The Åre