The Garn Formation (Bajocian-Bathonian) in the Kristin Field, Halten Terrace: its origin, facies architecture and primary heterogeneity model - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 517

Its base is sharp and mainly erosional and the
formation comprises littoral sandstones with a
deltaic or foreshore gravelly component to the
west. The Melke Formation (Bajocian to early
Oxfordian) consists of heterolithic sublittoral
deposits and bioturbated neritic mudstones. In
the Trøndelag Platform to the east, the Melke
Formation is a time-stratigraphic equivalent of the
Garn Formation (Brekke et al ., 2001) but it is time-
transgressive, onlapping the latter formation west-
wards in the Halten Terrace (Gjelberg et al ., 1987;
Corfield et al ., 2001).
This muddy lower unit of the Viking Group
(Fig. 3) marked the incipient tectonic collapse and
inundation of the western hinterland in the late
Middle Jurassic. The Late Jurassic rifting pulse
and eustatic sea-level rise then dramatically
increased this palaeogeographic change, with
neritic sedimentation proceeding in the Trøndelag
Platform and an anoxic, deep neritic to bathyal
environment extending from the Halten Terrace
westwards to the evolving Vøring Basin (Spekk
Formation; Brekke et al ., 2001). Local submarine
erosion of the uplifted footwalls of rotated fault
blocks (Fig. 2) occurred until the Kimmeridgian.

show a helium-based porosity of 18% to 20%,
which is twice greater than expected from the
general porosity/depth trend in the region
(Ehrenberg, 1993; Chuhan et al ., 2001). The rela-
tively high preservation of primary porosity is
attributed to the early diagenetic coating of sand-
stone grains with illite and mixed illite-chlorite
(Ehrenberg, 1993; Chuhan et al ., 2001; Storvoll
et al ., 2002).

THE GARN FORMATION

The Garn Formation is 95 m to 120 m thick in the
studied wells in the Kristin Field but thins gradu-
ally to the north and south (Quin et al ., 2010) and
is also considerably thinner or locally eroded on
the Halten Terrace horsts (e.g. on the Sklinna High
at the western margin of the terrace, Fig. 2B). The
formation consists mainly of fine-grained to
medium-grained subarkosic arenites (Chuhan
et al ., 2001) with coarse to very coarse sandstone
intercalations. Its base is a regional erosional
unconformity of earliest Bajocian age, recording
fault-block tilting accompanied by a relative sea-
level fall (Corfield et al ., 2001). Biostratigraphic
data from the adjacent Smørbukk and Smørbukk
South fields indicate that the base, although a sub-
tle angular unconformity with onlap in seismic
sections, can be regarded as approximately isoch-
ronous. Erosional truncation of the underlying
Not Formation was very limited and probably did
not exceed 5 m to 10 m even on structural highs
(Corfield et al ., 2001).
The most voluminous lower sandstones of the
Garn Formation were deposited in synclinal
troughs, mainly incipient grabens and half-
grabens bounded by fault ruptures and/or blind-
fault flexures. The uppermost sandstones, in
contrast, were deposited as progradational wedges
back-stepping onto adjoining structural highs and
interfingering with the heterolithic to muddy
deposits of the Melke Formation (Corfield et al .,
2001). The littoral zones of sand deposition pro-
gressively migrated from the depressions onto the
adjoining highs, giving way to the neritic environ-
ment of the Melke Formation.
The marked regional diachroneity of the upper
boundary of the Garn Formation was first recog-
nised by Gjelberg et al . (1987), who suggested that
the formation was interfingering with the Melke
Formation while shrinking in lateral extent. This
interpretation remained largely overlooked by

The Kristin Field

The siliciclastic sandstones of the Garn and Ile
formations (Fig. 3) are the main hydrocarbon res-
ervoirs in the Kristin Field (Helgesen et al ., 2000;
Martinius et al ., 2005). The hydrocarbon trap is
structural, formed by a north-trending horst block
tilted towards the east (Fig. 2), as is characteristic
of the Halten Terrace fields. The hydrocarbon
source rocks are the coals and coaly shales of the
underlying Åre Formation (Helgesen et al ., 2000;
Martinius et al ., 2005; Quin et al ., 2010). The
field's estimated original in-place hydrocarbon
volume includes ~ 100 GSm 3 of gas and ~ 100 MSm 3
of condensate, with a dew point at ~ 400 bar (Quin
et al ., 2010). The reservoirs are the depth of 4600 m
to 5600 m, with high temperature (170 °C) and
high initial fluid pressure (911 bars). The field was
discovered by Saga Petroleum in 1996 and pro-
duction commenced in November 2005, with
Statoil as the operator.
The porosity of the deeply buried reservoirs in
the Halten Terrace generally tends to be reduced
by quartz cementation (Bjørlykke et al ., 1986;
Ehrenberg, 1990; Walderhaug, 1994, 1996).
However, the sandstones of the Garn Formation in
the Kristin Field at the depth of 4500 m to 5000 m