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 518

researchers until Corfield et al . (2001) had
documented it from seismic sections, well logs
and biostratigraphic data in the Smørbukk area,
demonstrating also strong diachroneity on a
lateral scale of a few kilometres. This evidence
has important implications for the palaeogeography
and stratigraphic development of the Garn
Formation.
Corfield et al . (2001) suggested a three-stage
depositional scenario for the Garn Formation:
(1) an early Bajocian stage of differential erosion
due to syntectonic forced regression, followed by
lowstand accumulation of voluminous sand in
belt-shaped depocentres; (2) a transgressive stage
of 'pulsed progradation', when the littoral zones
stepped back onto structural highs as normal-
regressive wedges; and (3) a stage of final drowning,
when the littoral sand was generally covered by
neritic mud in the early Bathonian.
Despite these important insights, the depositional
environment of the Garn Formation remained
poorly understood. Gjelberg et al . (1987) originally
suggested a shoreface environment with tidal cur-
rents and high-energy waves, passing westwards
into mouth bars of a fan-deltaic system, whereas
Dalland et al . (1988) postulated east-advancing
lobes of braided-river deltas dominated by fluvial
and wave processes. Subsequent interpretations
generally invoked a high-energy shallow-marine
environment with deltaic, coastal spit, shoreface
and shelfal sand accumulations (Doré, 1992;
Brekke et al ., 2001; Corfield et al ., 2001; Elfenbein
et al ., 2005; Quin et al ., 2010). However, no fluvial
or other terrestrial deposits are documented.
Corfield et al . (2001) recognised elongate sand-
stone bodies with subtle positive relief and sug-
gested that the fault-related physiography and
regional sea-level fall probably focused tidal
currents in belt-like sand depocentres parallel to
the main faults. Tidal dunes were recognised
(Elfenbein et al ., 2005) and possible tidal sand
ridges were suggested (Corfield et al ., 2001) but no
attempt was made to recognise these sandstone
bodies in cores and take them into account in res-
ervoir models. Quin et al . (2010) attributed deposi-
tion of the Garn Formation to a structurally
controlled, eastward-advancing upper shoreface
environment or possibly tidal deltas, but noted
that the abundance of relatively coarse-grained,
cross-stratified sandstones in the central zone of
the Kristin Field, many kilometres away from the
Grip High palaeoshoreline, was difficult to explain
in this scenario.

The earlier stratigraphic interpretations gener-
ally considered the Garn Formation to be a uni-
form, tabular sandstone unit deposited during a
pre-rifting period of 'nearly total tectonic quies-
cence' (e.g. Ehrenberg et al ., 1992; Doré, 1992). On
this stratigraphic premise was also developed the
original layer-cake zonation model for the Garn
Formation reservoir in the Kristin Field, extended
further to adjacent fields to the north and east
(Elfenbein et al ., 2005; Quin et al ., 2010, fig. 2).
However, it has been pointed out by Corfield et al .
(2001) that a simple layered reservoir model for
the Garn Formation, as used in the Smørbukk,
Smørbukk South, Heidrun, Kristin, Trestakk and
Tyrihans fields, is probably unreasonable.

METHODS AND TERMINOLOGY

The present study is based on cores and geophysi-
cal data from six wells located in the axial zone of
the Kristin Field, with lateral well spacing of
1.9 km to 5.1 km (Fig. 4). The Garn Formation was
almost fully cored in these wells; and a total of
~ 700 m of cores has been logged for this study.
Conventional sedimentological logging was
used, with a bed-thickness measurement accuracy
of 0.5cm. In the measurement of bed thicknesses
and identification of sedimentary structures, spe-
cial care was taken to account for the regional
structural dip derived from resistivity imaging
logs and the inclination of some of the wells.
Wells 6406/2-5AT2, 6406/2-R-4H and 6506/11-N-
3H (Fig. 4) are non-vertical and the apparent
thickness of the Garn Formation in these wells
was corrected accordingly (i.e. multiplied by a
correction factor of 0.87, 0.72 and 0.62, respec-
tively). The top contact of the Garn Formation
with the overlying Melke Formation was not cored
in the three other wells (6406/2-5, 6406/2-3 T3
and 6506/11-6, Fig. 4). The basal contact with the
underlying Not Formation was cored in all wells.
The occurrence of large-scale cross-stratification
was verified and its dip direction assessed on the
basis of dip-picking data using resistivity imaging
logs from two wells.
The descriptive sedimentological terminology
used in this study is after Collinson & Thompson
(1982) and Harms et al . (1982). The term 'sedi-
mentary facies' refers to the basic types of sedi-
mentary deposits distinguished on the basis of
their bulk macroscopic characteristics and attrib-
uted to different modes of sediment deposition