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 514

INTRODUCTION of tidal deposits in sand-prone rift basins and the

record of relative sea-level changes in thick
subtidal sandstone successions.

The Halten Terrace (Fig. 1) is the most prolific
hydrocarbon province in offshore mid-Norway,
hosting more than a dozen fields with gas, con-
densate and light to medium oil in the Lower to
Middle Jurassic siliciclastic reservoirs (Spencer
et al ., 1993; Koch & Heum, 1995). The hydrocar-
bon-bearing formations are mainly tidal and del-
taic deposits of the Early Jurassic Båt Group and
the Middle Jurassic Fangst Group (Gjelberg et al .,
1987; Dalland et al ., 1988). Large parts of the res-
ervoirs consist of heterolithic deposits comprising
irregularly interlayered sandstones, siltstones and
mudstones. The extreme heterogeneity of these
deposits renders reservoir characterisation and
hydrocarbon recovery difficult and has been
addressed by numerous studies (Dreyer, 1992;
Wen et al ., 1998; Jackson et al ., 1999; Kjærefjord,
1999; McIlroy et al ., 1999; Martinius et al ., 2001,
2005; McIlroy, 2004; Brandsæter et al ., 2005;
Klefstad et al ., 2005; Nordahl et al ., 2005; Ringrose
et al ., 2005; Rivenæs et al ., 2005). Little research
has thus far been done on the heterogeneity of the
sandy and relatively monolithic uppermost unit
of the Fangst Group, the Garn Formation (Dalland
et al ., 1988), although the type, scale and spatial
configuration of sedimentary structures in sand-
stone reservoirs are generally considered to be
important to the fluid flow and hydrocarbon
recovery (Emmett et al ., 1971; Weber, 1982, 1986;
Weber & Van Geuns, 1990; Zweigel, 1998; Jackson
et al ., 1999; Elfenbein et al ., 2005).
The present study of the Garn Formation focuses
on its development in the Kristin Field (Fig. 1),
where this sandstone succession is ~ 100 m thick
and its sedimentological characteristics suggest
strong tidal influence accompanied by wave
action. Tidal currents are thought to have been
enhanced in the area by fault-related topography
combined with a relative sea-level fall in the early
Bajocian. Sedimentological analysis of well cores
is used to analyse the facies anatomy of the Garn
Formation and thereby provide a better under-
standing of its origin and sedimentary heterogene-
ity. Dune cross-strata sets appear to be an important
element of the primary heterogeneity and a statis-
tical analysis of their thickness frequency distri-
bution is used to predict the frequency distribution
of their volumes as an important component of an
improved reservoir model.
The study also contributes to the sparse existing
knowledge concerning the accumulation patterns

THE HALTEN TERRACE

Tectonics and palaeogeography

The tectonic structure of the Norwegian
Continental Shelf evolved through the Permo-
Triassic, Late Jurassic, Middle Cretaceous and
Palaeocene rifting phases (Bukovics et al ., 1984;
Doré, 1992; Doré et al ., 1999; Brekke et al ., 2001).
The Cretaceous phase had the greatest magnitude
of extension (Pascoe et al ., 1999; Corfield et al .,
2001) and the Palaeocene phase eventually sepa-
rated the Fennoscandian and Greenland cratons
and opened the North Atlantic. However, it was
probably not until the early Pliocene that the
hydrocarbons were generated and reservoir over-
pressure developed in the Halten Terrace (Skar
et al ., 1999).
The rifting with the influence of Triassic salt
resulted in listric detachment faults and deep pla-
nar faults at the continental margin (Jackson &
Hastings, 1986; Withjack et al ., 1989; Pascoe et al .,
1999; Corfield & Sharp, 2000; Marsh et al ., 2010).
The Halten Terrace structure is a stairway of tilted
fault blocks striking SW-NE and down-thrown
towards the NW (Fig. 2A), with internal horsts,
grabens and half-grabens (Fig. 2B). The Jurassic
physiography of the Halten Terrace is not reflected
in its present-day structure but can be recon-
structed from sedimentary facies distribution and
structural restoration (Corfield et al ., 2001).
The base of Jurassic syn-rift deposits in the
Halten Terrace was originally placed between the
shallow-marine sandstones of the Fangst Group
and the neritic mudstones of the Viking Group
(Fig. 3; Dalland et al ., 1988; Ehrenberg et al .,
1992; Koch & Heum, 1995) but subsequent stud-
ies have indicated that the rifting phase com-
menced earlier and extended into the earliest
Cretaceous. Syndepositional growth faults and
fault-propagation flexures formed in the Early to
Middle Jurassic (Blystad et al ., 1995; Corfield &
Sharp, 2000; Corfield et al ., 2001; Marsh et al .,
2010) and this deformation had an important
effect on basin physiography, on the location of
hanging-wall depocentres and on the develop-
ment of sediment fairways and accommodation
for the Garn Formation (Corfield et al ., 2001).