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during crustal cooling from the Early Cretaceous
to Recent (Dahl & Solli, 1993; Nøttvedt
et al
.,
1995). The rocks of the Lunde Formation and the
Statfjord Group were then buried to their present
depths of about 2500 m to 3000 m.
Detailed information from mudrock units comes
from continuously cored intervals with high
recovery (Table 1). Standard polished thin sec-
tions were prepared from selected samples and
employed to quantify fine material and non-clay
minerals. Palaeosol features encompass colour,
pedogenic aggregates (peds) and cutans, roots and
other bioturbation structures, striated pedogenic
fault surfaces, desiccation cracks and various types
of calcrete nodules and coalesced nodules (Müller
et al
., 2004).
The fluvial channel sandstones of the Lunde
Formation and the Statfjord Group have an aver-
age porosity of about 25% according to core
plug analyses. The mineralogical composition of
the sandstones was made by modal analyses in
thin sections (Sørlie 1996) (Table 2).
The mineralogy of 124 mudrock samples was
studied from seven wells (Tables 1, 3 and 4). Clay
mineralogy was determined by XRD. Powder
samples were prepared by crushing in a hand
mortar to a particle diameter of approximately
1 mm or less, then transferred to a beaker, covered
with distilled water and 10 ml of the deflocculant
Calgon and placed in an ultrasonic bath for 2 min-
utes. The resulting suspension was poured into a
settling column and allowed to stand until the
required size fraction of ≤ 2 µm had separated and
could be removed. Samples were made by sedi-
mentation under vacuum onto a porous ceramic
tile. Samples were treated with MgCl
2
(aq) so that
all the smectites would have the same
d
spacing
during the analyses. Air-dried samples were ana-
lysed with a Philips PW 1710 X-ray diffractom-
eter in the interval 3
o
to 50
o
2θ, with Cu (Kα)
radiation. Five major clay mineral groups were
identified by their basal spacing (Table 4). Smectite
was distinguished from chlorite by subsequently
treating the sample with ethylene glycol, which
expands the basal spacing to about 17 Å. Heating
to 550
o
destroyed kaolinite and certain chlorites
(Hardy & Tucker 1995). The iron oxides hematite
and goethite were also determined by XRD. The
computer program Mac Diff 4.2.5 and correlation
factors for qualitative and quantitative analysis of
the different clay minerals (Ferrell & Dypvik, 2009)
were applied.
Subdivision of the Lunde Formation
and the Statfjord Group
The Lunde Formation and the Raude and Eiriksson
formations in the Statfjord Group are subdivided
into allostratigraphic units (cf. North American
Commission on Stratigraphic Nomenclature, 1983,
p. 865). These are numbered stratigraphically from
below: L12 to L01 in the Lunde Formation and S5
to S1 in the Statfjord Group (Diesen
et al
., 1995;
Nystuen & Fält, 1995) (Fig. 3).
The allostratigraphic units in the Lunde, Raude
and Eiriksson formations are generally fining-
upwards (FU) successions with a lower part
dominated by multi-storey channelised sand-
stone bodies and an upper part with a decreasing
frequency of channel sandstone bodies and
increasing abundance of overbank sandstones
and floodplain mudstones. Individual allostrati-
graphic units are some tens of metres thick. The
internal organisation of individual units com-
monly changes laterally due to facies variation
across the depositional basin. Two or more
allostratigraphic units form higher-order fining-
upwards and coarsening-upwards (CU) sets
(Fig. 3). We refer in the description and discus-
sion below to an informal subdivision of the
Lunde Formation in a lower (LLF), middle (MLF)
and upper part (ULF) and the fluvial part of the
Statfjord Group into a lower (LSG) and upper
part (USG) (Fig. 3). ULF is subdivided into ULF
1 and 2. This subdivision of the Lunde Formation
should not be confused with the former informal
member subdivision of Nystuen
et al
. (1989) (see
above).
Material and methods
This study presents data from 36 wells from the
Snorre Field and adjacent oil fields in the Tampen
Spur area (Table 1, Fig. 1). Approximately 3500 m
of core from 22 selected wells have been studied
in detail with respect to stratigraphic variation
in fluvial style, using traditional sedimentologi-
cal logging techniques. Additional wells have been
used for correlation within the Tampen Spur area
(Fig. 1, Table 1).
Sedimentary facies and alluvial architecture
The Lunde Formation and the Statfjord Group
consist of repeated successions of (1) channelised
sandstone bodies interbedded with (2) proximal
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