Channel development in the chalk of the Tor Formation, North Sea: evidence of bottom current activity - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 563

The margins of the channel have been drilled in
its western segment, with two wells at the north-
ern margin (2/4-A8 and 2/4-A6) and one well at
the southern margin (2/7-14). Well 2/7-4 located
in the Edda Field at 7 km from the channel has
also been studied and is included for reference.

density; a distinction between clay-rich and
clay-poor autochthonous chalks can be based on
the gamma ray log responses with clay-rich beds
showing high gamma ray values. Although several
studies have noted that allochthonous chalk is
generally more porous than autochthonous chalk
(Hatton, 1986; Kennedy 1987a, 1987b; Taylor &
Lapré, 1987; Brasher & Vagle, 1996), other factors
such as diagenesis, pore fluid type, sediment
texture and composition also undoubtedly influ-
ence the degree of porosity preservation. Some
studies take the view that there is no straightforward
relationship between high porosity development
and re-deposition of chalk (Maliva & Dickson,
1992; Fabricius et al ., 2007).
The channel fill is subdivided into a lower and
upper unit based on seismic characteristics and
calibrated well-log signatures (e.g. Figs 6 and 7).
The lower unit represents 40% of the channel
succession whilst the upper unit makes up the
remaining 60%. The following sections describe
the facies associations which make up the
depositional successions of the lower and upper
channel-fill units and the channel margins,
respectively.

Chalk facies

Facies have been distinguished based on
depositional texture (Dunham, 1962), diagnostic
sedimentary structures and biogenic features.
Several facies classifications have earlier been
proposed for the chalk (Kennedy, 1987b; Crabtree
et al ., 1996; Røgen et al ., 1999). The classification
used in this study (Table 2) is after Bramwell et al .
(1999) (Table 3). The mode of deposition is inter-
preted following a similar approach to that of
Kennedy (1987b). Grain size is not generally
recorded in chalk since this classification of core
samples would allow recognition of little more
than a fairly monotonous succession dominated
by mudstone and wackestone lithologies. To bet-
ter differentiate these lithologies, an estimation of
argillaceous content has been made, indicating
the relative amount of carbonate vs. terrigenous
material. Pebble floatstone and grain-supported
sediments are recorded as pebble-grade deposits
in the sedimentological logs. Short descriptions
and depositional interpretations of the seven
chalk lithofacies that have been recognised are
given in Table 2.
Recognition of facies in non-cored well intervals
is tentative and based upon relationships between
petrophysical log response and core facies. Hatton
(1986), Kennedy (1987b) and Campbell & Gravdal
(1995) have demonstrated that log responses
reflect different chalk facies: (1) Allochthonous
facies are generally characterised by intervals
with low density, high porosity and low gamma
ray values. These facies were deposited by a
variety of depositional processes: (a) debris flow,
mud flow, low density turbidity currents and mud
cloud deposits show distinct low density/high
porosity signals, commonly with sharp bases and
tops; (b) thick slump and slide deposits show low
density/high porosity but with a gradual decrease
of porosity and increase in density at the top;
(c) coarse turbidites and winnowed lags are
distinguished by thin spikes of high porosity and
low density response; (2) Autochthonous facies;
commonly correspond to broad and less sharply
delineated log signals with low porosity and high

Facies association of the lower channel-fill unit

Description: The basal part of the lower channel-fill
unit, calibrated to seismic section, is characterised
by thin, high porosity deposits that are interpreted
from wireline logs to be either packstone and
grainstone chalk of facies PGC (see Table 2)
(e.g. 3 m thick in well 2/8-15, where the channel
base is at 3475 m according to the wireline logs
and seismic calibration).
No core samples are available from either these
basal deposits or from the overlying thick
succession of inferred bioturbated homogeneous
chalk (facies BHC; Table 2) or laminated chalk
(facies LC) that make up ~ 80% of the lower
channel-fill unit. Density and porosity logs of this
succession show high-frequency facies alternation
at decimetre-scale to metre-scale. These deposits
pass upwards into a bioturbated argillaceous
chalk (facies BAC) that makes up the remaining
~ 20% of the lower channel-fill unit and that is
represented by a 5 m core interval in well 2/8-15
(Figs 7, 12A and 12B).
Interpretation: The lowermost 3 m of the lower
channel-fill unit (Fig. 9, well 2/8-15 at 3475 m to
3472 m) is a packstone/grainstone (PGC facies,
Table 2) chalk deposit formed by erosive reworking