Lange Formation (redefined)
Cromer Knoll Group
Introduction
The Lange Formation is redefined in this study to represent a relatively uniform sequence of
marine mudstones with sporadic limestone stringers. It is only equivalent to
the lower part of the Lange Formation as originally defined by Dalland et al. (1988). The upper part
of the original Lange Formation is defined as a new formation, the Blålange
Formation. In the type area of the
Halten Terrace, the Lange and Blålange Formations are separated by a significant stratigraphic break that
appears to correspond to the Lower/Upper Cretaceous boundary. With this new definition, it is now
practical to
redefine the Cromer Knoll Group in the Norwegian Sea with a similar
stratigraphic extent to that in the
North Sea. The Lange Formation likely being correlative in time to the Rødby and
Sola Formations in the North Sea area.
Sandstones within the redefined Lange Formation are generally rare, although one new sandstone member, the
Langebarn Member, is recognised.
Name
English/ Norwegian and any previous names:
This is equivalent to the informal 'Middle' and 'Lower' Lange Formations of Swiecicki et al. (1998)
and 'lower' Lange Formation of Færseth and Lien (2002).
Derivatio nominis:
From the Norwegian name for the fish species Molva molva or Ling (Dalland et al., 1988).
Lithology
The Lange Formation consists predominantly of mudstones containing occasional stringers of limestone and
rare
sandstones. The mudstones are similar to those of the overlying Blålange
Formation but are generally more
uniform with fewer limestone and sandstone interbeds. The mudstones are light to medium grey, grey brown,
occasionally green-brown, soft, plastic, amorphous, non- to moderately calcareous, blocky, firm, locally
subfissile, micromicaceous and slightly silty in part.
Sample depository
Palynological preparations (organic matter depository)
Type well
6506/12-1:
21 slides from dc samples and 2 swc samples at 3724 m and 3804 m covering the interval 3710 m - 3810 m
(Stratlab, RRI and OD preparations) deposited at the Norwegian Petroleum Directorate.
Reference well
6506/12-4:
11 slides from dc samples covering the interval 3757 m - 3835 m (Stratlab and RRI preparations) deposited
at the Norwegian Petroleum Directorate.
Core photographs
No cores were taken in the designated type
6506/12-1 and reference
6506/12-4 wells.
Thickness
The Lange Formation ranges in thickness from 3 m to 467 m.
Regional isochore of the Lange Formation thickness in the Norwegian Sea based on released well data.
The isochore map is generated from Norlex data using thin plate splines (thickness constrained to original
range).
Thicknesses in metres.
Circled wells contain both top and base horizons.
The red wells have Norlex biostratigraphy. Click for large version.
Interactive Function:
Note that this map is only a regional interpretation and the user can generate more specific, local area
isochore maps
interactively within Norlex using the link below.
Interactive Norlex isochore map for the Lange Formation
|
Geographical distribution
The (revised) Lange Formation is generally regionally extensive, and is only absent from parts of the
Nordland Ridge and on local highs along the western flank of the Trøndelag Platform. However it has
generally not been penetrated in the deep wells of the Vøring Basin so data from this area is lacking. It
reaches a maximum thickness of 467 m in well
6507/7-12. This well was drilled in an Early
Cretaceous
mini basin, developed in the hanging wall of the Revfallet Fault complex separating the Dønna Terrace from
the Nordland Ridge to the east. The formation is generally thicker in wells on the Dønna Terrace e.g.
6507/2-3 (432 m). Elsewhere, on the Halten
Terrace it is generally between 50-150 m thick although it may
reach 300 m in some wells e.g.
6406/3-2. In the Helgeland Basin, it locally
reaches over 150 m e.g.
6610/7-1.
Occurrences of formation tops in wells
Type well (revised)
Well name: 6506/12-1
WGS84 coordinates: N 65°10'07.58, E 06°43'44.07
UTM coordinates: 7229359.52 N 393591.68 E
UTM zone: 32
Drilling operator name: Den norske stats oljeselskap a.s. (Statoil
a.s.)
Completion date: 06.02.1985
Status: P & A
Interval of type section & thickness in type well:
3812.5 m - 3705 m, 107.5m thickness.
The lower boundary follows the original definition of Dalland et al. (1988). Previously, the Lange
Formation extended from 3190 m. The interval 3705 m to 3190 m is now assigned as the new formation:
Blålange Formation.
Type well 6506/12-1, former definition (click to enlarge) |
Type well 6506/12-1, redefined (click to enlarge) |
Reference well (revised)
The top of the reference well is revised from 3150 m to 3755.5 m to accommodate the new
Blålange Formation.
Well name: 6506/12-4
WGS84 coordinates: N 65°12'46.97, E 06°43'30.37
UTM coordinates: 7234298.14 N 393591.29 E
UTM zone: 32
Drilling operator name: Den norske stats oljeselskap a.s. (Statoil
a.s.)
Completion date: 13.08.1985
Status: P & A
Interval of type section & thickness in reference well:
3835 m to 3738 m, 97 m thickness. The lower boundary follows the
original definition of Dalland et al.
(1988). The upper boundary of the Lange Formation is redefined in this
study following the creation of the
new Blålange Formation.
It is unfortunate that Dalland et al. (1988) defined the 6506/12-4
as the
reference section for the Lange Formation as the base of the formation
with the underlying Lyr Formation has an incomplete
log profile (on both
gamma ray and sonic logs) due to the tool sticking during wireline logging
(see figure).
Reference well 6506/12-4, former definition (click to enlarge) |
Reference well 6506/12-4, redefined (click to enlarge) |
Upper and lower boundaries
Upper Boundary
In the type area of the Halten and Dønna Terraces, the boundary between the Lange Formation (as redefined
in this study from Dalland et al., 1988) and overlying Blålange
Formation is characterised by a
down-section decrease in sonic velocities representing a stratigraphic break corresponding to the
Lower/Upper Cretaceous boundary.
Lower Boundary
The base of the Lange Formation is typically with the underlying
Lyr Formation and is characterised by an
increase in gamma-ray values and decrease in velocity. The boundary has been sampled in shallow cores from
the eastern part of the Trøndelag Platform (Bugge et al., 1984).
Well log characteristics
The Lange Formation is generally characterised by a relatively uniform gamma ray and sonic velocity
response reflecting the monotonous nature of the mudstone sequence.
Type seismic section
Biostratigraphy
In the type well 6506/12-1 the redefined Lange Formation is middle Aptian (Zone 30) to late Albian age
(Zone 29) and has unconformable lower and upper boundaries with the
Lyr and Blålange Formations
respectively. In this well the lower boundary of the Lange Formation lies between the LO Gardodinium
trabeculosum, LO Cerbia tabulata and LO Aptea polymorpha dinocyst markers and increase in
Hedbergella spp.
defining a middle Aptian age and above the LO Valensiella magma typical of the late Barremian (Zone
31). In
the type well, sediments of early Aptian age are absent at the base of the Formation. In other wells,
including the reference well 6506/12-4, the lower boundary with the underlying Lyr Formation is complete
and a thin early Aptian interval characterised by LO Sirmiodinium grossii is developed at the base
of the Lange Formation.
In the type and reference wells, the upper boundary with that of the overlying Blålange Formation is
associated with the late Albian dinocyst LO Apteodinium grande and the planktonic foraminiferal
event LAO
Hedbergella planispira. In these wells Cenomanian sediments (Zone 28) of the basal Blålange
Formation are absent.
Age (revised)
Early Cretaceous, early Aptian to late Albian.
Dalland et al. (1988) had the oldest extent of this formation as ranging into the Ryazanian Boreal
stage
and as a partial time equivalent of the Lyr Formation which they considered to be
Valanginian to early
Aptian. This interpretation is not followed in this study with the Lange being stratigraphically younger
than the Lyr Formation.
Correlation
Biostratigraphy (this study) indicates the redefined Lange Formation is correlatable to the
Cromer Knoll Group, Rødby and Sola Formations of the northern North Sea.
Depositional environment
The Lange Formation was deposited in a relatively deep marine bathyal palaeoenvironment with variable oxic
or dysoxic bottom conditions (Gradstein et al., 1999).
Remarks
The Lange Formation represents deposition with the K2 sequence of Swieciciki et al.
(1998), K35 to K40
sequences of Vergara et al. (2001) and the K20 - K30 sequences of Færseth and Lien (2002).
References
Bugge, T., Knarud, R. and Mørk, A. 1984. Bedrock geology on the mid Norwegian Continental shelf. In:
Spencer, A. M. et al. (eds), Petroleum Geology of the North European margin. NPD/Graham and Trotman,
London, 271-283.
Dalland, A., Worsley, D. and Ofstad, K. 1988. A lithostratigraphic scheme for the Mesozoic and Cenozoic
succession offshore Mid and Northern Norway. Bulletin of the Norwegian Petroleum Directorate,
Bulletin No. 4, 1-65.
Færseth, R. and Lien, T. 2002. Cretaceous evolution in the Norwegian Sea - a period characterized by
tectonic quiescence. Marine and Petroleum Geology, 19, 1005 -1027.
Gradstein, F., Kaminski, M.A. and Agterberg, F.P. 1999. Biostratigraphy and paleoceanography of the
Cretaceous seaway between Norway and Greenland. Earth-Science Reviews, 46, 27-98.
Swiecicki, T., Gibbs, P.B., Farrow, G.E. and Coward, M.P. 1998. A tectonostratigraphic framework for the
Mid-Norway region. Marine and Petroleum Geology, 15, 245-276.
Vergara, L., Wreglesworth, I., Trayfoot, M., Richardsen, G. 2001. The distribution of Cretaceous and
Paleocene deep-water reservoirs in the Norwegian Sea basins. Petroleum Geoscience, 7, 395-408.
