Blålange Formation (new)
Shetland Group
Introduction
The original Lange Formation of Dalland et al. (1988) has been replaced by two new formations:
The Langebarn Formation and the Blålange Formation.
The Blålange Formation is a new formation that is equivalent to the upper part of the
original Lange Formation as defined by Dalland et al. (1988). The new underlying Langebarn
Formation is proposed for the lower part of the original Lange Formation.
A two fold distinction was necessary since in many wells on the Halten and Dønna Terrace the
two units are separated by an unconformity that is regionally identifiable on wireline logs. The
definition of the Blålange Formation follows the main criteria used by Dalland to recognise the original,
mudstone dominated formations and is primarily based on a change in average sonic velocity. This new
sub-division also provides an opportunity to re-define the Shetland /Cromer Knoll Group boundary in the
Norwegian Sea at a similar stratigraphic level to that of the North Sea.
The Blålange Formation typically comprises a thick, relatively uniform sequence of Upper Cretaceous
(Coniacian - Cenomanian) mudstones with sporadic limestone stringers and interpreted tuffaceous horizons.
The Blålange Formation is of Late Cretaceous age.
The formation contains of a series of sandstones units, defined as members, that are locally developed on the
Halten-Dønna Terrace area, in the Vøring Basin and along the Møre margin.
The Lysing is redefined as a member of the Blålange Formation.
In total eight detached sandstone members are defined within the Blålange Formation in three distinct
geographical areas: Skolest, Gapeflyndre and Tunge Members (Møre margin), Breiflabb, Smørflyndre and Lysing
Members (Halten and Dønna Terrace areas), Sandflyndre and Skrubbe Members (Vestfjorden basin area).
These eight members are described later in the following section:
The new members range in age from Cenomanian through Coniacian.
In addition to these eight new members, two intervals of sandstones are retained with informal status due
to a lack of information or isolated occurrences.
An interval of interbedded sandstones and mudstones developed below the Turonian/Coniacian boundary (and
below the Coniacian Lysing Member) are sporadically developed in the deeper water
Vøring Basin e.g.
6505/10-1. These have informal designated as
the 'lower Lysing unit' (and appear to be of late Turonian age) since they can
currently only be distinguished on the basis of biostratigraphy rather than independent log or lithological
data.
Isolated thin bedded sandstones within the upper part of the Blålange Formation of 'mid' Turonian age are
commonly developed in Quadrant 6507 e.g. 6507/5-2. These sandstones have not been cored and do not reach
any significant thickness although there is potential for these to be more extensively developed in deeper,
undrilled locations. These have been informally called 'intra Blålange sandstones' (mid Turonian).
Name
English/ Norwegian and any previous names:
The Blålange Formation is equivalent to the informal 'upper' Lange
Formation of Swiecicki et al. (1998) and 'upper' Lange Formation of Færseth and Lien (2002).
Derivatio nominis:
The name Blålange is derived from the Norwegian name for the fish species
Molva dypterygia. The Blålange or blue Ling is appropriately a close relative of
the Lange fish (cod family) and lives in deep waters along the Norwegian
coast and fjords. It is therefore considered an appropriate name for a
lithostratigraphic unit formerly part of the Lange Formation.
Lithology
The Blålange Formation mainly consists of mudstones with subordinate beds of sandstones and stringers of
limestones, dolomites and marls. The mudstones are medium dark grey to medium brown grey, blocky,
occasionally sub-fissile to fissile, predominantly firm, occasionally moderately hard, generally non
calcareous or occasionally slightly calcareous. The formation contains subordinate sandstones as beds or
members, siltstones and stringers of dolomite, limestones and marls. The siltstones are light brown-light
brown grey, argillaceous and slightly calcareous. The calcareous stringers are white to light brown or red
brown, moderately hard to hard.
Sample depository
Palynological preparations (organic matter depository)
Type well
6506/12-1: 102 slides including 5 swc samples
covering the interval 3179.5m swc - 3705m dc
(Stratlab, RRI and OD preparations) deposited at the Norwegian Petroleum Directorate.
Reference well
6506/12-4: 94 slides including 20 core and 5
swc samples covering the interval 3129.1m core
(3135m corrected depth) - 3754m dc (Stratlab, RRI and Saga preparations) deposited at the Norwegian
Petroleum Directorate.
Reference well
6610/3-1R: 67 samples including 6 core and 29
swc samples covering the interval 2670m dc -
3411m dc (Statoil, RRI and Saga preparations) deposited at the Norwegian Petroleum Directorate.
Core photographs
Type well
6506/12-1: no cores taken in this formation
Reference well
6506/12-4: core#1 interval 3129m - 3150.9m
(uncorrected mMD RKB core
depths). Note there is a +5.9m core correction to this core. The core was cut in the lower part of the
Lysing Member and across the lower boundary.
Reference well
6610/3-1R: core#2 interval 3315m-3324.25m
(uncorrected mMD RKB core depths). The core was
cut within the middle part of the Sandflyndre Member.
Thickness
The Blålange Formation ranges in thickness from 13 m to 1573 m.
Regional isochore of the Blålange 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 Blålange Formation
|
Geographical distribution
The Blålange Formation reaches a maximum thickness of 1573 m in the Helland Hansen well
6505/10-1 where the
well had not penetrated the base of the formation at the well TD (5031 m). It generally reaches in excess
of
1000 m in wells in quadrants 6406 and 6506 on the Dønna Terrace e.g.
6506/6-1 (1114 m). Thicknesses in the
Vøring Basin are difficult to establish since many wells have a TD within this formation but in well
6606/5-1 located on the Utgard High, 849 m of
Blålange Formation was penetrated prior to the well TD at
3817 m. The Blålange Formation is generally thinner on the Trøndelags Platform and in the Halten and Dønna
Terrace areas it is between 300 m and 900 m thick.
Occurrences of formation tops in wells
Type well
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:
3705 m - 3175 m, 530 m thickness.
Logs of the type well 6506/12-1 (click to enlarge)
Reference wells
Two reference wells are designated, namely
6506/12-4, the original reference well used by
Dalland et al.
(1988) to illustrate their Lange Formation (replaced herein) and in the upper part
illustrating the (new)
Blålange Formation, and well
6610/3-1R to show variation in the formation
in the Norwegian Sea.
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 reference section & thickness in reference well:
3738 m - 3132.5 m, 605.5 m thickness.
Well name:
6610/3-1R
WGS84 coordinates: N 66°55'29.70, E 10°54'06.28
UTM coordinates: 7424470.29 N 583170.04 E
UTM zone: 32
Drilling operator name: Den norske stats oljeselskap a.s (Statoil a.s.)
Completion date: 11.12.1993
Status: suspended, re-entered later.
Interval of reference section & thickness in reference well:
3412 m - 2655 m, 757 m thickness.
Reference well 6506/12-4 (click to enlarge) |
Reference well 6610/3-1R (click to
enlarge) |
Upper and lower boundaries
Upper Boundary
The top of the Blålange Formation normally coincides with the top of the
Lysing Member in the Halten and
Dønna Terraces and in deep-water wells in the Vøring Basin. In these instances, the top of the Blålange
Formation marks a downward change from mudstones of the
Kvitnos Formation to sandstones and interbedded
mudstones of the Lysing Member. This is typically defined by a marked downward decrease in gamma ray values
and an increase in sonic velocity. In wells in quadrants 6406, 6407 and 6507, where the boundary represents
a continuous sequence of mudstones, the boundary can be recognised by a slight downward decrease in average
gamma ray values, an increase in average resistivity values and/or slight increase in average sonic
velocity e.g.
6507/3-4. The latter probably related to
compactional differences of the mudstones.
Lower Boundary
The base of the Blålange Formation in the type area of the Halten and Dønna Terraces, and the boundary with
the underlying Langebarn Formation (new) is characterised by a down-section
decrease in sonic velocities.
This represents an unconformity that appears to correspond to the Lower/Upper Cretaceous boundary.
Well log characteristics
The gamma-ray and sonic log profiles of the Blålange Formation are less uniform than those of the
underlying Lange Formation, due to the frequency of thin bedded limestone stringers
e.g. 6506/12-1. In
addition, there are several high gamma spikes that are considered to represent thinly bedded tuffs. These
provide a means of correlation between wells and are identified (in ascending order) as k54a to k58a (the
'a' suffix denoting presumed ash bed). On gamma logs these tend to have sharp, well defined bases and
gradually decreasing upper values e.g. k55a, 6506/12-1, 3417m. These high gamma features appear to be
restricted to the Blålange Formation and Turonian sediments, being absent in the overlying
Kvitnos Formation. The k58a marker, e.g. 6506/12-4, 3168m is a regional marker
that is typically developed towards
the top of the Blålange Formation but below the Lysing Member and provides an
independent stratigraphic
means of distinguishing this unit from underlying, Turonian-restricted
sandstones.
Type seismic section
Biostratigraphy
The main stratigraphic control of the Blålange Formation is provided by dinoflagellate cysts. Towards the
top of the formation and generally below the base of the
Lysing Member are the LO Stephodinium coronatum
and LO Cyclonephelium membraniphorum Turonian markers (Zone 27). These are typically associated with
the
not age-diagnostic but characteristic increase in numbers of Palaeohystrichophora infusorioides and
Heterosphaeridium difficile although these can extend in numbers into the overlying sediments. In
the type
area on the Halten and Dønna Terraces where the Blålange Formation rests unconformably upon the Lower
Cretaceous Lange Formation (redefined) the oldest dinocyst markers are diagnostic
of the early Turonian
and include LAO Surculosphaeridium longifuratum, LCO C. membraniphorum and rare
Litosphaeridium
siphoniphorum. The FO H. difficile in sidewall or core samples provides a valuable datum to
identify an age
no older than Turonian and the degree of truncation e.g. 6506/12-1, 3696.5m (swc). It also provides a datum
to demonstrate any Cenomanian - Albian reworking from the underlying sediments. In the more complete
sections of the Vestfjorden Basin, the Blålange Formation extends older into the early Cenomanian and
contains the Sandflyndre Member. In this area, and represented by well 6610/3-1R, a more complete
and
typical Cenomanian succession (Zone 28) of LO (few) Litosphaeridium siphoniphorum, LO Rhombodella
paucispina (late Cenomanian), LO Epelidosphaeridia spinosa (middle Cenomanian), LAO
'Sidridinium borealis'
and LO Ovoidinium verrucosum (early Cenomanian) are present in the lower part of the Blålange
Formation.
Micropalaeontologically, the Blålange Formation is generally devoid of calcareous foraminifera including
planktonic types. Small, smooth Hedbergella spp, such as the LO Hedbergella delrioensis may
be present in
the upper part of this formation but generally planktonic foraminifera increase in numbers in the
underlying Lange Formation (redefined), notably LCO Hedbergella. The
exception being the stratigraphically
more complete wells in the Vestforden area e.g. 6610/3-1R where planktonic foraminifera are frequent in the
older i.e. Cenomanian part of the Blålange Formation. Influxes of 'cenosphaerid-type' radiolarians are
typical of levels within the formation. Generally, the planktic bivalve Inoceramus is common in the
overlying Kvitnos Formation and decreases in numbers above the Blålange Formation
although this is probably
related to post sedimentary, diagenetic processes rather than biostratigraphy.
Age
Late Cretaceous, early Cenomanian - 'earliest' Coniacian.
The Blålange Formation is predominantly of Turonian age. In the type well 6506/12-1 the base of the
Blålange Formation is typically unconformable with the Lower Cretaceous
Lange Formation (redefined). This
unconformity is regionally significant in the Halten-Dønna Terrace area and early Turonian sediments of
the Blålange Formation progressively onlap parts of the
Cromer Knoll Group.
In the reference well
6610/3-1R, in the Vestfjorden area, the
Blålange Formation extends older to the base of the Cenomanian.
Correlation
The Blålange Formation is laterally equivalent to the lower Kyrre Formation and
more calcareous rich
Tryggvason Formation in the Shetland Group of the
northern North Sea. Time equivalents to the North Sea
Cenomanian Blodøks and Svarte Formations are generally
not developed, being within the unconformity at the
Blålange/Lange boundary, but equivalent sediments are sporadically present in the Vestfjorden Basin
(Quadrant 6610) as sandstones of the Sandflyndre Member, on parts of the
Halten and Dønna Terraces where
they are represented by interbedded sandstones and mudstones of the
Smørflyndre Member, and along the
south east Møre margin, as Gapeflyndre Member.
Depositional environment
The Blålange Formation was deposited in a relatively deep but restricted marine palaeoenvironment with
mainly dysoxic bottom conditions developed in a semi-restricted basin. Connections with the North Atlantic
oceanic realm were limited (Gradstein et al., 1999). An increase in palaeobathymetric gradient from
east (Halten Terrace) to west (Vøring Basin) is inferred.
Remarks
The Blålange Formation represents deposition with the K3 sequence of Swieciciki et al. (1998), K50
to lower
K80 sequences of Vergara et al. (2001) and the K40 - lower K60 sequence of Færseth and Lien (2002).
References
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.
