Upper Palaeozoic carbonate reservoirs on the Norwegian Arctic Shelf:
delineation of reservoir models with application to the Loppa High
1
Lars Stemmerik1, Geir Elvebakk2 and David Worsley3
Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen NV, Denmark
IKU Petroleum Research, N-7034 Trondheim, Norway (Present address: Saga Petroleum ASA, Postboks 1134,
N-9401 Harstad, Norway)
3
Saga Petroleum ASA, Postboks 490, N-1301 Sandvika, Norway
2
ABSTRACT: The reservoir potential of the Upper Palaeozoic carbonates in the
Barents Sea area is primarily controlled by early diagenetic processes. Upper
Bashkirian to Asselian shallow platform carbonates deposited in warm, arid to
semi-arid climates were dominated by aragonitic organisms and mineralogically
unstable aragonite and high-Mg calcite cements and mud. A reservoir model for
these carbonates involves extensive dolomitization and dissolution of metastable
carbonate during repeated subaerial exposure. The reservoir model is confirmed by
drilling and is accordingly regarded as low risk. Artinskian and Upper Permian
shallow water carbonates deposited in a cold temperate climate were dominated by
calcitic organisms and silica sponges, and associated with calcite cements and mud
and chert. A reservoir model for these carbonates involves either preservation of
primary porosity in carbonate build-ups or extensive dissolution of build-up marine
cement during prolonged subaerial exposure. This model is not confirmed by drilling
and is regarded as high risk.
KEYWORDS: Barents Sea, carbonate reservoir, Carboniferous, Permian, carbonate diagnesis
INTRODUCTION
This paper discusses potential Upper Palaeozoic carbonate
reservoirs in the western Barents Sea based on data from 9
deep wells and studies of outcrop and subcrop analogues in
the uplifted, marginal parts of the depositional basin and in
more distant areas such as the Timan–Pechora Basin of
Russia, the Sverdrup Basin of Arctic Canada and the East
Greenland Basin (Fig. 1). These data have been applied to the
Loppa High area in the westernmost part of the Barents Sea.
Seismic mapping of the high has outlined the depositional
facies distribution and the late Palaeozoic structural evolution
in considerable detail, and it has been possible to define
several carbonate plays using the reservoir models outlined
here.
Several wells were drilled on the Loppa High during the late
1980s and the results of these led to a general downgrading of
the area. A general view has been that both source rock and
migration are problems in the area. Geochemical re-evaluation
carried out simultaneously with the present study shows clearly
that this is not the case: potential source rocks of several ages
have contributed to multiphase hydrocarbon migration in the
area from the late Triassic and onwards, the most relevant
sources being lower to middle Triassic marine shales. Dry holes
drilled previously have been unfavourably located in terms of
migration and reservoir properties and the one find in the area
previously classified as containing ‘dead’ or ‘residual’ oil may in
fact have been inadequately tested. In our view, a new phase of
exploration must address the challenge of finding a viable
reservoir as opposed to commonly tight carbonate formations.
Not least, but outside the scope of the present paper, potential
Petroleum Geoscience, Vol. 5 1999, pp. 173–187
leakage as a result of fault rejuvenation and Cenozoic uplift may
be the main threat to the reservoirs envisaged by the present
work.
From earlier studies it is evident that there is a shift in
carbonate sedimentation through time related to the northward
movements of the Laurasian plate during the late Palaeozoic
(Steel & Worsley 1984; Stemmerik 1997). Carbonate deposition
started during the late Bashkirian when the Barents Sea area
was located around 25N and by the end of the Permian it
had moved to 35–40N (Scotese & McKerrow 1990). The
Moscovian to early Sakmarian carbonates were deposited in a
warm and arid setting. These sediments contain abundant
calcareous algae, were mainly composed of aragonitic material
and are associated with common evaporites. The late
Sakmarian–Artinskian succession is dominated by cooler water
carbonates mainly composed of calcitic organisms and calcite
cements and finally the Kungurian–Kazanian deposits are
typical cool-water carbonates with abundant chert in the
deeper, basinal areas.
There is, therefore, a very direct link between the age, the
depositional facies, diagenesis and consequently the reservoir
potential of these carbonates. Well exposed outcrop analogues
for the Moscovian–early Sakmarian carbonates have been
studied on Bjørnøya, in North Greenland and in the Timan–
Pechora Basin (Fig. 1). The late Sakmarian–Artinskian sediments have their best outcrop analogues in the Sverdrup Basin
of Arctic Canada (Beauchamp 1993), additional information
comes from the Timan–Pechora Basin and the Kazanian of
East Greenland. The Kungurian–Kazanian succession is best
represented by outcrops on Bjørnøya, Spitsbergen and in North
Greenland.
1354-0793/99/$15.00 1999 EAGE/Geological Society, London
174
L. Stemmerik et al.
Fig. 1. Pre-drift reconstruction of the
Barents Sea region with major structural
elements. Locations and position of
offshore wells mentioned in the text is
shown.
STRATIGRAPHY
The North Greenland–Barents Sea region formed a complex,
rift-related system of rapidly subsiding basins and more stable
platforms during the late Palaeozoic (Fig. 1) (Stemmerik &
Worsley 1989). Rifting patterns seem to follow lineaments
established by earlier Caledonian and Ellesmerian basement
trends (Doré 1991; Gudlaugsson et al. 1998), and during the late
Palaeozoic two linked rift arms developed. The Atlantic rift arm
started to form between Greenland and Norway and extended
towards the northeast across the central Barents Sea, whereas
the Arctic rift arm extended westwards between Greenland and
Spitsbergen and eventually linked with the Sverdrup Basin rift
in the far west (Fig. 1) (Gudlaugsson et al. 1998).
The most important late Palaeozoic rift phases took
place during the mid-Carboniferous (Bashkirian) and mid- to
late Permian (Artinskian–Kazanian) (Stemmerik et al. 1991;
Stemmerik & Worsley 1989). The mid-Carboniferous rifting
event led to fault-controlled subsidence and depocentres
formed along the rift axis as in the case of the Tromsø and
Nordkapp basins (Fig. 1). This rifting was followed by a marine
transgression in the latest Bashkirian and these rapidly subsiding basins became sites of deeper water deposition during the
late Palaeozoic (Figs 1, 2) (Stemmerik & Worsley 1989). The
more slowly subsiding platform areas, such as the Finnmark
Platform, and tectonically active blocks, such as the Loppa
High, were sites of carbonate platform deposition from the
mid-Carboniferous until the late Permian. Although the gross
depositional evolution of these carbonate platforms is similar
throughout the region, individual platforms and structural
blocks show different depositional trends related to their
tectonic history.
The mid-Carboniferous to Upper Permian succession in the
western Barents Sea consists of three second-order depositional
sequences (Fig. 2). The late Bashkirian to early Sakmarian
second-order sequence was deposited in a warm and arid
climate during a period characterized by high frequency and
high amplitude glacioeustatic sea-level fluctuations (Stemmerik
& Worsley 1989; Stemmerik et al. 1996). Sedimentation was
dominated by warm-water carbonates, evaporites and local
siliciclastics (Steel & Worsley 1984). The upper, Sakmarian
boundary coincides with a shift from warm tropical carbonates
to cool-water carbonates and from high frequency and high
amplitude sea-level fluctuations to low frequency and low
amplitude fluctuations. Deposition of the Artinskian–early
Kungurian sequence took place during an overall rise in relative
sea-level that gradually led to flooding of the basinal margins.
Thick aggradational to slightly retrogradational successions of
bryozoan-dominated cementstones and crinoid-dominated
packstones and grainstones dominate the lower part of this
sequence in the offshore areas. The upper part is composed of
brachiopod and bryozoan-dominated shelf facies and basinal
shales and chert.
The latest Kungurian–Kazanian sequence is separated from
the underlying sediments by a subaerial exposure surface on the
platform areas (e.g. Ehrenberg et al. 1998a; Stemmerik et al.
1996). A very rapid rise in relative sea-level outpaced sedimentation in most areas and deep-water spiculites and shales
Carbonate reservoirs on the Norwegian Arctic Shelf
175
Fig. 2. Time distribution of mid-Carboniferous–Permian second-order depositional sequences in the Barents Sea region. KK: Kapp Kåre
Formation, KD: Kapp Duner Formation, HF Hambergfjellet Formation (modified from Stemmerik 1997).
commonly rest directly on the sequence boundary. In platform
areas such as North Greenland, two distinctive, 80–100 m thick
transgressive–regressive units occur, each composed of a thin
transgressive succession of black shales and spiculites and a
thick regressive succession of bryozoan-dominated rudstone
or biogenic floatstone. A similar twofold division is seen in
the deep water, shale- and spiculite-dominated successions
in the offshore areas (Ehrenberg et al. 1998a).
DEPOSITION
Upper Bashkirian–lower Sakmarian warm-water
carbonates
The Upper Bashkirian to Asselian carbonates in the Barents Sea
region are composed of a mosaic of algal and foraminiferdominated shelf facies with isolated lenticular build-ups and
linear trends of phylloid algal and Palaeoaplysina build-ups along
platform margins (Fig. 3) (Bugge et al. 1995; Ehrenberg et al.
1998a; Stemmerik et al. 1998). Bryozoan-dominated build-ups
are of minor importance and possibly limited to the Moscovian
interval (Pickard et al. 1996; Stemmerik 1996). Sabkha facies are
volumetrically unimportant and mainly confined to structural
highs and innermost platform areas (Stemmerik et al. 1995).
Oolitic grainstone deposits are extremely rare compared to
modern carbonate platforms and to age-equivalent platforms in
the Sverdrup Basin (Morin et al. 1994). The basinal deposits are
dominated by anhydrite and halite that were deposited during
sea-level lowstands (Stemmerik & Worsley 1989).
The platform successions consist of stacked up to 10 m
thick, shallowing upward cycles of mainly subtidal carbonates
capped by subaerial exposure surfaces. Moscovian platform
carbonates are well documented from North Greenland, on
Spitsbergen and Bjørnøya and in well 7120/2-1 from the Loppa
High (Pickard et al. 1996; Stemmerik 1996; Stemmerik et al.
1998). In well 7120/2-1 more than 200 m of late Bashkirian to
late Moscovian cyclic carbonates and mixed siliciclastics and
carbonates have been cored (Fig. 4; Table 1). Gzelian–Asselian
carbonates have been described from Bjørnøya, North
Greenland, the IKU shallow cores and well 7128/6-1 from the
Finnmark Platform (Stemmerik et al. 1994, 1995; Bugge et al.
1995; Ehrenberg et al. 1998a). In well 7128/6-1 c. 200 m of
Gzelian–Asselian carbonates have been cored. Gzelian–early
Asselian carbonate cycles consist mainly of inner shelf facies
and build-ups of Palaeoaplysina–phylloid algal packstone (Table
1) (Ehrenberg et al. 1998a). The middle–late Asselian cycles are
dominated by sabkha and lagoonal facies while the late Asselian
cycles are dominated by thick inner shelf units and thin
build-ups of phylloid algal–Palaeoaplysina wackestone (Table 1).
Mixed siliciclastic and carbonate cycles are locally found
along structural highs, like the Loppa High and along the
feather edge of the depositional basin in the Upper Bashkirian
to Moscovian part of the succession (Stemmerik 1996;
Stemmerik et al. 1998). Stacked phylloid algal – Palaeoaplysina
build-ups with numerous exposure surfaces occur in middle
and outer platform settings over footwall uplifts and hanging
wall flanks of the older rift structures. These build-ups are more
than 40 m thick along the west coast of Bjørnøya (Stemmerik
et al. 1994), and in North Greenland build-up-dominated
platforms are up to 100 m thick and several kilometres wide
(Fig. 5) (Stemmerik 1996).
Similar cyclic deposits are recognized throughout the Arctic
during the late Carboniferous and Asselian and most likely
represent the depositional response to high frequency and
high amplitude, glacioeustatic sea-level fluctuations triggered
by glaciations on the present day Southern Hemisphere
(Stemmerik & Worsley 1989).
Upper Sakmarian–Kazanian cool-water carbonates
The late Sakmarian–Artinskian carbonates are dominated by
three main rock types. In the lower part, the most prominent
176
L. Stemmerik et al.
Fig. 3. Generalized palaeogeographic maps of the Barents Sea region. (A) Gzelian–Asselian warm-water setting; (B) Artinskian cool-water
setting with carbonate build-ups; and (C) Kazanian cool-water platforms with chert.
features are large, up to 650 m thick, carbonate build-ups
dominated by bryozoans, Tubiphytes and early marine cement.
These build-ups form isolated mounds or more commonly
large barrier-like features along the margins of the deep-water
basins (Fig. 3) (Gerard & Buhrig 1990; Bruce & Toomey
1993). They have been penetrated by several wells, including
7121/1-1 from the Loppa High and the extensively cored
7229/11-1 from the Finnmark Platform (Blendinger et al. 1997).
Protected areas behind the build-ups and the comparable quiet
water offshore areas were dominated by deposition of
bryozoan- and crinoid-rich wackestones and packstones that
are interbedded with siliciclastic shales in more basinal settings
as documented by well 7228/9-1 on the flanks of the
Nordkapp Basin.
The upper part of the Artinskian succession consists of
crinoid-bryozoan grainstones or rarely packstones deposited
above storm wave base on an open cool-water shelf
(Stemmerik 1997). The Kungurian–Kazanian carbonates show
a distinctive lateral facies zonation from brachiopod-dominated
inner shelf facies (Fig. 6) to bryozoan-dominated outer shelf
and upper slope facies and pass basinwards into siliciclastic
shale and spiculitic chert (Stemmerik 1997). Carbonates of this
age are poorly known from well data and most information
comes from outcrops on Spitsbergen, Bjørnøya and in North
Greenland where they form 30 m to 160 m thick transgressive–
regressive packages (Stemmerik 1997). Core data from well
7128/6-1 on the Finnmark Platform show a dominance of deep
water facies whereas data from well 7128/4-1 document the
occurrence of a more than 50 m thick interval dominated by
cherty bryozoan–echinoderm wackestones and packstones
(Ehrenberg et al. 1998a).
DIAGENESIS
Published studies of core material from deep wells in the
Barents Sea are rare (Blendinger et al. 1997; Ehrenberg et al.
1998b). Studies of outcrop analogues on Bjørnøya, in North
and East Greenland and IKU’s shallow core material from the
Finnmark Platform indicate that most diagenetic processes of
importance to reservoir quality are of early diagenetic origin
(e.g. Scholle et al. 1991, 1993; Stemmerik & Larssen 1993). Here
we will use a detailed study of the Gzelian–Asselian carbonates
in IKU’s shallow cores from the Finnmark Platform to
illustrate the diagenesis of the warm-water succession, supplemented with published and unpublished data from deep wells
and outcrops. This information will be used later to develop
reservoir models for this stratigraphic interval. The diagenesis
of the cool-water carbonates is discussed more briefly on the
basis of well data and diagenesis of Upper Permian build-ups in
East Greenland.
Moscovian–early Sakmarian warm-water carbonates
Gzelian–Asselian carbonates in the IKU shallow cores from the
Finnmark Platform are preserved as either low porosity calcite
(less than 5% and m1 mD permeability) or highly porous
dolomite (10–30% and permeabilities from 1–500 mD)
(Elvebakk et al. 1993).
The dolomitized shelf and sabkha deposits are dominated by
dark grey to brownish microcrystalline dolomite (Figs 8a, 8c,
8e). This dolomite is less abundant in build-ups where mosaics
of xenotopic and sucrosic dolomite are the most important
types (Fig. 8c). The sucrosic dolomite is a pore-filling cement
Carbonate reservoirs on the Norwegian Arctic Shelf
177
Fig. 4. Sedimentological log of the late
Bashkirian and Moscovian cored
intervals in well 7120/2-1 showing
transition from cyclically interbedded
siliciclastics and carbonate to pure
carbonate. From Stemmerik et al.
(1998), NPF Special Publication 8.
that lines mouldic and solution-enlarged pores (Fig. 8a). The
isotopic composition of microcrystalline dolomite from inner
shelf and sabkha facies suggests that this dolomite type
may have formed from sabkha brines (Table 2; Fig. 7) (e.g.
McKenzie 1981; Pierre et al. 1984). In contrast, the xenotopic
and sucrosic dolomites appear to be precipitated from meteoric
modified marine water based on their isotopic composition
(Table 2; Fig. 7).
The calcite-dominated intervals include rare early diagenetic
calcite cements and carbonate mud is replaced by calcite
microspar.
Anhydrite occurs as early, syndepositional nodules in the
dolomitic rocks, and as a later pore-filling cement in moulds after
fossils and solution-enlarged pores. However, coarsely crystalline
calcite is the most important porosity destructive cement (Fig.
8d). It fills secondary pores particularly in the calcite-dominated
rocks and significantly influences the porosity in these rocks.
Based on the isotopic composition, two types of late calcite
cement can be distinguished (Table 2; Fig. 7). Coarsely crystalline
calcite with light oxygen and carbon values, and associated with
native sulphur was precipitated from hydrocarbon saturated
water at elevated temperatures, probably exceeding 80C.
178
L. Stemmerik et al.
Table 1. Typical warm-water carbonate facies
Dolomitic mudstone: Sabkha deposits with sand-size siliciclastic material and
anhydrite nodules. Mainly found in the Gzelian–Asselian of the Finnmark
Platform.
Palaeoaplysina–phylloid algal build-ups: Sub-wave base build-ups of
Palaeoaplysina and phylloid algal-dominated wackestones, packstones and
grainstones. Individual build-ups rarely exceed 10 m in thickness but may
stack to form >100 m thick complexes. Moscovian–Asselian (early
Sakmarian?).
Fusulinid-dominated facies: Open shelf deposits ranging from wackestones to
grainstones. This facies often contains a diverse fauna. Bashkirian–Sakmarian.
Foraminifer-dominated grainstones and packstones: Lagoonal to inner shelf
deposits. The lagoonal facies are commonly associated with peloids.
Bashkirian–Asselian.
Crinoid-dominated wackestones: Outer shelf facies typical for the maximum
flooding interval of Moscovian cyclic deposits.
Fig. 5. Outcrop examples of Upper Carboniferous–Lower
Permian stacked Palaeoaplysina–phylloid algal build-ups. (A)
Moscovian isolated platform (P) with several intervals of brownish
weathering, dolomitized build-ups in North Greenland. The
platform margin is onlapped by gypsum (G). Cliff is 400 m high.
(B) Upper part of stacked Gzelian–Asselian build-ups along the
east coast of Bøjrnøya. Note the lenticular shape of individual
build-ups. The build-ups are overlain by a laterally widespread,
fusulinid-rich unit. Persons for scale.
The paragenetic sequence, together with the relative changes
in porosity observed in the dolomite- and calcite-dominated
units, is outlined in Figs 9 and 10. It is evident that processes
retaining depositional porosity are mainly related to early
diagenesis and are the results of sabkha-related dolomitization
and dissolution of metastable carbonates in meteoric water.
Porosity destructive processes are mainly related to chemical
compaction, redistribution of anhydrite and cementation by
Fig. 6. Brachiopod-dominated cool-water carbonates, eastern
North Greenland. Brachiopods are all preserved as chert. Scale in
centimetres.
coarsely crystalline calcite; both cementation events seem to
post-date hydrocarbon migration and these two cement types
are believed not to have significantly influenced entrapment
reservoir quality.
The diagenesis of the time-equivalent shoaling-upward cycles
in well 7128/6-1 resembles that described from IKU’s shallow
cores. The sediments in the c. 200 m thick Gzelian–Asselian
succession are preserved as calcite or dolomite or most commonly a mixture (Ehrenberg et al. 1998b). They show a wide
porosity distribution with log-based values in the range 0–30%.
Generally, reservoir quality porosity is confined to the interval
from 1870–2030 m where the net/gross ratio is 0.6 using a
cut-off value of 15% porosity. Average porosity and permeability values, based on core plug analysis are given for the
most common facies in Fig. 11 and a cross-plot of porosity
versus permeability is shown in Fig. 12. The porosity within the
calcite-dominated rocks represents primary, sheltered porosity
inside fossils, mouldic porosity after dissolved fossils and
solution-enlarged vuggy porosity. The porosity constructive
processes are related to early fresh water diagenesis while
porosity destructive diagenesis includes early calcite cementation, anhydrite cementation and cementation by late diagenetic
calcite and dolomite. In the dolomite-dominated rocks, porosity
occurs as sheltered porosity inside fossils, mouldic porosity
after dissolved fossils, vuggy porosity and intercrystalline
porosity. The porosity is related to early dolomitization and
dissolution of metastable minerals, whereas the porosity
destructive diagenesis follows the same trends as those seen in
the calcite-dominated rocks.
The Finnmark Platform examples illustrate the diagenetic
controls on the Gzelian–Asselian platform deposits. The diagenesis of older, Moscovian, platform deposits is documented
in well 7120/2-1 from the Loppa High where middle to inner
shelf sediments are completely dolomitized by microcrystalline
and sucrosic dolomite. Porosities of these dolomites range from
5–30% (mostly 8–14%); their matrix permeability is low but
high fracture permeabilities are recorded. Matrix porosity
includes intercrystalline, mouldic and vuggy pores, and generally the diagenesis and porosity history of these deposits
corresponds to that outlined for the dolomitic units on the
Finnmark Platform.
Stacked successions of Palaeoaplysina build-ups have not been
drilled so far and the best example of diagenesis of these
deposits is from the Kapp Duner Formation on Bjørnøya
(Stemmerik & Larssen 1993; Stemmerik et al. 1994). Present day
porosities in these rocks range from 1–16% as a result of
Carbonate reservoirs on the Norwegian Arctic Shelf
179
Fig. 7. Cross-plot of ä13 and äO18
values of the different types of calcite
and dolomite. For further discussion
see the text.
late-stage calcite cementation. Prior to calcite cementation,
calculated porosities range from 7–39%, corresponding to a
situation prior to hydrocarbon migration (Stemmerik, unpublished data).
Late Sakmarian–Kazanian cool-water carbonates
Late Sakmarian–Artinskian bryozoan–Tubiphytes build-ups have
been drilled in several deep wells, including 7121/1-1 from the
Loppa High, 7124/3-1 and 7226/11-1 from the margins of the
Nordkapp Basin and most recently 7229/11-1 from the northern Finnmark Platform (Fig. 13). All drilled build-ups are
preserved as calcite and have porosities, usually less than 2%,
due to pervasive cementation of early marine radial calcite.
However, this type of cement did not completely occlude pores
in the build-up in well 7229/11-1 and, based on visual core
estimates, 10–20% porosity was present prior to cementation
by a later coarsely crystalline calcite cement. This cement mainly
fills Stromatactis-like cavities and fractures; timing of this
cementation event in relation to hydrocarbon migration is
critical in evaluation of effective reservoir potential.
Age-equivalent bryozoan–Tubiphytes build-ups from the
Sverdrup Basin in Arctic Canada follow the same general
diagenetic pathway and have no reservoir potential (Beauchamp
1993). The diagenesis of bryozoan-cement build-ups of
Kazanian age in East Greenland follows a more promising
reservoir pathway with an average porosity of more than 10%
prior to hydrocarbon migration (Scholle et al. 1991).
In well 7128/6-1, the Artinskian cool-water carbonates are
mainly composed of low-Mg calcite with little potential for
development of secondary porosity. They are cemented by
widespread syntaxial calcite cement, coarsely crystalline calcite
and chert, and they are tight. Cementation was completed very
early, and these rocks have remained diagenetically inert during
burial except for replacement of some carbonate mud by
rhombic dolomite. Studies of comparable facies in IKU’s
shallow cores and onshore Bjørnøya and North Greenland
confirm that this is a regional pattern.
RESERVOIR MODELS
The Late Palaeozoic carbonates in the nine wells drilled so far
in the Barents Sea follow the same diagenetic and depositional
patterns as their subcrop and outcrop equivalents on the
Finnmark Platform, Bjørnøya, Spitsbergen and in North
Greenland (see Figs 8–11, 14). The very different burial
histories of these regions seem to have limited effects on the
diagenesis and, consequently, porosity history of the rocks, and
reservoir quality seems primarily to be stratigraphically controlled. Based on the diagenetic and depositional models
presented above two overall reservoir models are discussed (see
Fig. 14a, b).
Moscovian–early Sakmarian warm-water carbonates
The porosity preserved in the Moscovian carbonates in well
7120/2-1 from the Loppa High, the Gzelian–Asselian carbonates in well 7128/6-1 from the Finnmark Platform and timeequivalent deposits in IKU shallow cores, and Bjørnøya and
North Greenland outcrops represents a combination of intercrystalline porosity related to early dolomitization and
dissolution-generated vuggy porosity (see Figs 8, 11). Depositional porosity is of minor significance in these rocks and is
mainly found inside fossils. Similar relationships are seen in
producing fields from the Timan–Pechora Basin in northern
Russia (Stemmerik, unpublished data). The main porosity
destructive processes are partial cementation with remobilized
gypsum and anhydrite, and late calcite (Fig. 9). The latter event
post-dates hydrocarbon migration and therefore did not affect
entrapment reservoir quality.
Several studies indicate that dissolution of aragonite was early
and related to subaerial exposure. Repeated exposure was
controlled by high frequency, glacioeustatic sea-level fluctuations as well as by local tectonic movements (Stemmerik &
Larssen 1993). Karstification and fresh water dissolution was
most pronounced during low, second- and third-order sea-level.
In North Greenland, up to 40 m deep karst pipes occur in
association with prolonged subaerial exposure of Moscovian
180
L. Stemmerik et al.
Fig. 8. Microphotographs of characteristic diagenetic components and porosity types in the Kapp Duner Formation of Bjørnøya and
IKU shallow cores from the Finnmark Platform. (A) Dolomitized Palaeoaplysina build-up where the original carbonate mud is replaced
by microcrystalline dolomite (dark). Note partial filling of secondary porosity by rhombic dolomite. Kapp Duner Fm, Bjørnøya.
(B) Dolomitized Palaeoaplysina build-up. Note well developed secondary porosity. Kapp Duner Fm, Bjørnøya. (C) Dolomitized mudstone.
The darkest areas are microcrystalline dolomite, the lighter brown areas (arrow) are xenotopic dolomite while the lightest areas consist
of sucrosic dolomite. Note well developed secondary porosity (green). IKU 7029/03-U-02. (D) Phylloid algal–peloidal packstone with
encrusting foraminifers (arrow). Moulds of phylloid algal fragments (P) and secondary vugs have been filled by blocky calcite cement leaving
no porosity. IKU 7030/03-U-01. (E) Dolomitized mudstone with silt and sand-size siliciclastic grains (white). The original sediment is
replaced by xenotopic dolomite. Note pronounced secondary porosity (green). IKU 7030/03-U-01.
phylloid algae–Palaeoaplysina build-ups (Stemmerik 1996), and
deep karstic features are also seen in the Kapp Kåre and Kapp
Duner formations of Bjørnøya, where they are associated with
tectonic uplift (Worsley et al., in press). Dissolution was less
pronounced during shorter periods of exposure and it may not
have affected the more fine-grained transgressive deposits at the
base of the cycles. Dissolution is most pronounced on structural highs and in inner and middle shelf settings where it
contributed significantly to the porosity of bioclastic packstones
and grainstones and in build-ups dominated by phylloid algae
and Palaeoaplysina.
Dolomitization is pervasive both in inner platform sabkharelated deposits and in open marine and build-up deposits.
Dolomitization is early and in most cases caused by migration
of hypersaline fluids or, more rarely, fresh water modified
hypersaline fluids. Dolomitization is most pervasive during
second- and third-order lowstands when the main accumulation
of evaporites occurred in the basins.
The best overall reservoir quality is expected in the
Gzelian–Asselian succession but reservoir quality intervals may
be found throughout the upper Bashkirian–Asselian succession.
This conclusion is based on a sequence stratigraphic model in
181
Carbonate reservoirs on the Norwegian Arctic Shelf
Table 2. Characteristic diagenetic components of the warm-water carbonates
Calcite-dominated intervals
Fibrous calcite cement
Peloidal calcite cement
Bladed calcite
Calcite microspar
Coarse calcite cement A
Dolomite-dominated intervals
Microcrystalline dolomite
Xenotopic
Sucrosic dolomite
Anhydrite cement
Coarse calcite cement A
Coarse calcite cement B
Early marine
Early marine
Post-aragonite dissolution
Replacing carbonate mud
Late, post-HC migration
Rare
Rare
Rare
Common
Common
Early, sabkha-related
Recrystallized microcrystalline dolomite
Pore-filling cement
Late
Late, post-HC migration
Late, associated with HC
Common in
Common in
Common in
Rare, locally
Common
Rare, locally
inner shelf deposits
build-ups
build-ups
common
common
äO18
(per mil PDB)
äC13
(per mil PDB)
5 to 10
3 to 9
5 to 9
0 to 5
3 to 18
2 to +4
3 to 8
3 to 8
4 to 6
2 to 7
2 to 7
5 to 9
14 to 18
3 to 18
5 to 18
Fig. 9. Paragenetic sequence of the
diagenetic events in the
Gzelian–Asselian sediments in IKU
shallow cores from the Finnmark
Platform. Note different porosity
evolution in calcite- and
dolomite-dominated units.
which the Moscovian to Kasimovian succession forms a
second-order transgressive unit related to rifting whereas the
Gzelian and Asselian succession formed during a second-order
highstand and therefore underwent more frequent and longer
subaerial exposure (see Fig. 2). There are local modifications to
this overall pattern. For example, the Moscovian succession
is dolomitized and porous in well 7120/2-1 from the Loppa
High and Moscovian phylloid algal–Palaeoaplysina build-ups
are dolomitized in the uppermost part of the Kapp Kåre
Formation on Bjørnøya. Also, Moscovian platform carbonates
associated with intra-shelf evaporites in North Greenland are
dolomitized and porous.
Observations in well 7128/6-1 indicate that some degree of
reservoir heterogeneity may be expected, with the highest
porosities in the upper part of the shallowing upwards cycles
and in the most grain-rich deposits.
This warm-water reservoir is regarded as low risk for
reservoir quality as the presence of connected porosity has been
documented in several deep wells.
Late Sakmarian–Kazanian cool-water carbonates
The upper Artinskian crinoid–bryozoan grainstones are tight
throughout the region. In most cases this is because of
cementation by syntaxial calcite and they are not expected to
have any significant reservoir potential, rather they may form a
potential regional seal. Also, the late Sakmarian–Artinskian
intra-reef packstones and wackestones are tight due to cementation by syntaxial calcite and chert, and they also form
potential lateral seals.
Potential reservoirs within this succession are thus limited to
the late Sakmarian–Artinskian bryozoan–Tubiphytes build-ups
182
L. Stemmerik et al.
Fig. 10. Summary of diagenetic changes seen in limestone (left)
and dolomite (right) in IKU’s shallow wells from the Finnmark
Platform. Note the lack of pressure solution features in the
dolomite and the abundance of late diagenetic calcite cement in
the limestone compared to the dolomite.
(see Figs 13, 14b). All drilled build-ups are tight because of
calcite cementation. However, the 7229/11-1 build-up core had
between 10% and 20% porosity before cementation by a late
coarsely crystalline calcite cement suggesting that more
shallowly buried build-ups may have retained some primary
porosity. Alternatively, secondary porosity related to fresh
water dissolution, as described from the Kazanian build-ups in
East Greenland could be present in some build-ups. The lack of
secondary vuggy porosity in the drilled build-ups makes the
East Greenland analogue unlikely, and only build-ups subjected
to prolonged subaerial exposure can be expected to contain
significant amounts of secondary porosity. Build-ups that have
been subjected to prolonged late Permian to early Triassic
exposure occur locally, for example as the result of differential
uplift of the Loppa High during the latest Permian (Fig. 14b).
These build-ups may have undergone as much as 25 Ma of
subaerial exposure in a temperate, and possibly relatively
humid, climate and consequently may represent the most
likely candidates for build-ups with karst-related porosity.
Karstification related to post-Artinskian exposure is seen on
Bjørnøya where karstic fractures filled by Kungurian or younger
Permian sediments are seen cutting down into the underlying
formations. The karst event apparently did not affect the
crinoid–bryozoan grainstones of the Hambergfjellet Formation
but certainly did affect the underlying Kapp Duner Formation.
The critical factor in this reservoir scenario seems to be the
primary mineralogy of the marine cements, where aragonite
cements will be more easily dissolved than high-Mg calcite
cements.
The occurrence of cool-water reservoirs is regarded as high
risk, and porosity has so far not been documented from this
stratigraphic interval.
APPLICATION TO THE LOPPA HIGH
The Loppa High is located near the southwestern margin of the
Norwegian Barents Sea (Fig. 1). The high is separated from the
Hammerfest Basin to the south and the Bjørnøya Basin to the
west by major faults and grades eastwards into the Bjarmeland
Platform. The principal structural elements and depositional
units are shown in Fig. 15. Upper Carboniferous to midPermian sediments infill and drape a mid-Carboniferous rift
topography but pinch out westwards as the result of late
Permian and early Triassic uplift and erosion. The western part
Fig. 11. Microphotographs illustrating
the most important facies and porosity
types in well 7128/6-1. The porosity
and permeability values are average
values for this facies type. (A)
Palaeoaplysina build-ups; (B) Bioclastic
packstone; (C) bioclastic wackestone;
and (D) dolomitic mudstone.
183
Carbonate reservoirs on the Norwegian Arctic Shelf
Fig. 12. Cross-plot of permeability
versus porosity for the
Gzelian–Asselian warm-water
carbonates in well 7128/6-1.
of the Loppa High developed as a positive structural element in
the late Palaeozoic but its present structural expression is the
result of late Jurassic–early Cretaceous rifting.
The overall late Palaeozoic stratigraphy and depositional
evolution follows the patterns outlined above and the fringing
carbonate platforms east of the high show several phases of
build-up development (Fig. 15). The internal facies distribution
of these platforms has been outlined in considerable detail by
mapping of seismic facies within several stratigraphic intervals.
Here, we have chosen to present seismic facies maps and
palaeogeographic maps of the Gzelian–Asselian and midArtinskian stratigraphic intervals to illustrate the two types of
carbonate deposition in the region and to provide additional
details to the palaeogeographic overview maps (Fig. 16). The
facies indicate that the Loppa High represented an easterly
dipping platform area during late Bashkirian to Moscovian
times. A regional transgression drowned Bashkirian continental
to marginal marine siliciclastics and caused a westwards and
updip migration of carbonate deposition during the early
Moscovian and Kasimovian. While carbonate deposition
dominated in the eastern downflank areas, the up-dip crestal
areas were characterized by mixed siliciclastics and carbonates
until the late Moscovian – as documented by well 7120/2-1
(Fig. 4). The Upper Moscovian–Kasimovian succession over
eastern areas is dominated by shallow marine subtidal to open
marine limestones with an expected limited reservoir potential
due to the increased possibility of early marine calcite cementation. In up-dip areas towards the crest, the lithofacies are
dominated by dolomites and partially dolomitized limestones
that represent an inner shelf with carbonate build-ups, periodically with restricted marine hypersaline lagoons between the
build-ups. A transgression in the earliest Gzelian led to
deposition of evaporites in shallow basinal areas with increasing
salt content towards the Nordkapp Basin. In the Loppa High
area, lithofacies consist of inner platform carbonates with
abundant phylloid algal–Palaeoaplysina build-ups in up-dip flank
and crestal areas whereas a shallow evaporite basin was located
over the downflank eastern areas (Figs 14, 16a). This overall
pattern persisted until latest Asselian or earliest Sakmarian times
(Fig. 14).
Following a Sakmarian fall in sea-level a subsequent transgression introduced more open marine conditions and a distally
steepened ramp was developed on the eastern flank of the
Loppa High. This transgression coincided with a change in
hydrographic regime and possibly climate and the upper
Sakmarian and Artinskian carbonates are typical cool-water
carbonates. Flooding of the ramp from the east resulted in reef
growth and as the transgression continued, stratigraphically
younger build-ups formed further westwards until the transgression slowed down in the mid-Artinskian. Most build-ups
are isolated ‘keep-up’ structures with a classical mound morphology. Laterally extensive build-ups of bryozoans, Tubiphytes
and echinoderms formed along the shelf break where up to
several hundreds of metres thick, stacked build-up complexes
occur. East of this trend, even larger composite build-ups occur
over pre-existing highs. The build-ups are overlain by uppermost Artinskian crinoid–bryozoan grainstones or, in up-dip
areas are truncated by the top Artinskian and Permian–Triassic
unconformities.
Prolonged late Permian subaerial exposure of the Loppa
crest is documented in Figs 14 and 15. According to the facies
maps this subjected mainly grain-rich inner and middle shelf
facies and build-ups to karstification. According to our proposed reservoir models, enhanced reservoir potential should be
expected along the crest.
Data from the seismic facies maps have been evaluated
within the framework of the overall reservoir models to predict
the reservoir potential in different play trends within the area.
Seismic facies mapping indicates that inner to middle shelf
deposits dominate in the mid-Bashkirian to mid-Kasimovian
interval in crestal areas. This suggests that better reservoir
properties are to be expected there than in well 7120/2-1. Also,
the up-dip position of these crestal segments compared to the
well increases the potential for dissolution. The Kasimovian to
Asselian interval is dominated by inner to middle shelf facies
and carbonate build-ups in the crestal areas possibly resembling
those seen in well 7128/6-1. The prolonged subaerial exposure
of the Loppa crest may have caused significantly more fresh
water dissolution in this area than in well 7128/6-1 and even
better reservoir qualities than those recorded in this well can be
expected.
Sakmarian–Artinskian bryozoan–Tubiphytes build-ups in the
crestal areas are overlain directly by Triassic sediments. Prolonged subaerial exposure during the latest Permian and earliest
Triassic may have led to carbonate dissolution and development of karst-related vuggy porosity following the cool-water
reservoir model. Geophysical studies of a flat event in one
Artinskian build-up suggest porosity values of the order of 20%
184
L. Stemmerik et al.
Fig. 13. Sedimentological log of cored
Artinskian bryozoan–Tubiphytes build-up
in well 7229/11-1 from the Finnmark
Platform.
in contrast to less than 2% porosity in drilled build-ups that
have not been affected by this exposure event.
CONCLUSIONS
+ Palaeoclimate had a major control on facies development
and early diagenesis, and consequently reservoir potential of
the Upper Palaeozoic carbonates in the Barents Sea area.
+ Warm, arid to semi-arid climates dominated in the Barents
Sea region during the late Bashkirian to Asselian and the
shallow platform carbonates were dominated by aragonitic
organisms, such as calcareous algae and Palaeoaplysina,
whereas associated carbonate cements and mud consisted of
mineralogically unstable aragonite and high-Mg calcite. The
carbonates are associated with evaporites both in nearshore
settings as seen in IKU shallow cores from the Finnmark
Platform and in deeper water settings in the Nordkapp
Basin.
+ Cool temperate climates dominated in the Barents Sea
region during the Artinskian and Late Permian and shallow
water carbonates were dominated by calcitic organisms such
as crinoids, bryozoans, brachiopods and silica sponges,
whereas associated carbonate cements and mud consist of
calcite or more rarely high-Mg calcite. The carbonates are
associated with spiculitic chert, particularly in the younger
parts of the succession.
+ The Moscovian–Asselian reservoir model implies extensive
dolomitization and dissolution of metastable carbonates
during repeated subaerial exposure related to high frequency
and high amplitude sea-level fluctuations. The existence
of reservoirs is confirmed by drilling and is accordingly
regarded as low risk in terms of probability of occurrence.
Good analogues to this type of reservoir are found in well
7120/2-1 from the Loppa High and 7128/6-1 from the
Finnmark Platform and from age-equivalent rocks on
Bjørnøya and in North Greenland.
+ The Sakmarian–Artinskian reservoir models imply either
preservation of primary porosity in stromatactis cavities in
bryozoan–Tubiphytes build-ups or extensive dissolution of
marine cement in these build-ups during prolonged subaerial
Fig. 14. Upper Palaeozoic geological models for the central Loppa High emphasizing depositional and diagenetic data important for reservoir
properties. The intra-Bashkirian–Asselian model exemplifies the warm-water reservoir model and the late Sakmarian–Artinskian and Late
Permian models are examples of the cool-water reservoir model.
186
L. Stemmerik et al.
Fig. 15. Composite seismic line crossing the Loppa High area from west to east and the general stratigraphic interpretation based on well data
and seismic facies mapping. Important tectonic events are indicated.
Fig. 16. Palaeogeographic maps of the Loppa High area based on mapping of seismic facies: (A) Gzelian–Asselian; (B) Late Artinskian.
exposure. The models are based on data from well 7229/
11-1 on the Finnmark Platform and Upper Permian
bryozoan–marine cement build-ups from central East
Greenland combined with seismic data. These reservoirs are
not confirmed by drilling and are regarded as high risk in
terms of occurrence.
Carbonate reservoirs on the Norwegian Arctic Shelf
We would like to than IKU Petroleum Research for permission to
use data from their cores on the Finnmark Platform. The paper is
published with the approval of the Geological Survey of Denmark
and Greenland and Saga Petroleum ASA.
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Received 3 March 1998; revised typescript accepted 18 January 1999.