Structural and stratigraphic overview of Barents Sea North

The Barents Sea is divided into geological basins, platforms and highs, collectively designated structural elements (figure 6). This region contains a more or less continuous succession from the Upper Palaeozoic to the Eocene. In the mapped eastern part of Barents Sea North, the succession has been eroded from the Upper Cretaceous, and the Quaternary sedimentary package is very thin. Triassic strata outcrop on the largest highs.

 

Figure 6.
Map of the main geological structural elements in the Norwegian sector of the Barents Sea. It is worth noting that the structural elements in Barents Sea North have yet to be formally approved by the Norwegian Committee on Stratigraphy. Structural elements mentioned in the text are named on the map. The yellow line shows the location of the geoseismic profile presented in figure 7. The area opened for petroleum activities in Barents Sea South is shown by quadrants.

 

MAIN STRUCTURAL ELEMENTS

The north-easterly orientation of the basins and highs in Barents Sea South, the anticlines on the Kong Karl platform and the Storbanken high, and much of the Sentralbanken high are assumed to be controlled primarily by the reactivation of older zones of weakness. Important tectonic events which affected the eastern parts of Barents Sea North are summarised in figure 13. During the Late Palaeozoic and Mesozoic, this part of the Barents Sea was a relative quiet area in tectonic terms.

Regional extensions in the form of rifting occurred in the Middle Carboniferous. Later movements in the Late Permian, Late Jurassic and Early Cretaceous are dominated by compression. Development of the present structural highs in the eastern part of Barents Sea North initiated in the Late Jurassic. Regional subsidence began in the Late Permian and continued in the Triassic, when a large delta system originating in the Urals to the south-east prograded gradually north-westwards over the Barents Sea and reached Svalbard in the Late Triassic (Lundschien et al, 2014). The area has been subjected to strong compression in later times, probably in the Palaeocene/Eocene. Since the Upper Cretaceous and overlaying packages are missing today, however, this/these episode(s) cannot be documented directly from the area. The whole area was glaciated during the Quaternary, and glacial erosion removed most of the Upper Cretaceous and overlaying sediments.

Figure 7 presents a geoseismic profile which shows the most prominent structural elements in the eastern part of Norway’s Barents Sea sector. Sediments are mainly of Late Devonian/Carboniferous to Cretaceous age, and more eroded in the north than to the south. Areas which were basins/grabens in the Carboniferous are generally associated with evaporite deposition and in some places, such as the Nordkapp Basin, with salt diapirism.

A number of these old basins/grabens have been inverted and appear today as highs, such as the Sentralbanken high and anticlines on the Kong Karl platform. The profile in figure 7 shows the anticlines on the Kong Karl platform and the Storbanken high. Furthermore, the figure shows that the largest structural elements – such as the Storbanken and Sentralbanken highs – are deeply eroded, probably because of compressional movements in the Palaeogene and later glacial erosion.

 

Figure 7.
Geoseismic profile showing the main structural elements in the eastern part of Norway’s Barents Sea sector. The location of the profile is shown in figure 6.

 

The main structural features are illustrated below by a number of seismic profiles whose locations are presented in figure 8.

The Storbanken high is of Palaeozoic age, with a thin sedimentary package from the Upper Carboniferous to the Permian (figure 9). In its eastern part, the Sentralbanken high has a Palaeozoic core with a number of horst and graben structures formed in the Middle Carboniferous but covered – unlike the Storbanken high – by a thick package from the Upper Carboniferous to the Permian (figure 10). Large parts of the Kong Karl platform are dominated by compression anticlines, which probably reflect a reversal of older normal faults and folding located in areas of the platform which also contained highs during much of the Palaeozoic (figure 11). The Olga basin subsided during the Cretaceous, but was a deep basin in the Palaeozoic as well (figure 12).

 

Figure 8.
Location of seismic profiles.

 

 

Figure 9.
A profile across the Storbanken high. Thin packages date from the Late Carboniferous/Permian above the high, and from the Lower and Middle Triassic in the north. Upper Triassic (Carnian) sediments dominate in the north, and the whole Jurassic package is eroded. Much of the Storbanken high looks as if it might be a basement high. Figure 8 shows the location of the profile.

 

 

Figure 10.
A profile across the Sentralbanken high, with the Olga basin in the north. On a large scale, the profile shows compression of the Sentralbanken high. Half-graben systems assumed to be from the Early Carboniferous and mobilisation of evaporites deposited in the Late Carboniferous and Early Permian can be seen in the lower part of the profile. Small compression anticlines show up on a smaller scale, and probably reflect an inversion of normal faults formed earlier. These movements appear to cut through the entire succession from basement to seabed. Figure 8 shows the location of the profile.

 

 

Figure 11.
A profile across the Kong Karl platform, with the southern part of the Storbanken high to the south-east. The platform is dominated by compressional anticlines large and small. The package under the Carboniferous reflector has poor reflectivity and could indicate that much of the platform area is part of a basement plateau. To the north-west is a distinctive Palaeozoic basin area with a large salt-related anticline and a very thin Lower and Middle Triassic package. Figure 8 shows the location of the profile.

 

 

Figure 12.
A profile across the Olga basin, with the flank of the Sentralbanken high to the south and the Storbanken high to the north. The succession from the upper part of the Late Jurassic and the Early Cretaceous thins out towards both highs. Clear reflectors beneath the Carboniferous show that the basin was formed in the Early Carboniferous or the Devonian. Figure 8 shows the location of the profile.

 

STRATIGRAPHIC DESCRIPTION

Figure 13 provides a schematic presentation of the chronostratigraphy and lithostratigraphy in the eastern part of Norway’s Barents Sea sector correlated with eastern Svalbard. This correlation is based on geological information from wells in the south, shallow boreholes in the north, seismic surveying and fieldwork in Svalbard. The lithostratigraphic framework in the Barents Sea and on land in Svalbard shows great similarities, and can be divided into regional mappable units (groups and formations).

The sedimentary succession reflects both short-term and length variations in relative sea level and a constantly changing climate. Climatic variations reflect a general northwards movement for the whole area, from equatorial conditions at the Devonian-Carboniferous boundary to a northern temperate climate during the Palaeogene and Neogene. The sedimentary succession is dominated by siliciclastic rocks, but deposition of carbonate rocks dominated in the Late Carboniferous and Early Permian when the region was located in the northern dry climatic belt.

 

Figure 13.
Chronostratigraphic and lithostratigraphic diagram for the eastern part of Norway’s Barents Sea sector and for eastern Svalbard, modified from http://www.npd.no/en/Topics/Geology/Lithostratigraphy/. Tectonic events which have affected the area and the plays established in the eastern part of Barents Sea North are specified to the right of the diagram.

 

DEVONIAN AND EARLY CARBONIFEROUS

Devonian rocks have not been proven by drilling in Norway’s Barents Sea sector. The development of the Devonian east of Svalbard is unknown.

During the Early Carboniferous, the Barents Sea was subject to a tectonic phase with extension and the development of rift basins. Places where growth faults associated with this rifting are clearly visible include the Sentralbanken high (figure 14).

 

Figure 14.
Growth faults in rotated half-graben systems assumed to be from the Early Carboniferous in eastern parts of the Sentralbanken high. Note that the Late Carboniferous-Permian package thickens across the whole Sentralbanken high and appears today as an inverted structural element.

 

Large thicknesses of clastic sediments, including conglomerates, have been observed in Billefjorden on Spitsbergen in a well-defined graben system from the Early Carboniferous. Coal measures have also developed here, which were exploited by the Pyramiden mining community. Similar graben systems are found in a number of locations in Spitsbergen, where the largest are the Billefjorden, the Indre Hornsund and St Jonsfjorden troughs. Based on seismic observations from Barents Sea North, similar graben systems are also likely to be present at many places in the evaluated area (figure 15).

 

Figure 15.
A horst/graben system from the north-eastern Storbanken high. The horst top represents an unconformity surface and was probably exposed subaerially at the Permian-Triassic transition.

 

These sediments are likely to be similar in character to those in Spitsbergen, and could be suitable as both source and reservoir rocks where the sediments have not been too deeply buried.

 

LATE CARBONIFEROUS-PERMIAN

Extension and rifting also continued during the transition from the Early to Late Carboniferous. A dry climate combined with frequent changes in sea level led in part to the deposition of evaporites such as gypsum/anhydrites and salt in the central parts of the graben structures (figure 16). The evaluated area is dominated by a marine environment far from clastic input systems. Various types of carbonates could have been deposited laterally to the deepest evaporite basins and on platforms and highs. Magnesium-rich dolomites dominate in the Late Carboniferous and Early Permian, while limestones and shales dominate in the Late Permian. Reef-building could have been an important factor for forming reservoir rocks in the Late Carboniferous and Permian. That applies particularly to the horsts and highs around the evaporite basins.

 

Figure 16.
Section from the Sentralbanken high flattened on the top Permian reflector. Evaporite deposits (strong reflectors) in the largest graben, and carbonate deposits (homogenous and low-reflective packages) on the highs and platform areas. As the graben was filled with evaporites, its area also became part of the carbonate platform. Note that the top Permian surface creates a strong multiple internally in what are assumed to be carbonate deposits.

 

During the Late Carboniferous, the Barents Sea was located in the northern dry climatic belt but an ice age prevailed globally. Repeated global glacials and interglacials led to large global variations in sea level, which also affected the Barents Sea. These frequent changes meant that the deposited carbonates time and again experienced subaerial exposure with the possibility for karstification. Collapse breccia resulting from the leaching of evaporites may also have played a role in the formation of reservoir rocks.

Parts of Barents Sea North were during the last part of the Permian subjected to a new tectonic period, which led in the Palaeozoic package to a general uplift of highs and subsidence of basins. Towards the end of the Permian, changes in relative sea level exposed a number of areas – including to such an extent that they experienced subaerial exposure and were subject to erosion. Land areas developed on exposed fault blocks where carbonate rocks could have been subject to further karstification as well as possible coastal processes (figure 17).

 

Figure 17.
Late Carboniferous and Permian sediments thin over the Storbanken high. This could reflect little sedimentation and/or erosion following subaerial exposure. Note the significant thinning of the Induan, Olenekian and Anisian northwards.

 

The age range – hiatus – between the Triassic and the Permian varies in magnitude. In areas where carbonates have been subaerially exposed to possible karstification, the gap in time is greater than where carbonates have been less exposed or have possibly remained submerged throughout the Permian-Triassic transition (figure 15). Large parts of the Early Triassic are missing in Edgeøya and Wilhelmøya, which indicates that the carbonates have been exposed and eroded over a lengthy period.

 

TRIASSIC

Generally speaking, the Triassic is quiet tectonically in Barents Sea North. The whole period was dominated by a thick delta and floodplain system, which built up gradually westwards and north-westwards towards Svalbard with sources in the south-east from the Urals and in the east from Novaya Zemlya. This system can be traced in the seismic data as different clinoform build-outs of varying age which become younger towards the north-west (figure 7). The clinoforms observed in the seismic data are on a scale, which probably represents a marked continental shelf progradation in the form of enormous delta and floodplain build-outs (figure 18). This system reached the Sentralbanken high and the Olga basin during the Early Triassic (late Induan). During the transition between the Induan-Olenekian, the basin subsided faster that the supply of new sediments, and the established deltas in the Induan transgressed. Seismic interpretation indicates that this episode was regional across the whole Barents Sea.

 

Figure 18.
Two sets of clinoforms build-out in the Early Triassic (indicated by arrows) in the Olga basin, one in the Induan and the other in the Olenekian. Repeated downlap on the top Permian and Induan reflectors.

 

Based on the seismic interpretation in this study, it is difficult to find indications of large tectonic episodes in the Middle Triassic. However, well data show that small transgressions and regressions probably occurred. These could be caused by global sea-level variations or local lobe subsidence and build-out. Lobe shifts are a natural process in all progressive delta building, and have probably been a very important factor in the distribution of sand and silt together with clay throughout the Triassic in the Barents Sea.

Progradation, represented by new clinoforms, continued in the Late Triassic, and sediment flux appears to have increased substantially in Barents Sea North (figure 19). This probably reflects an increased input of sediments from source areas to the east and southeast, which could be caused in turn by tectonic episodes in Novaya Zemlya and the Urals during the Triassic and/or climate variations which might also have speeded up erosion in the mountain regions. Some 1 500-1 800 metres of sediment have been deposited near Kvitøya in the Carnian, which could suggest deep water depth initially here or increased subsidence in these areas.

 

Figure 19.
Triassic sequences in the northernmost part of the mapped area flattened on the Induan reflector. The Triassic has an erosional contact with the Permian sediments.

 

Preliminary results from shallow drilling in 2015 suggest that the supply of clastic sediments continued in the Norian. This trend was broken by the regional marine transgression in the Norian, represented by a dated shale from the shallow wells which could be equivalent to the Flatsalen Formation in Svalbard and the (lower part of the) Fruholmen Formation in the Barents Sea. Towards the end of the Norian, seismic interpretation indicates that a erosional boundary (unconformity) exists between the Norian and the Rhaetian south of Kvitøya (figure 20).

 

Figure 20.
Well 7934/8-U-1 has been drilled through Late Triassic sediments, and dating shows a Norian marine shale (Flatsalen Formation). The overlying and underlying sandstones (sst) are undated.

 

JURASSIC AND UPPERMOST TRIASSIC (RHAETIAN)

About 230 metres of clastic sediments are exposed on Kong Karls Land (figure 2) in the Realgrunnen subgroup (the base of the subgroup is not subaerial; Larsen et al, in preparation). The bulk of these sediments belong to the Svenskøya and Kongsøya Formations from the Early and Middle Triassic. A large proportion of the clastic sediments consist here of tidal deposits. Seismic interpretation in the mapped area south of Kvitøya shows an estimated thickness of about 250 metres of sediments with the same age.

Attribute mapping of three-dimensional seismic on the Haapet Dome in Barents Sea South shows channel systems in the Realgrunnen subgroup running from south-east to north-west, in line with the trend throughout the Triassic, and the geomorphological characteristics indicate that these are fluvial channels. Plans call for the Korpfjell prospect on the Haapet Dome to be drilled in 2017 (production licence 859). The directional trend for the channel system on the Haapet Dome is a good indication that the provenance area lies either on Novaya Zemlya or even further south in the Ural mountain chain. Extrapolating the trend of the channel systems in the Realgrunnen subgroup on the Haapet Dome towards the northwest allows them to be correlated with delta deposition and fluvial channels in the Wilhelmøya subgroup on Hopen.

From Spitsbergen, it is known that sedimentation changed in the Middle Jurassic. A characteristic condensed stratum (the Brentskardhaugen) developed and marks the transition between Late Triassic and thick marine shales in the Late Jurassic Agardhfjellet Formation (corresponds to the Fuglen and Hekkingen Formations in the Barents Sea). In Kong Karls Land, this shale shows a marked thinning at the top of a big anticline, which indicates that the tectonic episode which led to the formation of the anticline must predate the overlying Early Cretaceous sandstones. The sandstones in the Early Cretaceous Helvetiafjellet Formation have an erosional contact with the underlying Late Jurassic shale (Larsen et al, in preparation). A corresponding thinning of the organically rich Late Jurassic shale can be observed on the seismic lines in virtually all the large anticlines and highs where this strata is not eroded (figure 21). This shows that the earliest phases in the formation of these structures occurred in the Late Jurassic and Early Cretaceous.

 

Figure 21.
Thinning of Late Jurassic shale and deposits from the lowest Cretaceous on the flank of one of the big anticlines on the Kong Karl platform are shown to the left. The underlying package does not appear to thin out towards the anticlines.

 

CRETACEOUS

Extensive volcanic activity occurred in Barents Sea North during a brief period of the Early Cretaceous. Vulcanites on Kong Karls Land are developed both as basaltic lava and intrusives. Intrusions from this period are common on both Svalbard and large parts of Barents Sea North, where they are easy to identify in the seismic data (figure 22). They occur as both dykes and sills.

 

Figure 22.
Intrusions following the bedding plane (sills) on the Kong Karl platform in the Lower Jurassic succession to the left. Note that the reflection pattern representing the Early Cretaceous onlaps the base Cretaceous reflector on the flank of the anticline.

 

Fold structures in the northeastern Barents Sea were strongly reactivated and acquired their present form in a tectonic phase which cannot be dated stratigraphically because the Late Cretaceous and overlying sediments have been eroded. A Palaeogene age has previously been proposed for this phase, coinciding with mountain formation on Spitsbergen. The new mapping of Barents Sea North shows that the graben structures in the Carboniferous-Permian have to some extent determined the fold direction of the anticlines.

When the evaporites began to move, the overlying sediments were pushed up (figure 23). Dome formation probably began in most places during the Late Jurassic, but has important phases in the Early Cretaceous (figure 24).

 

Figure 23.
The largest salt structure in the evaluated area, just east-south-east of Kong Karls Land. This forms a large anticline about 15 kilometres wide and 75 long. Given the geometry of the rim synclines, salt mobilisation appears to have begun after the Late Triassic depositions.

 

Figure 24.
Prograding Early Cretaceous fan deposit in the eastern part of the Sentralbanken high. Sediments in the fan deposit probably originate locally.

 

PALAEOGENE

As described above, one of the main theories on the formation of large anticlines in the northeastern part of the mapped area was that these occurred at the same time as compression on Svalbard during the Palaeogene. However, no Palaeogene sediments have been proven in the evaluated area which can confirm this. They are only to be found in Spitsbergen or the western part of the Barents Sea, where the fold direction is different from that east of Svalbard.

At the same time, seismic interpretation indicates that tectonic forces related to regional movements in the Late Jurassic and Early Cretaceous, and doming related to evaporites, have been an important factor in the formation of the highs and anticlines in the Early Cretaceous before the final compression phase or phases occurred.

 

NEOGENE-QUATERNARY

During the Pliocene-Quaternary, the mapped area has been subject to repeated glaciations. These can be divided into three main phases – the first 3.6 million years ago, followed by a growth stage and then the final period of large-scale glaciation about a million years ago (Knies et al, 2009). Erosion related to glaciation was extensive throughout the Barents continental shelf. Large quantities of sediment were deposited in submarine areas (depocentres) along the western and northern margins. Total erosion was greatest on northern platform areas and the areas around Svalbard. Some two-three kilometres of sediments are estimated to have been removed on Svalbard (Smelror et al, 2009). This extensive erosion was both glacial and related to the tectonic episodes in the Palaeogene. According to Ramberg et al (2007), most of the Palaeogene and Cretaceous strata in Barents Sea North were eroded away during the Neogene.