Main geological features

24.06.2010
 

 

Land areas and the seabed in the areas covered by this evaluation. Heights and depths have been scaled up. Only the unopened part of  Nordland V is included.

Figure 3.
Land areas and the seabed in the areas covered by this evaluation. Heights and depths have been scaled up. Only the unopened part of  Nordland V is included.


Structural geology

The sea areas off Nordland and Troms, which embrace the West Fjord, Nordland VI/VII, Troms II and the Egga Edge (figure 3), have a varied and interesting geology. With the NCS at its narrowest here – less than 20 kilometres wide in some places – the water depth goes down to about 400 metres. From the edge of the continental shelf, the seabed plunges to the abyssal plain more than 2 500 metres beneath sea level.

In Nordland VI/VII, the bedrock is divided by marked basement ridges separating basins filled with sedimentary deposits. The most prominent ridge is the Lofoten Ridge (figure 4 and figure 5), which extends northwards towards Andøya. Between this ridge and the mainland lies the West Fjord Basin. The Ribbe and Harstad Basins lie west and north respectively of the Lofoten Ridge. Sedimentary rocks of Jurassic and Cretaceous age (figure 7) fill the Ribbe Basin. The Harstad Basin is characterised by a deep syncline in the Jurassic and Cretaceous which means that particularly thick Cretaceous sediments have been deposited there. A number of prospects have been identified in both basins (figure 10). The Utrøst Ridge rises west of the Ribbe Basin, with basement rocks reaching right to the seabed in places.

Basement rock is buried much more deeply in the northern part of Nordland VII than in Nordland VI, and the geological strata have been sharply tilted. Fault blocks which could provide possible oil and gas traps have formed in a number of places.

Sedimentary rocks from the Jurassic and Lower Cretaceous are preserved in a small area on land in Andøya. Drilling for both coal and oil has been conducted in these rocks. Jurassic coal measures were found as early as the late 19th century, but no commercial deposits have been proven.

Extensive faulting which forms structures in Jurassic and Cretaceous rocks has been identified close to land off Andøya. This faulting reflects extensive slumping in the Late Cretaceous.

While the mainland and continental shelf comprise continental crust, the abyssal plain is composed of basalts and sediments on oceanic crust. Large petroleum deposits are unlikely to have formed in the areas of oceanic crust. The boundary between oceanic and continental crust lies in very deep water (figure 4). A zone of sedimentary rocks occupies the area between the continental shelf edge and the oceanic crust, and is covered in many areas by basalts and other volcanic rocks. This area lies partly within and partly west of Nordland VI/VII. Interpreting the geology there is very challenging because seismic signals are sharply attenuated when they penetrate the basalts, and accordingly provide weak images of the sub-surface. While no prospects have been identified there for this reason, plays take account of the possibility that it could be a potential resource.

 

Figure 4. Map of the structural geological elements in the Lofoten-Vesterålen area. A-A1 shows the position of the seismic cross-section presented in figure 5.

Figure 4.
Map of the structural geological elements in the Lofoten-Vesterålen area. A-A1 shows the position of the seismic cross-section presented in figure 5.

 

Figure 5. A seismic and geological cross-section south of Lofoten, oriented north-west to south-east. Its location is shown in figure 4.

Figure 5.
A seismic and geological cross-section south of Lofoten, oriented north-west to south-east. Its location is shown in figure 4.



The Egga Edge is a collective term for the outer edge of the NCS. In this report, it designates the area within the transitional zone between the continental shelf and the deep ocean in the south-west Barents Sea (figure 6). Rocks from the Palaeogene period dominate in this area (figure 7). Geological knowledge of Triassic and Cretaceous rocks in the area is limited, but these sediments generally lie too deep for oil and gas discoveries to be made there. The most important reservoir rock is thought to be Eocene sandstones deposited in deep water.

The West Fjord Basin is a deep sedimentary basin between the Lofoten Ridge and the mainland (figure 4). It was formed in the Cretaceous and has a base of Triassic/Jurassic and older sedimentary rocks. The basin is surrounded by large faults, particularly towards the Lofoten Ridge, with upthrusts as high as 4-5 000 metres or more. Large volumes of sediment were deposited in the West Fjord Basin during the Cretaceous. Whether reservoir rocks are present in this level is uncertain, but seismic data show that sandstone fans could have built out into the basin from the south-east. A belt of small fault blocks and structures of Triassic/Jurassic age is found along the south-eastern side of the West Fjord, and petroleum could be present there. The data coverage is insufficient to identify prospects in the area.


Reservoir rocks

The potential reservoir rocks in the surveyed area consist of sandstones deposited in the Triassic, Jurassic, Cretaceous and Palaeogene periods. The NPD also sees possibilities that fractured and weathered basement rock could function as reservoirs. Large parts of Nordland VI/VII were probably highlands right up to the Jurassic. Triassic sediments can only be expected in local basins, and older sediments have not been deposited.

Triassic and Jurassic sandstone reservoirs form parts of the most extensive deposition systems on the NCS. Sandstones of Early and Middle Jurassic age provide good reservoirs in both Norwegian and Barents Seas, and corresponding properties are expected in this area. However, local variations could occur – particularly in the relatively enclosed Ribbe Basin – which mean that the properties differ somewhat from those observed in better-explored areas. Scientific drilling by the IKU in the 1990s confirms that Triassic and Jurassic sandstones are present. Seismic data show that Jurassic sediments were eroded on some of the big fault blocks before these were buried under younger rocks. The reservoir rocks will then lie as sedimentary wedges on the flanks of the structure. In many cases, that would reduce opportunities for making large discoveries in such structures.

Sandstones were deposited on the seabed during the Cretaceous and Palaeogene as a result of mainland uplift and erosion. Such sediments can have very varying reservoir properties, and have not been deposited everywhere. The stratigraphic package of Cretaceous age could be up to 2 000 metres thick, and the thin sandstones assumed to exist within it cannot be identified with any certainty from the seismic data. The NPD’s resource evaluation incorporates possible reservoir levels in Lower and Upper Cretaceous. That builds on knowledge from other parts of the Norwegian Sea. The potential for discoveries in Cretaceous sandstones could be substantial, but the uncertainty is high. A large fan of thick Palaeocene sandstone deposits, originating from the landmass to the north-east, has been identified in Nordland V/VI.

 

Figure 6. Map showing structural geological elements in Troms II and the south-western Barents Sea, with the location of the Egga Edge area.


Figure 6.
Map showing structural geological elements in Troms II and the south-western Barents Sea, with the location of the Egga Edge area.


 

Source rocks

The main source rock for oil and gas in the area is of Late Jurassic age. It has been documented to the south, in Nordland III, and by scientific drilling in the actual area. The source rock is an organically rich shale which can form both oil and gas. These shales are likely to be found in all the basins, including west of the Utrøst Ridge.

The source rock is through to be buried deep enough to form oil in parts of the Ribbe and Harstad Basins. It lies so deep that it will form gas west of the Utrøst Ridge and Andøya, and in deeper parts of the Træna and Harstad Basins. At even greater depths, the source rock is so hot that it can probably no longer yield petroleum. A narrow zone close to land in the West Fjord, particularly on the southern side, could have been relevant for oil or gas formation. Another possible source rock is coal and shale from the Triassic and the Early and Middle Jurassic. Nor can Cretaceous source rocks be excluded, but these have not been assessed in detail. The volume and type of petroleum which the source rock is able to produce represent substantial uncertainty factors, because they depend on the quality and buried depth of the rock.

Coastal areas in northern Nordland and southern Troms counties were subject to extensive uplift and subsequent erosion in the Late Cretaceous and Palaeogene periods. That also applies to the Nordland VII, Troms II and parts of the Nordland VI areas of the NCS. The biggest erosion following the uplift is in the order of 1 500-2 000 metres over parts of the mainland and by the Utrøst Ridge. Uplift was substantial west of the Utrøst Ridge and Vesterålen.

In sedimentary basis where the source rocks reached their maximum burial several million years ago, and have since been uplifted, hydrocarbon formation can be slow or cease. That could mean that gas leaking slowly from the traps is not being replaced, and increases the risk that hydrocarbons fail to be retained.

Experience from the Barents Sea confirms that erosion can lead to increased leakage from the traps. The identified erosion and uplift in the assessed areas show that the Ribbe Basin due west of Lofoten probably offers the biggest risk of leakage.

 

Figure 7. Stratigraphic levels for plays in the areas assessed. Reservoir levels for a number of fields and discoveries (with informal discovery names) in the Norwegian and Barents Seas are also shown.

Figure 7.
Stratigraphic levels for plays in the areas assessed. Reservoir levels for a number of fields and discoveries (with informal discovery names) in the Norwegian and Barents Seas are also shown.

 

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