Lofoten and Vesterålen: Promised Land or Fata Morgana?

In 2010, the Norwegian Petroleum Directorate estimated (P50) that the Norwegian Atlantic areas of Nordland VI, VII and Troms II, near the Lofoten and Vesterålen Islands, could hold as much as 1.3 Bboe while the industry players’ P50 figure is even larger, at 3.5 Bboe. Well defined Jurassic rotated fault blocks, similar to the play that holds giants like Åsgard, Skarv and Norne, make the area potentially attractive.

However, concerns for the environment and fisheries have so far kept most of the area closed to exploration. The southern part of Nordland VI was open from 1996 to 2001, with one dry well drilled into the Cenozoic. The main Jurassic play is still unexplored, but there are clear indications of an active petroleum system.

Four stratigraphic wells in the Ribban Basin and two offshore Vesterålen suggest a sedimentology similar to that found along the Nordland coast. The basement is Triassic fluvial coarse conglomerates, sand and shale. The Lower to Middle Jurassic consists of shallow marine sandstone, the main reservoir, followed by upper Jurassic shale, including the well-known rich Kimmeridgian source rock. The Lower Cretaceous consists of mainly shale in the wells, but seismic anomalies may indicate turbidite sands. From the Upper Cretaceous and the early Cenozoic, sands up to a hundred metres thick – the targets of the sole commercial well – are possibly the result of erosion of the uplifted mainland. They are overlain by Upper Cenozoic shale and Quaternary glacials.

Enigmatic Structure

Geological and structural map of the Lofoten and Vesterålen area, showing that Lofoten is the only exposed basement high on the Norwegian Continental Shelf. Source: NPDAnd yet, Lofoten and Vesterålen are different; enigmatic even. The Lofoten Ridge itself is unique, being the only exposed basement ridge on the Norwegian continental shelf. It consists of crystalline rocks up to 1.3 billion years old. The ridge is separated from the mainland by the Vestfjorden Basin, an asymmetric graben which opens towards the south, like a fan. It is filled with sediments interpreted to be of Palaeozoic to Recent age, with the main deposition through the Cretaceous. A similar feature, the Ribban Basin, lies on the ridge’s west flank, and is in turn separated from the shelf edge by the less pronounced, submarine Utrøst basement ridge. The Ribban Basin is split in two by a major fault block, the Marmæle spur. Towards the north, the Lofoten Ridge merges into the large islands of Vesterålen, while the shelf narrows to a thin band, only 40 km wide at the narrowest. Curiously, faults on this thin margin consistently dip to the east in the south and to the west in its northern part, and in both areas fault blocks are strongly rotated over (interpreted) low-angled detachment faults.

The major syn-rift phase in the Ribban Basin is early Cretaceous, followed by late Cretaceous and Paleocene post-rift sediments. In the deeper basin, a pre-rift sequence of Triassic and Jurassic age rests on crystalline basement. The Base Cretaceous Unconformity and the bright amplitude of the late Jurassic source rock are prominent on seismic data, but the unconformity becomes less pronounced updip on the western flank, and this succession seems to be eroded on fault crests. The deeper part of the basin is obscured on seismic, especially in the Vestfjorden Basin, where seismic coverage is sparse and of old vintage. Deep parallel reflections suggest that older, Palaeozoic sediments reside in the deep Vestfjorden Basin, although this is uncertain. Seismic character in the western Ribban Basin may suggest the presence of a local, deep Palaeozoic sub-basin there as well.

In contrast, the narrow Vesterålen margin exhibits narrow, fault-defined half grabens filled with a Mesozoic succession, similar to further south, but which is heavily eroded, due to a major uplift.

Origins of the Lofoten Ridge

Map showing estimated total erosion in metres in the Lofoten and Vesterålen region, expressed as the difference between the current burial and maximum burial. Source: NPDWhy is the Lofoten Ridge there, surrounded by deep basins? Why does the shelf narrow, and why did Vesterålen lift two kilometres up? The reasons stretch 400 million years back in time.

During the clash of Europe with Greenland and North America, which created the Caledonian mountain chain, the mountains probably collapsed along strike, a phenomenon known from the Himalayas today. Major detachment zones developed, and in north Norway these are consistently spoon-shaped, with axes plunging gently to the south-west or north-east.

It is likely that the Vestfjorden and Ribban Basins represent similar spoon-shaped troughs, developed as the mountain chain stretched along strike, with the basement ridge emerging between. Seismic interpretation suggests that the basin-bounding faults flatten out into low-angled detachments, and within the basement are prominent reflections that may be downthrown imbricated thrust nappes. Gravimetric-magnetometric data reveal a very prominent positive anomaly along the Lofoten Ridge, which fits with a shallow Moho. The basement below the basins is modelled to be low-magnetic, indicating that they are buried metasedimentary thrust nappes, rather than the high-magnetic basement of the Lofoten Ridge. Notably, gravity modelling also supports the suggestion that older sedimentary basins may be present deep beneath Ribban and Vestfjorden, possibly analogues to the Devonian post- Caledonian basins in western Norway. On the mainland, the Steigen detachment forms a spoon-shaped detachment parallel to Vestfjorden, the ‘Sagfjorden shear zone’, with Precambrian basement in the footwall and lower allochton phyllites in the hangingwall.

Different Tectonic Styles

The major part of the Lofoten Ridge consists of gneisses and igneous rocks, but the outermost islands of Værøy and Røst exhibit highly deformed mylonites. Røst is entirely flat, defined by a pervasive foliation. There is evidence for both north-east to south-west and north-west to south-east oriented stretching of the foliation, which is thought to reflect the initial orogen-parallel extension, followed by perpendicular, post-orogen extension.

Ar/Ar dating of mica by Steltenpohl and co-workers indicates that Lofoten was successively exhumed from the north-east to the south-west during the Carboniferous and Permian. A probable mechanism is reactivation of the footwall between the Caledonian troughs, which was simultaneously exhumed as the Lofoten Ridge. Thereafter, during the Mesozoic and Cenozoic, steep faults developed along the basin flanks to form the present basin and ridge configuration.

The tectonic style in Vesterålen is different from Lofoten. Seismic sections show fault blocks heavily rotated above low-angled detachments. Substantial stretching took place during the Mesozoic, followed by up to 2 km of uplift and thus erosion into the sediments. Widespread fractured and weathered basement onshore, often transformed to gravel, demonstrate that subaerial weathering took place in the basement. The structural style thus resembles a basin and range-style metamorphic core complex.

Prospectivity

As a result of this uplift, hydrocarbon retention will be a major issue offshore Vesterålen. The northward uplift of Lofoten may create migration paths from traps to the surface, although this is less likely further south. Migration from source to trap generally has low risk in Lofoten, with the reservoirs just below the source rock and some with large catch areas on basin flanks.

Due to the overall spoon shape of the Ribban and Vestfjorden Basins, the Upper Jurassic source may have matured during the Cretaceous across large areas, but may today possibly be over-mature in the deep basins, and within the generation window today only along the gently dipping flanks of the Ribban Basin and the Marmæle Spur. In Vesterålen, burial and maturity is uncertain due to uplift.

The quality of seismic in the Lofoten and Vesterålen area is variable. At prospect level it is generally good down to the Base Cretaceous Unconformity, but the intra-Jurassic architecture and section to basement is more uncertain. For prospects on highs or protruding fault blocks, the sediments appear to wedge out towards the crest, either due to block rotation during deposition, or as a result of erosion, and only a thin sedimentary reservoir will be available.