Geological knowledge of the area of the Norwegian Continental Margin around the remote Lofoten Islands has remained limited, with significant exploration activity confined to the south in the Vøring and Møre Basins and in the deep offshore to the west.
The Lofoten-Vesterålen region shows an extension of the Vøring Basin with similar geological setting and exploration potential. Key targets are tilted fault blocks and horst structures containing Jurassic and Triassic reservoirs. Significant exploration potential also exists within Cretaceous and Tertiary fans, which have been proven in the area to the south-west. However, the critical factors to hydrocarbon exploration in this area will be the identification and mapping of reservoir distribution, thickness and quality, identification of stratigraphic pinch-outs, proper imaging of Jurassic/Triassic tilted fault blocks buried below thick sequences of Cretaceous and Tertiary strata and the retention of hydrocarbons due to uplift and erosion.
In order to help elucidate this promising area, CGGVeritas has acquired more than 5,740 km of 2D seismic lines of various vintages. The earliest data, shot in 1993/94, which form nearly 40% of the total data set, have been mostly reprocessed in 2010. The rest of the lines were acquired in 2001.
The Lofoten-Vesterålen region, comprising Nordland VI, Nordland VII and Troms II blocks, is almost unexplored, with only two wells. However, there has been significant exploration activity south of this area, culminating in the discovery of giant fields such as Aasgard, Victoria, Norne, Linerle and Lange. These fields produce mainly from Jurassic fault blocks related to the Jurassic/Cretaceous rifting of the Norwegian Continental Margin.
The Aasgard field, which lies in water depths of 240 to 300m, contains estimated recoverable reserves of 1.12 Bbo and 6.5 Tcfg. The structure is a rotated fault block with reservoirs in the Jurassic Garn, IIe, Tofte, Tilje and Åre formations. The Heidrun field also contains reserves in Early-Middle Jurassic sandstone reservoirs of the Garn, IIe, and Åre formations, estimated at about 1.1 Bbo and 1.4 Tcfg. The Norne field, located 80 km north of Heidrun, in blocks 6608/10 and 6508/1 of the Nordland II area, was discovered in 1992 by Statoil and is the most northerly producing field in the Norwegian Sea. It contains estimated recoverable reserve of 616 MMbo and 388 Bcfg in Jurassic sandstones, with oil reservoired in the IIe and Tofte Formations and gas in the Not Formation.
To the south and south-west of the Lofoten Islands blocks there has also been a number of recent significant gas discoveries, including Hvitveis, Luva, Asterix and Stetind. These are mainly located within structurally/stratigraphically trapped Cretaceous and Palaeocene fan reservoirs.
The Hvitveis gas discovery was made by ExxonMobil in 2003, with well 6706/6-1, which was drilled to test the hydrocarbon potential of the Cretaceous Hvitveis prospect at the Naglfar Dome in the northern part of the Voring Basin. This seismically defined reservoir interval was interpreted to be a Nise formation equivalent. Luva, found by BP in 1997 in 1,262m water depth on the Nyk High in the Norwegian Sea, had 70m gas in good quality reservoir with a net-gross ratio of 77%, confirming the reservoir properties of the Nise Formation.
The Stetind gas discovery is located further to the south-west of the Lofoten-Vesterålen region. It was found by Norsk Hydro in 2005, targeting a Lysing Formation prospect on the western flank of the Fles North rotated fault block structure in the Vøring Basin. Gas was encountered in 70m thick turbidite sands and further shows were recorded in the Nise Formation.
So, while the area immediately round the Lofoten Islands is relatively unexplored, significant gas deposits have been found within the area to the south-west.
The Lofoten and Vesterålen region lies between the northern end of the Vøring Basin and the southern edge of the Barents Sea Margin. The area is made up of a series of north-east to south-west trending basins and ridges with a complex tectonic history spanning the Mesozoic to Cenozoic. The basement ridges include the Lofoten Ridge, Utrøst Ridge, and Nordland Ridge. The Ribban and Harstad Basins lie west and north respectively of the Lofoten Ridge whilst the Traena and Nagrind Basins lie farther south west of both the Lofoten and Utrøst Ridges. These sedimentary basins, which contain thick sequences of Mesozoic strata, are thought to have the ingredients for an excellent petroleum system in this part of the Norwegian Continental Margin.
Potential reservoir rocks in the region may consist of Triassic, Jurassic, Cretaceous and Palaeogene age sandstones.
As noted above, Jurassic reservoir rocks have been proved to the south-west of the Lofoten-Vesterålen area. The reservoir interval in the Jurassic is mainly Early-Middle Jurassic and is made up of heterolithic successions consisting of a series of extensively developed regressive-transgressive, fluvial, tide-and wave-influenced, tide-dominated and marginal marine sandstone wedges of the Åre, Tilje, IIe and Garn Formations. These sandstone wedges are interpreted to fill in the accommodation space created during rifting. They are separated by marine mudstones of the Lower and Upper Ror, the Not and the Melke Formations respectively.
An example of a similar play is found in the in the Heidrun field, about 200km south-west of the islands, where hydrocarbons are trapped within tilted fault blocks containing Jurassic sandstones. The seal is provided by intra-formational shales and overlying Lower Cretaceous shales. This system of tilted Jurassic fault blocks and associated shallow marine deposits has also been interpreted in the Lofoten-region.
Seismic brings better understanding
The key challenges to good imaging in this region include the presence of a hard sea floor, which sets up several multiple sea floor reflections, distorting clear discrimination between multiples and primary reflections at the affected levels. The Mesozoic fault blocks are hard to visualise and pre-stack migration and careful velocity analyses have been used to enhance the images overall.
As a result fault blocks are clearly imaged, and a boost in frequencies has made it possible to recognise shallow (Upper Cretaceous and Tertiary) clastic fan deposits as can be seen from the examples shown here.
With the combination of the newly reprocessed data and the 2001 data, new interpretation will lead to a better understanding of the geology of this little known area.