geologyofthenorwegiancontinentalshelf · petroleum geology remains enigmatic – partly due to...

Chapter 22

Geology of the Norwegian Continental Shelf

Jan Inge Faleide, Knut Bjørlykke, and Roy H. Gabrielsen

22.1 Introduction

In the preceding chapters we have included only a fewregional examples and case studies because of spacelimitations. The present chapter will, however, pro-vide some examples. The North Sea and other partsof the Norwegian continental shelf contain several dif-ferent petroleum provinces which can illustrate someof the general principles of petroleum geology andgeophysics. The geological evolution of these sedi-mentary basins provides a necessary background tounderstand the distribution of source rocks and the tim-ing of petroleum migration. The structural history ofrifted basins, passive margins and also uplifted basinssuch as the Barents Sea is critical to the trapping ofoil and gas. These basins are very well documentedby seismic and well data. The Norwegian PetroleumDirectorate also provide information about explorationand production on their web pages (www.npd.no).

22.1.1 Regional Geological Setting

The Norwegian continental shelf comprises three mainprovinces (Figs. 22.1 and 22.2):

• North Sea• Mid-Norwegian continental margin• Western Barents Sea

J.I. Faleide (�)Department of Geosciences, University of Oslo, Oslo, Norwaye-mail: [email protected]

Prior to continental break-up and the onset ofseafloor spreading in the Norwegian-Greenland Sea(Fig. 22.3) these provinces were part of a muchlarger epicontinental sea lying between the continen-tal masses of Fennoscandia, Svalbard and Greenland.Thus there are many similarities in the stratigraphy(Fig. 22.4) and geological evolution of the variousprovinces but also important differences – in particularduring Cretaceous-Cenozoic times.

The sedimentary basins at the conjugate continentalmargins off Norway and Greenland and the adja-cent shallow seas, the North Sea and the westernBarents Sea, developed as a result of a series of post-Caledonian rift episodes until early Cenozoic time,when complete continental separation took place. TheNorwegian Margin comprises the mainly rifted vol-canic margin offshore mid-Norway (62–70◦N) and themainly sheared margin along the western Barents Seaand Svalbard (70–82◦) (Fig. 22.2). Physiographically,the Norwegian margin consists of a continental shelfand slope that vary considerably in width and morphol-ogy (Fig. 22.1).

22.1.2 Petroleum Exploration

When gas was found onshore in the Groningen Fieldin the Netherlands in 1958 it soon generated inter-est in the North Sea itself. However, it was difficultto say what lay off the Norwegian coast beneath theQuaternary sediments. Moraine material which hadbeen recovered from the seabed during marine geo-logical investigations in the 1950s contained clasts thatalmost without exception were basement rocks.

467K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,DOI 10.1007/978-3-642-02332-3_22, © Springer-Verlag Berlin Heidelberg 2010

468 J.I. Faleide et al.

–5° 0° 5° 10° 15° 20°

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75°

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NorthSea

Barents Sea

Norwegian-Greenland

Sea

VP

EB

Fig. 22.1 Regional setting (bathymetry/topography) of theNorwegian Continental Shelf and adjacent areas. EB = EurasiaBasin, VP = Vøring Plateau

The pre-Quaternary geology of the NorwegianContinental Shelf was virtually unknown prior to theearly 1960s, as there had been no seismic measure-ments and few other indications of what was beneaththe cover of Quaternary, partly glacial, sediments.Before any seismic or borehole data were obtained inthe North Sea, one had to extrapolate from the geol-ogy of the surrounding land areas. The Mesozoic rocksof northeast England and Denmark were assumed tocontinue beneath at least parts of the North Sea, andCarboniferous coal seams and Permian sediments werealready well known in northeastern England. There

were also a few indications that younger sedimentaryrocks were present beyond the Norwegian coast. Themoraines on Jæren contained small amounts of prob-ably Cretaceous material, there were erratic blocks ofJurassic age on Froan and pieces of coal from Tun inVerran, off Trøndelag. Much farther north, the fairlythin Jurassic and Cretaceous sequences on Andøya hadlong been known. It was difficult, though, to knowhow much reliance could be placed on these scatteredobservations; not least, one had no idea of how thicksuch Mesozoic sediments would be.

A confirmation of the presence of thick sedimentsequences on the shelf was obtained in seismic refrac-tion studies in the Barents Sea and the central NorthSea in the late 1950s, indicating accumulations in theorder of several kilometres. Seismic refraction studiessupported by potential field data in the Skagerrak, per-formed in 1963–1964, confirmed the presence of morethan 3 km of sediments also in this area. The early seis-mic data that was shot in the North Sea had very poorquality and it was not until drilling on the Norwegianshelf commenced in 1966 that it was possible to havean informed opinion on the potential for oil and gas.

The Norwegian continental shelf, extending fromthe baseline to the limit approved by the UNCommission on the Limits of the Continental Shelf,amounts to 2.2 million km2. About half of this acreagehas bedrock in which petroleum may be found, andhalf of that has been opened for petroleum explo-ration. The first licences were awarded in 1965 andthe first commercial discovery in the Norwegian partof the North Sea (block 2/4) was made in 1969.The motivation for this drilling was to test a possiblePermian (Rotliegendes sandstone) reservoir. Instead,however, an unexpected discovery of hydrocarbonswas made in a domal structure in Upper CretaceousChalk. Continued exploration on this play type resultedin a number of discoveries in the Ekofisk area of thecentral North Sea.

Large fields in the North Sea, such as Sleipner,Statfjord and Gullfaks, were discovered during the first10 years after the Ekofisk Field was proven, and allfour are still in production. Most of the other largefields on the shelf were found between 1979 and 1984(see Sect. 22.2.3 for more details on the North Seaexploration history).

Following the exploration success in Norwegianwaters south of 62◦ N, the areas to the north wereopened for exploration in 1979. Results and experiencegained from the North Sea were utilised when areas

22 Geology of the Norwegian Continental Shelf 469

Late Cretaceous -PalaeoceneLate Jurassic -Early Cretaceous

Late Palaeozoic–5° 0° 5° 10° 15° 20°

EurasiaBasin

JMMC

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Finland

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70°

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Fig. 22.2 Main structuralelements of the NorwegianContinental Shelf andadjacent areas related todifferent rift phases affectingthe NE Atlantic region(modified/updated fromFaleide et al. 2008). For moredetails see close-up maps inFigs. 22.5, 22.10 and 22.14.JMMC = Jan Mayenmicro-continent

further north were opened, and areas with a simi-lar geological setting to the northern North Sea weretested first (Haltenbanken offshore Mid-Norway andTromsøflaket in the southwestern Barents Sea). Theexploration histories of the areas north of 62◦ N aremore complex, and even today important aspects of thepetroleum geology remains enigmatic – partly due togeological complexities.

Large parts of the Norwegian continental shelf arenow well explored. Better knowledge of the geol-ogy and advances in technology lead to a higherdiscovery rate, but the finds have mostly been small.Consequently, the growth in resources has beenrelatively low for the last 20 years. The largest dis-covery made in this period was Ormen Lange in the

Norwegian Sea in 1997, but the growth in resourcessince 1997 does not equal the production in the sameperiod.

Technological advances have led to development ofdiscoveries in areas of great water depth and far fromshore. The technological advances have also made ahigher proportion of the resources profitable to recover.On average, fields on the Norwegian shelf have arecovery factor of 46% for oil. This is high com-pared with oil provinces in other parts of the world.Continuous research and development of technologyare required to raise this further.

Significant volumes remain to be found and pro-duced on the Norwegian continental shelf. Only athird of the total resources have so far been produced,


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Eurekan Orogeny(not restored)

(a) Present

JMMC

JMM

C

(b) Chron 13 (c. 33 Ma) (c) Breakup (c. 55 Ma)

Fig. 22.3 Plate tectonic reconstructions of the NE Atlantic.(a) Present (same as Fig. 22.2), (b) Reconstruction to chron13 (c. 33 Ma) using the rotation pole of Gaina et al. 2009,

(c) Reconstruction to time of breakup (ca. 55 Ma) based onunpublished work of Faleide et al.

and an estimated quarter of them have still not beendiscovered. It may still be possible to make largediscoveries in less explored areas, such as in deepwaters in the Norwegian Sea, in the Barents Sea andin areas that are not yet open. Oil production reachedits peak in 2000–2001, but gas production is stillincreasing. For more facts about the petroleum activ-ity on the Norwegian shelf see http://www.npd.no/en/Publications/Resource-Reports/2009/.

22.2 North Sea

22.2.1 Structure

The North Sea is an example of an intracratonicbasin; that is to say a basin which lies on continentalcrust. The prerequisite for forming major sedimen-tary basins on continental crust is that the crust (andmantle lithosphere) is thinned, resulting in subsidenceto maintain isostatic equilibrium. The North Sea hasbeen subjected to periods of stretching/thinning and

subsidence during late Carboniferous, Permian-EarlyTriassic and Late Jurassic times. Each rift phase wasfollowed by a thermal cooling stage, characterised byregional subsidence in the basin areas.

The Northern North Sea province is dominated bythe Viking Graben, which continues into the SognGraben towards the north (Fig. 22.5). These grabensare flanked by the East Shetland Basin and the TampenSpur to the west, and the Horda Platform to the east(Figs. 22.6 and 22.7). These are Jurassic-Cretaceousfeatures, and the main crustal thinning took place inthe late Middle to Late Jurassic, followed by thermalsubsidence and sediment loading in the Cretaceous.However, the Viking Graben and its margins are under-lain by an older major rift basin of assumed Permian-Early Triassic age. The axis of this rift system isthought to lie beneath the present Horda Platform. It isbounded by the East Shetland Platform in the west andthe Øygarden Fault Zone in the east. Structures withinthis area are characterised by large rotated fault blockswith sedimentary basins in asymmetric half-grabensassociated with lithospheric extension and crustalthinning (Fig. 22.6). The area was presumably also


MandalTau

Sandnes

Bryne Sleipner

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BurtonCookDrake

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rent

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jord

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cree

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Kviting

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MaastCampSantConiTurCen

AlbAptBarrHautValangRyaz

VolgKimmOxfCallBathBajocAalenToarcPliencSinemHettRhætNorCarnLadinAnisScythTatarKazan

KungArtinSakmAsselGzelKasimMoscovBashSerpukViseTour

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tiary

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ous

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assi

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amSt

eph

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SouthernNorth Sea

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SouthernViking Graben

TampenSpur

Trøndelag Platform

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SvalbardNordkapp B.,Bjarmeland P.

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Continental clastics, unspecified

Continental clastics, mainly sands

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Halite

Marginal evaporite deposits, sabkha

Coastal-, delta- and flood plain deposits

Marginal marine and shallow marine deposits, mainly sands

Shallow marine deposits, mainly shales

Deeper marine deposits, mainly shales

Shallow marine carbonates

Volcanic deposits

Carbonaceous shalesClastic mixing in carbonates,sands in shales

Spiculite

Coal

Coarse clastics

Source rocks

Chalk Group

Fig. 22.4 Lithostratigraphic summary for the Norwegian Continental Shelf and adjacent areas (modified from Brekke et al. 2001)

strongly affected by post-orogenic (post-Caledonian)extension in Middle to Late Devonian times.

The Norwegian Central North Sea Province encom-passes the northwestern part of the Central Graben(Figs. 22.5, 22.8 and 22.9), which is mainly a Jurassic-Cretaceous structure, and its northeastern margin. Thestrata of the Central Graben area were affected byhalokinesis already in the Triassic, and major structur-ing accordingly occurred already in pre-Jurassic times.However, salt movements have locally continued intothe Tertiary. Jurassic rifting and generation of large,rotated fault blocks resulted in extreme erosion inplaces. The complex structural pattern established dur-ing this stage was further complicated by Cretaceous

inversion. The Norwegian-Danish Basin, east of theCentral Graben, also contains many salt structures buthas not been affected by significant rifting (Figs. 22.8and 22.9).

The geometric shape of the sedimentary basin isinfluenced by the structure in the underlying rocks(basement) and by the thickness of the continen-tal crust. Most of the North Sea is underlain bya Caledonian basement. Long-lived zones of weak-ness inherited from the Caledonian Orogeny playeda role in the later evolution of the North Seabasin. The southeastern North Sea and Skagerrak areunderlain by a Precambrian basement covered by aLower Palaeozoic sedimentary succession. In the south


OG

SkG

HP

VG

ESP

NDB

STZCG

Late Cretaceous -Palaeocene

Late Jurassic -Early Cretaceous

Late Palaeozoic

60

58

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SB

SH

UH

ÅGWG

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FB

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SGMgBMrB

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Fig. 22.5 Main structural elements in the North Sea and adja-cent areas (close-up of Fig. 22.2 – modified/updated fromFaleide et al. 2008). Location of interpreted regional profiles(black), crustal transects (red) and seismic examples (blue)shown in Figs. 22.4–22.9. CG = Central Graben, ESB =East Shetland Basin, ESP = East Shetland Platform, HG =Horn Graben, HP = Horda Platform, MgB = Magnus Basin,

MNSH = Mid North Sea High, MrB = Marulk Basin, NDB= Norwegian-Danish Basin, OG = Oslo Graben, RFH =Ringkøbing-Fyn High, SB = Stord Basin, SG = Sogn Graben,SH = Sele High, SkG = Skagerrak Graben, STZ = Sorgenfrei-Tornquist Zone, TS = Tampen Spur, UH = Utsira High, VG =Viking Graben, WG = Witchground Graben, ÅG = Åsta Graben

the North Sea basin is bounded by the Hercynian(Variscan) mountain range which runs E-W throughGermany, northern France and southwest England.The contraction occurred during the Carbonifererous-Permian. Uplift in this area resulted in vast amounts ofsediment being deposited in the area to the north andthis initiated the formation of the North Sea basin.

The offshore part of the Oslo Rift in the Skagerrakis characterised by NE-SW striking half-grabens(Fig. 22.9). Cross-sections across the Skagerrak showtilted half-grabens filled with down-faulted UpperCarboniferous-Lower Permian and Lower Palaeozoicstrata that are unconformably overlain by Triassicsedimentary rocks. Although a thick succession ofLower Palaeozoic strata has been preserved in theSkagerrak Graben, a substantial part of this pre-riftunit has been eroded in the central part of the rift.From strata preserved in the onshore Oslo Graben we

know that the Lower Palaeozoic succession consistsmainly of Cambrian-Ordovician and Silurian platformseries and Upper Silurian-lowermost Devonian clas-tic sedimentary rocks, with the latter being depositedin the Caledonian foreland basin. In the western partof the Skagerrak, the late Palaeozoic structures ofthe Oslo Rift link up with the Sorgenfrei-TornquistZone (also termed the Fennoscandian Border Zonein Denmark). The Sorgenfrei-Tornquist Zone strikesin a NW-SE direction from the western Skagerrakacross northern Jutland and the Kattegat into Scania(Fig. 22.2). If the effects of post-Permian structur-ing along the Sorgenfrei-Tornquist Zone are restored,late Palaeozoic rift structures similar to those seenin the Oslo Rift are observed. The major NW-SEtrending faults are associated with dextral strike-slipmovements during Late Carboniferous-Early Permiantimes.


E. ShetlandBasin

VikingGraben

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ØygardenFault Zone

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Crystalline basement

High velocity lower crustal body

Fig. 22.6 Interpreted regional deep seismic line and crustal transect across the northern North Sea (modified from Christianssonet al. 2000). See Fig. 22.5 for location

22.2.2 Stratigraphy/Evolution

A stratigraphic summary for the North Sea is given inFig. 22.4.

22.2.2.1 Devonian

The Caledonian Orogeny led to uplift and formationof a major mountain chain along western Scandinaviaand Scotland, East Greenland and a southern branchinto Poland. Thick red continental sediments, whichin Britain are known as Old Red Sandstone, weredeposited in Devonian time in response to the exten-sional collapse of the Caledonides. On the Norwegian

mainland we have Devonian basins in western Norway(Hornelen, Håsteinen, Kvamshesten, Solund) whichare filled with thick conglomeratic sediments. Thereare also Devonian deposits further north, for exampleon the island of Smøla. The Devonian sandstones ofwestern Norway are metamorphosed and can not bereservoir rocks, but they are somewhat more porous insouthern England.

Although Devonian sediments in the northern NorthSea have been reached in only a few wells, thereare reasons to believe that Devonian sediments arepresent regionally in the deeper parts of the pre-Triassic half-grabens beneath the Horda Platform,Viking Graben and East Shetland Basin. The pres-ence of Upper Palaeozoic rocks, of both Devonian


1

0

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4

525 km

s tw

tNW SEHorda PlatformViking GrabenTampen SpurEast Shetland Basin

2

Fig. 22.7 Regional seismic line across the northern North Sea (courtesy Fugro and TGS). See Fig. 22.5 for location and Fig. 22.6for stratigraphic information

and Lower Permian age (Rotliegendes), has also beenconfirmed by drilling on the East Shetland Platform.Seismic data reveal large sedimentary basins beneaththe platform, thought to contain Upper Palaeozoic(Devonian-Carboniferous) rocks. In this context it isimportant to note that oil is produced from Devoniansandstones in the Embla Field of the Central Graben.

The Caledonian plate movement changed from sub-duction to Late Devonian lateral (strike-slip) move-ment between Greenland and Fennoscandia, includingalong the Great Glen Fault. In Scotland this wasmarked by active volcanism, especially in the MidlandValley Rift.

22.2.2.2 Carboniferous

Following the markedly dry climate which prevailedthrough Devonian time in the North Sea region, theCarboniferous period gradually became more humid.Northwest Europe moved northwards from the arid

belt of the southern hemisphere into the humid equa-torial belt. This provided the basis for all the coaldeposits. There was also a marked transgression overlarge areas. The strike-slip movements along theGreenland/Fennoscandia plate boundary ceased at thetransition from Devonian to Carboniferous, and eversince this has been an area of diverging plate move-ment and rift formation until final continental break-upand onset of seafloor spreading in earliest Eocene time.There was rifting in the Midland Valley of Scotland,continuing along the Forth Approahes into the WitchGround Graben which was a volcanic centre in theNorth Sea. Black mud deposited in these graben struc-tures forms source rocks for oil.

There was no rifting in England AT that time andthe Carboniferous Limestone was deposited as a shelfcarbonate during the Early Carboniferous. North ofthe carbonate platform and in much of the North Sea,deeper water shales predominated, with some carbon-ate. Sandstones are encountered higher up in the LowerCarboniferous and these can be important reservoir


1

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4

525 km

s tw

tSW NENorwegian-Danish BasinCentral GrabenMid North Sea High

3

Fig. 22.8 Regional seismic line across the central North Sea (courtesy Fugro and TGS). See Fig. 22.5 for location

rocks in the southern North Sea (Fell Sandstone).At the Lower/Upper Carboniferous boundary is theYoredale Formation, consisting of cyclic deposits ofmarine carbonates and shales, and fluvial sandstones.These have been interpreted as the result of eustaticchanges in sea level due to glaciations on the southernhemisphere, but local tectonics and even sedimentationprocesses can also produce cyclicity.

The Hercynian (Variscan) mountain range formedalong the subduction zone through Germany and north-ern France close to south England. It was upliftedas a marked topographic feature, and at its foot(the Variscan foredeep) major sedimentary units weredeposited, derived from erosion in the mountains.Thick beds of coal developed from the swamp areason the deltas. In Britain these sandy delta depositsare known as the Millstone Grit. Contemporary coaldeposits are found in the southern North Sea and inthe Netherlands, and it is these which provided thesource of the gas in this region. Fluvial sandstonesbetween the coal seams can be reservoirs both for

oil and gas. Black marine shales were also depositedin the mid-Carboniferous in the southern part of theNorth Sea. In the Oslo Region there are thin sand-stones and carbonate sediments of the Asker Groupyielding Upper Carboniferous fossils, demonstrating aconnection with the North Sea basin.

22.2.2.3 Permian

During the early Permian, uplift of the Hercynian(Variscan) mountain range continued and sedimentarybasins developed in front of it in the southern NorthSea and within subsided areas of the range itself. Atthe same time, northwest Europe was pushed farthernorthwards from the equator, into the dry belt of thenorthern hemisphere. The high mountain range to thesouth also contributed to severe aridity in the North Seabasin and most of northwest Europe. This region wasthen located in the middle of a large continent in a sim-ilar position to the dry areas to the north of the presentHimalaya.


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Fig. 22.9 Regional profiles across the SE North Sea, Skagerrak and Kattegat (modified from Heeremans and Faleide 2004).See Fig. 22.5 for location

The latest Carboniferous-Permian extension in theNorth Sea region (Fig. 22.2) was linked to the Variscanfold belt by strike-slip movements on a series of NW-SE trending zones of weakness (among others theSorgenfrei-Tornquist Zone). The rifting was associatedwith widespread and massive igneous activity. This iswell known from the onshore part of the Oslo Rift(Oslo Graben) but similar rocks, both extrusives andintrusives, are present in the subsurface offshore (e.g.Skagerrak Graben, Sorgenfrei-Tornquist Zone, CentralGraben; Fig. 22.9).

Sedimentation in the North Sea region was dom-inated by two E-W aligned basins separated by theMid-North Sea and Ringkøbing–Fyn highs. The basinswere infilled with Rotliegend continental sediments,most rapidly in the southern basin which was near-est the mountains and subsided fastest. Alluvial fansand aeolian dune sand accumulated most of the sedi-ment supply from the south. These types of sedimentare well-exposed in southwest England near Exmouthbut also in northeast England. In the North Sea basinthe Upper Rotliegend contains an extensive, mostlyaeolian, sandstone called the Auk Formation.

The climate behind the mountain range was dry,and a marine evaporite basin eventually developed,possibly with a narrow passage through the VikingGraben to the open ocean in the north to a seawaybetween Norway and Greenland. There was, however,an important connection eastwards through Poland.The deposition of the Zechstein salt commenced inthe southern Permian basin and spread across mostof the North Sea basin during Late Permian time butis absent in the northern North Sea. Sabkha depositsaccumulated in areas marginal to the evaporite basins.

22.2.2.4 Triassic

Rifting continued into earliest Triassic time and theTriassic to Middle Jurassic succession reflects a pat-tern of repeated outbuilding of clastic wedges fromthe Norwegian and East Shetland hinterlands withina generally evolving post-rift basin. There was stillconsiderable sediment supply from the Variscan moun-tains to the south and there is also evidence of anuplift of Scandinavia. A broadly similar geometry of


the megasequences in both continental Triassic andmarine Jurassic successions was related to subsidence-rate variations. Differential subsidence across faultsthroughout Triassic time has also been reported. TheØygarden Fault Zone, forming the eastern margin ofthe Permo-Triassic basin, was active throughout mostof the time interval. The sedimentation rate was in mostcases high enough to keep up with the subsidence,resulting in a rather flat landscape with gently flowingrivers. If the sediment supply had been less, these riftbasins would have been marine. Along the rift struc-ture along the middle of the North Sea the thickness ofthe Triassic sediments may exceed 5 km. The under-lying Permian salt started to form diapirs and Triassicsediment was eroded, or not deposited, at the top of thestructures.

The climate in northwest Europe during the Triassicwas still arid, and continental red beds continuedto be predominant. In England these are referred toas the New Red Sandstone because they are simi-lar to the Devonian continental sediments (Old RedSandstone). The equivalent facies in Germany is theBunter sandstone of Lower Triassic age (sandstonesof the Brent Group and Statfjord Formation). In southEngland east of Exmouth there are also good exposuresof Triassic deposits with sandstones and conglom-erates (Sherwood Sandstone Group) and mudstones(Mercia Mudstone Group). The Sherwood Sandstoneis a very important groundwater reservoir in large areasof England. In Southern Norway north of Hamar, theBrumundal sandstone is an outlier of a more extensivecover of Permian red bed facies.

In the Upper Triassic we find carbonates (Muschel-kalk) and salt deposits (Keuper salt) in the southernpart of the North Sea. In the central and northern areas,however, continental clastic sedimentation continuedright up to the end of the Triassic (Rhaetian). Sabkhaenvironments fringed the evaporite basins and caliche,that is carbonate precipitation in soil profiles, is typical.Towards the end of the Triassic the climate became lessarid with more normal fluvial sedimentation and grad-ually also marine sedimentation when the StatfjordFormation was deposited.

22.2.2.5 Jurassic

The transition from Triassic to Jurassic approximatelycoincides with a change from continental to shallow

marine depositional environments. The climate alsogradually became more humid in the Jurassic asnorthwest Europe was pushed northward out of thearid belt at about 30◦ N. A transgression in theEarly Jurassic (Lias) led to the accumulation of blackshales over large parts of NW Europe and they areexposed in Yorkshire in northeast England and Dorsetin southern England. This is because a transgres-sion over an uneven land surface results in a shallowsea with poor vertical circulation in the water. TheUpper Lias (Toarcian) in particular contains goodsource rocks for oil and gas in southern parts of theNorth Sea. In Britain and much of the North Sea theLias deposits consist of black shales with thin car-bonate and sand beds. Iron-rich layers with siderite(FeCO3) and chamosite (Fe-chlorite) are common.Beds rich in these two minerals were the basis foriron mines which were especially important duringthe industrial revolution in England, Germany andFrance.

In the northern part of the North Sea fluvial andpartly marine sandstones of Lower Jurassic age (Lundeand Statfjord formations) are important reservoir rocksin the Viking Graben. The Statfjord Formation issucceeded by the Dunlin Group, which is a darkmarine shale but normally without enough organiccontent to become a significant source rock. Thenfollows the Brent Group sandstone, a prograding deltasequence which forms the main reservoir rock in thenorthern North Sea. This sandstone was depositedin a delta that drained the central part of the NorthSea towards the marine embayment to the north,between the Shetland and Horda platforms. It wassourced from an uplifted area in the south associatedwith Middle Jurassic (Bajocian-Bathonian) volcanicactivity. The volcanic centre was located south of theViking Graben and east of Scotland (Moray Firth). Onthe other side of the uplifted area, the sediments weretransported southwards, exemplified in good coastalsections in Yorkshire.

The lower part of the Brent Group consists ofupward-coarsening beds of mica-rich sandstones thatrepresent prograding delta deposits, especially dis-tributary mouth bars. This is the Etive and Rannochformations. The middle part is the Ness Formation,comprised mostly of fluvial delta facies with chan-nels, crevasse sand deposits, lagoonal deposits and coalbeds. These delta top facies were deposited during anorthward progradation of the Brent Delta. At the top


of the Brent Group, the Tabert Formation is a gener-ally well-sorted sandstone formed by the reworkingof deltaic deposits during a transgression. The sedi-ment supply from the south was no longer able to keepup with the basin subsidence, resulting in a gradualdrowning of the Brent Delta. Each of these delta facieshas its characteristic reservoir properties, which are afunction both of primary sorting and mineralogy andsubsequent diagenetic modification.

In the Late Jurassic, volcanism was very muchreduced and the areas within and surrounding the riftsystems subsided in response to lower geothermal gra-dients. At the same time, normal faulting along theViking Graben led to the rotation of basement blocksand their overlying sediments. The shoulders of thetilted fault blocks were exposed to erosion, remov-ing Lower-Middle Jurassic and locally even UpperTriassic strata. The Late Jurassic (Oxfordian) trans-gression covered the Viking Graben with a thick drapeof clayey sediments of the Heather Formation, whilethe coarser clastics (sand) were deposited as turbiditesand in deltas along the basin margins. Some of thedeltas appear to have been controlled by the samestructures that have determined the location of thefjords in western Norway.

The uppermost Jurassic Kimmeridge ClayFormation is transgressive and often forms a sev-eral hundred metres thick rich source rock which onthe Norwegian side is called the Draupne Formation.The rift topography produced numerous, locallyoverdeepened, basins with poor bottom water circula-tion. Only a relatively small proportion of the organicproduction was oxidised while the sedimentation ratewas fairly high. The organic-rich shales of the UpperJurassic are thus the prime source rock in the NorthSea, and provided the main petroleum source in boththe Statfjord and Ekofisk areas. The thickness of theUpper Jurassic sediments along the rift axis may reach3,000 m. The deposition of organic-rich shales contin-ued into the Early Cretaceous in some of the basins.The edges of the rotated blocks suffered erosion anda few were not buried until the Late Cretaceous. Themajority of the faults die out before the Cretaceousbut a few continue up into younger beds. The riftingresulted in rotated fault blocks containing sandstonereservoirs of Lower and Middle Jurassic age (sand-stones of the Brent Group and Statfjord Formation).Small fan deltas developed along the rift and alsodeepwater sandstones including debris flows. Some

of these are good reservoir rocks occuring within theUpper Jurassic source rocks (see Chap. 12).

22.2.2.6 Cretaceous

The last phase of rifting in the North Sea in theLate Jurassic was followed by a major transgression,but the uplifted rift stuctures remained dry and wereislands for most of the Early Cretaceous. There is amajor unconformity between the Cretaceous and theJurassic except in the deep parts of the rifts where theremay have been continuous sedimentation (Figs. 22.6and 22.7). The Base Cretaceous Unconformity is verywell marked on most seismic sections from the NorthSea. Fault activity diminished during the Cretaceous,and the Cretaceous subsidence was due primarily tocrustal cooling after the Jurassic rifting. The transi-tion from syn- to post-rift configuration was stronglydiachronous, suggesting that the thermal state of thesystem was not homogeneous at the onset of thepost-rift stage.

Three stages can be identified in the post-riftCretaceous development of the northern North Sea:(1) The incipient post-rift stage (Ryazanian–latestAlbian) was characterised by different degrees of sub-sidence. The major structural features inherited fromthe syn-rift basin (e.g. crests of rotated fault blocks,relay ramps and sub-platforms) had a strong influenceon the basin configuration and hence the sediment dis-tribution. (2) In the middle stage (Cenomanian–lateTuronian) the internal basin relief became graduallydrowned by sediments. This is typical for basins wheresediment supply outpaces or balances subsidence, aswas the case in the northern North Sea. Thus, theinfluence of the syn-rift basin topography becamesubordinate to the subsidence pattern determined bythe crustal thinning profile, which in turn relied onthermal contraction and isostatic/elastic response tosediment loading. (3) The mature post-rift stage (earlyConiacian–early Palaeocene) was characterised by theevolution into a wide, saucer-shaped basin where thesyn-rift features were finally erased. Since thermalequilibrium was reached at this stage, subsidenceceased, and the pattern of basin filling became, to alarger degree, dependent on extra-basinal processes(see Chap. 12).

The Lower Cretaceous shales are black, but onlylocally do they form good source rocks. Conditions


subsequently became more oxidising as bottom cir-culation improved. In the Early Cretaceous grey toreddish oxidised shales were also deposited, whichbecome increasingly calcareous upwards. The LowerCretaceous shales (Cromer Knoll Formation) are shal-low to deep marine mudstones with little sand.

In the Late Cretaceous the sea attained its trans-gressive maximum and clastic sedimentation almostceased across large areas of northwest Europe. Partsof Scandinavia were probably also covered by theCretaceous sea. Sedimentation was dominated byplanktonic carbonate algae (coccolithoporids) whichformed a lime mud, the main component of Chalk,though it also includes some foraminifera and bry-ozoa. The main development of the Chalk was in theCampanian and Maastrichtian, but sedimentation con-tinued up into the Danian of the Palaeocene, for themost part through resedimentation of earlier Chalkdeposits which had been uplifted along the centralhighs. In the Viking Graben the carbonate contentdiminishes northwards and we do not have pure lime-stone (Chalk) facies like that in the southern andcentral part of the North Sea. Instead, shales pre-dominate, though often with a significant carbonatecontent.

At the close of the Cretaceous and beginning ofthe Tertiary, compressive movements were felt fromthe Alpine Orogeny to the south. Part of this move-ment was accommodated along diagonal fault zones,such as the Sorgenfrei-Tornquist Zone in Scania andthe Kattegat. The Polish Basin and parts of the NorthSea area were uplifted (inversion) and eroded.

22.2.2.7 Cenozoic

The early Cenozoic rifting, break-up and onset ofseafloor spreading in the NE Atlantic gave rise todifferential vertical movements also affecting theNorth Sea area. The sedimentary architecture andbreaks are related to tectonic uplift of surrounding clas-tic source areas, thus the offshore sedimentary recordprovides the best age constraints on the Cenozoicexhumation of the adjacent onshore areas.

Major depocentres sourced from the upliftedShetland Platform and areas along the incipient plateboundary in the NE Atlantic formed during LatePalaeocene-Early Eocene times. A local source areaalso existed in western Norway. Tectonic subsidenceaccelerated in Palaeocene time throughout the basin,

with uplifted areas to the east and west sourcing pro-grading wedges, which resulted in large depocentresclose to the basin margins. Subsidence rates outpacedsedimentation rates along the basin axis, and waterdepths in excess of 600 m are indicated. Uplift alongthe Sorgenfrei-Tornquist Zone caused erosion intotop Chalk along the southeastern flank of the NorthSea basin.

Prominent ash layers of earliest Eocene age arefound throughout the entire North Sea and also furthernorth. Onshore in Denmark the volcanoclastic sedi-ments are known as “moler”. The extensive volcan-ism was related to the opening of the North Atlanticand both the Eocene and Oligocene mudstones aredominated by smectitic clays formed from volcanicash. These smectitic mudstones are characterised bylow seismic velocities also compared to the overlyingNeogene sediments. Ash layers rich in volcanic glass(hyaloclastites or tuff) may, however, form hard lay-ers due to diagenesis. They will then be characterisedby high seismic velocities, particularly after burial tomore than 2 km depth and quartz cementation, giv-ing rise to strong seismic marker horizons (top BalderFormation).

In Eocene times progradation from the EastShetland Platform was dominant and major depocen-tres developed in the Viking Graben area, with deepwater along the basin axis. The Palaeocene and Eocenesubmarine fans were built up with turbidite sands car-ried out into the central part of the North Sea withthe Utsira High limiting their eastward extent. Partsof Fennoscandia were probably covered by sea duringMiddle-Late Eocene times.

At the Eocene-Oligocene transition, southernNorway became uplifted. This uplift, in combinationwith prograding units from both the east and west,gave rise to a shallow threshold in the northern NorthSea, separating deeper waters to the south and north.The uplift and shallowing continued into Miocene timewhen a widespread hiatus formed in the northern NorthSea, as revealed by biostratigraphic data. Miocene out-building from the north into the southeastern North Seawas massive (Fig. 22.8). Coastal progradation of theUtsira Formation in the Northern North Sea reflectsLate Miocene-Early Pliocene uplift and erosion ofmainland Norway. This relatively thick sandstone isa good aquifer and is used for injection of CO2 inthe Sleipner Field. It was still relatively warm in theEarly Pliocene, compared to the Late Pliocene whenmountain glaciation started to develop.


The Late Pliocene basin configuration was dom-inated by the progradation of thick clastic wedgesin response to uplift and glacial erosion of easternsource areas (Figs. 22.7 and 22.8). Considerable LatePliocene uplift of the eastern basin flank is docu-mented by the strong angular relationship and tilt-ing of the complete Cenozoic succession below thePleistocene unconformity. The Plio-Pleistocene sed-iments are partly glacial and partly marine repre-senting reworked glacial sediments and are typicallypoorly sorted. Such sediments compact readily andperiods of glacial advances may contribute to thecompaction.

The ice sheets advanced repeatedly out onto theshelf, but often deposited much of their debris load rel-atively near to land. Only during relatively short peri-ods did the glaciers cover most of the North Sea basin.When major ice sheets developed on the Baltic Shieldmuch of the ice flowed southwards along the valleysto the Oslo region and deepened the Oslofjord. Theice flow continued along the south coast of Norwayand eroded a trough in the Tertiary and Mesozoic sedi-ments. This is up to 700 m deep and continues up alongwestern Norway. Thick Pleistocene sedimentary fanswere deposited at the slope in front of the bathymetrictrough.

At most places on the shelf the Holocene (the last10,000 years) is represented by only a thin layer ofsilty sediment, chiefly reworked from Quaternary orolder sediments exposed on topographic highs or alongthe margins of the North Sea. Very little “new” sedi-ment has been supplied from land during the Holocene.This is because the fjords act as very efficient sed-iment traps, collecting the sediment from the rivers.Because fjords are deep but have shallow thresholds,not much clastic sediment reaches the shelf. On theseafloor there are in many areas frequent depressionswhich are typically a few tens of metres across and sev-eral metres deep. These are called pockmarks and haveformed by fluid, generally gas, seeping from deeperlayers to emerge at the seafloor.

The Cenozoic sedimentation was relatively rapidand the clayey sediments had little time to compactsufficiently to reduce the water content. Some bedstherefore display plastic folding and diapir structuresdue to the under-compacted clays, especially in theEocene. Polygonal faults are also common in thesemudstones. They form a network which are fromseveral hundred metres to 1 km across.

22.2.3 Exploration History and PetroleumProvinces/Systems

Exploration has been taking place in the North Seafor over 40 years and most of the large Norwegianfields are situated here. This is a mature part of thecontinental shelf and most of the plays are confirmed.The Norwegian part of the North Sea can be dividedinto two petroleum provinces characterised by differ-ent petroleum systems/plays: (1) the Central Grabenarea, and (2) the northern North Sea (Fig. 22.5).

The most important source rocks in the North Seaare shales in the Upper Jurassic. Thick beds of shalerich in organic matter were deposited over most of theNorth Sea area in Late Jurassic time. In the Norwegiansector, these belong to the Draupne Formation inthe Viking Group or the Mandal Formation in theTyne Group. Middle Jurassic coal is another impor-tant source rock, mainly for gas. In the southern part ofthe Norwegian sector, this coal is found in the BryneFormation in the Vestland Group. In the northern partof the North Sea, these coal seams are found in theBrent Group. Source rocks may possibly be found inCarboniferous or older strata in the southern part of theNorth Sea, but these have so far not been confirmed.

22.2.3.1 Central Graben

In the Central Graben area rifting took place dur-ing Permian-Early Triassic and Middle-Late Jurassictimes, and Zechstein salt was deposited north of theMid North Sea High (Figs. 22.8 and 22.9). Whenthis area was first drilled, the target was the Permiansandstones beneath the salt. It was by accident that oilwas discovered in the Upper Cretaceous rocks (Chalk).Ekofisk was the first field to be discovered, but nowthere is a string of fields with Upper Cretaceous reser-voirs, both on the Norwegian side and in the DanishSector.

There are other sandstone reservoirs too, such asthose of Upper Jurassic age in the Ula and Gydafields. This lead is found south of the Brent delta,which excluded the Brent Group from forming reser-voir rocks there. The area was uplifted and MiddleJurassic sediments are mostly absent. Later, oil hasbeen found in Upper Palaeozoic reservoir rocks whichhad been the original prospecting target in the area.


In the Embla Field the reservoir rock is a sandstonebelieved to be of Devonian age.

The Ekofisk Field, found in 1969, was the first largeoilfield in Europe. The Chalk in the Ekofisk reservoiris of the same type as we have onshore in Denmarkand eastern England in stratigraphic levels approach-ing the Cretaceous/Tertiary boundary (Maastrichtianand Danian). Chalk lithologies had previously beenassumed to be far too fine-grained to be reservoir rocks,and Ekofisk was the first large oilfield of this type. TheAustin Chalk in Texas is one of the few other occur-rences where Chalk forms a reservoir rock, but thefields there are fairly small by comparison.

Chalk is comprised of microscopic (0.001–0.005 mm) skeletons of planktonic algae which floatedin the surface waters. After they died they sank tothe seafloor to accumulate as a calcareous ooze. Firstwhen the scanning electron microscope (SEM) wasdeveloped was it possible to study these algae. Thecoccolithoporid skeleton consists of calcite, whichmakes the sediment more stable during diagenesis.We do not find as much solution and reprecipitationas would have been the case if the fossils had beenaragonitic. There was uplift and some erosion of theChalk in the Danian (earliest Tertiary) with a degreeof resedimentation of Chalk beds along the slopes ofthe shallower areas, i.e. slumping. These redepositedChalk sediments have proved to be the ones with thebest reservoir characteristics.

The Chalk beds were then buried by Palaeoceneand Eocene clays which sealed the Chalk and pre-vented the freshwater circulation which we otherwiseoften find in carbonate rocks along continental shelves.These clays have high contents of expanding clayminerals (smectite) and form a layer with very lowpermeability, so that porewater could not be forced outquickly enough with respect to the subsidence rate. Theeffective stresses and hence compaction have thereforebeen strongly reduced, and this is an important fac-tor explaining why the porosity can still be as highas 30–35%. Both mechanical compaction and chemi-cal pressure solution are reduced by the overpressure.The Ekofisk reservoir lies at over 3 km depth and with-out overpressure there would not have been such goodporosity. In the shallower Valhall reservoir the effectivestresses are even lower.

The Ekofisk structure is a dome-shaped anticlinalwhich has formed in response to an underlying saltdiapir (Upper Permian). The structure covers c. 50 km2

and has a closure height of 244 m. The oil column is306 m in thickness, which means there is oil belowthe structure’s lowest point (spill point). This situationrequires an additional diagenetic trap, i.e. low perme-ability coupled with the capillary forces in the Chalkhinder the oil from seeping out laterally from below thestructural trap. The porosity within the reservoir rockvaries from 30 to 35% to tighter layers with 0–20%.However, in addition to this primary porosity there isa secondary porosity provided by fissures. Fissures areparticularly important in connecting the small pores,so that the permeability increases to circa one to afew millidarcy. Without the fissuring the permeabilitywould be much lower and it would have been impossi-ble to produce the reservoir. The fissures are probablyrelated to the horizontal tension which developed whenthe beds were bent upwards above the salt structure,so that the cracks are mostly vertical. The high over-pressure also aids the fissuring process. The sourcerock for the hydrocarbons at Ekofisk is the underlyingKimmeridge Clay which attains its optimal maturity atthis depth.

Gas injection and water injection have been impor-tant for maintaining reservoir pressure and prevent-ing the fissures from closing. The reservoir rock hasalso been deliberately fissured by hydrofracturing, toincrease the permeability. The platform at Ekofiskhas sunk several metres during production, and thesubsidence has continued despite the pressure beingmaintained with water injection. According to the lawsof soil- and rock-mechanics the subsidence shouldcease when the effective stresses are not increasing,but here there has probably also been a chemical com-paction which is not simply a function of effectivestresses. This may be due to water substituting for theoil during production. There are several satellite fieldsround Ekofisk, including Cod, Albuskjell, Tor, Eldfisk,West Ekofisk and Edda.

22.2.3.2 Northern North Sea

The northern North Sea contains the majority ofthe giant fields discovered so far in the Norwegiancontinental shelf. The area is, however, still underintensive exploration, and the existing infrastructuremakes even relatively small and complex traps inter-esting. The province is extensively drilled, particularlyalong the margins of the basin, and a whole series


of trap types have been investigated. The most com-mon trap type is provided by rotated fault blocks alongboth margins of the Viking Graben (Figs. 22.6 and22.7), generated during Bathonian-Ryazian extension.The reservoirs are commonly found within sandstonesof the Rhaetian-Sinemurian Statfjord Formation, inthe Aalenian-Bathonian Brent Group, and locally inthe Pliensbachian-Toarcian Cook Formation of theDunlin Group. Sealing faults are frequently importantfor this trap type. Also Upper Jurassic (Bathonian-Kimmeridgian) sandstones provide good reservoirs,perhaps particularly in the platform areas, where faultblock rotation is moderate (e.g. the Troll Field), and inmixed structural-stratigraphic traps along fault blockcrests. Post-rift marine sands (Cretaceous and Tertiary)add to the prospectivity of the northern North Sea.Indeed, the Tertiary marine sands of the Frigg type wasthe major target of early exploration activity.

In the northern North Sea there are no Permian saltdeposits, but there are large thicknesses of Permianand Triassic sediments at depth (Fig. 22.6). Thelarge oil fields in the northern part of the NorthSea have Brent Group sandstones, and often alsoStatfjord Formation sandstones, as reservoir rocks.This applies to Statfjord, Gullfaks, Vigdis, Visund,Snorre and Oseberg. The traps consist of rotated faultblocks formed during rifting in the Late Jurassic. Theystood as islands in the sea, and much of the UpperJurassic and Lower Cretaceous is absent through non-deposition or erosion. At the top of the Gullfaksstructure we only find a thin shale from the UpperCretaceous, and the structure probably remained abovesea level until then.

The source rock is the Kimmeridge Clay fromthe Upper Jurassic (Draupne Formation), and the oilhas migrated up to the top of the fault blocks, butstratigraphically downwards from Upper to Middleor Lower Jurassic. In the Snorre Field the erosionat the top of the rotated fault blocks has reachedright down to sandstones in the Upper Triassic (LundeFormation) which is the reservoir rock together withthe Statfjord Formation. The Statfjord Field was dis-covered in 1974 and was the first giant oil field onthe Norwegian shelf after Ekofisk. The field lies inblocks 33/9 and 13/12, and extends into the BritishSector. The Statfjord Field is large, estimated to con-tain about 550 mill. tons oil and 70 · 109 m3 gas andNLG. Since the sand has such high porosity and per-meability, the production rate was very high (330,000barrels a day in 1997). It is part of the platform-like

Tampen Spur which is situated north of the VikingGraben and east of the East Shetland Basin, and whichincorporates the Statfjord Field (Figs. 22.6 and 22.7).The Tampen Spur is one of several fault blocks whichsubsided and rotated along low-angle faults in Middleto Upper Jurassic time. The reservoir rocks consist ofthe Statfjord Formation (Lower Jurassic) and the BrentGroup (Middle Jurassic).

The Statfjord Formation is comprised of fluvialsandstones which are replaced towards the top by shal-low marine deposits. The Dunlin Shale succeeds theStatfjord Formation, and has enough organic materialto be a source rock even if its contribution is verymodest compared to the Kimmeridge Clay (DraupneFormation). During the Middle Jurassic there was con-siderable volcanic activity in the central part of theNorth Sea (Rattray Formation). This volcanism andthe high geothermal gradients caused this area to beuplifted. The Triassic and Permian sandstones werethen eroded and the sediments were for the mostpart transported northwards in a fluvial system whichdebouched in the Brent delta. The delta built outacross almost the entire area between west Norway andShetland, and sediment was also supplied from theseareas to the delta. A several hundred metre thick seriescalled the Brent Group was deposited, consisting offive formations that represent different facies. First theBroom Formation was deposited, mostly on the Britishside, and the Oseberg Formation on the Norwegianside. These were deposited as local fans related tofaults along the sides of the initial rift. The RannochFormation consists of a mica-bearing sandstone whichrepresents an upwards-coarsening sequence - from“offshore” to “shoreface” - and represents the actualprogradation of the Brent delta. The mica settled outof suspension outside the fluvial channels. The EtiveFormation consists of sandstones deposited on thedelta front above wave base and is therefore bet-ter sorted. The Ness Formation represents the fluvialdelta-top facies. Here there are meandering fluvialchannels and crevasse splays. The clay and mica con-tents are higher in the levée and overbank deposits. TheNess Formation represents the maximum northwardextent of the delta. The Tarbert Formation is much bet-ter sorted, having been deposited as marine delta-frontsediments while the delta was being transgressed bythe sea. The sediment supply from the Mid-North SeaHigh was reduced as this area was gradually trans-gressed and sediment supply to the delta could thenno longer keep pace with the subsidence. Part of the


Ness Formation was probably eroded and redepositedas marine sediments as part of the Tarbert Formation.

The reservoir properties are a function of thesedepositional conditions, both on account of primarysorting and because diagenetic changes are usuallydetermined by the primary mineralogical compositionand sorting. Mica, and especially biotite, is mostlyaltered to kaolinite and then expands, blocking muchof the porosity. Carbonate-cemented beds are oftenassociated with marine environments with aragoniticfossils which have dissolved and formed cement.

The Brent delta was transgressed by the mudstonesof the Heather Formation towards the end of the LateJurassic. The main source rock for the area is theUpper Jurassic Kimmeridge Clay. It was depositedover a large part of the North Sea basin and alsoonshore present Britain. In the northern North Seait is referred to as the Draupne Formation and theMandal Formation in the Norwegian Central Graben.It may be more than 500 m thick along the rotatedfault blocks which emerged as islands. The irregularbottom topography in the basin gave poor circulationand stagnant conditions with the deposition of a goodsource rock. The crests of many of the blocks were soheavily eroded during rotation that most of the Jurassicsequence is missing from them.

Shale at less than 3.5 km depth on the shallowerparts of the structures is not sufficiently mature tohave generated oil or gas in significant quantity. Wherethe Kimmeridge Clay is downfaulted deeply enoughalong the listric faults, oil has migrated up into thetectonically overlying but stratigraphically underlyingStatfjord and Brent Groups. Most of the oil comes fromthe area north of the Snorre Field where the source rock(Draupne Formation) is buried to 4–5 km, but there isalso some from the areas south of Gullfaks. Such areasthat have generated a lot of oil are called kitchen areas.

In the Troll Field the reservoir rock is of UpperJurassic age and contains mostly gas formed in thedeepest parts of the basin (>4.5 km).

The oil began to migrate into the reservoir dur-ing early to mid-Tertiary times, when the source rock(Kimmeridge Clay) began to be mature. The oil/watercontact in the Statfjord Formation and the Brent Groupis at different levels which relate to two different pres-sure cells. The reservoir lies at 2.5–3 km depth and thesandstones have little quartz cement, such that loosesand is often recovered instead of solid core samples.There is also a danger of some sand coming up duringoil production.

During recent years, an increasing focus has beenset on the prospectivity of sands in the post-riftsequences, and some discoveries in different settingshave been made (e.g. the Agat and Grane fields).

Some of the reservoirs in the northern North Sealeak oil or gas, and the gas in particular is seen inseismic reflection data. The leakages are probablythe result of pressure having built up to the pointwhere the cap rock has fractured. This is the caseat Gullfaks (block 34/10) where fracture pressure hasbeen reached. This means that some of the oil hasmigrated up through the Cretaceous shale cap rockand into the Cenozoic succession. Parts of this oiland gas have accumulated in Palaeocene and Eocenesandstones that were deposited as turbidites emanatingfrom the Shetland Platform to the west. After depo-sition, the eastern side towards the Norwegian coastwas uplifted, imparting a gentle westward dip to thetop of these sandstone beds. They now form traps, asin the Frigg Field where the reservoir is Eocene tur-bidite sands. Heimdal, Balder and Grane fields alsocontain Palaeocene and Eocene sandstones, which aregood reservoir rocks. Eocene clays rich in smectitemake good cap rocks. These clays are lightly com-pacted, have low seismic velocities and often formsmall diapirs. During the Palaeocene a lot of sed-iment was supplied from the west, but these bedsalso dip westwards and rarely have closure towardsthe east.

In the northern North Sea a gas field (Peon) is foundin the Quaternary glacial sediments only about 160 mbelow the seafloor. This shows that compacted glacialsediments may serve as a cap rock and capable oftrapping gas.

22.3 Mid-Norwegian Shelf/Margin

22.3.1 Structure

Along strike the mid-Norwegian margin comprisesthree main segments (Møre, Vøring and Lofoten-Vesterålen), each 400–500 km long, separated bythe East Jan Mayen Fracture Zone and BivrostLineament/Transfer Zone (Fig. 22.10).

The Møre Margin is characterised by a narrowshelf and a wide/gentle slope, underlain by the wideand deep Møre Basin with its thick Cretaceous fill(Figs. 22.11 and 22.12). The inner flank of the Møre


0 5 10 15

68

66

64

62

Trøndelag Platform

VøringBasin

LofotenBasin

Norway Basin

MøreBasin MTFC

MMH

VMH

HT

NR

FH

HB

FB

RsB

TBVS

NS

HG

GR

Late Cretaceous-Palaeocene

Late Jurassic-Early Cretaceous

Late Palaeozoic7

8

9

11

13

12

10

NH

DT

UH

VH

RiB

VB

UR

VE

Fig. 22.10 Main structural elements of the mid-NorwegianMargin and adjacent areas (close-up of Fig. 22.2 – modi-fied/updated from Faleide et al. 2008). Location of interpretedregional profiles (black), crustal transects (red) and seismicexamples (blue) shown in Figs. 22.11, 22.12 and 22.13.DT = Dønna Terrace, FB = Froan Basin, FH = Frøya High,GR = Gjallar Ridge, HT = Halten Terrace, HB = Helgeland

Basin, HG = Hel Graben, MMH = Møre Marginal High,MTFC = Møre-Trøndelag Fault Complex, NH = Nyk High,NR = Nordland Ridge, NS = Någrind Syncline, RiB =Ribban Basin, RsB = Rås Basin, TB = Træna Basin, UH =Utgard High, UR = Utrøst Ridge, VE = Vøring Escarpment,VH = Vigra High, VB = Vestfjorden Basin, VMH = VøringMarginal High, VS = Vigrid Syncline

Basin is steeply dipping basinward and the crystallinecrust thins rapidly from >25 km to <10 km. Thesedimentary succession is thickest along the west-ern part of the basin, 15–16 km, decreasing land-wards to 12–13 km. The Møre Basin comprises sub-basins separated by intrabasinal highs formed duringLate Jurassic-Early Cretaceous rifting. Most of thestructural relief was filled in by mid-Cretaceous time.Sill intrusions are widespread within the Cretaceoussuccession in central and western parts of the MøreBasin, and lava flows cover the western part. Seawardof the Faeroe-Shetland Escarpment, at the MøreMarginal High, thickening of the crystalline crust andshallowing of the pre-Cretaceous sediments and topcrystalline basement occur near the continent-oceantransition.

The ∼500 km wide Vøring Margin comprises, fromsoutheast to northwest, the Trøndelag Platform, the

Halten and Dønna terraces, the Vøring Basin andthe Vøring Marginal High (Figs. 22.10, 22.11, 22.12and 22.13). The Trøndelag Platform has been largelystable since Jurassic time and includes deep basinsfilled by Triassic and Upper Palaeozoic sediments. TheVøring Basin can be divided into a series of sub-basinsand highs, mainly reflecting differential vertical move-ments during the Late Jurassic–Early Cretaceous basinevolution.

The Vøring Plateau is a distinct bathymetric feature(Fig. 22.1), and includes the Vøring Marginal Highand the Vøring Escarpment. The Vøring MarginalHigh consists of an outer part of anomalously thickoceanic crust, and a landward part of stretched conti-nental crust, covered by thick Early Eocene basalts andunderplated by mafic intrusions.

The Bivrost Lineament separates the Vøring andLofoten-Vesterålen margins, marking the northern


?

?

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10 km

10 km

10 km

NW SEUR RiB VfBLRPlio-PleistoceneGlacial Sediments

Cenozoic

Upper Cretaceous

Lower Cretaceous

Intrusion

Jurassic-Triassic-Upper Palaeozoic

Breakup Lava

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Trøndelag PlatformVøring Basin

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Vøring Marginal High

NR HB

VE SEVøring BasinVøring Marginal High Halten TerraceNW

Møre BasinMøre Marginal HighNW SE

7

8

9

10

Trøndelag Platform

Fig. 22.11 Regional profiles across the Mid-Norwegian Margin (profiles 7–8 modified from Blystad et al. 1995 and profiles 9–10modified from Tsikalas et al. 2005). See Fig. 22.10 for location and abbreviations

termination of the Vøring Plateau and the VøringMarginal High, as well as the Vøring Escarpment. TheBivrost Transfer Zone is a major boundary in terms ofmargin physiography, structure, break-up magmatismand lithosphere stretching; break-up related magma-tism is more voluminous south of it while the less mag-matic Lofoten-Vesterålen margin was more susceptibleto initial post-opening subsidence.

The Lofoten-Vesterålen margin is characterised bya narrow shelf and steep slope (Figs. 22.10, 22.11and 22.12). The sedimentary basins underneath theshelf are narrower and shallower than on the Vøringand Møre margins. Typically they form asymmet-ric half-graben structures with changes in polaritybounded by a series of basement highs along theshelf edge. Beneath the slope, break-up-related lavasmask a sedimentary basin whose detailed mappingis hampered by poor seismic imaging. The continen-tal crust on the Lofoten-Vesterålen margin appears to

have experienced only moderate pre-break-up exten-sion, contrasting with the greatly extended crust in theVøring Basin farther south.


A stratigraphic summary for the Mid-NorwegianShelf/Margin is shown in Fig. 22.4. The pre-opening,structural margin framework is dominated by the NEAtlantic-Arctic Late Jurassic–Early Cretaceous riftepisode responsible for the development of majorCretaceous basins such as the Møre and Vøring basinsoff mid-Norway, and the deep basins in the SWBarents Sea (Fig. 22.3). Prior to that, Late Palaeozoicrift basins formed between Norway and Greenlandand in the western Barents Sea along the NE-SWCaledonian trend. It has been suggested that the main


Lofoten rifted margin

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km

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Extrusives

NW VMH SEVE

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NW SEVHMMHNB

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Vøring Basin Trøndelag PlatformFB

Møre Basin

Fig. 22.12 Regional crustal transects across the Mid-Norwegian Margin (modified/updated from Faleide et al. 2008). See Fig. 22.10for location and abbreviations

Late Palaeozoic–Early Mesozoic rift episodes tookplace in mid-Carboniferous, Carboniferous–Permianand Permian–Early Triassic times. Sediment packagesassociated with these movements are poorly resolved,mainly because of overprint by younger tectonism andburial by thick sedimentary strata.

On the mid-Norwegian margin, the TrøndelagPlatform (Froan Basin) and Vestfjorden Basin recordsignificant fault activity in Permian–Early Triassictime. Permian–Triassic extension is generally poorlydated, but is best constrained onshore East Greenlandwhere a major phase of normal faulting culminated inthe mid-Permian and further block faulting took placein the Early Triassic. The later Triassic basin evolutionwas characterised by regional subsidence and depo-sition of large sediment volumes. The Lower-MiddleJurassic strata (mainly sandstones) reflect shallowmarine deposition prior to the onset of the next majorrift phase.

A shift in the extensional stress field vector toNW-SE is recorded by the prominent NE Atlantic-Arctic late Middle Jurassic–earliest Cretaceous riftepisode, an event associated with northward propa-gation of Atlantic rifting. Considerable crustal exten-sion and thinning led to the development of majorCretaceous basins off mid-Norway (Møre and Vøringbasins) and East Greenland, and in the SW BarentsSea (Harstad, Tromsø, Bjørnøya and Sørvestsnagetbasins). These basins underwent rapid differen-tial subsidence and segmentation into sub-basinsand highs.

By mid-Cretaceous time, most of the structuralrelief within the Møre and Vøring basins had beenfilled in and thick Upper Cretaceous strata, mainlyfine-grained clastics, were deposited in wide basins.Pulses of coarse clastic input with an East Greenlandprovenance appeared in the Vøring Basin from earlyCenomanian to at least early Campanian times.


0

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825 km

s tw

t

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Trøndelag PlatformVøring Basin

NR HBHHA DTRBVS

13

Fig. 22.13 Regional seismic line across the Mid-Norwegian Margin (courtesy Fugro and TGS). See Fig. 22.10 for location andabbreviations and Fig. 22.11 for stratigraphic information

Break-up in the NE Atlantic was preceded byprominent Late Cretaceous–Palaeocene rifting. At theonset of this rifting, the area between NW Europeand Greenland was an epicontinental sea coveringa region in which the crust had been extensivelyweakened by previous rift episodes. The main periodof brittle faulting occurred in Campanian time fol-lowed by smaller-scale activity towards break-up. TheCampanian rifting resulted in low-angle detachmentstructures that updome thick Cretaceous sequences andsole out at medium-to-deep intracrustal levels on theVøring and Lofoten-Vesterålen margins.

Late Cretaceous–Palaeocene rifting at the VøringMargin covers a ∼150 km wide area bounded onthe east by the Fles Fault Complex and the UtgardHigh (Fig. 22.10). Along the outer Møre and Lofoten-Vesterålen margins, most of the Late Cretaceous–Palaeocene deformation is masked by the lavas, but thestructures appears to continue seawards underneath thebreak-up lavas. On the Møre and Vøring margins, thePalaeocene epoch was characterised by relatively deepwater conditions. Depocentres in the western Møreand Vøring basins were sourced from the uplifted riftzone in the west. The northwestern corner of south-ern Norway was also uplifted and eroded, and the

sediments were mainly deposited in the NE North Seaand SE Møre Basin.

Final lithospheric break-up at the Norwegian mar-gin occurred near the Palaeocene–Eocene transition at∼55 Ma. It culminated in a 3–6 m.y. period of mas-sive magmatic activity during break-up and the onsetof early seafloor spreading. At the outer margin (e.g.Møre and Vøring margins), the lavas form character-istic seaward-dipping reflector sequences (Fig. 22.11)that drilling has demonstrated to be subaerially and/orneritically erupted basalts. These have become diag-nostic features of volcanic margins. During the mainigneous episode at the Palaeocene–Eocene transition,sills were intruded into the thick Cretaceous succes-sions throughout the NE Atlantic margin, includingthe Vøring and Møre basins. Magma intrusion intoorganic-rich sedimentary rocks led to formation oflarge volumes of greenhouse gases that were ventedto the atmosphere in explosive gas eruptions formingseveral thousand hydrothermal vent complexes alongthe Norwegian margin.

The mid-Norwegian margin experienced regionalsubsidence and modest sedimentation since MiddleEocene time and developed into a passive rifted mar-gin bordering the oceanic Norwegian-Greenland Sea.


Mid-Cenozoic compressional deformation (includingdomes/anticlines, reverse faults and broad-scale inver-sion) is well documented on the Vøring margin, but itstiming and significance are debated. The main phaseof deformation is clearly Miocene in age but some ofthe structures were probably initiated earlier in LateEocene–Oligocene times.

There is increasing evidence on the Norwegianmargin for Late Miocene outbuilding on the innershelf (Molo Formation) indicating a regional, mod-erate uplift of Fennoscandia. Over the entire shelfthere is a distinct unconformity, which changes on theslope to a downlap surface for huge prograding wedges(Fig. 22.13) of sandy/silty muds sourced on the main-land areas around the NE Atlantic and the shelf. Thishorizon marks the transition to glacial sediment depo-sition during the Northern Hemisphere glaciation sinceabout 2.6 Ma. Pliocene sedimentation is interspersedwith ice-rafted debris signifying regional cooling andformation of glaciers. Large Plio-Pleistocene depocen-ters formed fans in front of bathymetric troughsscoured by ice streams eroding the shelf.


The mid-Norwegian continental shelf was opened forexploration in 1980, when a limited number of blockson Haltenbanken were announced in the fifth licens-ing round, and it has had fields in production since1993. These are situated within the major fault com-plexes which separate the platform area to the eastfrom the deep Cretaceous basins in the west, and alongthe margin of the Trøndelag Platform (the Halten andDønna terraces) (Figs. 22.10 and 22.11). In recentyears blocks have also been awarded in greater waterdepths within the deep Vøring and Møre basins.

22.3.3.1 Haltenbanken

The Haltenbanken area (Halten and Dønna terraces)has good source rocks and reservoir rocks, and alsostructural traps (Fig. 22.11). On the eastern side ofthe main fault (the Klakk Fault), water has drainedeastwards through the Jurassic sandstones up to thecoast so that pressure has not built up much above

hydrostatic pressure; this has been an important fac-tor in hindering leakage. The main problem in the areais that many of the reservoirs are very deep and oilcan have escaped due to overpressure. Typical traptypes drilled so far on the mid-Norwegian continen-tal shelf are rotated fault blocks of Jurassic age in thehorst-and-graben terrane of the Halten Terrace, andcomplex fault blocks within the major fault zones.Here too the source rock formed during rifting in theLate Jurassic. It is called the Spekk Formation, whichapproximately equates with the Draupne Formationin the North Sea and the Kimmeridge Formation inEngland. In the Early Jurassic Åre Formation there arecoal beds, which may be the source for the gas andpossibly some of the oil.

The most important reservoir rocks are of Earlyand Middle Jurassic age, with alternating sandstonesand shales from the Åre, Tilje, Tofte, Ile and Garnformations (Fig. 22.4). The fields in the east, for exam-ple Heidrun Field, are not so very deep, but in theSmørbukk Field the reservoir rocks lie at more than4 km. Also here it was block rotation during riftingwhich created the majority of the structures.

West of the Smørbukk Field there is a large fault,and to the west of that there is high overpressure inthe Jurassic sandstones. Several wells have been drilledhere that show signs of there having once been oilpresent, and that it has leaked out. This is presum-ably due to the high pressure having reached fracturepressure. In the Kristin Field, however, some of theoil is still in place. The Lavrans and Kristin fieldslie at more than 5 km, where the temperature canreach 170–180◦C. Here the porosity and permeabil-ity in the reservoir rocks are critical, and it is thedegree of quartz cementation which essentially deter-mines whether there is sufficient porosity to producepetroleum. The sand grains are coated with chloritein much of the Tilje Formation, and in places also inthe Garn Formation. This hinders quartz precipitationand thus helps maintain the relatively high porosities(20–25%) despite the great burial depth. Where thechlorite is absent and the sand is pure quartz, as in otherparts of the Garn Formation, the porosity is quite low(<10–12%).

The Draugen Field has Upper Jurassic sandstonereservoirs and is fairly shallow, just 1.5 km. The sourcerock in this area is not mature and the oil found atDraugen has actually had a relatively long migrationpath from deeper-lying areas to the west.


The subsidence and sedimentation rates were highduring the Pliocene and Quaternary, providing asequence about 1 km thick (Fig. 22.13). This is con-siderably greater than in most of the North Sea. Thusneither the reservoir nor source rocks have been deeplyburied for a geologically long period. This has left thesource rocks somewhat immature for their depth, butalso means that the reservoir rocks have less quartzcement.

22.3.3.2 Vøring and Møre Basins

This area was first opened for exploration at the end ofthe 1990s, with great expectation because of the largestructures that could be seen on the regional seismic.Inversion structures (mainly huge, gentle anticlineswith wavelengths in the order of tens of kilometres)are common in the deep Cretaceous basins, and pro-vide traps of considerable size. These were generatedduring Tertiary inversion.

The geology proved to be very different from theNorth Sea and Haltenbanken. There had been consid-erable magmatic activity associated with the break-upbetween Norway and Greenland and the initiation ofseafloor spreading. Tertiary lavas had flowed out acrossgreat tracts of land and intrusive dykes and sills pen-etrated the Cretaceous sediments. There was someconcern that this could have raised the temperature somuch that all the oil had become gas. However, it wasfound that this heating had been quite local and hadhad little overall effect.

More significant was the extreme thickness of theCretaceous sequence, up to 6–7 km. This meant thatthe Upper Jurassic source rocks had already maturedby the mid-Cretaceous. This was before the reservoirrocks, of Upper Cretaceous and Lower Tertiary age,had even been deposited, and before the structures wenow see (e.g. Ormen Lange Dome, Helland HansenArch, Gjallar Ridge) had formed. It thus appears thatthe structures have only trapped some of the gasformed at depth from the oil. Gas has been found inseveral prospect in the western Vøring Basin, in UpperCretaceous sandstones. It has been possible to map thedistribution of sands from amplitude analysis of theseismic data.

The large Ormen Lange gas field was proven in1997. It is located at water depths of 800–1,100 m

in the eastern Møre Basin within the Storegga Slidearea. The gas is found in Palaeocene and uppermostCretaceous reservoir rocks (sandstones).

22.4 Barents Sea

22.4.1 Structure

The Barents Sea covers the northwestern corner of theEurasian continental shelf. It is bounded by young pas-sive margins to the west and north that developed inresponse to the Cenozoic opening of the Norwegian-Greenland Sea and the Eurasia Basin, respectively(Figs. 22.1 and 22.2).

The western Barents Sea is underlain by largethicknesses of Upper Palaeozoic to Cenozoic rocksconstituting three distinct regions (Figs. 22.14, 22.15and 22.16). (1) The Svalbard Platform is covered bya relatively flat-lying succession of Upper Palaeozoicand Mesozoic, mainly Triassic, sediments. (2) Abasin province between the Svalbard Platform andthe Norwegian coast is characterised by a number ofsub-basins and highs with an increasingly accentu-ated structural relief westwards. Jurassic-Cretaceous,and in the west Palaeocene-Eocene, sediments arepreserved in the basins. (3) The continental marginconsists of three main segments: (a) a southern shearedmargin along the Senja Fracture Zone; (b) a centralrifted complex southwest of Bjørnøya associated withvolcanism and (c) a northern, initially sheared andlater rifted margin along the Hornsund Fault Zone. Thecontinent-ocean transition occurs over a narrow zonealong the line of Early Tertiary break-up and the mar-gin is covered by a thick Upper Cenozoic sedimentarywedge.

The post-Caledonian geological history of thewestern Barents Sea is dominated by three majorrift phases, Late Devonian?-Carboniferous, MiddleJurassic-Early Cretaceous, and Early Tertiary, eachcomprising several tectonic pulses. During LatePalaeozoic times most of the Barents Sea was affectedby crustal extension. The later extension is charac-terised by a general westward migration of the rift-ing, the formation of well-defined rifts and pull-apartbasins in the southwest, and the development of abelt of strike-slip faults in the north. Apart from


SvalbardPlatform

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Fig. 22.14 Main structuralelements in the westernBarents Sea and adjacentareas (close-up of Fig. 22.2 –modified/updated fromFaleide et al. 2008). Locationof interpreted regional profiles(black), crustal transects (red)and seismic examples (blue)shown in Figs. 22.15, 22.16,22.17, 22.18 and 22.19.BB = Bjørnøya BasinFSB = Fingerdjupet Sub-basinGH = Gardarbanken HighHB = Harstad BasinHfB = Hammerfest BasinHFZ = Hornsund Fault ZoneKFC = Knølegga Fault ComplexKR = Knipovich RidgeLH = Loppa HighMB = Maud BasinMH = Mercurius HighMR = Mohns RidgeNB = Nordkapp BasinNH = Nordsel HighOB = Ottar BasinPSP = Polheim Sub-platformSB = Sørvestsnaget BasinSFZ = Senja Fracture ZoneSH = Stappen HighSR = Senja RidgeTB = Tromsø BasinTFP = Troms-Finnmark PlatformVH = Veslemøy HighVVP = Vestbakken Volcanic Province

epeirogenic movements which produced the presentday elevation differences, the Svalbard Platform andthe eastern part of the regional basin have been largelystable since Late Palaeozoic times.


The Barents Sea is underlain by a thick succession ofPalaeozoic to Cenozoic strata showing both lateral andvertical variations in thickness and facies – charac-terised by Upper Palaeozoic mixed carbonates, evap-orites and clastics, overlain by Mesozoic-Cenozoicclastic sedimentary rocks (Fig. 22.4). Direct infor-mation on the nature of the crystalline crust beneaththe Barents Sea sedimentary basins is scarce, but

the available, mostly indirect, evidence indicates thatthe basement underlying much of its western partwas metamorphosed during the Caledonian Orogenyand that the structural grain within the Caledonianbasement may have influenced later structural devel-opment. The Caledonian crystalline basement has aNE-SW grain and was folded during Silurian time.

22.4.2.1 Upper Palaeozoic

Within the Caledonian domain Devonian molasse sedi-ments were deposited in intermontane basins undergo-ing extensional collapse. A Devonian tectonic regimecomprising both extensional and compressional eventsis so far only known on Svalbard, located on thenorthwestern corner of the Barents Shelf. Here, thick


NW SENordkapp BasinOttar BasinLoppa High

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Fig. 22.15 Regional profiles across the western Barents Sea (modified from Faleide et al. 1993, Breivik et al. 1995). See Fig. 22.14for location and abbreviations

Devonian strata are found in a north-south trendinggraben structure and the graben fill is discordantlyoverlain by Carboniferous strata (Fig. 22.17).

Lower to lower Upper Carboniferous strata, mainlyclastics, were deposited in extensional basins rangingfrom wide downwarps to narrow grabens. A 300 kmwide rift zone, extending at least 600 km in a north-easterly direction (Fig. 22.14), was formed mainlyduring mid-Carboniferous times. The rift zone wasa direct continuation of the northeast Atlantic riftbetween Greenland and Norway, but a subordinatetectonic link to the Arctic rift was also established.The overall structure of the rift zone is a fan-shapedarray of rift basins and intrabasinal highs with orien-tations ranging from northeasterly in the main riftzone to northerly at the present western continentalmargin. The structural style is one of interconnectedand segmented basins characterised by half-grabengeometries.

The Carboniferous rift phase resulted in the for-mation of several interconnected extensional basinsfilled with syn-rift deposits and separated by fault-bounded highs. Structural trends striking northeastto north dominate in most of the southwesternBarents Sea (Fig. 22.14) where the Tromsø, Bjørnøya,Nordkapp, Fingerdjupet, Maud and Ottar basinshave been interpreted as rift basins formed at thistime (Figs. 22.15, 22.18, 22.19 and 22.20). Faultmovements ceased in the eastern areas towards theend of the Carboniferous and the structural relief wasgradually infilled and blanketed by a platform succes-sion of Late Carboniferous-Permian age. The lowerpart of this succession passes upwards from cycli-cal dolomites and evaporites to massive limestones.It includes a widespread evaporite layer of latestCarboniferous-earliest Permian age mapped regionallyin the SW Barents Sea (also on the NE Greenlandshelf/margin).


Hornsund sheared margin

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Fig. 22.16 Regional crustaltransects across the westernBarents Sea-Svalbard margin(modified/updated fromFaleide et al. 2008).See Fig. 22.14 for locationand abbreviations

The thickness of the salt layer in the NordkappBasin (Fig. 22.18), which is inferred to have reached4–5 km locally, implies that a substantial fault-generated depression was in place or developed duringearly Late Carboniferous times. Not all of this thick-ness corresponds to a fault-defined relief, however,because salt was also deposited in the basin during thesubsequent phase of differential thermal subsidence.Still, a considerable fault-bounded basin must haveexisted.

From Bashkirian to Artinskian/Early Kunguriantimes, carbonate deposition took place on a broadshelf. Carbonate build-ups were common at basinmargins and intrabasinal highs controlled byunderlying older structures. Following Gzelian-Asselian/Sakmarian deposition of basinal evaporitesand growth of marginal carbonate build-ups, a regionalshallow-water carbonate platform was establishedduring Sakmarian-Artinskian times. Carbonate sed-imentation occurred in the entire region until lateEarly Permian time when clastic deposition started to

dominate the area (Fig. 22.4). The change to platformtype sedimentation marked the initial developmentof a regional sag basin which continued to subsidein the Late Permian during the deposition of chertylimestones and shales.

The western part of the rift system was affectedby renewed faulting, uplift and erosion in Permian-Early Triassic times. Normal faulting along the westernmargin of the Loppa High (Figs. 22.19 and 22.20) aswell as the uplift, tilting and erosional truncation ofthe high itself are of sufficient magnitude to indicate asignificant Permian-Early Triassic rift phase affectingthe N-S structural trend. Evidence of fault movementsis found as far north as the Fingerdjupet Sub-basin.The erosional surface associated with this rift phaseextends from the Loppa High to the Stappen High, andPermian tectonic activity is known to have occurredon Bjørnøya and on the Sørkapp High in the southernpart of Spitsbergen. Therefore the rift phase probablyaffected a narrow northerly trending zone along theentire present western margin.


ISFJORDEN

W Festningen ColesbuktaGrumantbyen

Fault

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Devonian

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adgøhsIW0

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s twt

VAN MIJENFJORDENE

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Fig. 22.17 Svalbard geology in seismic lines from fjords (modified from Faleide et al. unpublished)

0

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s tw

t

NNW SSEBjarmeland PlatformMaud Basin Nordkapp BasinNordsel HighOttar Basin

25 km

21

Fig. 22.18 Regional seismic line across the SW Barents Sea (Nordkapp Basin – Bjarmeland Platform – Maud Basin) (courtesyFugro and TGS). See Fig. 22.14 for location and Fig. 22.15 for stratigraphic information


0

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s tw

tSW NEBjarmeland PlatformLoppa HighVeslemøy High

22

25 km

Fig. 22.19 Regional seismic line across the SW Barents Sea (Bjarmeland Platform – Loppa High – Veslemøy High) (courtesyFugro and TGS). See Fig. 22.14 for location and Fig. 22.15 for stratigraphic information

0

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s tw

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NNW NEHammerfest BasinLoppa HighBjørnøya Basin

Snøhvit Goliath

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25 km

23

Fig. 22.20 Regional seismic line across the SW Barents Sea (Hammerfest Basin – Loppa High – Bjørnøya Basin) (courtesy Fugroand TGS). See Fig. 22.14 for location and Fig. 22.15 for stratigraphic information

22.4.2.2 Mesozoic

In Early Triassic time a regional deepwater basincovered much of the Barents Sea. During the rela-tively short Triassic period large amounts of clastic

sediments were deposited. The Uralian highland tothe east was an important sediment source. Sedimentswere also shed into the Barents Sea from the BalticShield and from other local source areas. Differentialloading triggered salt diapirism in the Nordkapp Basin


(Fig. 22.18) and possibly later also in other salt basinsin the SW Barents Sea.

The Triassic strata are dominated by shales andsandstones, the vertical and lateral distribution ofwhich is complex. There seems to be an increased con-tent of coarse clastic rocks in the younger intervals. Asimilar increase is also observed in an easterly and asoutherly direction towards the main sediment sourcearea. Marine conditions prevailed in Late Permian andEarly Triassic times followed by a shallowing and par-tial exposure of some areas. The continental regimeprevailed in large areas in Middle Triassic time andthe northward and westward prograding deltaic systemcontinued to infill the regional basin.

In the central and northern basin areas marine con-ditions existed throughout the Middle Triassic. A goodMiddle Triassic source rock is known from Svalbardand this is probably present in large parts of the west-ern Barents Sea. In the Late Triassic the shoreline wasmoved back to the southern and eastern borders ofthe SE Barents Sea basin. The Triassic ended withregression and erosion. Late Triassic sediments werealso derived from other source areas, mainly in thenorthwest.

The Lower-Middle Jurassic interval is dominated bysandstones throughout the Barents Sea. These sand-stones form the main reservoir in the SW BarentsSea (Hammerfest Basin; Fig. 22.20). They probablyalso covered the Loppa High and Finnmark Platformbut were partly eroded during later tectonic activity.The late Middle Jurassic sequence boundary marksthe onset of rifting in the southwestern Barents Sea,whereas unconformities within the Upper Jurassicsequence reflect interplay between continued faultingand sea-level changes. The sequence is so thin that it isgenerally impossible to resolve these unconformitieson the seismic data. The shales and claystones containthin interbedded marine dolomitic limestone and raresiltstones or sandstones toward the basin flanks, reflect-ing relatively deep and quiet marine environments.

The Late Jurassic-earliest Cretaceous structur-ing in the SW Barents Sea was characterised byregional extension accompanied by strike-slip adjust-ments along old structural lineaments, developing theBjørnøya, Tromsø and Harstad basins as prominentrift basins (Fig. 22.15). The evolution of these basinswas closely linked to important tectonic phases/eventsin the North Atlantic-Arctic region. Rifting continuedin Early Cretaceous time and an important phase of

Aptian faulting is documented in the SW Barents Sea.Several phases of Late Mesozoic and Early Cenozoicrifting gave rise to very deep basins in the SW BarentsSea. A tentative Middle Jurassic horizon correspond-ing to the top of the shallow marine sandstones, alsomarking the onset of late Mesozoic rifting, can be fol-lowed from the seafloor down to 9 s twt (16–17 km)over a relatively short distance (Fig. 22.15).

The Lower Cretaceous comprises three sedimen-tary units from Valanginian to Cenomanian. Shalesand claystones dominate, with thin interbeds of silt,limestone and dolomite. These strata make up themain basin fill in the deep SW Barents Sea basinsand they dominate the regional subcrop pattern. Themarine environments throughout the deposition ofthese units are dominated by distal conditions withperiodic restricted bottom circulation.

In Early Cretaceous time the northern Barents Seawas characterised by widespread magmatism withoutany signs of faulting. Extrusives and intrusives (sillsand dykes) belonging to a regional Large IgneousProvince (LIP) in the Arctic are documented from bothonshore (Svalbard and Franz Josef Land) and offshoreareas in the northern Barents Sea. The magmatismwas most extensive during Barremian?-Aptian timesbut both older and younger dates have been reported.The present distribution in the northern Barents Seais modified by later uplift and erosion. The EarlyCretaceous magmatism within the Arctic LIP hadimportant palaeogeographic implications. It causedregional uplift and the Neocomian in the north and eastwas characterised by southward sediment prograda-tion. This depositional regime is difficult to documentin the western Barents Sea due to later uplift and ero-sion. The magmatism and regional uplift was relatedto rifting and break-up in the Amerasia (Canada) Basinand the formation of the Alpha Ridge.

Little or no Upper Cretaceous sediments weredeposited in the Barents Sea except in the SW BarentsSea which continued to subside in response to fault-ing in a pull-apart setting. The Wandel Sea Basin isa NE Greenland equivalent where Late Cretaceousstrike-slip faulting and pull-apart basin formation iswell documented (Figs. 22.2 and 22.3). The UpperCretaceous succession varies in thickness and com-pleteness. In the Tromsø Basin a 1,200 m shale suc-cession has been drilled while seismic data indicatethat the sequence reaches 2,000–3,000 m in rim syn-clines in the central basin. Whereas the Tromsø and


Sørvestsnaget basins were depocentres through mostof this period, areas further east were either trans-gressed only during maximum sea-level and/or displayonly condensed sections.

22.4.2.3 Cenozoic

The Cenozoic structuring was related to the two-stageopening of the Norwegian-Greenland Sea and the for-mation of the predominantly sheared western BarentsSea continental margin (Fig. 22.3). Continental break-up and onset of sea-floor spreading were precededby a phase of rapid late Palaeocene subsidence.The Palaeogene succession rests unconformably onthe Cretaceous and this depositional break at theCretaceous-Tertiary transition (Maastrichtian-Danian)occurs throughout the southwestern Barents Sea.

The western Barents Sea-Svalbard margin devel-oped from a megashear zone which linked theNorwegian-Greenland Sea and the Eurasia Basin dur-ing the Eocene opening. The first-order crustal struc-ture along the margin and its tectonic developmentis mainly the result of three controlling parameters:(1) the pre-break-up structure, (2) the geometry ofthe plate boundary at opening and (3) the direc-tion of relative plate motion. The interplay betweenthese parameters gave rise to striking differencesin the structural development of the different mar-gin segments of sheared and/or rifted nature. Thecontinent-ocean boundary is clear along the shearedSenja margin. The central rifted margin segmentsouthwest of Bjørnøya was associated with magma-tism in the Vestbakken Volcanic Province both dur-ing break-up at the Palaeocene-Eocene transition andlater in the Oligocene. In the north, strike-slip move-ments between Svalbard and Greenland gave riseto compressional (transpressional) deformation withinthe Spitsbergen Fold and Thrust Belt (Fig. 22.17).Compressional deformation is also observed in theBarents Sea east of Svalbard, showing that stressrelated to transpression at the plate boundary west ofSvalbard was transferred over large distances. Domalstructures observed in the eastern Barents Sea may berelated to this compressional regime.

The opening of the Greenland Sea was complex,involving jumps in the location of the spreading axisand the splitting off of microcontinents. Since earliestOligocene time (magnetic chron 13) Greenland moved

with North America in a more westerly direction rela-tive to Eurasia (Fig. 22.3). This gave rise to extension,break-up and onset of seafloor spreading also in thenorthern Greenland Sea west of Svalbard, and a deep-water gateway between the North Atlantic and Arcticwas finally established sometime in the Miocene. Thishad large implications for the palaeo-oceanographyand climate in the region.

The late Cenozoic evolution was characterised bysubsidence and burial of the margins by thick sed-iments in a clastic wedge derived from the upliftedBarents Sea area. Late Cenozoic uplift and erosion ofthe Barents Sea has removed most of the Cenozoic sed-iments, and even older strata. The subcrop pattern isdominated by Mesozoic units. The erosion seems tohave been most extensive in the western Barents Sea,especially in the northwestern part including Svalbardwhere more than 3,000 m of sedimentary strata havebeen removed. In the SW Barents Sea the erosionestimates are in the range 1,000–1,500 m. In the east-ern Barents Sea, little work has so far been doneon Cenozoic uplift and erosion, however, erosion hasbeen calculated to be between 250 and 1,000 m. Highseismic velocities at the top of the bedrock in theNW Barents Sea, decreasing south- and eastwards,reflect the pattern of differential uplift and erosion.The Neogene and Quaternary, resting unconformablyon Palaeogene and Mesozoic rocks, thickens dramat-ically in the huge sedimentary wedge at the margin.The glacial sediments are dated as Late Pliocene toPleistocene/Holocene in age.


More than three decades of research and petroleumexploration in the Barents Sea have revealed a deepand complex sedimentary basin system affected bya variety of geological processes. A large petroleumpotential has been proven including multiple sourceand reservoir intervals. However, there are still manyuncertainties and problems linked to understanding theprospectivity of the Barents Sea. Areas in the southernBarents Sea were opened for exploration in 1980 andthe first discovery was made in 1981.

A great variety of traps (fault and salt structures,stratigraphic pinchout) and sealing mechanisms existin the Barents Sea area, and several different play


models have proven hydrocarbon accumulations. Thediscoveries are dominated by gas and gas condensateaccumulations (e.g. Snøhvit Field). Potential reser-voirs are distributed across several stratigraphic lev-els from Devonian to Tertiary and comprise bothsandstone and carbonate lithologies. Jurassic sand-stones are the main proven reservoir but hydrocarbonshave also been found in sandstones of Triassic andCretaceous age. Several proven and potential sourcerock units are present in the Barents Sea. The mostimportant are the Upper Jurassic and Triassic organic-rich (anoxic) shales. Lower Cretaceous, Permian andCarboniferous shales, as well as Lower Permian evap-orites and Lower Jurassic coals also have sourcepotential.

About 25 discoveries have been made in theBarents Sea, most of them in the Hammerfest Basin(Fig. 22.20) where their reservoirs are in sandstones,mainly Jurassic, as in the Snøhvit Field. Deeper dis-coveries have also been made, such as in Triassicsandstones in 7122/7-1 (Goliath Field) and 7125/4-1(Nucula Field). Oil and gas have also been found inTriassic sandstones in the Nordkapp Basin, which isdominated by salt tectonics (Fig. 22.18). Gas and oilhave been found in carbonates of Carboniferous toPermian age on the Finnmark Platform.

The most significant exploration problem in theWestern Barents Sea relates to the severe uplift anderosion of the area that took place during the Cenozoic.The quantity of sediments removed, and the timingof this removal, are still a matter of debate, but it isagreed that the uplift and erosion have had importantimplications for oil and gas exploration in the BarentsSea. Residual oil columns found beneath gas fields inthe Hammerfest Basin indicate that the structures wereonce filled, or partially filled, with oil. The removal ofup to 2 km of sedimentary overburden from the areahas had critical consequences for these accumulations:exsolution of gas from the oil, and expansion of thegas due to the decrease in pressure, resulted in expul-sion of most of the oil from the traps. Seal breachingand spillage probably also occurred as a result of theuplift and tilting. A further consequence of these latemovements was the cooling of the source rocks inthe area, which effectively caused most hydrocarbongeneration to cease. Thus, little new oil was avail-able to fill available trapping space. These mechanismsmay explain the predominance of gas over oil in theBarents Sea.

Further Reading

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Blystad, P., Brekke, H., Færseth, R.B., Larsen, B.T., Skogseid,J. and Tørudbakken, B. 1995. Structural elements of theNorwegian continental shelf, Part II: The Norwegian SeaRegion. Norwegian Petroleum Directorate Bulletin 8.

Breivik, A., Gudlaugsson, S.T. and Faleide, J.I. 1995. OttarBasin, SW Barents Sea: A major Upper Paleozoic riftbasin containing large volumes of deeply buried salt. BasinResearch 7, 299–312.

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Brekke, H. 2000. The tectonic evolution of the NorwegianSea continental margin with emphasis on the Vøring andMøre basins. In: Nøttvedt, A. et al. (eds.), Dynamics of theNorwegian Margin. Geological Society Special Publication167, pp. 327–378.

Brekke, H., Sjulstad, H.I., Magnus, C. and Williams, R.W. 2001.Sedimentary environments offshore Norway – An overview.In: Martinsen, O. and Dreyer, T. (eds.), SedimentaryEnvironments Offshore Norway – Paleozoic to Recent, NPFSpecial Publication 10, pp. 7–37 (Norwegian PetroleumSociety and Elsevier Science B.V.).

Christiansson, P., Faleide, J.I. and Berge, A.M. 2000. Crustalstructure in the northern North Sea – An integrated geophys-ical study. In: Nøttvedt, A. (ed.), Dynamics of the NorwegianMargin. Geological Society Special Publication 167, pp.15–40.

Doré, A.G., Lundin, E.R., Jensen, L.N., Birkeland, O., Eliassen,P.E. and Fichler, C. 1999. Principal tectonic events in theevolution of the northwest European Atlantic margin. In:Fleet, A.J. and Boldy, S.A.R. (eds.), Petroleum Geologyof Northwest Europe: Proceedings of the 5th Conference.Geological Society, pp. 41–61.

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