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N R . 2 & 3 O K T O B E R 1 9 9 6

Oil and gas exploration in the North Sea:OUR UNQUENCHABLE THIRST FOR ENERGY

THE ABC’S OF OIL

THE ABC’S OF THE NORTH SEA

“PLAYS” AND RESOURCES IN THENORTH SEA

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6O I L A N D G A S E X P L O R A T I O N I N T H E N O R T H S E A

The natural background and ourunquenchable thirst for energy

Kai Sørensen

We in the western world have developedan unrealistic attitude towards energy.Our entire culture is based on accessibleand inexpensive energy. There is, how-ever, a price to be paid. Combustion offossil fuels, particularly oil and coal pro-duce atmospheric pollution and CO2.Today, no one can afford to close theireyes to the consequences.

Politicians attempt to alter our energyconsumption habits, with both whips andcarrots, but the results are unimpressive.The majority of people are not preparedto pay the price, a higher cost of fossil fu-els or a change in our lifestyle. Ourconsumption of fossil fuels can thus beused as an indicator of our true will tochange our existing habits to become mo-re environmentally conscious. In Den-mark, as in other western countries, the fi-gures for fossil fuel consumption speak forthemselves: our consumption of oil, coaland gas has increased year by year sincethe Second World War (WW II).

While there has been a general trend ofsteadily increasing consumption sinceWW II, the consumption curve fell twiceduring the 1970’s (see figure 1: *1 and *2).This was the result of the creation of OP-EC together with OPEC’s aim to use oil asan economic weapon. Most people are fa-miliar with these events.However, few are aware that after a steepincrease in prices immediately followingthe formation of OPEC, much to the de-

light of oil com-panies and oil-producing co-untries, the spi-ralling pricesquickly lostmomentum,and in the 10years since thehuge drop inprices at theend of 1986,prices actuallyfell. The expla-nation for thisphenomenomcan be found inthe North Seaand Alaska. The

period during the 1970’s when the crisiswas becoming serious for non-OPEC co-untries coincided with increasing fossil fu-el production in the North Sea and Alaska.During the 1980’s production from thesetwo regions was so large that OPEC’s we-apon was virtually ineffective.In the future, however, our ability to holdoil-producing countries and oil prices incheck may be limited, as can be observedfrom figure 2 which depicts the remaining,known oil reserves in the North Sea com-pared to those of the Opec countries andRussia.

The development and expansion of thehuge North Sea oil and gas production inthe course of a decade is the subject ofthis theme issue. It has a lot to do withgeology. Bear in mind that geology is “theheart of the matter” in the world’s largestindustry.

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*1 *2

5550 60 65 70 75 80 85 90 94

0

5

10

15

20

25

Oil

Coal

NG

SE

SE: Sustainable energyNG: Natural gas

T.O

.E.*

106

Year

Kuwait*

Venezuela*

Libya*

Russia

Nigeria*

North Sea

Iran*Iraq*Abu Dhabi*Saudi Arabia*

0 10000 20000 30000 40000

Million tons* OPEC - member country

Figure 1.The curve depicts the Danish energyconsumption since WW II, calculated in tons of oilequivalents t.o.e).

Figure 2.A column chart depicting OPEC’s oil reserves compared to the North Sea re-serves. (Gas is not included). Source: BP Statistical Review of World Energy, June 1995)

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Oil and gas are hydrocar-bons, formed through ananoxic transformation ofalgae and more develop-ed plants, which, throughfortunate circumstances,have been preserved inthe underground’s sedi-mentary deposits. If theconditions are suitable,if temperatures are suffi-ciently high and largequantities of organic ma-terial are available, firstoil and thereafter gas canbe produced. Gas canalso form directly fromcarbon-rich organic ma-terial. These transforma-tions are complicatedprocesses, for example:the most hydrogen-richorganisms (algae) aretransformed into oil, theproduct with the fewest hydrogen atoms (ageneral atomic formula of CH2), while themost carbon-rich material (land plants)forms gas, the most hydrogen-rich final pro-duct (atomic formula CH4).

Rocks in which oil and gas form are calledsource rocks, and the sites where organicmaterial is transformed into hydrocarbonsare referred to as “kitchens”. The porespaces which are found between the sedi-mentary rock’s mineral particles are nor-mally filled with groundwater.The oil and/orgas migrate from a kitchen up into the over-lying rocks by displacing this water. If thepore spaces in the overlying rock are large,numerous, (a porous rock type) and evenlydistributed (the rock is permeable, meaningthat fluids can flow easily through the porespaces/fractures) then it is possible forhydrocarbons to accumulate, and they canbe produced (pumped or piped) to the sur-face. A porous and permeable rock typewhere oil or gas accumulates is called a re-servoir. Hydrocarbons can be trapped hereif the natural geometric configuration of thereservoir is suitable and if the reservoir hasbeen sealed by an impermeable formationcalled the cap rock so that the hydrocar-bons cannot migrate further.

Nature has now done its work – transfor-ming the organic material into hydrocar-bons, causing the oil to migrate and creatinga suitable reservoir to act as a trap for theoil or gas.The rest is up to the geologist whomust find the reservoir. Once the reservoiris discovered it is referred to as a discovery.When production starts of a discovery, it isthen referred to as a field.

Under natural conditions (figure 3) hydro-carbons in a reservoir are present in the fol-lowing three ways:as a liquid capped by gas;as a liquid; or as a gas.Liquids which are pro-duced to the surface separate into threecomponents:oil, gas and water.There can belarge quantities of gas dissolved in the oil un-der the conditions which exist in the reser-voir.When the gas in a reservoir is extract-ed to a surface,several kilometres above thereservoir, some of the gas will condense toa liquid referred to as condensate whichresembles gasoline/petrol.Gas formed fromcarbon-rich source rocks, for example coal,

contains only small amounts of higherhydrocarbons and produces only a smallamount of condensate when produced tothe surface.This is referred to as “dry gas”.Gas which is formed in marine mudstone(shale) is rich in higher hydrocarbons, andlarge quantities of condensate are producedat the surface. Such a gas is described as be-ing “wet” or “rich”.Oil which is formed fromthe same type of source contains a large qu-antity of dissolved gas (light oil).The DanishNorth Sea fields contain both wet gas andlight oil.The southern part of the North Seacontains only dry gas.

3

The ABC’s of Oil

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Dep

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oil

gas

An oil kitchen located over a dry gas kitchen

Oil + condensate kitchen Gas condensate kitchen

Composition in the well stream at the surface

Oil

Water

Condensate

Gas

Coal (gas source rock)

Sandstone reservoir

Marine shale(oil source rock)

Migration route

Figure 3.The complexity of a hydrocarbon system. Both the source rocks and the reservoirs are found at va-rious depths.The migration route from the kitchen to a trap (or reservoir) can be short or long. Permutationsin the geologic configurations and hydrocarbon-water compositions presented here result in a complicated fi-eld situation, both in terms of finding and exploiting hydrocarbons.

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6O I L A N D G A S E X P L O R A T I O N I N T H E N O R T H S E A

It is not possible to describe the geologichistory of the North Sea briefly. Instead theten main events (in a petroleum geologysense of the word) will be summarised inthe following section. The numbers inparenthesis (1) in the text refer to thepictograms in figure 4.

In the early Permian period, northwestEurope was a hot, barren landscape like theSahara today.Ephemeral lakes which formedduring periods of precipitation turnedsaline. Briefly stated: it was a harsh environ-ment.There was, however, desert sand (2)which was rounded and well-sorted andwhich today is preserved in the sub-surfaceof the southern part of the North Sea .Thisdesert sand covered the remains of theCarboniferous fern-forests (1) preservingthem as a record of more humid times andas a reminder of the changeability of theearth’s climate.

The early Permian landscape was later inun-dated by the sea. In northwestern European inland sea formed and with the continu-ing warm and dry climate it became hyper-saline and salt precipitates formed (3).During the end of the Permian period,almost 1 kilometer of salt was deposited inthe central part of this basin. Coal depositslater formed gas, which migrated upwardsand accumulated in the desert sand whichwas sealed by the overlying salt deposit.Thiscombination of circumstances, a sourcerock, a reservoir and a seal or impermeablecap rock is called a “play”.This term will beuseful to our understanding of the geologi-cal history as well as the hydrocarbonexploration history in the North Sea.

The saline Permian basin was connected tothe ocean.There was an ocean north ofpresent day Scandinavia and Greenland (re-member, that the Atlantic ocean did notexist at this time) and an ocean south ofEurope called the Tethys Sea.This sea isanother important key in understandingthe geology of the North Sea, because du-ring much of the Mesozoic this sea inun-dated the North Sea area, bringing marinesediments with it.The Permian period en-ded with a continent-continent collisionwhich formed the Ural mountain chain, and

in front of this, a dry alluvial plaintowards the North.“Redbeds” weredeposited in this landscape.To thesouth, marine carbonates were depo-sited in the shallow shelf waters ofthe Tethys Sea.Throughout the Trias-sic period this sea sometimes inun-dated the land, time followed by a ret-reat over northwest Europe.At the end of the Triassic period a shelf seaspread over most of northwest Europe andit was first in the middle of the Jurassicperiod that land masses (where the Northsea is today) emerged from this sea in con-nection with a short volcanic pulse. Sand inthe form of huge delta complexes (4) wasdeposited along the coasts of this landwhich stretched from present-day Born-holm-Skåne northwestward to the ShetlandIsles.The delta plains spread seaward, onlyto be inundated again by the sea.This Mid-dle Jurassic sand is the most important re-servoir for North Sea oil, particularly in theNorwegian and English parts of thenorthern North Sea.

During the Upper Jurassic period largeparts of the North Sea were again belowsea level.The region under the middle of theNorth Sea became a fault zone (5) forminga deep depression in an extensive shelf sea.This fault zone, also called a rift zone inanalogy with the rift valley systems whichexist today in, for example east Africa, hadthree main rift branches (figure 8):

• The Viking Graben• The Central Graben• The Moray Firth

There were, however, a large number of in-dividual faults which played an importantrole in the formation of hydrocarbon trapsof the Jurassic reservoirs, as well as in-directly forming younger traps.At the end ofthe Jurassic period conditions changed inthe sea covering the three rift branches andin the deepest, or most isolated parts mudwith a high content of marine algae was de-posited.Today this mudstone (shale) is thedominant oil source rock (6) and is knownin England as the Kimmeridge Clay. Themost important “plays” in the North Seanorth of a line from Esbjerg to Hull (figure8) have this source rock in common.

During the Upper Jurassic period there wasrift activity, and deposits which would laterbecome excellent source rock were laiddown. For these two reasons the Upper Ju-rassic period stands out as an important ti-me in hydrocarbon history.The story, how-ever, is not finished. Sand, which later wouldbecome sandstone reservoir rock (7) wasalso deposited during the Upper Jurassic pe-riod. After formation of the large central

4

The ABC’s of the North Sea 180 MaBP

60 MaBP

Present

165 MaBP

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O I L A N D G A S E X P L O R A T I O N I N T H E N O R T H S E A

land mass (where the North Sea is today)during the Middle Jurassic,the sea inundatedthe entire North Sea area. However, due tosimultaneous tectonic activity, the deposi-tional conditions during the Upper Jurassicwere quite varied.There was a varied land-scape in the area close to the present dayNorth Sea with land areas located close toshallow ocean areas which were wide ornarrow. Marine sand, which later becamemarine sandstone was deposited in these“shelves”. Occasionally, sand was “pumped”out of these shallow regions into deeperbasin regions by turbidites as a result oftectonic activity.

During the Upper Jurassic and LowerCretaceous period the transport of erodedmaterial to the North Sea gradually decrea-sed and finally stopped completely duringthe Lower Cretaceous. At the end of theCretaceous period the Tethys Sea shelf ex-tended northward,and carbonates were de-posited over the majority of this shelf. InDenmark, these deposits are referred to as“skrivekridt” or white chalk. Parts of thischalk series can have reservoir characteri-stics (8).

In northwestern Europe this chalk depositi-on was suppressed during the transition

from the Cretaceous to the Tertiary becau-se the initial spreading of the ocean floor inthe North Atlantic created new land alongthe coasts of this newly formed ocean.Themost noticeable land mass was created nearthe present-day British Islands. Largequantities of erosional material derivedfrom this new land mass were washed outinto the North Sea where they formed theNorth Sea’s classic reservoir: sand from theearly parts of the Tertiary (Paleocene andEocene).This reservoir (9) was one of thefirst to be found during hydrocarbon explo-ration in the North Sea. Particularly theScandinavian peninsula, and perhaps thedistant Carpathians and the Alps suppliedhuge quantities of erosional material to theNorth Sea at the end of the Tertiary.Today,these deposits are kilometer-thick and con-tain large quantities of sand, but becausethere is no shales to seal these sands theyhave no commercial interest. Nevertheless,this late Tertiary (Neogene) history plays acrucial role for the oil geology in the NorthSea. Subsidence in this late part of theTertiary brought the North Sea’s sourcerocks down to depths where oil and gascould form in large quantities (10). Thissteady subsidence which continued duringthe Quarternary and which still occurstoday has resulted in an effective preserva-

tion of oil and gas in hundreds of establisheddiscoveries.The fact that the eastern part ofDenmark (figure 5) did not subside duringthe Neogene is the geological explanationfor the lack of success in hydrocarbon ex-ploration in the Danish region east of thecentral North Sea.With the use of models itis possible to relatively accurately “date” theformation of oil in the North Sea. A largepercentage of all the oil which has been fo-und was formed in the last 10 million years.The number of discoveries in the NorthSea (over 700) are not just the result of fort-unate coincidences regarding the formationof source rock,reservoirs and traps.The ex-planation lies in the fact that the North Seais still actively subsiding.This means that oiland gas continue to form, also today.At thesame time, the ability of the traps to retainhydrocarbons increases due to the continu-ally increasing pressure. It is the history ofthe Neogene which determines whetherexploration of sedimentary basins in theNorth Sea area will be a success or a fiasco.Neogene subsidence creates favorable con-ditions for the formation and trapping ofhydrocarbons while Neogene uplift doesthe opposite.The Norwegian-Danish basinis a fiasco of the latter type.

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Cretaceous

Upper Jurassic

Lower Jurassic

Triassic

Upper Permian

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550 km

Central gravenMid North Sea High Norwegian-Danish basin

England

Norway

A

B

A B

Figure 5. Profile through the Central Basin and the Norwegian-Danish Basin, based on a seismic profile.

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6O I L A N D G A S E X P L O R A T I O N I N T H E N O R T H S E A

6

It’s ideas that find oilA “play”, as defined previously, is a set ofcircumstances which can lead to the for-mation of an extractable deposit of oil or

gas in the underground.A play may be spe-culative, meaning that it has not yet beenproven by way of a discovery, or it may beestablished.The term play can also be un-derstood as a number of prospects in whi-ch the “the set of circumstances” are assu-med to be fullfilled.Thus the word play canbe used in two ways: to refer to somethingconcrete – a number of discoveries and/orprospects; or something abstract – a setof circumstances/geological conditions.Think of Plato and his distinction betweenthe idea of a chair and all chairs! Once thisconcept of duplicity is understood, then itis also possible to understand the centraldictum in oil exploration: “It’s the ideasthat find oil”.The North Sea’s exploration history canbe described with the help of half a dozenplays, five of which contain more than 95%of the hydrocarbon resources (figure 6).The following discussion will concentrateon these five plays and the regions(“fairways”) where the “set of circumstan-ces” of a play are met.Since 1964, approximately 2,700 explora-tion wells have been drilled in the NorthSea, resulting in 800 discoveries. Theextractable oil and gas resources in thesediscoveries (reserves), calculated in oilequivalents, are equal to 100 billion barrels(b.o.e.). Approximately 6 barrels equal 1cubic meter. These resources are produ-ced at a steadily increasing rate. Each yearapproximately 3 billion b.o.e. of oil and gasare produced at the surface, clearly sur-passing the rate at which new resourcescan be located.

The North Sea can be divided into two se-parate provinces, a southern provincecalled the “Carboniferous Gas Province”and a northern province which will be re-ferred to here as the “Jurassic Rift Provin-ce”. The carboniferous coal in the sou-thern province has been a source of a lar-ge number of gas discoveries including theregion’s largest, the Dutch Groningen field.In the northern province, which was affe-cted by tectonic activity in the Jurassicperiod, the Upper Jurassic KimmeridgeClay is a source of both oil and gas.

The Carboniferous Gas Province:The Permian PlayAll the plays in the Carboniferous GasProvince in the southern North Sea andHolland have the common characteristicthat gas was formed from Carboniferouscoal.The North Sea’s exploration historyreally starts in the early 1960’s with the re-cognition of the immense size of the Gro-ningen gas field and the possibility that thisplay’s fairway extended out into the NorthSea. At the same time technologicaladvances were making oil exploration andproduction at sea possible.Off-shore pro-duction of hydrocarbons was undertakenfirst in the relatively protected MaracaiboLake in Venezula and then in the Gulf ofMexico.Thus, hydrocarbon exploration inthe North Sea was only feasible due to thetechnological advances which evolvedfirst in the Gulf of Mexico, and later, as ini-tial exploration in the North Sea wasfruitful, also there. Today the challengesassociated with exploration and producti-on of oil from the North Sea are some ofthe driving forces behind technologicaldevelopments within the oil industry.

Although there are potentially more re-servoirs in the southern gas province, theEarly Permian sandstone contains themajority of the resources due to the fol-lowing: its thickness (up to 300 m); its idealreservoir characteristics (highly porousand permeable); and the reservoir’seffective seal by overlying imperviousPermian salt. Although there are goodsandstones from the Triassic age in thesouthern North Sea, these have been cutoff from gas migration from the Carboni-ferous coal by the overlying Permian salt.The southern North Sea’s dominant play isa Lower Permian sand with dry gas fromthe Carboniferous coal which is effective-ly sealed by salt.This salt cap rock is so ef-fective that gas fields in certain areas of thesouthern North Sea have remained intactduring periods with significant uplift.A lar-ge part of the gas reserves in this play areproduced by a consortium composed ofShell and Exxon, whereas BP and othercompanies first became aware of this playafter the largest discoveries had been ma-de. In the exploration history of a play, the

Plays and Resources in the North Sea

TERTIARY

Eocene

Zechstein

Paleocene

CRETA-CEOUS

JURASSIC

Neo

gene

Pale

ogen

eU

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Low

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50

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150

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Figure 4. Episodes in the North Sea’s oil geology.Details are found in the text.

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O I L A N D G A S E X P L O R A T I O N I N T H E N O R T H S E A

largest discoveries will be made early on,and thus it is important for oil companiesto constantly be aware of any new devel-opments. More than half of the approxi-mately 4,000 billion m3 of gas which hasbeen found in the Carboniferous Gas Pro-vince was located in the Groningen field,and more than 95% of the gas reserves ac-cumulated in early Permian sandstone (re-ferred to locally as Rotliegend). Discoveri-es are still made in the Rotliegend sand-stone, often in smaller structures whichwere on the exploration “waiting list”. Ex-ploration of new plays continues, with so-me success, but the gas reserves in theCarboniferous Gas Province are decliningmarkedly. In the European market thesedeclining supplies are replaced with gasfrom the northern North Sea’s JurassicRift Province where the Norwegian gas fi-elds are important.

The Jurassic Rift ProvinceInitial exploration in the North Sea led tothe belief that it was a gas province. Thisbelief sheds light on the statement “I willdrink all the oil there is in the North Sea”– which was attributed to a number ofpeople including the director for N.G.U. inNorway and BP’s head geologist inEngland. The statement was decidedlyapocryphal.

A few years after the initial exploration, at

the end of the 1960’s,oil was discovered inthe northern North Sea, first in the chalkformation in the Danish and Norwegianregions of the North Sea, and soon after inthe Paleocene sand in the English sector.Oil in the Jurassic sandstone was found in1971 and in the following years. Thus, itwould be correct to state that the mostimportant plays in the North Sea were es-tablished within a five year period fromthe end of the 1960’s to the beginning ofthe 1970’s.

The reason for this order of events is dueprimarily to the advances in the quality of

seismic data. In the early days of oil explo-ration it was not possible to “see” muchdeeper than the top of the chalk, and con-sequently it was this formation and theoverlying Paleogene section which wereexplored first. In the following section thestratigraphy will be used as a frameworkto discuss the plays, starting with theMiddle Jurassic play followed by the UpperJurassic, Cretaceous and the Paleogeneplays.

The Middle Jurassic PlayAs mentioned previously, it was not possi-ble to “see” the deep-lying Jurassic layer

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Dry well Gas discovery

Play Other FindsUK N DK NL Sum

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97

38

124

119

294

127

799

1037

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1034

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5324

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1207

2502

4091

4524

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14592

Figure 6. Resources per play per country, calculated in t.o.e..The Permian play is exclusively gas.The otherplays are gas and oil.The values are the sum of the total reserves found.The produced volumes are not de-ducted from the reserves (as is the case in figure 2). Source: Spencer, Leckie & Chew, 1996).

Figure 7.The Permian gas play.

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8

Northern limit of the Lower Permian sand

Southern limit of the Upper Permian salt

Germany

Great Britain

Denmark

Holland

Norway

Groningen

Th

eJ u

r a s s i cR i f t

Pr ov i n

c e

Ce n t r a l

G r a b e n

T h e C a r b o n i f e r o u s G a s P ro v i n c e

M o r r a y F i r t hR i f t

Troll

Viking Graben

Esbjerg

Hull

Shetland

Magnus

Brent

Oseberg

Frigg

Piper

Ula

Ekofisk

Tyra

Dan

Fulmar

Miller

Brae

Forties

Statfjord

Figure 8.Discoveries in the North Sea.The discoveries (approximately 700 in the 5 main plays)are distributed among the main fairways: the Jurassic Rift Province and the Carboniferous GasProvince.The source rocks are the Kimmeridge Clay and the Carboniferous coal, respectively.Source: Spencer, Leckie & Chew, 1996

125 km

Discovery in Paleogene sandstoneCretaceous ChalkUpper Jurassic sandstoneMiddle Jurassic sandstonePermian sandstone

Faults/fault zone (Jurassic Rift)

Limit of the Kimme-ridge Clay

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with the seismic techniques available in the1960’s. In the most northern region of theNorth Sea it was possible to see to thebottom of the Cretaceous section (Creta-ceous deposits in that region are compo-sed of shale.) Although the topography ofthis surface was very pronounced, therewas no indication of the nature of the un-derlying formations. The most widelyaccepted theory was that the bottom ofthe Cretaceous surface was underlain bybedrock. In other words, the pronouncedstructures were a type of Shetland islandsoverlain by shale deposits. Petroleum geo-logists’ and oil companies’ interest in thisregion was already high because there hadbeen some huge discoveries for example,the Ekofisk discovery in a chalk formation,and the Forties discovery in the Paleocenesand. When Shell drilled an exploratorywell in 1971 and found oil, the dream ofthe North Sea as a significant oil provincebecame a reality.The well, called the BrentDiscovery Well, was drilled through morethan 200 m of sandstone, and oil was pre-sent throughout the reservoir. Shell sealedthe well without testing it. It was not ne-cessary, Shell knew it had a gigantic disco-very.Shell waited for the next round of licens-ing and applied for licences to all theblocks into which this play might extend.Today, no oil company involved in NorthSea exploration would be permitted to

behave so tactically. The Middle Jurassicsandstone was named the Brent Formati-on (in the northern-most region of theNorth Sea). In the following few years alarge number of discoveries were establis-hed in this play including the following:Statfjord, Oseberg, Snorre, and Gullfaks inthe Norwegian region; Brent, Beryl,Cormorant, Lyell, and Ninian in the Englishregion. Smaller discoveries were also ma-de southwest of Norway in the Danish re-gion (Harald and Lulita) and deep in the

Moray Firth.This play had the characteri-stic that the source rock was younger thanthe reservoir rock, and it was only possi-ble for the oil to migrate into the reservo-ir because fault tectonics during the UpperJurassic brought the source and reservoirrocks into contact with each other, as illu-strated in the Middle Jurassic play figure(figure 9). Thirty billion b.o.e. have beenfound in this play, the majority of it oil.Thetwo largest fields, Brent and Statfjord, con-tain a third of these resources.The remai-ning hydrocarbon resources are distribut-ed among 117 discoveries.

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Viking Graben

Lyell Ninian

Cretaceous (shale)

Lower Jurassic, Triassic & older sediments

Kimmeridge Clay

Tertiary

Brent

Oil discovery Oil and gas discovery

Osebjerg

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Dep

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Figure 9.The Middle Jurassic play.The yellow layer represents the “Brent” sandstone.

Steep, narrow shelf

Turbidite

Troll

Brae Miller &Magnus

Wide, sand-dominated shelf

Viking Graben

Figure 10.The Upper Jurassic play.The three main types of reservoirs are found in the fields named in the dia-gram.

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10

Only one percent of these resources areDanish. Only after the Danish Under-ground Consortium was forced torelinquish exploration areas in the begin-ning of the 1980’s exploration of the Ju-rassic formations in Denmark started.

Upper Jurassic PlaysSea level rose after deposition of theMiddle Jurassic deltaic sandstone aroundthe uplifted central land mass (in the pres-

ent-day North Sea).The entire land masswas inundated by the sea during the UpperJurassic period.At the same time a numberof faults were reactivated. Thus, the geo-logic picture of Upper Jurassic period iscomplex with regions subsiding whileother areas experienced uplift. One couldrefer to this time as a “geologist’s para-dise” because the tectonic activity, deposi-tion of reservoir sand as well as sourcerocks all took place during the Upper Ju-rassic. It could also be said that this geo-logic time period has a special place in thehearts of all North Sea oil geologists.

In every oil province there is at least onemain challenge. In the Northern NorthSea the main challenge is to find reservoirrocks. In the southern North Sea area thechallenge is to find structural closuresbelow the impervious cap rock of the Up-per Permian salt. In the Northern NorthSea the stratigraphic focus of the mainproblem is the Upper Jurassic.The reser-voirs in the other plays are relatively sim-

ple to understand and thus also simple tofind and to explore.The Upper Jurassic re-servoirs are complex. In fact, there is nosingle Upper Jurassic play, but rather se-veral, because the reservoirs are so diffe-rent from each other.The number of playsis the result of the geomorphology of theUpper Jurassic.

The two main elements are the following:1.The Upper Jurassic coastlines2.The morphology of the sea bed

(bathymetry)

The significance of these two elements isillustrated in the Upper Jurassic play figure(figure 10).

Since the Upper Jurassic reservoirs are allmarine in origin, there is some justificati-ons for regarding the Upper Jurassicdiscoveries as belonging to one play. Butresearchers and exploration geologistsrequire a more nuanced understanding ofthis period. In some areas the marine shelf

FairwaySource rocknot matured

Paleogene sand hinders

sealing

gas

Tertiary

Oil-saturated chalk

Gas cap

Triassic

Salt

0

2

4

6

8

p

Oil discovery Dry wellOil & gas discovery

Sealing shale

Tertiary

Impermeable chalk

High oil saturation

W. Ekofisk Ekofisk

Chalk2 km 20

0 m

Figure 12. High porosity and saturation under thestructural closure in one of the Norwegian chalkfields

Figure 11.The Chalk play.

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was narrow and tectonically connected toa fault.This results in one type of reservoir.In other areas the marine shelf was wideand experienced long-term inundationswhere fluctuations in sea level producedpronounced short-term variations incoastline location. Finding the latter typeof sandstone reservoir is extremely diffi-cult. In addition to the tectonic activity du-ring the Upper Jurassic period, the NorthSea area also experienced recurringearthquakes. Earthquake activity can be in-ferred from faults as well as the presenceof a particular type of sediment, called tur-bidites. Turbidite sediments begin as shal-low water sediments which are loosened,often by earthquakes, and transported inthe form of suspensions into deeper wa-ter. In principle then, there are three maintypes of reservoirs: marine sandstones de-posited on narrow shelves associated withfaults; sandstones deposited on wide shel-ves with labile coastlines; and turbiditesdeposited in deep water. The majority ofresources in the Upper Jurassic play werediscovered in sandstone deposited on wi-de labile shelves. These include the fol-lowing: the Norwegian Troll field, which isthe most productive field in the play and

also the world’s largest producing offsho-re gas field; the English Piper and Fulmar oilfields; as well as the Norwegian Ula andGyda fields. These play’s total resourcesare estimated to be 20 billion b.o.e., ofwhich almost one half, approximatley 1250

billion cubic meters of gas are found in theTroll field. Three discoveries have beenmade in Upper Jurassic sandstone withinthe Danish North Sea area, the Gert, Ravnand Elly.The reserves in these discoveriesare, however, by North Sea standards very

Tertiary UK

Cretaceous N, DK + UK

1500

1000

500

0

Year90 958580757065

Mill

ion

t.o.

e.

Shetland’s bedrock platform

a la Forties

a la Frigg

Neogene

Paleogene

Creraceous

Middle/Upper Jurassic Triassic & older

Fairway No migrationNo cap rock over reservoir

West

Discovery in a stratigraphic trap

East 0

2

4

Dep

th in

km

Gas discovery Dry wellOil discovery

Figure 14. Discovery curve for the Cretaceous and Tertiary.The figure illustrates that the large discoveries aremade early in the history, and that the majority of discoveries are made within about five years. Politically im-posed limitations on exploration can shift this picture, but at that time in the exploratory history of the NorthSea when the large chalk reservoirs in Norway and Denmark was found there was little political interferen-ce.The Tertiary discovery curve for England is completely different from the above curve. Initially, the EnglishTertiary curve has a “typical” steep shape indicating a large number of discoveries over a short period, followedby a flattening out.Then in the 10-year period between 1985-95 the curve rose steeply as the volume of foundresources doubled, the result of a number of moderate sized discoveries.This increase in discoveries was dueto new technology (3D seismic) and an improved understanding/interpretation of the geology (seismic- andsequence stratigraphy). Source: Spencer, Leckie & Chew, 1996.

Figure 13.The Paleogene play.The majority of the sand (with the exception of thaton the Shetland’s platform) was deposited as turbidites.There is also sand in Neoge-ne deposits, but this contain no hydrocarbons due to lacking cap rock.

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small. Many of the Brae Trend discoverysare associated with narrow shelves. Theturbidite sediments which were transpor-ted long distances out to sea, form the ma-in reservoirs in discoveries such as theClaymore, Miller and Magnus Fields in theMoray Firth and the Viking Graben.

The Upper Cretaceous PlayOil and gas discoveries were first made inthe chalk formations in the Danish regionof the North Sea (Anne in 1966, Roar andTyra in 1968) and oil was found in theNorwegian region (Valhall) in 1967. InDecember 1968 the first gigantic oil fielddiscovery in the North Sea was made, theEkofisk field, located in Upper Cretaceouschalk.These events were epoch-making intwo respects. First, the discoveries provedthat, at least in the northern part of theNorth Sea, there was an oil province withthe potential to be extremely profitable.Second, the discoveries indicated thatparts of the chalk formation has reservoircharacteristics. The reservoir in this playwas deposited after formation of the manyfaults which play a definitive role in forma-tion of traps in nearly all of the Jurassic di-scoveries in the North Sea. Nevertheless,this play is dependent upon structuringthrogh salt deformation as depicted in fi-gure 11. These strange pillar-like featureswere formed out of Permian salt.This ext-remely mobile salt became even more soduring the intense tectonic movement du-ring the Upper Jurassic period and wasforced upwards forming a large number ofsalt pillars (diapirs). Other diapirs whichwere formed during the Triassic period be-came active again.The rapid growth of thediapirs ceased prior to deposition of thechalk, but their tops ended close to thesurface.The majority of the oil discoverysin chalk were trapped in a structure whicharose from compression over nearly im-mobile salt structures.The discoveries inchalk are concentrated in two relativelynarrow fairways (see figure 8), one locatedentirely in the Norwegian area and the ot-her located entirely in the Danish region.The chalk play’s fairway is limited to theCentral Graben where the salt was thickenough to be deformed and the sourcerocks were thick. In addition, the salt mo-

vement created a migration route throughthe lower region of the chalk, and probablyalso created the reservoir characteristics.Not all the upper region of the chalkwithin the play’s fairway is of reservoirquality. Figure 12 illustrates the variation inoil saturation and porosity in a Norwegianchalk field.

The oil saturation in the two fields is highdirectly below the structural closures.Thisindicates that the reservoir quality decrea-ses with distance from the structure andthat this deterioration is so pronouncedthat the chalk changes from being a reser-voir to functioning as its “side seal” awayfrom the structure. The narrow fairwaysand the restriction of reservoir qualitychalk to the areas over the salt diapirs canbest be explained with the model discus-sed in the following section.One of the characteristics of a good chalkreservoir is believed to be the early migra-tion of oil into the chalk. Thus, the earlyformation of oil is a prerequisite, and this ismost likely in the deepest parts of theCentral Graben. All of the commercialdiscoveries in chalk are located in the im-mediate vicinity of the deepest parts of theCentral Basin. A good transport corridoris also a precondition to ensure that thechalk’s total volume (matrix) can be filledwith oil. Fracture formation in connectionwith the deformation in the units over thediapirs created this network of “highways”for migration. Nearness to the deepestpart of the Central Graben limits this playin the east-west direction and towards thesouth. The play does not continue to thenorth because a new reservoir (next secti-on) formed on top of the chalk, so that inthis area the chalk lacks a seal.The majori-ty of the Danish oil and gas reserves arefound in chalk.

The Tertiary PlayUntil about 1995 there were no discove-ries in the Tertiary formations in theDanish region of the North Sea, but Stat-oil’s Siri discovery changed this situation.Almost one hundred discoveries, locatedin Tertiary sands of Paleocene and Eoceneages have been made in the English andNorwegian regions of the North Sea.

Discoveries continue to be made in thisplay. Thus 4 out of 9 discoveries in theNorwegian region in 1995 were in thisplay.The most important geologic precon-dition for this play was uplift of thenorthwestern part of England-Scotlandand the “Shetlands region” in connectionwith formation of the North Atlantic.Hugequantities of sand which were eroded fromthese uplifted land masses were depositedin a relatively narrow shelf east of the pre-sent-day Shetland islands. Sand from thisshelf was pumped out in the form of turbi-dites into the deep regions of the NorthSea both into the Viking Graben andthrough the Moray Firth out into the Cen-tral Graben.This play’s fairway is thus easyto understand: the area where the playworks is limited to the extent of the sandin combination with access to a “kitchen”.The Tertiary discoveries (see figure 13) canbe divided into three types of traps.The first type is a structural trap where thePaleocene sand is draped over an under-lying raised feature. The gigantic Fortiesfield, which together with the Ekofisk fieldheralded the oil bonanza of the North Sea,are found in this type of trap. The thicksand in the Forties field is composed ofturbidites.The entire Tertiary section com-pressed under its own weight and thisthick sand was folded over a underlyingbedrock ridge, in the same way that thechalk structures were formed by com-pression over salt diapirs.The second typeof trap is composed of some of the Tertia-ry turbidites which are stacked on top ofeach other, so that they, together with theoverlying clay seal form structural traps.The largest gas field in this play, the Friggfield, was found in this type of trap.Whilethe sand in Forties is from the Paleocene,the sand in the Frigg is from the Eocene. Inthe northern North Sea the quantity ofsand decreases through the oldest part ofthe Tertiary (Paleogene).The third type ofdiscovery in the Paleogene are found instratigraphic traps where a lateral sealforms via lithological changes from the re-servoir (sandstone) to the non-reservoirrock (shale).This last type of discovery isdifficult to locate, as evident from the cur-ve of Tertiary discoveries in the English re-gion of the North Sea (figure 14).

Dan

Dan

Ekofisk

Statfjord

Troll

filled reservoir volume within these areashas seldom been attributed to insufficientsource rocks.A second condition of a largeoil field is that the reservoir must be thick.The thickest reservoirs in the North Seaare found in Upper Jurassic sandstone andconglomerates in structural traps createddue to faulting (Brae type, figure 10). Inthese fields the reservoir thickness is oftengreater than the height of the column of oil.Thus, in the fields with an exceptionallythick reservoir the bottom reservoir regi-ons are water-filled. The reservoirs of theNorth Sea giants can be 200-300 metersthick, while some reservoirs are only a fewmeters thick. Many fields on land, where asmall discovery can be commercially viablehave a thickness under 10 meters.Additio-nally, the area of the trap can vary widely.The Troll field which is the largest gas fieldin the North Sea, covers an area of 700km2.

An oil field with an area of less than 10 km2

can be large if the reservoir is thick and ofgood quality. Fulmar is a good example ofthis type of field.Before production started,the Fulmar field was estimated to containreserves of 80 million cubic meters of oil,although the field area is less than 10 km2.This apparently improbable situation can beexplained with a few simple calculations. If atotal reservoir volume in the field is calcu-lated by multiplying the area (8 km2) by the

reservoir thickness (200 m) and then sub-tracting the deduction for edge effects:8,000,000 x 200 = 1.6 billion cubic meters– edge effects (where the reservoir thinstowards the field limit) = 1.2 billion cubicmeters.The average porosity is over 20%,which is a good porosity for a sandstonefield.This gives a pore volume of 250 millioncubic meters.A good reservoir has a saturation of over80%, meaning that more than 80% of thefluid in the pore spaces is oil. Based on thisassumed oil saturation, the oil volume inthe reservoir would be over 200 millioncubic meters.With today’s technology it ispossible to recover more than 50% of theoriginal “in place” reserves. Finally, the factthat the volume of oil shrinks during theprocess of extraction to the surface mustbe considered. These calculations demon-strate that the Fulmar oil field could conta-in 80 million cubic meters of oil eventhough it’s surface area was so limited.

The North Sea’s largest oil field, the Stat-ford, originally had “in place” ressources ofmore than 1,000 million cubic meters andcovered an area of approximately 140 km2.Prior to production, it was expected thatabout 40% of these reserves could be ex-tracted. However, after production com-menced the expected degree of recoveryhas been increased upwards to 60%

Everyday 100,000 cubic meters of oil areproduced from the Statfjord field which ismore than twice the volume of the totalDanish oil production in the North Sea.

The Ekofisk chalk reservoir contains the sa-me volume of “in place” ressources as theStatfjord field. The degree of recovery atthe Ekofisk field is 40%, and although this isexceptionally high for a chalk reservoir, itmeans that in practical terms the pro-ducible Ekofisk reserves are less than thoseof the Statfjord field.The largest Danish oilfield, the Dan,has “in place” reserves of 500million cubic meters, but with an antici-pated degree of recovery of only 20%, theextractable resources will be approximate-ly 100 million cubic meters.

There are some areas in the North Seawhere “infrastructure” is in place. Infra-structure refers to the production plat-form, where the well steam can be separa-ted into its various components (water, oiland gas) as well as an oil and gas transportsystem (for example a pipe or loadingsystem to a tanker vessel). If these facilitiesare in place, then it may be profitable to ex-ploit a small discovery with reserves of 1-2million cubic meters. Thus, this discussionmakes it clear that size is a very relativeterm when considering the commercialprofitability of an oil field.

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Figure 15. Size of a gas field and three oil fields in the North Seacompared to the size of the island of Funen in Denmark.

Something about Size

What makes an oil field large? There arethree simple elements. First, there must bea suitable quantity of source rock present.This condition is more than met in the cen-tral and northern North Sea and in thesouthern gas province. An incompletely

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Oil, the sisters and …

Denmark started oil produc-

tion from the Dan field in 1972 and became

self sufficient early in the nineties. Coal is still

imported, but today Denmark exports both

gas and oil.The degree of total energy self suf-

ficiency in Denmark is now over 100% of

which 8% is supplied by renewable resources,

and the remainder is supplied by oil and gas. In

terms of the national economy, Denmark’s in-

creasing energy self sufficiency together with

falling oil prices has put Denmark in the good

company of other countries with strong eco-

nomies.

One could ask why the multi-national oil com-

panies, the survivors of “The seven sisters”

and the new oil moguls have such a prominent

role in the industry.Wouldn’t it be possible for

a country such as Denmark to create a state-

owned oil company, the profits of which

would supply the national coffers? In order to

answer these questions we must understand

the undertaking of a large oil company. The

large multi-national oil companies find the hy-

drocarbons.They finances construction of the

production plants, where raw oil and gas are

refined, as well as construction of storage and

transportation networks. The oil companies

earn money by selling the processed hydro-

carbons to consumers.This chain of activities

requires large amounts of capital and is asso-

ciated with numerous financial risks. It is the

element of risk which protects the oil compa-

nies from being absorbed into the public

sector. Politicians in capitalistic countries refu-

se to take responsibility for the financial risks

associated with hydrocarbon exploration,

production and transportation. The multi-na-

tional oil companies distribute their risk

throughout the world, and this is an economic

necessity in the oil industry.Today, even coun-

tries with many years experience with state-

owned oil companies such as Venezuela, for-

mer USSR countries and Iran, to name a few,

invite multi-national companies to participate

in exploration and production.This trend may

change, but it is unlikely until oil reserves are

severely reduced. In the meantime we must

learn to live with the multi-national oil com-

panies.The task of each nation is to do this in

a way which is most beneficial to its citizens.

The discussion regarding the

final disposal of the Brent Spar platform, and

similar oil-related issues in the news media

may have lead to a belief that oil activities in

the North Sea are associated with huge envi-

ronmental risks. In fact, the countries surro-

unding the North Sea have regulated activities

to such an extent that adverse environmental

effects associated with exploration and pro-

duction are minuscule in comparison to many

other environmental threats. Similarly, oil con-

tamination on beaches can, almost always, be

linked to tanker vessels which clean their

holds en route to the North Sea. Now and

then a ship runs aground,and crude oil washes

ashore.Again, the problem is not hydrocarbon

recovery, but rather the limited possibilities

available to politicians to regulate ships and

shipping traffic.The major environmental pro-

blem associated with oil and gas arises when

these hydrocarbons are combusted. In the

North Sea region and in the hydrocarbon-

consuming parts of the world this environ-

mental issue is, in reality, a lifestyle issue (see

figure 1) and not a problem which can be

solved by demanding that oil companies chan-

ge their practices. In other parts of the world,

where public authorities lack the power to

impose or enforce environmental legislation,

hydrocarbons may be produced under less

stringent environmental control.Thus, we can

help the environment in two ways:

1. By decreasing our energy consumption

2. By helping hydrocarbon-producing

developing nations to formulate environ-

mental laws and assist them in enforcing

them against oil companies.

By decreasing our energy consumption, parti-

cularly of fossil fuels we will also extend the

period in which oil can be purchased for a

reasonable price. Falling oil production in the

North Sea and Alaska must be expected in the

near future. We and our governments must

decide whether we will act now and plan for a

future with expensive oil, or whether we will

wait passively until the energy crisis comes.

It is our duty as a society to

ensure that precious non-renewable hydro-

carbon resources are utilised in a way that is

best for the society as a whole.What is good

economy for an oil company is not necessari-

ly the same for a society. It may be better to

produce 20 million tons of oil from a field with

a few production wells over 15 years rather

than to produce 30 million tons over 20 years

with more wells. In order to enter into a con-

structive discussion with multi-national oil

companies regarding exploitation of these re-

sources society must develop knowledge and

competence within a number of fields. GEUS

is the Danish authorities’ “professional right

hand” in these discussions. We have earned

this role partly through work undertaken on

behalf of the Danish Energy Agency (Energi-

styrelsen) and through scientific investigations

and research. Furthermore GEUS provides

consulting services to oil companies.This last

activity is important because it gives GEUS the

opportunity to follow both the technological

and scientific developments within the fields

of hydrocarbon exploration and production.

The relevance of our consulting services

would quickly deteriorate without these

updates.Additionally, these consulting projects

uncover problems or points of dispute which

may be relevant research projects. Clearly the

triangle of interconnected activities - research

– expert advisor – consulting work is not just

an economic necessity, it also ensures that

GEUS’s competence evolves simultaneously

with that of the oil industry.This is crucial to

GEUS’s ability to assist authorities in regula-

ting the hydrocarbon industry. It also means

that GEUS has a certain independence to

refuse projects which are not relevant to our

fields of interest, allowing us to select projects

which are both relevant and which will further

develop our competence. This is an ongoing

process.There are still some areas of petrole-

um geology, exploration and production

where our knowledge is lacking, but we are

constantly striving to keep our scientific

knowledge current, not only for ourselves, but

also for the sake of society.

SOCIETY ENVIRONMENTTHE

RESEARCH

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Cap rock: a low permeability rock formation through which hydrocar-bons are unable to migrate.The cap rock acts like a seal, trapping the hydrocarbons in the underlying forma-tion.

Deltaic: deposited in a delta environment.Dry gas: natural gas with a low content of

liquid hydrocarbons.Exploration well: a well which is drilled

to determine whether a prospect con-tains producable hydrocarbons. Often an exploration well recovers only wa-ter.

Fairway: an area where all the compo-nents of a play are present.

Fault: a fault or fault zone along which there has been displacement of the sides relative to one another parallel tothe fracture.

Hypersaline: a liquid having a salt contentwhich is much higher than that of normal ocean water.

Hydrocarbons: a general term referring to organic material composed exclusi-vely of carbon (C ) and hydrogen (H).

Kitchen: refers to the area in the un-derground where hydrocarbons form.

Light oil: oil with a low specific gravity.Mesozoic: formed during the earth’s

middle age (Triassic, Jurassic and Cretaceous).

Migration: the movement of hydrocarbons from their source rock (the kitchen) through permeable rocksto a reservoir.

Migration route: the path taken by the majority of the hydrocarbons from thekitchen to a reservoir.

Morphology: the shape and structure of any given surface.

Play: as an abstract concept a play refers to a set of conditions which must metin order for hydrocarbons to accumu

late in a reservoir rock.As a concrete term a play refers to any number of established discoveries or prospects where existing geological evidence indicates that the requirements for a playare present, or are likely present.The set of conditions include a sourcerock, a porous and permeable reser-voir covered by a seal and with a suitable natural configuration so that hydrocarbons cannot migrate out of the trap.

Prospect: A hydrocarbon trap which has not yet been investigated by explora-tory drilling.

Red beds: sedimentary strata composed primarily of sand and clay with a characteristic red colour derived fromthe presence of the iron-rich mineral hematite, which coats the individual grains.

Reflector: refers to a surface which sepa-rates two rock formations each with different seismic characteristics.A re-flector is found during a seismic inves-tigation where sound waves are shot through the underground and reflec-ted back from these surfaces.

Reservoir: a porous and permeable geological layer which contains oil andgas.

Rift zone: a regional scale system of down-throwing faults in the earth’s crust.

Salt diapir: a column of salt which has ri-sen through the overlying rock forma-tions from a salt layer which is often lo-cated 2-6 km under the top of the saltcolumns.The diapirs are typically 1-5 km in diameter and the driving force for their movement is buoyancy due tothe low density of salt

Shelf: the slightly-sloping, underwater re-gion of a continent located between

the shoreline and the continental slo-pe.The width of the continental shelf isnormally delineated by an ocean depthof 200 m, or by the edge of the conti-nental slope.

Shelf sea: a shallow sea situated on the continental shelf which rarely exceedsa depth of 2-300 m, for example the North Sea.

Source rock: all the types of rock in which oil and gas can form.

Stratigraphic trap: a trap for oil or gas which is the result of lithologic changesin a rock formation or changes in its extent rather than structural configu-ration.

Structural trap: a trap for oil or gas which is the result of folding, faulting orother deformation.

Tectonic activity: movement in the earth’s crust for example earthquakes,faulting and other deformations whichoccur due to forces involved in tecto-nics.

Test: an experimental production of oil orgas to the surface after a discovery has been made during exploratory drilling.

Trap: any type of barrier (lithological or structural) to upward movement (mi-gration) of oil or gas allowing these toaccumulate in underlying formations.Atrap includes reservoir rock and an im-permeable cap rock.

Turbidite: a sediment deposited by a turbidity current, which is a tongue-shaped current of suspended material which flows from shallow waters to deeper water after being loosened,often by an earthquake.

Wet/rich gas: a natural gas containing liquid hydrocarbons.

Dictionary

This theme issue, Oil and Gas Exploration in the North Sea, was written by head of department Kai Sørensen. Priorto his employment at GEUS, he was employed at Statoil, first as a senior geologist in Stavanger (Norway) and lateras Exploration Manager in Denmark. He has also taught at Århus University and Denmark’s Technical University.He has been a visiting researcher at Imperial College in London, at M.I.T. in Boston and at Cornell University.Kai Sørensen is the head of the geophysical department at GEUS.

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The Geological Survey of Denmark andGreenland is a research and advisory in-stitute in the Ministry of the Environment.The primary goal of the institute is toconduct scientific and practical investiga-tions of the environment and energy-re-lated subjects, as well as geological map-ping of Denmark and Greenland.GEUS can also be contracted to conductinvestigations on behalf of others undernormal terms of business.

A subscription to GEOLOGY – Newsfrom GEUS can be ordered free of char-ge by contacting Knud Binzer.The magazi-ne has 4 issues each year.Only selected is-sues are available in English.

GEUS is pleased to provide further infor-mation regarding the topics discussed inthis issue, or other topics of a geologicalnature.It is permitted to reproduce sections ofthis issue on the condition that the infor-mation is properly referenced.

GEOLOGY –News from GEUS is editedby geologist Knud Binzer and Klaus Hins-by in collaboration with an editorialgroup at the institute.Consultant: Marianne Vasard Nielsen.

Write, telephone, or email:GEUSGeological Survey of Denmark and GreenlandThoravej 8DK-2400 Copenhagen NV, DenmarkTel: + 45 38 14 20 00Fax: + 45 38 14 20 50Email: geus@geus.dk Website: www.geus.dk

GEUS Publications All GEUS publications can be purchasedat Geografforlaget (the Geographic Pub-lishing Company).They may be contacted via telephone:+45 63 44 16 83 or telefax: +45 63 44 1697, email: go@geografforlaget.dk, orwebsite: www.geografforlaget.dk

Their address is: Geografforlaget,DK-5464 Brenderup

ISSN 1396-2353

Production: H.K. Pedersen, GEUSPrinting: From & Co.Cover photo: Statoil A/SIllustrations: Carsten ThuesenTranslation: MapleLeaf MiljøtekniskeOversættelser.

G E U S

The following articles contain statisti-cs regarding the number of discove ri-es and hydrocarbon resources:A.M. Spencer, G.G. Leckie & K.J. Chew:North Sea hydrocarbon plays and their re-sources (an article in a Geological Society

Special Publication on northwestern Euro-pe’s hydrocarbon industry by K.W. Glennieand A. Hurst, 1996. Figures 6, 8 and 13 arebased on numbers found in this article.

An in-depth description of the petro-

leum geology and exploration historyof the North Sea:K.W. Glennie (ed.) Introduction to the Pe-troleum Geology of the North Sea. Black-well Scientific Publications, Oxford 1990(3rd edition).

Further Reading

New publication from GEUS

The annual Review of Greenland acti-vities is a special bulletin regarding re-search in Greenland and off-shore areas,including the north Atlantic and Arctic. Itcontains review articles on primary acti-vities written in a style that enables otherthan professionals to get an all-round im-pression of GEUS research in Greenland.

Articles in Review of Greenland activitiesbulletins are available as pdf-files from1996 to present at the GEUS website.

www.geus.dk/publications/publ-dk.htm

GEOLOGICAL SURVEY OF DENMARK AND GREENLANDMINISTRY OF ENVIRONMENT AND ENERGY

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