regional velocity-depth anomaly

8/13/2019 Regional Velocity-Depth Anomaly

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ABSTRACT

A normal veloci ty -depth trend for the UpperCretaceousDanian Chalk Group is determined byidentifying interval-velocity data that representmaximum burial in areas unaffected by overpres-suring; these data are derived from 845 wellsthroughout the North Sea Basin. Data from pelagiccarbonate deposits on a stable plateau constrainthe trend for shallow depths. Positive velocityanomalies relative to the trend are mapped alongthe western and eastern margins of the North SeaBasin, and reflect regional Neogene uplift and ero-sion of up to 1 km along the present-day limit ofthe Chalk. A hiatus at the base of the Quaternaryincreases in magnitude away from the basin center,

where a complete Cenozoic succession is found.This hiatus is consistent in size with the missingsection estimated from Chalk velocities whenallowance is made for the Quaternary reburial ofthe Chalk. Negative velocity anomalies in the cen-tral and southern parts of the basin outline an area

within which overpressures in the Chalk exceed10 MPa, equivalent to a bur ial anomaly greaterthan 1 km relative to the normal trend. The Chalk

pressure system is primarily dependent on overbden properties because retention of overpressgenerated by the load of the upper overburddepends on the thickness and sealing quality of lower overburden; therefore, the Chalk is conered to represent a regional aquitard, and hydrodynamic model of long-distance migrat

within the Chalk is rejected. The Neogene upand erosion of the margins of the North Sea Baand the rapid, late Cenozoic subsidence of its cter fit into a pattern of late Cenozoic vertical moments around the North Atlantic.

TABLE OF CONTENTS

IntroductionVelocity Anomaly and Burial AnomalyDatabaseDerivation of the Normal Velocity-Depth TrendReduction of Chalk Porosity With Depth

Areas of Velocity Anomaly in the North Sea BasiNeogene Exhumation of the North Sea BasinOverpressuring of the North Sea Chalk AquitardConsequences for Depth ConversionDiscussionConclusions

Appendix 1: List of SymbolsAppendix 2: Comparison of Compaction

Trends for ChalkAppendix 3: Velocity-Porosity Conversion

for ChalkReferences Cited

INTRODUCTION

The Upper CretaceousDanian Chalk Group foa coherent body in the North Sea region covermore than 500,000 km2, with an average thicknesabout 500 m (Figures 1, 2). Clastic influx into North Sea Basin was low during the deposition ofChalk, which is composed mainly of coccoliths, debris of planktonic algae (Kennedy, 1987; Zieg1990). Today, the Chalk crops out in most countin northwest Europe, but is buried at depths grea

2

Copyright 1998. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received June 20, 1996; revised manuscript received March16, 1998; final acceptance April 15, 1998.

2Geological Survey of Denmark and Greenland (GEUS), Thoravej 8,DK-2400 Copenhagen NV, Denmark; e-mail: [email protected]

This study was made possible through the generous support of theCarlsberg Foundation and GEUS. Petroleum Information (Erico) is thankedfor giving me permission to use Chalk pressure data from its British andNorwegian pressure studies, and for placing the British and most of theNorwegian well velocity data at my disposal; without the backing of PeterSheil and Stuart Thomas, both Petroleum Information (Erico), this studywould not have been possible. Statoil is thanked for giving me access to welldata. Christian Hermanrud, Erik Vik, and Lars Wensaas at the StatoilResearch Center in Trondheim, Norway, helped me with many basicquestions. The Geological Survey of the Netherlands is thanked for giving meaccess to pressure data. Per Knudsen, National Survey and Cadastre-Denmark, advised me on the kriging technique, and Ida Lind, DanishTechnical University, took part in many considerations. I thank colleagueswho have supported me in many ways, in particular Torben Bidstrup, JimChalmers, Anders Mathiesen, and Jens Jrgen Mller. Jens Clausen, Dopas;Finn Surlyk, University of Copenhagen; and Claus Andersen, Thomas Dons,Jon Ineson, Peter Konradi, and Birger Larsen, all GEUS, provided valuablecomments on different parts of the manuscript. Finally, editors and journalreferees are thanked for their penetrative and constructive reviews.

Regional Velocity-Depth Anomalies, North Sea Chalk: ARecord of Overpressure and Neogene Uplift and Erosion

Peter Japsen2

AAPG Bulletin, V. 82, No. 11 (November 1998), P. 20312074.


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than 3 km in the central North Sea (Figures 3, 4). TheCenozoic North Sea Basin gently bends from its mar-gins to its depocenter, aligned over the mainly Late

Jurassic Central and Viking grabens (Figure 2C).

Nature thus provides a spectacular laboratory to studythe effect of large depth variations on the compactionof the Chalk, and hence on velocity, which varies overa range of 3000 m/s.

The compaction of the Chalk does not every-where correspond to it s present buria l depths(Scholle, 1977). Along the margins of the North SeaBasin, the thickness of the Chalk overburden hasbeen reduced. The regional character of thisexhumation was recognized only recently, and themagnitude (up to 1 km), timing, and causes of thisexhumation are still disputed (e.g., Bulat andStoker, 1987; Jensen et al., 1992; Japsen, 1993a,1997; Hillis, 1995a; Rohrman and van der Beek,

1996). In the central North Sea, the Chalk is over-pressured (up to 20 MPa at 2600 m depth), mainlydue to rapid, late Cenozoic burial (Carstens, 1978;

Japsen, 1994). Major commercial interests are relat-

ed to the porosity of the Chalk because giant hydro-carbon accumulations are trapped in the Chalk inthe Norwegian and Danish sectors of the centralNorth Sea (e.g., the Ekofisk and Dan fields).

From the observation that the Chalk over largeareas is far from normally compacted, I here pres-ent a new normal velocity-depth trend for theChalk Group based on North Sea data constrainedby data from pelagic carbonate deposits on a stableplateau (Shipboard Scientific Party, 1991; Urmos etal., 1993). Normal compaction curves for the Chalkhave been suggested by previous workers (Scholle,1977; Sclater and Christie, 1980; Bulat and Stoker,1987; Hillis, 1995a). The trend presented here,

2032 Velocity-Depth Anomalies, North Sea Chalk

Figure 1Lithostratigraphic nomenclature for the Upper CretaceousCenozoic in the North Sea Basin. This studyfollows the nomenclature used in the Danish, Dutch, and United Kingdom sectors by using the term Chalk Groupto refer to the chalky limestone facies as opposed to the mudstone facies of the Shetland Group (Johnson and Lott,1993). The Post Chalk Group (Nielsen and Japsen, 1991) is subdivided into an upper and lower part at the mid-Miocene unconformity (Japsen, 1994).

Figure 2Late CretaceousCenozoic geology of the North Sea Basin. The Chalk crops out along the basin margins,but is buried below more than 3000 m of Cenozoic cover in the center of the basin. (A) Isopach of the Post ChalkGroup. (B) Isopach of the Chalk Group. (C) Late CretaceousCenozoic structural elements. In (A and B), Edb. (Edin-

burgh) and Cph. (Copenhagen) mark the location of the depth profile on Figure 3. Modified after Ziegler (1990),with corrections from well data and Andrews et al. (1990), Britze et al. (1995a, b), Cameron et al. (1992), Day et al.(1981), Gatliff et al. (1994), Isaksen and Tonstad (1989), Japsen and Langtofte (1991), Johnson and Lott (1993), Knoxand Holloway (1992), Kockel (1988a, b), and Ter-Borch (1990).

This study

north centr.

PostChalkGroup

upper

lower

N o r t h S e a B a s i n

Mesozoic

Cenozoic

Up

perCretaceous

Tertiary

Qua.

Paleogene

Neog.

Chronostratigraphy

Danien

Maastrichtian

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Campanian

Santonian

Coniacian

Turonian

Cenomanian

ChalkGroup

ShetlandGroup

Danishsector

Michelsen(1982)

north centr.

UKsector

Knox & Holloway (1992)

Johnson & Lott (1993)

NordlandGroup

Westray Group

Stronsay GroupMoray Group

Montrose Group

ChalkGroup

ShetlandGroup

Norwegiansector

Isaksen & Tonstad(1989)

centr.north

NordlandGroup

HordalandGroup

ShetlandGroup

Rogaland Group

central

ChalkGroup

ChalkGroup

Upper

Lower

Middle

NorthSeaSupergroup

southeast

Dutchsector

NAM & RDG(1980)


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Japsen 2

UTM zone 31

200 km

0 44 8 12

0 4 8 12

59

57

55

53

59

57

55

53

N

0 44 8 12

0 4 8 12

59

57

55

53

59

57

55

53

Cph.Edb.

?

Sh

etl

and

Group

2W 2E 6 10 14

58

56

54

58

56

54

1062E2W

(A)

2W 2E 6 10 14

58

56

54

1062E2W

54

56

58

(B)

DANISH B.: Danish Basin

CENTR. GR.: Central Graben

FENNOSCAN. H.: Fenno-

MF: Moray Firth

MNSH: Mid North Sea High

N SEA BASIN: North Sea Basi

RFH: Ringkbing-Fyn High

VG: Viking Graben

scandien Hi

DK: DenmarkG : GermanyN : NorwayNL: NetherlandsS :SwedenUK: United KingdomCph.

Edb.

Thickness (m)

Thickness (m)

Post Chalk GroupCenozoic excl. Danian

IsochoreC.i. 500 m

Fault trace

Thin Quat. coverBelow 500

Above 3500

UTM zone 31

200 km

50010001500

- 1000- 1500- 2000

5001000

1500200025003000

- 1000- 1500

- 2000- 2500- 3000- 3500

UK

DK

GNL

N

S

Index map

Chalk GroupUpper Cretaceous - Danian

IsochoreC.i. 500 m

Fault trace

Limit of Tertiary sediments (excl. Danian)

Overlap of Chalk Group and Shetland Group Inversion axis

N S E A B A S I N

(C)M F

M N S H

FENNOSCANVG

DANISRFH

CENTR

.

GR

.

Below 500

Above 2000


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however, is the first that is derived from data cover-ing the full depth range and from the full lateralextent of the Chalk. If overpressure and regionalerosion are not taken into account, the increase of

velocity with depth is underestimated, resulting inerroneous depth conversions. The trend isexpressed as four linear segments because a singlemathematical function fails to reflect the depth

variations in the compaction process.I interpret velocity anomalies relative to the nor-

mal trend (Japsen, 1993a) to be related on a region-al scale to the burial history of the Chalk (Japsen,1993b). Consequently, I introduce the correspond-ing concept of burial anomaly relative to a normal

velocity-depth trend. Estimates of maximum burialbased on Chalk velocity anomalies are comparableto estimates based on other methods; furthermore,North Sea pressure data confirm the level of over-pressure, as well as the areal extent of the overpres-sured zone, predicted from Chalk velocities in the

central and southern North Sea. Estimates of max-imum burial and overpressure frequently havebeen based on shale data, due to the uniformityof shale over large distances (e.g., Herring, 1973;Carstens, 1978; Magara, 1978; Chiarelli andDuffaud, 1980; Hansen, 1996). The exhumationof the North Sea Basin also has been estimatedfrom Chalk data because the Chalk is widespreadand relatively homogeneous (Bulat and Stoker,1987; Hillis et al., 1994; Hillis, 1995a). Estimatesof overpressure based on Chalk data have not pre-

viously been presented.

VELOCITY ANOMALY AND BURIAL ANOMALY

The normal velocity-depth trend, VN(z), for asedimentary rock expresses the increase of velocityas porosity is reduced during normal compaction,

where pore pressure is hydrostatic and the burial


Figure 3Burial of the Chalk Group across the North Sea with indication of both Mesozoic and LateCretaceousCenozoic structural elements. Depths below sea bed (water depth


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Japsen 2

Figure4Pre-Quate

rnarygeologyoftheNorthSeaBasin.Notethesymmetryacrossthebasin.BasedonAndrewsetal.(1990),Bidstrup(1994),

Cameronetal.(1987

,(1992),ChoubertandFaure-Murat(1976),Gatliffetal.(1994),HkanssonandPedersen(1992),Johnsonet

al.(1993),Jordtetal.

59

57

55

53

UK

N

57

N

DK

DK

S

55

53

NL

D

DDK

UK

NL

4

0

4

8

12

2W

2E

6

10

14

58

56

54

54

56

58

Edb.

Cph.

12

13

14

15

16

15

16

17

18

20

21

22

7

8

9

10

3

139

383

023

29

37

362

8

27

42

43

44

AE

FB

K

M

L

49

47

48

P

Q

5610

5606

5607

5608

5609

5604

5605

5508

5507

5506

5505

5504

5509

5510

551

1

5

411

19

2

52

53

5709

5710

5512

8

12

0

4

10

2E

2W

6

200km

UTMz

one31

Pre-Quaternary

Geology

Pliocen

e

Miocene-Pliocene

Miocene

(Pliocenethinorabsent)

Miocene

Paleogeneexcl.Danian

Paleogeneexcl.Danian

(Pliocenethinorabsent)

UpperC

retaceous-Danian

Mesozo

icexcl.L.Cret.

Pre-Me

sozoic

?

?

?

??

?

?


6/44

depth of the rock is not reduced {V= the instanta-neous velocity [in meters/second (m/s)] measuredover a thin unit at depthz[in meters (m)]; see listof symbols in Appendix 1}. The velocity trendshould be constrained by knowledge about the

veloci ty of the rock at the surface and at infinitedepth. Let VNbe given by a linear approximation,

where V0 = the velocity at the surface and k = thevelocity-depth gradient [in meters/second/meters(m/s/m)]. Based on this approximation, an expres-sion can be developed for the interval velocity, Vi,measured over a layer of thickness z(in meters)and two-way traveltime thickness T[in seconds(s)] (Slotnick, 1936; Japsen, 1993a). The velocity

anomaly, dV(m/s), has been defined as a correctionto the linear velocity model to calibrate the modelto well data (Japsen, 1993a):

(1)

where z, T, and depth to the top of layer,zt,are well data. Lateral variations of dVare thecombined expression of lateral variation in V0and k caused by differences in both lithology andpore fluids, and in the burial history of the rock(Figure 5) (Japsen, 1993a, 1994). A velocity-depth model given by linear segments must bedefined for intervals of Vrather thanzbecause

veloci ty i s the i r reversible parameter. Theveloci ty anomaly for a segmented model may be

dV = kz(e kT/2- 1)-1- V0- kzt

VN= V0 + k z


Figure 5Velocity anomaly (dV) and burial anomaly (dZB) as expression of reduced burial and undercompactiondue to overpressure (equations 1, 2). Retarded compaction due to rapid burial and low permeability causes over-pressure, and hence velocities low relative to depth (negative dVand positive dZB) (equation 5). Uplift and erosionmay reduce the overburden and cause overcompaction expressed as velocities that are high relative to depth (posi-tive dVand negative dZB). The normalized depth, zN, is the depth corresponding to normal compaction as predicted

by the normal velocity-depth trend for the measured velocity.

Overpressuring

Steady burial

Normal compaction

Vi

Rapid burial,

low permeability

Undercompaction

Uplift and erosion

Overcompaction

Origin Reduced overburden

dV

Vi

Vnorm

al

V=V

o+kz

Z

Velocity

Depth

Z

Z

ZNZN

dZB= - dV/k dZB = - dV/k

dZB

dZB

Vi

dZB

dZB

Vi

dV

{{


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approximated by calculating dVrelative to thesegment given for the actual interval velocity.

The buri al anomaly, dZB (m), is introducedhere as the difference between the present burialof the rock,z, and the normalized depth,zN, cor-responding to normal compaction as predictedby the normal velocity-depth trend for the mea-sured velocity (Figure 5). The velocity-depth gra-

dient, k, expresses the relation between depthand velocity along the linear trend; consequentlywe get

(2)

where the minus indicates that a positive velocityanomaly corresponds to a reduction in depth.The term burial anomaly is neutral as to whatcaused the anomaly, and indicates only that thedepth of the rock is anomalous relative to a refer-ence trend. Burial anomaly, as well as velocityanomaly, is zero for a normally compacted rock;

that is, a rock at maximum burial and hydrostaticpore pressure.

Negative Burial Anomaly Due to OverburdenReduction

A negative burial anomaly may indicate overcom-paction due to a reduction in burial depth but, aspointed out by Bulat and Stoker (1987), factorsother than burial influence velocity. The termuplift [apparent uplift of Bulat and Stoker (1987)or net uplift of Riis and Jensen (1992)] is not usedhere, as uplift of rocks (relative to the geoid) mustbe distinguished from exhumation of rocks (rela-tive to the Earths surface) (England and Molnar,1990). A geological formation can be consideredexhumed only when it has been returned to thesurface, not when the overburden has been partial-ly removed; consequently, the term apparentexhumation (Hillis, 1995a) is inappropriate (com-pare Japsen, 1997). The quantity estimated inexhumation studies is thus the reduction in over-burden thickness, or the burial anomaly, for a for-mation. Overburden reduction may be estimatedby physical methods based on measurements of

velocity, density, vitrinite ref lectance, or fissiontracks, or by inference from known geology in adja-cent areas. All methods require comparison withsome absolute standard, which is particularly diffi-cult to establish in the method based on knowngeology. To infer actual exhumation from velocitydata, the geological unit in question should be ofrelative homogeneous lithology, and should begeographically widespread to determine wherenormal compaction is present. If a lithologically

homogeneous unit is thick and known from mwells, statistical uncertainties are reduced.

The missing overburden section, zmiss,moved by erosion only equals the magnitudethe burial anomaly if no burial took place subquent to exhumation (Figure 6A). Any poexhumational burial,BE, will mask the magnituof the missing section, and we get (Figure (Hillis, 1995a)

Consequently, a pre-Quaternary erosion of 500will be masked by a subsequent Quaternary buof 500 m. Equation 3 implies that only when timing of the exhumation is known do we knowand are able to infer zmiss. The timing thbecomes a critical aspect not only for understaing the succession of events, but also for undstanding their true magnitude and for identifythe age of the eroded succession.

Positive Burial Anomaly Due to Overpressu

A posi tive burial anomaly may indicate undcompaction due to overpressure. Overpressure[in Pascals (Pa)] (1 MPa = 145 psi), is the differe

zmiss = - dZB + BE

dZB = - dV/k

Japsen 2

(A)

(B)

Depth(km)

Time (Ma)

Zmiss= dZ

Zmiss= BE

BE

dZB

dZB

Figure 6Schematic burial diagrams illustrating that

magnitude of the missing overburden section (Dzmwill be less than the magnitude of the measured buanomaly (dZB) in the case of post-exhumational bu(BE) (equation 3). (A) Exhumation followed by no desition; (B) exhumation followed by burial.


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between the measured formation pressure,P, andthe calculated hydrostatic pressure,PH, at depthz:

where f[in kilograms per cubic meter (kg/m3)] isthe mean pore fluid density of the overburden, and

gis the gravitational acceleration (9.807 m/s2). Thelithostatic pressure, S(Pa), at depthzis the stressexerted by the weight of the overburden:

where b is the mean bulk density (wet). Terzaghisprinciple states that the weight of the overburdenper unit area, S, is borne partly by the rock matrixand partly by the pore fluid:

where (Pa) is the effective stress that is transmit-ted through the matrix (Terzaghi and Peck, 1968).

Overpressure is generated by disequilibriumcompaction when the weight of the overburden isincreased by addition of sediments at the surface,and the pore fluid in the formation is sealed in theformation (Dickinson, 1953; Rubey and Hubbert,1959; Osborne and Swarbrick, 1997). The rock isunable to compact because the pore fluid cannotescape at the same rate as load is added to the over-burden of the rock. Consequently, the additionalload is carried by pore fluids, and pressures higherthan hydrostatic pressure result. The rock is said tobe undercompacted because porosity becomeshigh relative to depth.

Let the overburden to an overpressured unit (e.g.,the North Sea Chalk) be divided into a normallycompacted upper unit and an overpressured lowerunit, and let the burial rate accelerate during thedeposition of the upper unit. The maximum over-pressure generated by disequilibrium compactionthus may be approximated by the effective load, up,of the upper unit that initiated the overpressure byrapid burial (compare Rubey and Hubbert, 1959):

(4)

where up is the density contrast (wet bulk densi-ty minus pore fluid density) in the upper part ofthe overburden, and zup is its thickness. In thecentral North Sea, up is slightly above 1 103kg/m3, so up zup/100 MPa when zup is inmeters, meaning that deposition of 1000 m of sedi-ment may generate an overpressure of 10 MPa.

The overpressure of an undercompacted rock,Pcomp, is proportional to the burial anomaly, dZB,if the effective stress is increasing with time (com-pare Hubbert and Rubey, 1959; Magara, 1978):

(5)

Equation 5 is based on Terzaghis principle andstates that if a rock is shifted to a greater depth bydZBwithout change in the effective stress (indicat-ed by unchanged velocity), the effective stress ofthe added load is carried by an increase in porepressure (Figure 5). We get Pcomp dZB/100 MPaif we substitute up = 1 103 kg/m3, and dZB is inmeters. This means that a burial anomaly of 1000 mreflects overpressure due to undercompaction of10 MPa.

The effective stress, however, may be reducedwith time even during continuous sedimentation.Such unloading may take place if overpressureincreases due to transference (redistribution of

overpressure) or by buoyancy when brine is substi-tuted by hydrocarbons. Unloading leads to a higheroverpressure, P, than the paleo-overpressure,Pcomp, that prevailed at the time when the effec-tive stress was at maximum. As compaction is large-ly irreversible, the burial anomaly reflects thepaleo-overpressure and not the actual overpressureafter unloading.

Net Drainage CapacityIf a rock were completely sealed off when over-

pressure was induced by rapid burial, the porefluid of the rock would carry the effective stress ofthe weight added:

Thus, from equations 4 and 5 we get zup = dZB,meaning that the burial anomaly equals the thick-ness of the load added since the onset of overpres-sure. The burial anomaly thus is generally a fractionof zup depending on the efficiency of the rock todewater. The net drainage capacity,DC(%), is thusintroduced as

(6)

The net drainage capacity expresses how closethe rock is to compaction equilibrium relative tothe rapid, late loading, zup. If no drainage, andconsequently no compaction, has taken place, dZB= zup, and the drainage capacity for the rock is0%. If the rock is normally compacted, dZB = 0, and

DC = (1 - dZB/zup) 100

Pcomp = up

Pcomp = upg dZB

up = upg zup

s = + P

s = bg z

P = P PH= P f g z



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DCbecomes 100%. The drainage capacity does,however, only account for the net drainage if porefluid is added to the rock from above or below.

The drainage capacity,DC(equation 6), dependson the maximum effective stress exerted on therock and not on the present overpressure. Adrainage capacity relative to overpressure,DCP,can be calculated by substituting the burial anoma-

ly, dZB, in equation 6 by P 100 (Pin MPa):

(7)

Because Pcomp (equation 5) is only part of the totaloverpressure, we getDCPDC.

DATABASE

Chalk Velocity Data

The database for this study contains interval

velocities for the Chalk Group measured in 845wells that penetrate d the Chalk in the Brit ish(UK), Danish (DK), Dutch (NL), and Norwegian(N) sectors of the North Sea Basin (Figures 7, 8).Interval velocity is calculated by dividing thethickness of the Chalk by the corresponding tran-sit time determined from calibrated sonic logs. Inthe Viking Graben, the mudstone facies of theUpper Cretaceous Shetland Group substitutes forthe chalky limestone facies of the Chalk Group,

whereas the two fa cies overlap south of theViking Graben (Johnson and Lott, 1993) (Figures1, 2B). The range of the depth and velocity datafor the Chalk is considerable: The depth to thetop of the Chalk ranges from sea level to 3350 mbelow the sea bed; the Chalk thickness rangesfrom 50 to 1850 m with a mean of 520 m; and theChalk interval velocity ranges from 2290 to 5350m/s (Tables 1, 2).

All logs and reports for Danish wells were avail-able for this study [see Nielsen and Japsen (1991)for a detailed lithostratigraphic subdivision ofmost of these wells]. The only data availableabout the Chalk for the remaining wells weretime and depth readings to top and base of theChalk, water depths, and coordinates. The qualityof these data thus could be checked only by com-paring results from neighboring wells, makingthe results from isolated wells critical. Eleven

wells were considered to have er roneous da taand were excluded from the database. Sixty-six

wells with thin Chalk sections also were excluded(z< 50 m or T< 25 ms) because the uncertain-ty on the interval velocity and the velocity anoma-ly is considerable for a thin unit. Wells drilled onor near salt diapirs are included in the database,

however, to emphasize regio nal trends; dpoints from recognized diapirs are omitted frthe maps. Identification of all errors and diapirless important to regional mapping of velocanomalies and burial anomalies because the onary kriging procedure applied in the contour(see following paragraphs) produces a result wless variance than the data (Figures 8, 9, 10B).

Data from the North Sea Chalk is compareddata obtained from ODP Leg 130, Site 807,the Ontong Java Plateau (western Pacific Ocenear the equator) (Shipboard Scientific Pa1991; Urmos et al., 1993). An almost continusequence of pelagic carbonates of AlbianAptianPleistocene age was drilled over 1380 m. The solog becomes noisy for depths greater than 980below the sea bed, and is only used here above tdepth (Figure 11) (Urmos et al., 1993).

Chalk Pressure Data

Chalk formation pressures are available for study from 126 locations from producing Chfields and wells in the British, Danish, Dutch, Norwegian sectors in the North Sea (Figures 112; Table 3). The pressure evaluations are baon drill-stem and repeat-formation tests, and generally from the uppermost part of the Chalkfew test results indicating high pressure near base of the Chalk probably are related to

JurassicLower Cretaceous pressure regimes in Central Graben and are not included in the stuMud weights are used to give an upper limit for overpressure where overpressure is below 5 Mhowever, in Dutch waters one formation testthe Chalk is available and, consequently, m

weights are used to outl ine the area where overpressure is 510 MPa.

Chalk burial anomalies are assigned to nNorwegian Chalk fields for which pressure westimated by Caillet et al. (1997), and velocitknown for a well in the field. Corresponding valof formation pressure and interval velocity for Chalk are thus known in 68 cases. In wells whthe depth to the middle Miocene unconform(called the top overpressure) was not availabl

value was determined from a map, because topography of the unconformity is gentle apfrom over some diapirs.

Overburden Densities

Densities are rarely logged in the upper PChalk Group in the central North Sea (Figure 1). Tmean density of the upper Post Chalk Group is emated to be 2.05 103 kg/m3, based on a density

DCP= (1 100P/zup) 100

Japsen 2


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1000

2000

3000

4000

2000 3000 4000 60005000

Depth(mbelowtopsediments)

0

Elna - 1

VNormal

Velocity (m/s)

2700 5500

VN= 1600 + Z V N= 500 + 2 Z

a.

23/27-6

23/26A-3Karl-1

49/1-2L-1

15/16-1

15/28-2

L16-4

13/28-2

38/24-1

N4-1

16/6-1

16/3-2

44/19-3

S-1

Stenlille-6

22/9-1

7/11-1

CBA

1100

1750

2250

2875

Velocity (m/s)

Danish BasinVDB= 2421 + 1.07 Z

VCG

VNormal

1000

2000

3000

4000

2000 3000 4000 60005000

Depth(mbelowtopsediments)

0

Elna - 1

Danish well

Top Chalk truncated

Diapir

Omitted from regression analysis

VDB

(B)

VN= 937.5 + 1.75 Z

4875

D

Danish well

Dutch well

Norwegian well

UK well

Top Chalk truncated

Diapir

Stenlille-6

S-1

Karl-1

Fixpoint 1

Fixpoint 2

Fixpoint 3

L-1

VN= 2625 + Z

16/29-2

K8-9

15/21-1 21/6-2M-1

21/24-1

F3-6

38/25-1

(A)

Danish CentralGraben

VCG= 2199 + 0.72 Z


11/44

from 50 to 1200 m below the sea bed in the M-10well, quadrant DK 5505 (Foged et al., 1995). Themean bulk density of the lower Post Chalk Group istaken to be 2.11 103 kg/m3, the mean density cal-culated for the Post Chalk Group below 1300 mbased on 43 density (Formation DensityCompensated) logs in the area (Knudsen, 1993).The mean bulk density for the Post Chalk Group

thus becomes 2.08 10 3 kg/m3, assuming thethicknesses of the upper and lower Post ChalkGroup to be 1200 and 1300 m, respectively(Japsen, 1994).

Pore fluid density, f, equals 1.02 103 kg/m3 atdepth in the Viking Graben (Chiarelli and Duffaud,1980). This value is also applied to this study due tothe similar evolution of the Viking Graben andCentral Graben areas during the Cenozoic, and issupported by the trend of pressures vs. depth fornormally compacted Chalk (Figure 12). The meandensity contrast (difference between bulk densityand pore fluid density) for the upper part of theoverburden, up, thus equals 1.03 103 kg/m3 for

the upper Post Chalk Group.

Pre-Quaternary Geology

The pre-Quaternary geology is well known inareas where Mesozoic and older rocks subcropthe Quaternary, because these areas are mainlyonshore (Figure 4) (Ziegler, 1990). The Tertiarygeology, however, appears to be less well knownfor the younger sediments (Vinken et al., 1988)because the Neogene depocenter is situated off-shore and is divided by the five national sectors.The Neogene depocenter, furthermore, is of littledirect commercial interest. My mapping of thetransnational pre-Quaternary geology is based onpublications from the national sectors of theNorth Sea: the British sector (Andrews et al.,1990; Cameron et al., 1992; Johnson et al., 1993;Gatliff et al., 1994), the Dutch sector and parts ofthe German sector (Kreizer and Letsch, 1963;Choubert and Faure-Murat, 1976; Cameron et al.,1987; Zagwijn, 1989), the Danish sector and partsof the German sector (Hkansson and Pedersen,1992; Bidstrup, 1994; Srensen and Michelsen,1995; Michelsen et al., 1996), and the Norwegiansector (Sigmond, 1993; Jordt et al., 1995). In theDutch part of the mapped area, thin Pliocene

deposits extend far southward (Choubert aFaure-Murat, 1976; Cameron et al., 1987; Z

wi jn, 1989). The Miocene, or its upper partmissing in Dutch offshore and onshore arbelow the PliocenePleistocene cover (Kreiand Letsch, 1963; Cameron et al., 1987).

In the North Sea Basin, the base of Pleistocene generally is placed at approximat

2.4 Ma, at the first indication of cold clim(Zagwijn, 1989; Cameron et al., 1993). This ptice is followed likewise by the British and Dugeological surveys (Gatliff et al., 1994), evthough the boundary is at 1.6 Ma, accordingHaq et al. (1987).

DERIVATION OF THE NORMALVELOCITY-DEPTH TREND

A normal velocity-depth trend for the ChVN, is derived here to describe how velocityChalk with average composition increases dur

normal compaction. The normal trend is definqualitatively by identifying data that represmaximum burial in areas unaffected by overpsure, and by constraining the trend by likely ues near the surface (Figure 7A). The curved pof the normal trend is expressed in linear sments because a single mathematical function fto account for depth variations in the compactprocess. To arrive at a smooth normal trend, final choice of linear parameters is confined tnarrow interval, where round figures are pferred because the trend is only a model of complex geological reality.

Segment A, V


12/44


Cph.

Edb.

12

13

14

15

16

15

16

17

18

20

21

22

7

8

9

10

3

139

383

023

29

37

362

8

27

42

43

44

A E

FB

K

M

L

49

47

48

P

Q

5610

5606

5

607

5608

5609

5604

5605

5508

5507

5506

5505

5504

5509

5510

5511

5

411

5512

5710

2

52

53

59

57

55

53

57

55

53

4

0

4

8

12

8

12

0

4

2W

2E

6

10

14

58

56

54

10

6

2E

2W

54

56

58

15/28-2

21/6-2

16/29-2

16/6-1

Elna-1

Stenlille

Dan

Karl-1

L16-4

Ekofisk

S-1

44/19-3

38/24-1

L-1

Velocityanomaly

inm/s

Notmapped

Nat.quad.n

o.

Welldata

Oilfield

Welllocation

UTMzone31

100km

52

ChalkG

roup

UpperCretaceousDanian

Velocityan

omaly

C.I.1000

m/s

LimitofTertiarysediments(excl.Danian)

OverlapofChalkGroup

andShetlandGroup

UK

N

DK G

N

L

Below

-2500

-2500--1500

-1500-

-500

-500-500

500-1500

above1500

?

Figure8Velocityan

omalyfortheChalkGroupintheN

orthSeaBasinrelativetoVN

(equations1,8;Table2).NegativeChalkv

elocityanomaliesin

thecentralNorthSea

arecausedbyundercompactiondu

etooverpressure.Positiveanomaliesalongthebasinmarginindicatereducedburialofthe

Chalk.Contouringfromkrigedestimates.Oninset,UK=

UnitedKingdom,N=Norway,DK=

Denmark,G=Germany,andNL=H

olland.


13/44

the normal trend is taken as the group of wells at1400 m depth, where interval velocity is around3300 m/s (all from areas of normally pressuredChalk). Below fixpoint 1, the first wells from areasaffected by overpressure are indicated by a shift tosmaller velocities.

Rather than extrapolating the diffuse data trend

above fixpoint 1 to the surface, segment A of VNisestimated as V= 1600 +zbased on sonic logsdown to 980 m below the sea bed, obtained frompelagic carbonates, ODP Leg 130, Site 807 (Urmoset al., 1993) (Figure 11). Segment A is slightlybelow the data point for well UK 44/19-3, whichthus places a tight upper limit on normal Chalk

veloci ties for depths around 1 km, indicating aclose agreement between normal compaction forchalk in the North Sea and those drilled at ODPSite 807.

Segment B, 2700 < V< 4000 m/s, andSegment C, 4000 < V< 4875 m/s

In the deep parts of the North Sea Basin, theChalk is at maximum burial (except over diapirs)but frequently is overpressured. The overpressurecauses porosity preservation (e.g., Scholle, 1977;DHeur, 1986; Maliva and Dickson, 1992) andhence low velocities. For the deep data points,the higher velocities for a given depth are inter-preted to represent normal compaction. The

velocity level for normal compacted Chalk is thusdefined by the clear trend of data points below2000 m marking maximum velocities (40004900m/s). Fixpoint 2 for the normal trend is taken tobe 4800 m/s at a depth of 2200 m, and the veloci-ty-depth gradient between fixpo ints 1 and 2becomes 1.9 m/s/m. The velocity gradient, how-ever, is reduced with depth as the rock is com-pacted, and the velocity gradient is set to 2 m/s/maround 3300 m/s (fixpoint 1), and to 1.75 m/s/mfor 4000 < V< 4875 m/s, as indicated by the datatrend near fixpoint 2. Segment B becomes V= 500

+ 2 z, which crosses segment A for V= 27m/s. Segment C becomes V= 937.5 + 1.75

which crosses segment B for V= 4000 m/s apasses close to fixpoint 2.

Segment D, 4875 < V< 5500 m/s

At great depth the upper velocity levedefined by a single well, UK 16/29-2. Fixpoint taken to be 5375 m/s at 2750 m, and Segmenbecomes VN= 2625 +z(4875 < V< 5500) whthe velocity-depth gradient is reduced to 1 m/s

Above thi s velocity interval there are no dataindicate a further approximation of Chalk veloto the matrix velocity of calcite at 6400 m(Raiga-Clemenceau et al., 1988). The resultburial anomaly for all wells, except one, w

veloci ties above 5000 m/s indicates that thdeeply buried high-velocity chalks are overpsured, which is the case in the central North quadrants N 7, UK 23, and southernmost UK

where these wells are located.The normal velocity-depth trend, VN, for

Chalk Group gets the following form:

wherez= depth below top of the sediments. TNorth Sea Chalk has mainly velocities correspondto segments B and C, for which the fixpoints forupper depth interval are defined by minim

velocities and maximum burial, and by maximveloci ties and absence of overpressure for deeper interval. The trace of segments B and C

well defined by velocity data from the North Chalk, whereas the extrapolation along segmen

VN= 1600 + z V < 2700 m/s

VN= 500 + z 2 2700 m/s < V < 4000 m/s

VN= 937.5 + z 1.75 4000 m/s < V < 4875 m/s

VN= 2625 + z 4875 m/s < V < 5500 m/s

Japsen 2

Table 1. Statistics for the Well-Velocity Database*

Chalk SedimenGroup No. Vi(m/s) zt(m) zb (m) z(m) Top (mPenetrated Wells mean std. min. max. min. max. max. mean max. min. ma

Denmark 135 3708 678 2299 4903 2 3067 4139 572 1854 73 7Holland 41 3742 421 2746 4627 540 2360 3111 716 1651 7 5

Norway 104 4135 516 2618 5349 213 3129 4004 429 992 37 14UK 565 4043 605 2288 5333 0 3347 4403 503 1142 16 16All wells 845 4074 615 2288 5349 0 3347 4403 515 1854 73 16

*See Appendix 1 for list of symbols. This table does not include 66 wells penetrating thin Chalk ( z< 50 m or T< 25 ms) and 11 wells with erronvelocity data. Top of sediments indicates ground level/sea bed relative to mean sea level.


14/44


Table 2. Chalk Group Velocity Data from Selected Wells*

DanishQuadrant zm Vi dV dZ B z

Well Name No. (m) (m/s) (m/s) (m) (m)

DenmarkAdda 1 5504 2119 3558 1175 588 197Alma 1 5505 2202 4120 658 376 445Amalie 1 5604 3421 4899 1131 1131 948Arnum 1 5508 652 2957 1182 591 509Brglum 1** 5709 292 2704 1634 817 354C-1 5607 825 3150 1032 516 554Diamant 1 5603 3274 4655 1989 1136 647Elna 1 5604 2517 4587 752 430 284F-1 5706 951 3504 1135 567 583Fars 1** 5609 731 3244 1481 740 1395Felicia 1** 5708 344 2992 1844 922 597Fjerritsl. 1** 5709 145 2780 2001 1001 285Fjerritsl. 2** 5709 157 2828 2024 1012 287Fred.h. 1** 5710 267 2856 1824 912 127Gassum 1** 5610 511 2996 1578 789 972Glamsbj. 1 5510 499 2689 604 604 670Haldag. 1** 5609 217 2455 644 644 398

Hyllebj. 1** 5609 705 3243 1525 762 1372Ida 1 5606 1228 3818 877 439 423Inez 1 5606 1006 3808 1310 655 417

J-1** 5708 103 2372 670 670 134Jelling 1 5509 591 2976 1348 674 700K-1 5707 746 3184 1248 624 728Karl 1 5604 3518 4837 2190 1252 1125Kvols 1 5609 990 3534 1261 630 1488L-1 5605 2110 4495 131 75 301Liva 1 5503 3297 4504 2160 1234 883M-1 5505 1866 3228 994 497 289Mejrup 1 5608 981 3332 949 474 886Mors 1 5608 815 3584 1624 812 1357Nvling 1 5608 929 3384 1087 543 785Oddes. 1 5608 887 3333 1151 576 955

Olaf 1 5603 3525 4516 2505 1431 1228Rdby 1 5411 244 2472 634 634 430Rdding 1 5608 861 3297 1219 610 1200Rnde 1 5610 1049 3600 1314 657 1854S-1 5606 1147 3078 299 149 372Skive 1 5609 827 3396 1392 696 1245Skive 2 5609 600 3321 1707 853 920Stenlille 1 5511 691 3321 1541 770 1008Stenlille 6 5511 699 3346 1559 780 1062Thisted 2** 5608 434 2736 1447 724 803Tnder 1 5408 657 2680 431 431 480

Vemb 1 5608 1023 3389 896 448 727Voldum 1** 5610 632 3104 1499 749 1220rslev 1 5411 223 2299 483 483 407

rs 1 5609 955 3491 1342 671 1665

HollandA11-01 2585 4359 1089 622 449B13-02 1832 3434 695 347 589E16-01 1266 3571 574 287 607F02-01 1703 3540 362 181 177F03-06 1718 3169 762 381 206G17-01 1909 3993 216 108 1146K04-01 1679 4088 297 169 1155K08-09 2030 4401 42 24 1505


15/44

Japsen 2

Table 2. Continued.

DanishQuadrant zm Vi dV dZ B

Well Name No. (m) (m/s) (m/s) (m) (m

L08-02 2363 4429 514 294 149L16-04 1643 3840 288 144 165M07-01 1715 3956 71 35 72N04-01 1405 3312 77 38 86P04-01 1482 3690 383 192 132Q01-012 1343 4196 961 549 91

Norway1/5-2 3344 4267 2464 1408 992/1-1 3351 4500 2278 1302 643/5-1 2353 4444 598 341 487/11-1 3351 5349 622 622 577/3-1 1971 4170 207 118 368/1-1 1583 4027 325 186 299/4-3 1536 4177 578 330 6610/5-1 569 2950 1370 685 7111/10-1 1157 3981 1181 590 4115/6-3 2866 4305 1625 929 6016/1-2 2047 2914 1678 839 516/3-2 1485 3302 159 79 3116/6-1 1413 3287 19 10 4217/4-1 1171 3389 556 278 2818/10-1 907 2935 658 329 5625/8-1 1753 4044 41 23 9

United Kingdom12/29-1** 92 2288 597 597 1413/14-1 760 3585 1595 798 5713/28-2 1118 4243 1368 781 5413/30-1 1488 4498 975 557 5714/4-1 933 3740 1379 689 2315/3-1 1395 4246 869 496 1215/16-1 2335 4061 948 542 4615/18B-4A 2077 4254 313 179 2515/21-4 2077 4580 9 5 1515/28-2 2783 4229 1543 881 7716/16B-1 2222 4240 585 334 516/29-2 2713 5388 52 52 2219/4-1 1145 4144 1227 701 6320/1-1 1248 4080 975 557 5121/1-1 2243 4024 822 470 5121/6-2 2163 4810 93 53 3021/24-1 1800 3288 811 405 1222/1-2A 2775 4082 1709 977 1423/11-1 2698 4587 1068 610 2523/26A-3RE 3875 5053 1429 1429 10523/27-6 3325 5333 613 613 4827/3-1 448 2906 1526 763 3628/5-1 1763 3660 364 182 929/2-1 3146 4707 1714 980 6130/2-1 3697 4864 2487 1421 10331/21-1 2959 4583 1523 870 4036/15-1 910 3539 1245 622 5237/10-1 1344 3330 151 76 3038/1-1 1486 3621 153 76 2138/25-1 1643 3680 93 47 3539/2-1 2498 4133 1170 669 27


16/44

is based on limited data. Data from the North Seaalong segment A are not identified in this study.

REDUCTION OF CHALK POROSITY WITHDEPTH

Exponential decay of Chalk porosity, SC, withdepth as suggested by Sclater and Christie (1980)provides a general description of the compactionprocess (equation 9 in Appendix 2). A compari-son of SCwith VN(equation 8) is made based on atentative conversion of VN from a velocity-depthtrend to a porosity-depth trend by means of the

velocity-porosity convers ion for chalk given byequation 15 in Appendix 3. The normal com-paction curve defined by VN rather correspondsto a superposition of three different porosity-depth trends. In the upper kilometer, the porosityreduction estimated from VN is slower than SC(porosity is predicted to 42 and 35%, respectively,at a depth of 1000 m). Conversely, the porosityreduction estimated from VNis faster than predict-ed by SC in the interval from 1 to 2 km, and thetwo curves meet around a porosity of 15% at adepth of 2200 m. The porosity reduction predict-ed by the two curves is identical in the depthinterval from 2200 to 2875 m. This pattern corre-sponds to the interpretation of the porosity reduc-tion in carbonate deposits on the Ontong JavaPlateau by Borre and Lind (in press). According totheir interpretation, mechanical compaction isactive from the surface down to a porosity around20%, whereas cementation accelerates porosityreduction below about 1 km.

The steep increase of VNbelow 1100 m matchesthe sharply increasing velocity measured on car-bonate samples over the interval from about 1000to 1300 m below the sea bed on the Ontong JavaPlateau (Site 807) (Shipboard Scientific Party,1991). This increase in velocity around a porosityof 40% is the combined effect of accelerated poros-ity reduction due to cementation and the stiffergrain contacts created by cementation of the parti-cles (see Appendix 3). The onset of cementationbelow a depth of 1 km thus appears to take placefor the North Sea Chalk as has previously been sug-gested (Davis, 1987; Taylor and Lapre, 1987); how-ever, this depth is difficult to correlate betweenindividual wells in the North Sea Basin because theChalk in most wells is far from compaction equilib-rium relative to the present depth. The correctdepth scale for comparison of porosities and veloc-ities is obtained only if present depths are correct-ed by the burial anomalies.

AREAS OF VELOCITY ANOMALY IN THENORTH SEA BASIN

Chalk velocity anomalies calculated relative toV

Nconstitute geographically well-defined areas and

reflect the burial history of the North Sea Basin dur-ing the Cenozoic (equations 1, 8; Table 2; Figure 8).The velocity-anomaly map in Figure 8 is contouredfrom kriged estimates based on a spherical model(nugget = 5000; sill = 75,000, range = 35 km; sill =450,000, range = 400 km) and block kriging (4 4;grid increment = 10 km) (compare Hohn, 1988).The maps of Chalk burial anomalies are contoured


Table 2. Continued.

DanishQuadrant zm Vi dV dZ B z

Well Name No. (m) (m/s) (m/s) (m) (m)

42/13-1** 351 3497 2340 1170 67543/12-2 463 2618 564 564 53444/19-3 935 2668 145 145 61144/7-1 1075 3095 474 237 50347/3-1 349 3158 2012 1006 69848/10-2 977 3649 1257 629 82149/1-2 2245 4716 103 59 92950/16-1 1345 3382 319 159 113352/5-3 91 2394 704 704 8553/4-5 732 2893 951 476 44454/1-2 819 2826 696 348 260Cleethorpes 1** 124 3768 1598 1598 105

*First well in each quadrant (numerically or alphabetically); wells referenced in the text and in Japsen (1993b). See Appendix 1 for list of symbols.**Quaternary deposits overlying Chalk Group.Data source Hillis (1995a); upper and middle Chalk.Data source Petroleum Information (Erico).


17/44

Japsen 2

Figure9Chalkburialanomaliesrelativetothenormalvelocity-depthtrend(equations2,8;Table2).Overburdenreductionincreasesawayfromthe

lateCenozoicdepocenterinthecentralNorthSeaandreachesapproximately1kmalongthepresent-daylimitoftheChalk.Theuniformityofthis

mapwiththatofthe

pre-Quaternary(Figure4)suggeststhattheChalkburialanomalyisa

measureofexhumation.Thenegat

iveburialanomalies

?

?

?

59

57

55

53

57

55

53

4

0

4

8

12

8

12

0

4

2W

2E

6

10

14

58

56

54

10

6

2E

2W

54

56

58

Edb.

Cph.

1

2

13

14

15

16

15

16

17

18

20

21

22

7

8

9

10

3

139

383

023

29

37

362

8

27

42

43

44

A E

FB

K

M

L

49

47

48

P

Q

5610

5606

560

7

5608

5609

5604

5605

5508

55

07

5506

5505

5504

5509

5510

5511

5411

19

2

52

53

5709

5710

5512

ChalkGro

up


Burialanom

aly

(=--'apparentuplift')

C.I.250m

LimitofTertiarysediments

(excl.Danian)

Burialanom

aly

inm

UTMzone3

1

100km

52

Max.b

urial

0

-

-250

-250

-

-500

-500

-

-750

-750

--1000

>

--1000

Notmapped

Nat.quad

.no.

Well

data

Welllocation

UK

NDK G

NL

13/28-2

47/29a-1

Cleethorpes-1

S-1

Elna-1

L-1

Stenlille

Borg-1

Frederikshavn-1

Mona-1

Kim-1

Stevns

NorthS

eaBasin

MorayFirth

Mid-North

SeaHighC

entra

lGraben

Viking

Graben

Ringk.-

Fyn-

High

EMS


18/44

based on the same kriging parameters as the veloci-ty anomaly map (equation 2; Table 2; Figures 8, 10).

Three main areas are delineated on the map ofChalk velocity anomalies (Figure 8).

(1) Positive velocity anomalies along the basinmargin. The anomalies reflect Neogene erosion ofthe overburden; see also the corresponding map of

negative burial anomalies (Figure 9) and the sectionof this paper on the Neogene exhumation of theNorth Sea Basin.

(2) Velocity anomalies near zero in an intermedi-ate zone where the Chalk is normally compacted.

(3) Negative velocity anomalies in the centralNorth Sea. The anomalies are a measure of the over-pressure in the Chalk in the central and southernNorth Sea (equation 5), whereas to the north clasticcontent in the Chalk reduces velocity (see also thecorresponding map of positive burial anomalies inFigure 10, and the section in this paper on overpres-suring of the North Sea Chalk aquitard.)

These simple physical interpretations of thevelocity anomalies, discussed in detail in the fol-lowing section, provide evidence that the suggest-ed normal velocity-depth trend reflects the physi-cal properties for the Chalk Group.

NEOGENE EXHUMATION OF THENORTH SEA BASIN

The present-day limit of the Chalk Group followsthe trend of the British, Norwegian, and Swedishcoasts (Figure 4) and outlines the North Sea Basin

where Pl iocene deposits are present in the basincenter. The age of the pre-Quaternary rocks increas-es toward the coasts, and pre-Mesozoic rocks out-crop in Britain, Scandinavia, and central Europe. Thesymmetry in the pre-Quaternary subcrop patternacross the North Sea suggests a corresponding sym-metry in the burial history across the area. The fun-damental question is whether the pre-Quaternaryhiatus represents a period of nondeposition or aperiod of deposition followed by erosion.

In recent years, regional Cenozoic exhumation ofthe North Sea Basin has been documented by sedi-ment compaction studies. In the eastern North SeaBasin, exhumation was found to have happened dur-ing the Neogene/late Cenozoic (Jensen et al., 1992;

Japsen, 1993a; Jensen and Schmidt, 1993; Michelsenand Nielsen, 1993; Hansen, 1996), whereas in theUK southern North Sea (Bulat and Stoker, 1987;Hillis, 1995a) and the Inner Moray Firth (Hillis et al.,1994; Thomson and Hillis, 1995) the timing beyonda Tertiary age was unclear because no direct infor-mation about the timing can be deduced from sedi-ment compaction studies. The Tertiary age of the

exhumation was based on the fact that several strati-graphic units, including the Chalk Group, had expe-rienced similar magnitudes of exhumation (Bulatand Stoker, 1987; Hillis, 1995a). Bulat and Stoker(1987) found the exhumation of the southern NorthSea to be of late Tertiary age, whereas Hillis (1995b)suggested that it could be associated with eitherregional Paleocene or OligoceneMiocene unconfor-

mities. Hillis et al. (1994) found that the Chalk in theInner Moray Firth had been at maximum burialbefore the deposition of the thick Paleocene succes-sion encountered there today. Japsen (1997) suggest-ed that Britain and the western North Sea sufferedregional Neogene exhumation based on a compila-tion of exhumation studies from onshore and off-shore Britain.

Recognition of the differential Cenozoic subsi-dence, sedimentation, and exhumation in theNorth Sea Basin is important for understanding itspetroleum systems. The influence of exhumationof sedimentary basins on hydrocarbon prospectivi-ty was discussed by Dor and Jensen (1996). They

found that negative effects include spillage ofhydrocarbons, the potential for seal failure, andcooling of source rocks, but suggested that theseaspects have been overstated in the past, and thatmany of the worlds petroleum provinces havebeen recently uplifted. An indirect, but important,positive effect of uplift and erosion was found tobe redeposition of eroded material, thus con-tributing to the maturation of source rocks throughincreased burial; furthermore, mature source rocksat shallow levels, fracturing of tight reservoirs, andremigration of hydrocarbons to shallower reser-

voirs were found to be among the positive effects.Better paleogeographic constraints and understand-ing of burial history may be added as importantaspects of recognizing exhumation of sedimentarybasins.

Here it will be demonstrated that the map ofChalk burial anomalies relative to a normal veloci-ty-depth trend is consistent with the map of thepre-Quaternary geology of the North Sea Basin,and the burial anomalies on a regional scale aremeasures of the erosion (equations 2, 8; Figures 4,9). The exhumation was caused by uplift and ero-sion during the Neogene along both the westernand eastern margins of the basin, symmetric rela-tive to the basin axis, as is the age of the pre-Quaternary surface.

Magnitude of Exhumation Based onChalk Burial Anomalies

Negative chalk burial anomalies are mapped in abroad zone along the present margin of Chalkdeposits, whereas the Chalk is at maximum burial



19/44

Japsen 2

Figure10Correspo

ndingareasofoverpressuredChalk

outlinedfrompressuremeasureme

ntsandfromChalkburialanomaliescoincidentwiththe

lateCenozoicdepocenter.(A)Chalkformationoverpressure(Table3).(B)Chalkburialanom

aliesrelativetoanormalvelocity-d

epthtrend(Table2).

(C)LateCretaceousCenozoicstructuralelements.Theoverpressuredzonecorrespondstom

aximumthicknessoftheupperPos

tChalkGroup(Table

3),whereasPaleocenesandsoverlyingtheChalktothe

northwestcausebleed-offofoverpressure.SouthoftheVikingGraben

,shalyChalkcauses

positivevelocityano

maliesevenwheretheChalkisnor

mallycompacted.On(B),pointsAandBindicatethelocationofthede

pthprofileinFigure

19.InsetabbreviationsasinFigure9.

4

5

28

57

55

58

56

54

59

14

1

5

16

15

16

17

18

21

22

7

8

9

10

3

39

383

023

29

37

43

A E

FB

5604

5605

5506

5505

5504

2

59

57

55

54

56

58

0

4

2E

0

2E

6

4

6

44

36

42

750

1000

1250

Buriala

nomaly

(m

)

ChalkGroup


Burialanomaly

>500m

C.I.500

100km

B

1500

-2000

1000

-1500

500

-1000

B

elow500

Welldata

Nat.quad.no.

28

A

O

verlapofChalkGroup

a

ndShetlandGroup,

JohnsonandLott,1993

D

epthtomid-Miocene

u

nconformity>750m

C

.I.250m,

K

ockel,1988b

D

istributionofsandaboveChalk

(MaureenFm.),

K

noxandHalloway,1992,

S

pencer,1987

B

28

59

57

55

54

56

58

0

4

2E

0

2E

57

55

6

58

56

54

4

6

59

38

37

36

?

?

750

1000

1250

14

15

16

15

16

17

18

21

22

7

8

9

10

3

1

39

30

23

29

28

42

4

3

44

A E

FB

5604

5

605

5506

5

505

5504

2

4

5

(2175

(1450

(725

15-20

10-15

5-10

Below5

Overpressure

inMPa

(psi)

Pressurefromtest

Welllocation

Pressurefrommud

weight

PressuretestinSa

ndstone

aboveChalk(Elna-1)

-2

900)

-2

175)

-1

450)

(725)

ChalkG

roup

UpperCretaceo

usDanian

Overpres

sure

C.I.5MPa(7

25psi)

100km

A 2

8

Nat.quad.no.

Oilfield

Dan

Siri-1

T-1

Valhall

30/1C-3

16/28-1

16/2-1

16/1-3

Ekofisk

Elna-1

Mona-1

OverlapofChalkGroup

andShetlandGroup,

JohnsonandLott,1993

Depthtomid-Miocene

unconformity>750m

C.I.250m,

Kockel,1988b

DistributionofsandaboveChalk

(MaureenFm.),

KnoxandHalloway,1992,

Spencer,1987

30/6-3 J

oanne

UN

N

G

NL

DK

NorthSeaBasin

(c)

MorayFirth

MidNorth

SeaHighC

entr

alGrab

en

Viking

Graben

Ringk.-

Fyn-

High


20/44

in the central North Sea (Figure 9). This trend indi-cates that the burial depth of the Chalk Group inthis zone has been reduced from a maximum burialattained during the Cenozoic.

The zone of exhumation extends along the Britishcoast, as indicated by the continuous 500 m burial-anomaly contour. Erosion increases in a landwarddirection, and in the southwestern UK sector, amaximum of 1600 m is computed for theCleethorpes 1 well, whereas the burial anomaly iszero 250 km to the east. To the north, overburdenreduction reaches 1000 m in the Inner MorayFirth, and the transition zone between zero andthe 500 m anomaly is narrow, only 30 km. Farthernorth, the 500 m burial-anomaly contour turnsnortheast in the direction of the Viking Graben.On the Norwegian side of the Viking Graben, wellcontrol is poor, but farther south the area whereoverburden reduction exceeds 500 m follows thegeneral trend of the Norwegian and Swedishcoasts, and covers most of onshore Denmark. Innorthern Denmark, data from several wells indicateexhumation in excess of 750 m. Along the Dutchcoast, the Chalk appears to be close to normal com-paction for its present depth, and only a minorreduction in burial is indicated to the south. Lack ofdata close to the truncation of the Chalk makes con-touring of areas of maximum erosion difficult in theNorwegian and Danish sectors, and the in UK sec-tor between 55 and 57N. The course of the burial-anomaly contours east of the study area, in theBaltic Sea, awaits further investigation.

Comparison With Other Studies andEvidence of Exhumation

The map of the Chalk burial anomaly shows astriking resemblance to the map of the pre-Quaternary geology (Figures 4, 9). The deeper theerosion, the greater the span of the pre-Quaternaryhiatus. The Chalk is now at maximum burial in thecentral North Sea below the Pliocene depocenter,

whereas the estimated erosion reaches approxi-mately 1 km along the limitation of the Chalk. Theeast-west symmetry of both maps suggests symme-try in the causes generating this pattern across theNorth Sea.

The validity of the estimated overburden reduc-tion based on Chalk velocities is stressed by thesimilarity with estimates found by different meth-ods in studies of the North Sea Basin, such as stud-ies done on the eastern North Sea Basin (shale com-paction, density, and vitrinite ref lectance) (Jensenand Schmidt, 1992, 1993; Japsen, 1993a; Michelsenand Nielsen, 1993; Hansen, 1996)and the UKsouthern North Sea/East Midlands Shelf (fission-track analysis, vitrinite reflectance, chalk and shalecompaction) (Bulat and Stoker, 1987; Green, 1989;Bray et al., 1992; Hillis, 1995a). Regional exhuma-tion in the Inner Moray Firth also has been demon-strated, but with a smaller amplitude than the esti-mates presented here (Hillis et al., 1994; Thomsonand Hillis, 1995). A comparison with studies with

wells in common with this study is given in Table 4.Fission-track studies indicating kilometer-scale


Figure 11Comparison ofVNand sonic logs from (A)pelagic carbonate depositsof EocenePleistoceneage drilled in hole 807,ODP Leg 130 (ShipboardScientific Party, 1991), andthe Chalk Group in (B) theDanish Stenlille-6 and (C)

the Karl-1 wells (locationson Figure 8) (equation 8).Only after shifting the logstoward VNdo the soniclogs line up.

VNormal

+780 m

-1252 m

Karl-1 (Z-1252 m)

dZB= 0

Karl-1

Zm= 3518 m

Site 807

Stenlille-6

Stenlille-6 (Z+780 m)

dZB= 1252 m

dZB= 0

dZB= -780 m

Vi= 4837 m/s

Vi= 3346 m/s

1

4000 60005000

Depth(kmbelowtopse

diments)

0

Velocity (m/s)

4

3

2000 3000

2

Zm= 699 m


21/44

Japsen 2

Tertiary erosion over wide areas of the onshore UKalso are in line with this study (Green, 1986, 1989;Bray et al., 1992; Lewis et al., 1992).

Magnitude of Erosion in the EasternNorth Sea Basin

The course of the line of zero burial anomalyfound in this study is similar to that of Jensen andSchmidt (1993) (their hinge-line). At 56N, the loca-tions found in this paper and in Jensen and Schmidtare identical, just east of well DK L-1, whereas at58N this study suggests a position 30 km farther

west into the basin. The same relation applies for acomparison with the zero line indicated by Hansen(1996), only at 58N the shift is 60 km. The estimatesof Hansen (1996) are on average 196 m smaller thanthose presented here, but still within the averagestandard deviation of 260 m determined in that study.In northernmost Denmark, Chalk burial anomalies

appear to underestimate erosion due to relativhigh siliciclastic content in the basal Chalk, whhas avoided deep erosion in the area (quadrants 570910) (Sorgenfrei and Buch, 1964). A glaciinduced reduction in velocity commonly observethe upper 20 m of the Chalk below the Quaterncover in Denmark may add to this underestimat(C. Andersen, 1995, personal communication).

The area in Denmark where overburden redtion may be estimated is enlarged to the south byuse of Chalk velocities relative to previous wobased on deeper, but less extensive, strata (Japs1993a; Jensen and Schmidt, 1993; Michelsen Nielsen, 1993). A minimum of 300 m is estimed in southwesternmost Denmark (the Bor

well) , whereas est imates from the eastislands are above 500 m and reach a maximof approximately 750 m in the Stenlille weonly 100 m lower than the estimate of Jap(1993a) (Figure 9).

Figure 12Chalk formatpressures vs. depth belosea level, central North S(Table 3). Dashed lines

A, B, and C markgeographically coherentapparent pressurecompartments in theDanish Central Graben

with overpressures of 79 1, and 15 1 MPa,respectively. Theseapparent pressurecompartments may beexplained by the small,local variations of theoverburden rather thanhydraulic communicatioPEkofiskshows thepressure-depth trend foNorwegian Chalk fieldsin the Greater Ekofisk ar

Vertical pressurecommunication in the

hydrocarbon phase mayexplain the apparent drin overpressure with dein the Ekofisk area. mwemud weight equivalent.

2.0

20 30 40 50

2.5

3.0

3.5

Chalk Formation Pressure (MPa)

Depth(kmbelowsealevel)

AB

C

1.5

Valhall

Elna 1

Dan

PH=1.02g/cm

3mwe(0.44psi/f)

16/2-1

16/1-3

16/29C-7

16/28-1

S=2.08g/cm3mwe(0.90psi/f)

30/6-3

Nora 1

T-1

Ruth 1

Mona 1

PEkofisk

Danish well

Norwegian well

UK well

Ekofisk

Albuskjell

30/1C-3

7 MPa

13 MPa

9 MPa


22/44


Table 3. Chalk Group Pressure Data From Selected Wells*

Well or Mud Wt.Field Pressure Quadrant zup zlow z** P** P Equiv. dZB DCName Compartment No. (m) (m) (m) (MPa) (MPa) (Mg/m3) (m) (%)

DenmarkA-2 A 5505 1180 554 1790 25.6 7.7 1.46 Adda-1 B 5504 1143 877 2208 31.6 9.5 1.46 588 49Bo-1 5504 1274 708 2079 31.4 10.6 1.54 788 38Dan/Kraka A 5505 1155 613 1890 26.2 7.3 1.41 434 64E-1 B 5504 1205 773 2018 29.0 8.8 1.47 E-2 B 5504 1205 712 1992 29.5 9.5 1.51 891 26E-3 B 5504 1222 725 1984 29.3 9.4 1.50 624 49E-4 B 5504 1229 682 1981 29.3 9.4 1.51 838 32Elly-2 5504 1429 1345 2941 42.8 13.4 1.49 973 32Elna-1 5604 1261 1537 2438 28.4 4.0 1.19 430 64G-1 B 5505 1098 831 1997 28.6 8.6 1.46 410 63Gert-1 5603 1430 1618 3262 49.0 16.4 1.53 1432 0H-1 5504 1247 706 2020 30.5 10.3 1.54 859 31I-1 C 5604 1408 1262 2765 42.6 14.9 1.57 1439 2

John Fl.-1 A 5504 1250 230 1561 22.1 6.4 1.44 55 104Lulu-1 5604 1321 1335 2831 41.4 13.0 1.49 1496 9M-8 A 5505 1155 605 1803 25.7 7.7 1.45 434 62

M. Rosa-1 B 5504 1365 446 1951 28.2 8.6 1.47 161 88Mona-1 C 5604 1496 1427 3002 45.3 15.3 1.54 1271 15N-1 B 5504 1341 690 2108 29.9 8.8 1.45 699 48N-2 B 5504 1325 614 2017 28.8 8.6 1.45 N-3 B 5504 1316 657 2098 29.9 8.9 1.45 807 39Nils-1 A 5505 1157 494 1770 24.5 6.8 1.41 261 123Nora-1 C 5504 1340 1171 2574 41.2 15.9 1.61 910 32Otto-1 C 5604 1419 960 2521 39.8 14.6 1.61 914 36Roar-2 B 5504 1243 700 2098 30.6 9.6 1.49 762 39Ruth-1 B 5504 1138 374 1576 23.8 8.0 1.54 168 115S.E. Igor-1 A 5505 1059 885 1993 27.8 7.9 1.42 454 57T-1 C 5604 1328 801 2251 37.5 15.0 1.70 555 58Tove-1 A 5505 1178 351 1608 22.6 6.5 1.43 149 87

Vagn-2 A 5505 1104 352 1513 21.6 6.4 1.45 32 97

NorwayAlbuskjell 1, 2 1706 1364 3270 50.0 17.3 1.56 1492 13Edda 2 1450 1613 3285 49.6 16.7 1.54 1401 3Ekofisk 2 1624 1264 3288 49.9 17.0 1.55 Ekofisk W 2 1675 1390 3300 50.1 17.1 1.55 Eldfisk E 2 1502 1423 3015 48.6 18.5 1.64 1553 3Eldfisk N 2 1502 1298 3056 48.3 17.7 1.61 1553 3Eldfisk S 2 1502 1198 3027 48.0 17.7 1.62 1264 16Hod E 2 1382 1268 2755 47.1 19.5 1.72 1139 18Hod W 2 1440 1432 2653 46.1 19.5 1.77 1139 21Tommeliten A 1 1525 1500 3180 48.9 17.1 1.57 1505 1Tommeliten G 1 1525 1475 3290 49.2 16.3 1.53 Tor 2 1659 1213 3292 49.7 16.8 1.54

Valhall 2 1435 965 2588 45.7 19.8 1.80 1245 131/3-4 1480 2854 45.6 17.0 1.63 3/7 2 1339 1056 2573 39.5 13.7 1.56 6/3-2 1033 1261 2543 34.9 9.4 1.40 9/11-1 588 912 1608 17.6 1.5 1.12 269 14615/9-9 984 1330 2454 24.1 0.4 1.00 436 5616/2-1 876 717 1707 17.6 0.5 1.05 95 111

United Kingdom15/28-1 756 2392 24.6 0.7 1.05 16/28-1 1067 1636 2832 29.5 1.2 1.06 944 1116/29C-7 1149 2790 31.8 3.9 1.16


23/44

Japsen 2

On top of Danian carbonates found today atStevns Klint, eastern Denmark (Figure 9), a sedi-mentary column of about 750 m must have beendeposited and subsequently eroded, according tothe results presented here. This scenario agrees

with the occurrence of incipient stylolites in theMaastrichtian Chalk at this locality. The formationof stylolites is believed to be depth dependent, andincipient stylolites occur in the Chalk from 470 to830 m below sea bed on the stable Ontong JavaPlateau (Lind 1988, 1993).

Magnitude of Erosion in the WesternNorth Sea Basin

The estimates of overburden reduction based onChalk Group velocities presented here are, on aver-age, only 54 m smaller than estimates based on veloc-ity of TuronianMaastrichtian Chalk, UK southernNorth Sea (Hillis, 1995a) (38 wells in common, corre-lation coefficient 0.97, Table 4). The difference in theestimates of erosion is due to a slight shift betweenthe normal velocity-depth trends for Chalk used in

the two studies in the relevant depth interval(Appendix 2). The exclusion of the Cenomanian andDanian parts of the Chalk Group in Hilliss (1995a)investigation does not appear to affect the close simi-larity of the two studies. The estimated overburdenreduction for well UK 47/29a-1 of 1280 m basedon fission tracks and vitrinite data (Bray et al.,1992) is in good agreement with nearby wells cov-ered by this study (well UK 47/18-1; dZB =

1055 m) (Figure 9). Erosion of inversion zonethe southern North Sea was estimated relativesupposedly stable nearby areas in a numberstudies in the southern North Sea (Marie, 19Glennie and Boegner, 1981; Barnard and Coop1983; Allsop and Kirby, 1985; Cope, 1986).

In the Inner Moray Firth, moderate erosion emates (


24/44


25/44

timing may be determined. The regional erosion ofthe Chalk may have taken place at any time duringthe Cenozoic, because the timing of exhumationcannot be inferred directly from velocity data; how-ever, in areas where the Chalk Group is preservedtoday, it is more likely that relatively stable condi-tions prevailed immediately after deposition of theChalk, rather than that extensive Paleocene tecton-

ic activity and exhumation took place. High burialrates result if a kilometer-thick missing section isinterpreted to have been deposited over the fewmillion years between deposition of the preservedChalk and possible Paleocene erosion, as suggestedby some workers (Figure 13) (Green, 1989; Greenet al., 1993; Hillis et al., 1994). If maximum burialof the preserved Chalk occurred during theNeogene, a much longer time span is available forthe deposition of the removed overburden, andmoderate Cenozoic burial rates would be the result(Japsen, 1997).

Timing of Exhumation of the Eastern NorthSea Basin

In the eastern North Sea Basin, the regionalCenozoic exhumation was interpreted as beingNeogene in age by Jensen and Schmidt (1992,1993) and Japsen (1993a), whereas Michelsen andNielsen (1993) restricted their estimate to the lateCenozoic. The increasing erosion observed towardthe Norwegian and Swedish coasts matches anincrease in the age of the Quaternary subcrop inthe area, and only the pre-Quaternary unconformi-ty has an areal extent similar to the Cenozo icexhumation (Japsen, 1993a). Tectonism during thelate Oligocene and Miocene was suggested bySpjeldns (1975) to be a major factor in the shiftfrom open-marine to terrigenous facies in theTertiary of Denmark.

Timing of Exhumation of the WesternNorth Sea Basin

Tertiary exhumation of Britain and the westernNorth Sea was suggested by Japsen (1997) to havetaken place in two episodes, each with an ampli-tude of about 1 km. The first episode was aPaleogene phase that principally affected the pres-ent onshore Britain (west of the present extent of

the Chalk Group), and the second episode was aNeogene phase that affected both onshore areasand the western North Sea. Consequently, maxi-mum burial of Mesozoic and older rocks in thepresent onshore areas generally occurred in thePaleocene (60 Ma), as suggested by interpretationof fission tracks (e.g., Green, 1989). In the westernNorth Sea, however, maximum burial for theserocks was interpreted by Japsen (1997) generally to

have occurred in the Neogene. This suggestioconsistent with published estimates of overburreduction based on studies of sediment copaction (mainly offshore) (Bulat and Stoker, 19Hillis et al., 1994; Hillis, 1995a; Thomson and Hi1995) and of fission tracks (onshore) (Green, 191989; Bray et al., 1992; Lewis et al., 1992; Greenal., 1993), as well as with the concept of two p

ods of Tertiary exhumation (Lewis et al., 19Green et al., 1993).In the southern North Sea, regional exhu

tion was found to be of late Tertiary age by Buand Stoker (1987), and upper Paleocene argceous deposits on the east Midlands Shelf wsuggested to represent remnants of a more extsive Paleogene cover (Stewart and Bailey, 199The thick, shallow-marine sandstones of the mdlelate Miocene Utsira Formation may result frNeogene exhumation of northern Britain and surrounding shelf. This formation is present in

Viking Graben area between 58 and 62N aninterpreted to be sourced from the west (Isak

and Tonstad, 1989).In the Dutch sector, base Miocene, mid

Miocene, and base Quaternary unconformitiesprominent (van Wijhe, 1987; Zagwijn, 1989). TChalk may have been at maximum burial depththe time represented by any of the unconformithat separate the thin Neogene units. South of study area, the Rhenish Massif has been subjecuplift since the end of the Ol igocene, and ataccelerated rate since the end of the Mioce(Meyer, 1983). These observations agree witNeogene timing of Chalk maximum burial in southern Dutch sector of the North Sea.

Onset and Cessation of Neogene Exhumatiof the North Sea Basin

The dating of maximum Chalk burial to Neogene indicates a crude time interval (252 M(Haq et al., 1987) during which the exhumationthe North Sea Basin took place; however, whparts of the Neogene succession are preservthe timing of exhumation can be specifiedidentifying hiatuses in the stratigraphic recduring which deposition followed by exhumatmay have taken place. Where most of the Tertisediments are removed, the timing of erosion

be dated only indirectly by inference from lexhumed areas.Within the North Sea Basin, the middle Mioc

unconformity has been suggested to represent onset of pronounced uplift by several work(Bulat and Stoker, 1987; Jensen and Schmidt, 19

Jordt et al. , 1995; Riis, 1996; Stewart and Bai1996). The middle Miocene unconformity (14 M(Jordt et al., 1995) is a basinwide regional down

Japsen 2


26/44


surface (Jordt et al., 1995) that is present through-out the North Sea, apart from a narrow zone of

continuous late Cenozoic sedimentation in the cen-tral North Sea (see following sections) (Deegan andScull, 1977; Michelsen, 1982; van Wijhe, 1987;Isaksen and Tonstad, 1989). Thick deposits overliethe unconformity in the central North Sea, whereaccelerated Neogene sedimentation was found tobe evidence of Neogene uplift of the basin margin(Nielsen et al., 1986; Jensen and Schmidt, 1992). Inthe Danish Central Graben, the increased input ofterrigenous weathering products is indicated bythe abrupt increase in grain size at the transitionfrom the Paleogene to the Neogene, and by theincreasing amount of kaolinite (Nielsen, 1979).

Jordt et al. (1995) found that signi ficant basinaltectonic subsidence was initiated in the middleMiocene in the Norwegian North Sea coeval withthe uplift of southern Norway. They concludedthat the present geometry of the Cenozoicsequences is the result of tectonic uplift throughthe OligocenePliocene, and further uplift relatedto late PliocenePleistocene glacial erosion and iso-static adjustments.

Pleistocene sediments overlie older Cenozoicsediments unconformably throughout the NorthSea Basin (Figure 4) (Cameron et al., 1992; Laursen,1992; Gatliff et al., 1994; Jordt et al., 1995; Riis,1996). Thus, the upper Pliocene is missing as far

west as 4E in the Danish North Sea (the Mona-1well, Figure 9) (Laursen, 1992). Late Cenozoic sedi-mentation was continuous only in the narrow zoneof maximum Quaternary subsidence in the centralNorth Sea (Figure 14C) (Gatliff et al., 1994); e.g., inDanish well Kim-1, where the earliest Pleistoceneis represented by marine sediments (Figures 9, 16)(Konradi, 1995). The cessation of the Neogeneexhumation of the North Sea Basin thus appears to

be marked by the regional unconformity at thePliocenePleistocene transition (2.4 Ma)

(Zagwijn, 1989; Gatliff et al., 1994). Quaternaryerosion may be important where the Chalk is foundbelow a thin cover of primarily upper Quaternarysediments (e.g., onshore Denmark), in contrast to

whe re both the upper and lowe r par t of theQuaternary sediments are present, as in most of theNorwegian sector (Jordt et al., 1995).

Missing Section Removed During Exhumation

In the North Sea Basin, the post-exhumationalburial,BE, relative to the Neogene exhumation ishere approximated by the thickness of theQuaternary deposits, based on the assumptionthat the exhumation ceased by the end of theNeogene (equation 14 in Appendix 3; Figure 14B).The Quaternary thickness, however, is not wellknown in parts of the area due to uncertaintyabout the position of the PliocenePleistoceneboundary (Gatliff et al., 1994). The map of thecomputed estimate of the missing section (Figure14C) shows greater values toward the basin cen-ter than the burial-anomaly map because theQuaternary thickness increases toward the basincenter, whereas the Chalk burial anomaliesincrease toward the basin margins. At the basinmargin, where the Quaternary cover is thin, z

miss= dZB, and the removed sediments are likely tohave been of mainly Paleogene age (Figure 13)(Japsen, 1997).

Along the line of zero burial anomaly, a succes-sion of several hundred meters appears to havebeen removed (Figure 14C); however, the com-puted missing section exceeding 750 m along thiszero line in the southwestern North Sea where

Figure 13Burial diagramsfor the Chalk Group withPaleogene or Neogeneexhumation for well UK47/29a-1 (Figure 9). Note thedifferences in the derived

burial rates. (A) Maximumburial at 60 Ma (cf. Green,1989, his figure 9; Green et

al., 1993, their figure 4).(B) Maximum burial at 15Ma (cf. Hillis, 1995b, hisfigure 5). Well data andexhumation estimate fromBray et al. (1992). After

Japsen (1997).

(min.)

(A)

0

50

100

150

200

250

300

100 80 60 40 20 0

300

250

200

150

100

50

0

60

Time (Ma)

Depth(m

)

Burialrate(m/Ma)

L. Cret. Olig.P. Eo. Mio. P. Q.

Well UK 47/29A-1

Max. burial at 60 Ma

0

50

100

150

200

250

300

100 80 60 40 20 0

300

250

200

150

100

50

0

15

(B)

Time (Ma)

Depth(m

)

Burialrate(m/m.y.)

L. Cret. Olig.P. Eo. Mio. P. Q.

Maximum burial depth

Well UK 47/29A-1

Max. burial at 15 Ma

Known stratigraphy

Time of max. burial in Ma


27/44

the Quaternary is thick may well be an exaggtion due to a too easterly location of the zero lThe Danish L-1 well is close to the zero line, the Chalk is at maximum burial: dZB = +75(Figure 9). In this well a 37-m-thick lowPliocene unit is overlain by lower Pleistocene iments at the base of a 322-m-thick Quaternsuccession (Laursen, 1992). If we put dZB 0,

missing section becomes the thi ckness of Quaternary. About 300 m of PliocenelowermPleistocene sediments are thus likely to have beroded at the PliocenePleistocene transitiothis location.

Toward the basin center, the thickness of removed section is gradually reduced, and mustzero below the Quaternary depocenter, where simentation was continuous throughout the Cenozoic (Figure 15) (Gatliff et al., 1994). The ctral parts of the basin were thus affectedNeogene exhumation much later than the margin accordance with the notion of Stuevold Eldholm (1996), who suggested that increasin

greater shelf areas would become affected by tiltand erosion as uplift is progressing.

In the Inner Moray Firth, a Neogene timingexhumation means that no or only limited pexhumational burial took place, because Quaternary depocenter lies east of the erodarea (Figure 14B) and, consequently, zmidZB (>1000 m in quadrant UK 13). Hillis et(1994), however, assumed early Paleocene mmum burial of the Chalk in that area, and that post-exhumational burial equaled the thicknof the entire Cenozoic sequence (5001000 mquadrant UK 13). The model of Hill is et(1994) thus resulted in the unlikely prediction

removal of about 1 km of Danian deposits befdeposition of the Paleocene sandstones encotered today. At the estimated line of zero buanomaly (well UK 13/30-2), the missing sect

was found to exceed 1 km, and the possibilityerosion on this order east of the zero line, acrthe Viking Graben, was mentioned but later csidered unlikely (Hi l l i s et a l . , 1994; Hi l1995b). According to this study, the missing tion is less than 500 m near the zero line on bthe eastern and western sides of the VikGraben (Figure 14C), and probably zero belthe Pliocene depocenter in the Viking Grab(Figure 15).

Near the Dutch coast, the estimated misssection reaches 750 m due to the substanQuaternar y thickness even where the buanomaly is moderate. This finding seems to sgest that at least the southern part of the Duterritory was affected by the Neogene exhumtion, as indicated by tr uncation of the Chalong 53N, also on German territory.

Japsen 2

500

250

4 02W 2E 4 8 126 10 14

8 120 4 1062E2W

57

55

53

58

56

54

59

57

55

54

56

58

4 02W 2E 4 8 126 10 14

8 120 4 1062E2W

57

55

53

58

56

54

59

57

55

54

56

58

4 02W 2E 4 8 126 10 14

8 120 4 1062E2W

57

55

53

58

56

54

59

57

55

54

56

58

53

53

53

-

-

Chalkat max.burial

+

+

+

+

0

0Chalk

at max.burial

(A)

(B)

(C)

Chalk GroupBurial anomaly

C.i. 250 m

Chalk Group

Missing overburdensection

C.i. 250 m

Chalk GroupPost-exhumational burial

Quaternary thicknessC.i. 250 m

200 km

200 km

200 km

-1000

--

-750

-750

-500

-250

-250

-500

-750

750

?

?

?

500

750

1000

750

500

750

750

500

750

?

Figure 14Estimate of missing overburden section,zmiss, relative to the Chalk Group. (A) dZB, which isChalk burial anomaly (Figure 9). (B) BE, which is post-exhumational burial = Quaternary thickness (Caston,1977; Andrews et al., 1990; Cameron et al., 1992; John-son et al., 1993; Gatliff et al., 1994). Exhumation isassumed to be Neogene. (C) zmiss, which is missingoverburden section = BE dZB(equation 3).


28/44


Observations of Neogene Uplift and TectonicsAround the North Atlantic

The main vertical movements affecting theNorth Sea Basin since the middle Tertiary are sym-metrical about the basin axis (Figure 16). Rapid lateCenozoic burial of up to 1.5 km took place in thecentral part of the basin at an accelerating rate

(Nielsen et al., 1986), whereas Neogene overburdenreduction reached about 1 km along the present-daylimit of the Chalk. The accelerating subsidence ratein the late Cenozoic has long been known (e.g.,Sclater and Christie, 1980; Nielsen et al., 1986),

whereas the effect of Neogene exhumation of theeastern North Sea Basin has gained general recogni-tion within recent years (e.g., Jensen et al., 1992),but the timing of exhumation of the western NorthSea Basin has been disputed (Japsen, 1997). Theobservation that the base of the Quaternarydeposits is a major angular unconformity cuttingacross PliocenePaleozoic strata toward the British,Danish, and Norwegian coasts strongly indicatesthat uplift and erosion caused the Neogeneexhumation of the North Sea Basin (Figure 3)(Cameron et al., 1992; Jensen and Schmidt, 1993;Gatliff et al., 1994).

These observations are inconsistent with aninterpretation of the North Sea Basin following aMcKenzie model in which sedimentary basins wereformed by stretching and thermal subsidence

(McKenzie, 1978). Sclater and Christie (1980)found, on the basis of a McKenzie model, that thepost-middle Cretaceous sedimentation in the NorthSea Basin was uniform and created a saucer-shapebasin; however, Sclater and Christie (1980) notedthe high, late Cenozoic rate of sediment accumula-tion in the central North Sea. They suggested thehigh rate to be caused by shallowing water depthand high porosity of surface shales, whereas

Vejbk (1992) suggested phase transitions in theupper mantle played a role for the rapid subsi-dence. Such mechanisms alone, however, do notexplain the coincidence of uplift and rapid subsi-dence in the North Sea Basin during the lateCenozoic. Thermal rejuvenation and a renewed rift-ing phase were suggested by Thorne and Watts(1989) as possible causes for the high subsidencerates, whereas Kooi et al. (1991) found the occur-rence to be consistent with the present intraplatestress field. Rohrman et al. (1995) observed aNeogene domal upli ft of southern Norway, andfound it to be coincident with Oligocene andPliocene plate reorganizations in the North

Atlantic. Van Wees and Cloetingh (1996) foundthat accelerated subsidence as observed for theQuaternary in the North Sea Basin could be pre-dicted from a three-dimensional model, assumingan increase of compressive intraplate forces inagreement with observed stresses. Stuevold andEldholm (1996) considered the Fennoscandian

Figure 15Estimate of themissing section erodedduring the Neogeneindicated above the baseQuaternary unconformity.

The height of the exhumedwedge above the sea bed isthe burial anomaly, dZB,the part below the sea bed

is the post-exhumationalburial, BE. Mesozoic andLate CretaceousCenozoicstructural elements areindicated. Profile fromEdinburgh (Edb.) toCopenhagen (Cph.) isshown on Figure 9.

0

1

2

3

4

Thin Quaternarycover 0 m

Noerosionzmiss= 0 m

British

Massif North Sea BasinCentralGrabenMid North Sea High

Ringkbing - FynHigh Danish Basin


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Japsen 2

continental uplift to represent a flexural intraplate

deformation that was separated in time and spacefrom the uplift associated with the early Tertiarycrustal breakup.

An increasing amount of documentation hasemerged that suggests that Neogene exhumationand compressional tectonics have affected areasaround th

regional velocity-depth anomaly

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