A normal velocity-depth trend for the UpperCretaceous–Danian 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 areawithin which overpressures in the Chalk exceed10 MPa, equivalent to a burial anomaly greaterthan 1 km relative to the normal trend. The Chalk
pressure system is primarily dependent on overbur-den properties because retention of overpressuregenerated by the load of the upper overburdendepends on the thickness and sealing quality of thelower overburden; therefore, the Chalk is consid-ered to represent a regional aquitard, and thehydrodynamic model of long-distance migrationwithin the Chalk is rejected. The Neogene upliftand erosion of the margins of the North Sea Basinand the rapid, late Cenozoic subsidence of its cen-ter fit into a pattern of late Cenozoic vertical move-ments around the North Atlantic.
TABLE OF CONTENTS
IntroductionVelocity Anomaly and Burial AnomalyDatabaseDerivation of the Normal Velocity-Depth TrendReduction of Chalk Porosity With DepthAreas of Velocity Anomaly in the North Sea BasinNeogene Exhumation of the North Sea BasinOverpressuring of the North Sea Chalk AquitardConsequences for Depth ConversionDiscussion ConclusionsAppendix 1: List of Symbols Appendix 2: Comparison of Compaction
Trends for ChalkAppendix 3: Velocity-Porosity Conversion
for ChalkReferences Cited
INTRODUCTION
The Upper Cretaceous–Danian Chalk Group formsa coherent body in the North Sea region coveringmore than 500,000 km2, with an average thickness ofabout 500 m (Figures 1, 2). Clastic influx into theNorth Sea Basin was low during the deposition of theChalk, which is composed mainly of coccoliths, thedebris of planktonic algae (Kennedy, 1987; Ziegler,1990). Today, the Chalk crops out in most countriesin northwest Europe, but is buried at depths greater
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 Jørgen Møller. 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 Erosion1
Peter Japsen2
AAPG Bulletin, V. 82, No. 11 (November 1998), P. 2031–2074.
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 LateJurassic 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 its present burial 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 1—Lithostratigraphic nomenclature for the Upper Cretaceous–Cenozoic in the North Sea Basin. This studyfollows the nomenclature used in the Danish, Dutch, and United Kingdom sectors by using the term “Chalk Group”to 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 2—Late Cretaceous–Cenozoic 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 Cretaceous–Cenozoic 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.
Pos
t Cha
lk G
roup upper
lower
N o r t h S e a B a s i nM
eso
zoic
Cen
ozo
ic
Upp
er C
reta
ceou
sTe
rtia
ryQ
ua.
Pal
eoge
neN
eog.
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
Nor
th S
ea S
uper
grou
p
southeast
Dutchsector
NAM & RDG(1980)
Japsen 2033
UTM zone 31
200 km
0° 4°4° 8° 12°
0° 4° 8° 12°
59°
57°
55°
53°
59°
57°
55°
53°
N
0° 4°4° 8° 12°
0° 4° 8° 12°
59°
57°
55°
53°
59°
57°
55°
53°
Cph.Edb.
?
She
tland
Gro
up
2°W 2°E 6° 10° 14°
58°
56°
54°
58°
56°
54°
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(A)
2°W 2°E 6° 10° 14°
58°
56°
54°
10°6°2°E2°W
54°
56°
58°
(B)
DANISH B.: Danish BasinCENTR. GR.: Central Graben
FENNOSCAN. H.: Fenno-
MF: Moray FirthMNSH: Mid North Sea HighN SEA BASIN: North Sea BasinRFH: Ringkøbing-Fyn HighVG: Viking Graben
Fault traceLimit 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
F E N N O S C A N . H .
VG
DAN ISH B .R F H
CEN
TR.
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.
Below 500
Above 2000
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 ofvelocity with depth is underestimated, resulting inerroneous depth conversions. The trend isexpressed as four linear segments because a singlemathematical function fails to ref lect the depthvariations 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 normalvelocity-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; Chiarell i 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
2034 Velocity-Depth Anomalies, North Sea Chalk
Figure 3—Burial of the Chalk Group across the North Sea with indication of both Mesozoic and LateCretaceous–Cenozoic structural elements. Depths below sea bed (water depth <50 m). Location of profile from Edin-burgh (Ed.b), UK, to Copenhagen (Cp.h), Denmark, is shown on Figure 2A and B. Modified after Ziegler (1990), withcorrections 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), Knox andHolloway (1992), Kockel (1988a, b), Ter-Borch (1990), and Caston (1977).
0
1
2
3
4
Thin Quaternary cover <100 m
Edb. Cph.
200 km
W E
Upper Post Chalk Group
Lower Post Chalk Group
Chalk Groupkm
North Sea Basin
Mid-Miocene unc.
Base Pliocene
North Sea
UK Denmark
Base Quaternary
CentralGraben
Mid North Sea High Ringkøbing - Fyn High
Danish Basin
BritishMassif
Japsen 2035
Fig
ure
4—
Pre
-Qu
ater
nar
y g
eolo
gy
of
the
No
rth
Sea
Bas
in.
No
te t
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ss t
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(1
99
0),
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stru
p (
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),C
amer
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1987, (
1992),
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ou
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rat
(19
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), G
atli
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. (1
99
4),
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(1992),
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1993),
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rdt
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d L
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1963),
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(1988),
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an
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).
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57°
55°
53°
UK
N
57°
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°
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depth of the rock is not reduced {V = the instanta-neous velocity [in meters/second (m/s)] measuredover a thin unit at depth z [in meters (m)]; see listof symbols in Appendix 1}. The velocity trendshould be constrained by knowledge about thevelocity of the rock at the surface and at infinitedepth. Let VN be 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 dV are 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 V rather than z becausevelocity is the ir reversible parameter. Thevelocity anomaly for a segmented model may be
dV = k∆z(e k∆T/2 - 1)-1 - V0 - kzt
VN = V0 + k × z
2036 Velocity-Depth Anomalies, North Sea Chalk
Figure 5—Velocity 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 dV and positive dZB) (equation 5). Uplift and erosionmay reduce the overburden and cause overcompaction expressed as velocities that are high relative to depth (posi-tive dV and negative dZB). The normalized depth, zN, is the depth corresponding to normal compaction as predictedby 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
Vnormal
V = Vo + kz
Z
Velocity
Dep
th
•
Z
Z
ZNZN
dZB = - dV/k dZB = - dV/k
dZB
dZB
Vi
dZB
dZB
Vi
dV
{{
approximated by calculating dV relative to thesegment given for the actual interval velocity.
The burial 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 inf luence velocity. The term“uplift” [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 Earth’s 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 ofvelocity, density, vitrinite reflectance, 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 manywells, statistical uncertainties are reduced.
The missing overburden section, ∆zmiss, re-moved by erosion only equals the magnitude ofthe burial anomaly if no burial took place subse-quent to exhumation (Figure 6A). Any post-exhumational burial, BE, will mask the magnitudeof the missing section, and we get (Figure 6B)(Hillis, 1995a)
(3)
Consequently, a pre-Quaternary erosion of 500 mwill be masked by a subsequent Quaternary burialof 500 m. Equation 3 implies that only when thetiming of the exhumation is known do we know BEand are able to infer ∆zmiss. The timing thusbecomes a critical aspect not only for understand-ing the succession of events, but also for under-standing their true magnitude and for identifyingthe age of the eroded succession.
Positive Burial Anomaly Due to Overpressure
A positive burial anomaly may indicate under-compaction due to overpressure. Overpressure, ∆P[in Pascals (Pa)] (1 MPa = 145 psi), is the difference
∆zmiss = - dZB + BE
dZB = - dV/k
Japsen 2037
(A)
(B)
Dep
th (
km)
Time (Ma)
∆Zmiss = – dZB
∆Zmiss = BE – dZB
BE
dZB
dZB
Figure 6—Schematic burial diagrams illustrating that themagnitude of the missing overburden section (Dzmiss)will be less than the magnitude of the measured burialanomaly (dZB) in the case of post-exhumational burial(BE) (equation 3). (A) Exhumation followed by no depo-sition; (B) exhumation followed by burial.
between the measured formation pressure, P, andthe calculated hydrostatic pressure, PH, at depth z:
where ρf [in kilograms per cubic meter (kg/m3)] isthe mean pore fluid density of the overburden, andg is the gravitational acceleration (9.807 m/s2). Thelithostatic pressure, S (Pa), at depth z is the stressexerted by the weight of the overburden:
where ρb is the mean bulk density (wet). Terzaghi’sprinciple states that the weight of the overburdenper unit area, S, is borne partly by the rock matrixand partly by the pore fluid:
where σ (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, σup,of the upper unit that initiated the overpressure byrapid burial (compare Rubey and Hubbert, 1959):
(4)
where ∆ρup 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, ∆ρup is slightly above 1 × 103
kg/m3, so σup ≈ ∆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 Terzaghi’s principle andstates that if a rock is shifted to a greater depth bydZB without 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 ∆ρup = 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 ofoverpressure) 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 ref lects 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 = σup
∆Pcomp = ∆ρup × g × dZB
σup = ∆ρup × g × ∆zup
s = σ + P
s = ρb × g × z
∆P = P — PH = P — ρf × g × z
2038 Velocity-Depth Anomalies, North Sea Chalk
DC becomes 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, DC∆P,can be calculated by substituting the burial anoma-ly, dZB, in equation 6 by ∆P × 100 (∆P in MPa):
(7)
Because ∆Pcomp (equation 5) is only part of the totaloverpressure, we get DC∆P ≤ DC.
DATABASE
Chalk Velocity Data
The database for this study contains intervalvelocities for the Chalk Group measured in 845wells that penetrated the Chalk in the British(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 facies 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. Elevenwells were considered to have erroneous dataand were excluded from the database. Sixty-sixwells 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 regional trends; datapoints from recognized diapirs are omitted fromthe maps. Identification of all errors and diapirs isless important to regional mapping of velocityanomalies and burial anomalies because the ordi-nary kriging procedure applied in the contouring(see following paragraphs) produces a result withless variance than the data (Figures 8, 9, 10B).
Data from the North Sea Chalk is compared todata obtained from ODP Leg 130, Site 807, onthe Ontong Java Plateau (western Pacific Ocean,near the equator) (Shipboard Scientific Party,1991; Urmos et al., 1993). An almost continuoussequence of pelagic carbonates of Albian–Aptian toPleistocene age was drilled over 1380 m. The soniclog becomes noisy for depths greater than 980 mbelow the sea bed, and is only used here above thatdepth (Figure 11) (Urmos et al., 1993).
Chalk Pressure Data
Chalk formation pressures are available for thestudy from 126 locations from producing Chalkfields and wells in the British, Danish, Dutch, andNorwegian sectors in the North Sea (Figures 10A,12; Table 3). The pressure evaluations are basedon drill-stem and repeat-formation tests, and aregenerally from the uppermost part of the Chalk. Afew test results indicating high pressure near thebase of the Chalk probably are related to theJurassic–Lower Cretaceous pressure regimes in theCentral Graben and are not included in the study.Mud weights are used to give an upper limit for theoverpressure where overpressure is below 5 MPa;however, in Dutch waters one formation test forthe Chalk is available and, consequently, mudweights are used to outline the area where theoverpressure is 5–10 MPa.
Chalk burial anomalies are assigned to nineNorwegian Chalk fields for which pressure wasestimated by Caillet et al. (1997), and velocity isknown for a well in the field. Corresponding valuesof formation pressure and interval velocity for theChalk are thus known in 68 cases. In wells wherethe depth to the middle Miocene unconformity(called the top overpressure) was not available, avalue was determined from a map, because thetopography of the unconformity is gentle apartfrom over some diapirs.
Overburden Densities
Densities are rarely logged in the upper PostChalk Group in the central North Sea (Figure 1). Themean density of the upper Post Chalk Group is esti-mated to be 2.05 × 103 kg/m3, based on a density log
DC∆P = (1 — 100∆P/∆zup) × 100
Japsen 2039
2040 Velocity-Depth Anomalies, North Sea Chalk
1000
2000
3000
4000
2000 3000 4000 60005000
Dep
th (
m b
elow
top
sedi
men
ts)
0
Elna - 1
VNormal
Velocity (m/s)
2700 5500
VN = 1600 + Z VN = 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
Dep
th (
m b
elow
top
sedi
men
ts)
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-121/24-1
F3-6
38/25-1
(A)
Danish CentralGraben
VCG = 2199 + 0.72 • Z
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 Groupthus becomes 2.08 × 103 kg/m3, assuming thethicknesses of the upper and lower Post ChalkGroup to be 1200 and 1300 m, respectively(Japsen, 1994).
Pore fluid density, ρf, 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, ∆ρup, thus equals 1.03 × 103 kg/m3 forthe 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 (Håkansson and Pedersen,1992; Bidstrup, 1994; Sørensen 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 andFaure-Murat, 1976; Cameron et al., 1987; Zag-wijn, 1989). The Miocene, or its upper part, ismissing in Dutch offshore and onshore areasbelow the Pliocene–Pleistocene cover (Kreizerand Letsch, 1963; Cameron et al., 1987).
In the North Sea Basin, the base of thePleistocene generally is placed at approximately2.4 Ma, at the first indication of cold climate(Zagwijn, 1989; Cameron et al., 1993). This prac-tice is followed likewise by the British and Dutchgeological surveys (Gatliff et al., 1994), eventhough the boundary is at 1.6 Ma, according toHaq et al. (1987).
DERIVATION OF THE NORMAL VELOCITY-DEPTH TREND
A normal velocity-depth trend for the Chalk,VN, is derived here to describe how velocity ofChalk with average composition increases duringnormal compaction. The normal trend is definedqualitatively by identifying data that representmaximum burial in areas unaffected by overpres-sure, and by constraining the trend by likely val-ues near the surface (Figure 7A). The curved pathof the normal trend is expressed in linear seg-ments because a single mathematical function failsto account for depth variations in the compactionprocess. To arrive at a smooth normal trend, thefinal choice of linear parameters is confined to anarrow interval, where round figures are pre-ferred because the trend is only a model of thecomplex geological reality.
Segment A, V < 2700 m/s
For the shallow data points, the smaller veloci-ties for a given depth are closest to normal com-paction, whereas the higher velocities in generalref lect increasing burial anomalies (Figure 7A).The principal question, however, is whether theNorth Sea Chalk at shallow depths anywhere isfound at maximum burial. An envelope of mini-mum velocity for each depth can be traced fromzero burial to a depth of around 1400 m. At thismaximum depth, the present burial of the Chalkmust be closest to maximum burial. Fixpoint 1 for
Japsen 2041
Figure 7—Interval velocity vs. midpoint depth for the Chalk Group, and the normal velocity-depth trend, VN (equa-tion 8; Table 2). Shallow data points reveal high velocities relative to VN due to overburden reduction. Deeper datareveal low velocities relative to VN due to undercompaction and overpressure. See text for discussion of derivationof fixpoints and the normal velocity-depth trend. (A) Plot of 845 wells in the North Sea Basin. A, B, C, and D (alongbottom) indicate the four segments of VN. (B) Plot of 135 Danish wells and two semiregional trends with low appar-ent velocity-depth gradients.
2042 Velocity-Depth Anomalies, North Sea Chalk
Cp
h.
Ed
b.
1213
1415
1615
1617
18
2021
227
89
10
31
39383023
29 3736
2827
4243
44A E
FB
KM
L49
4748
PQ
5610
5606
5607
5608
5609
5604
5605
5508
5507
5506
5505
5504
5509
5510
5511 54
11
5512
5710
2
5253
59°
57°
55°
53°
57°
55°
53°
4°0°
4°8°
12°
8°12
°0°
4°
2°W
2°E
6°10
°14
°
58°
56°
54°
10°
6°2°
E2°
W
54°
56°
58°
15/2
8-2
21/6
-2
16/2
9-2
16/6
-1
Eln
a-1
Ste
nlill
e
Dan
Kar
l-1
L16-
4
Eko
fisk
S-1
44/1
9-3
38/2
4-1
L-1
Vel
oci
ty a
no
mal
y in
m/s
N
ot m
appe
d
Nat
. qua
d. n
o.
Wel
l dat
a
Oil
field
Wel
l loc
atio
n
UT
M z
one
31
100
km
52
Ch
alk
Gro
up
Upp
er C
reta
ceou
s–D
ania
n
Vel
oci
ty a
no
mal
yC
.I. 1
000
m/s
Lim
it of
Ter
tiary
sed
imen
ts (
excl
. Dan
ian)
Ove
rlap
of C
halk
Gro
up a
nd S
hetla
nd G
roup
UK
ND
K GN
L
Bel
ow
-250
0
-250
0 -
-150
0
-150
0 -
-50
0
-500
-
500
500
- 1
500
abov
e 15
00
?
Fig
ure
8—
Vel
oci
ty a
no
mal
y f
or
the
Ch
alk
Gro
up
in
th
e N
ort
h S
ea B
asin
rel
ativ
e to
VN
(eq
uat
ion
s 1
, 8
; T
able
2).
Neg
ativ
e C
hal
k v
elo
city
an
om
alie
s in
the
cen
tral
No
rth
Sea
are
cau
sed
by u
nd
erco
mp
acti
on
du
e to
ove
rpre
ssu
re.
Po
siti
ve a
no
mal
ies
alo
ng
the
bas
in m
argi
n i
nd
icat
e re
du
ced
bu
rial
of
the
Ch
alk
. Co
nto
uri
ng
fro
m k
rige
d e
stim
ates
. On
in
set,
UK
= U
nit
ed K
ingd
om
, N =
No
rway
, DK
= D
enm
ark
, G =
Ger
man
y, a
nd
NL
= H
oll
and
.
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 trendabove fixpoint 1 to the surface, segment A of VN isestimated as V = 1600 + z based 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 Chalkvelocities 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;D’Heur, 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. Thevelocity level for normal compacted Chalk is thusdefined by the clear trend of data points below2000 m marking maximum velocities (4000–4900m/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 fixpoints 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 = 2700m/s. Segment C becomes V = 937.5 + 1.75 × z,which crosses segment B for V = 4000 m/s andpasses close to fixpoint 2.
Segment D, 4875 < V < 5500 m/s
At great depth the upper velocity level isdefined by a single well, UK 16/29-2. Fixpoint 3 istaken to be 5375 m/s at 2750 m, and Segment Dbecomes VN = 2625 + z (4875 < V < 5500) wherethe velocity-depth gradient is reduced to 1 m/s/m.Above this velocity interval there are no data toindicate a further approximation of Chalk velocityto the matrix velocity of calcite at 6400 m/s(Raiga-Clemenceau et al., 1988). The resultingburial anomaly for all wells, except one, withvelocities above 5000 m/s indicates that thesedeeply buried high-velocity chalks are overpres-sured, which is the case in the central North Seaquadrants N 7, UK 23, and southernmost UK 22where these wells are located.
The normal velocity-depth trend, VN, for theChalk Group gets the following form:
where z = depth below top of the sediments. TheNorth Sea Chalk has mainly velocities correspondingto segments B and C, for which the fixpoints for theupper depth interval are defined by minimumvelocities and maximum burial, and by maximumvelocities and absence of overpressure for thedeeper interval. The trace of segments B and C iswell defined by velocity data from the North SeaChalk, whereas the extrapolation along segment D
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 2043
Table 1. Statistics for the Well-Velocity Database*
Chalk SedimentGroup No. Vi (m/s) zt (m) zb (m) ∆z (m) Top (m)Penetrated Wells mean std. min. max. min. max. max. mean max. min. max.
*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 erroneousvelocity data. Top of sediments indicates ground level/sea bed relative to mean sea level.
(8)
2044 Velocity-Depth Anomalies, North Sea Chalk
Table 2. Chalk Group Velocity Data from Selected Wells*
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, φSC, withdepth as suggested by Sclater and Christie (1980)provides a general description of the compactionprocess (equation 9 in Appendix 2). A compari-son of φSC with VN (equation 8) is made based on atentative conversion of VN from a velocity-depthtrend to a porosity-depth trend by means of thevelocity-porosity conversion 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 φSC(porosity is predicted to 42 and 35%, respectively,at a depth of 1000 m). Conversely, the porosityreduction estimated from VN is faster than predict-ed by φSC 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 VN below 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 toVN constitute geographically well-defined areas andreflect 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
*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).
Japsen 2047
Fig
ure
9—
Ch
alk
bu
rial
an
om
alie
s re
lati
ve t
o t
he
no
rmal
vel
oci
ty-d
epth
tre
nd
(eq
uat
ion
s 2
, 8; T
able
2).
Ove
rbu
rden
red
uct
ion
in
crea
ses
away
fro
m t
he
late
Cen
ozo
ic d
epo
cen
ter
in t
he
cen
tral
No
rth
Sea
an
d r
each
es a
pp
rox
imat
ely
1 k
m a
lon
g th
e p
rese
nt-
day
lim
it o
f th
e C
hal
k.
Th
e u
nif
orm
ity
of
this
map
wit
h t
hat
of
the
pre
-Qu
ater
nar
y (
Fig
ure
4)
sugg
ests
th
at t
he
Ch
alk
bu
rial
an
om
aly
is
a m
easu
re o
f ex
hu
mat
ion
. T
he
neg
ativ
e b
uri
al a
no
mal
ies
ind
icat
e a
red
uct
ion
in
bu
rial
. C
on
tou
rin
g f
rom
kri
ged
est
imat
es u
sin
g t
he
sam
e k
rigin
g p
aram
eter
s as
in
Fig
ure
8.
EM
S =
Eas
t M
idla
nd
s Sh
elf.
On
inse
t, U
K =
Un
ited
Kin
gdo
m, N
= N
orw
ay, D
K =
Den
mar
k, G
= G
erm
any
, an
d N
L =
Ho
llan
d.
?
?
?
59°
57°
55°
53°
57°
55°
53°
4°0°
4°8°
12°
8°12
°0°
4°
2°W
2°E
6°10
°14
°
58°
56°
54°
10°
6°2°
E2°
W
54°
56°
58°
Ed
b.
Cp
h.
1213
1415
1615
1617
18
2021
227
89
10
31
39383023
29 3736
2827
4243
44A E
FB
KM
L49
4748
PQ
5610
5606
5607
5608
5609
5604
5605
5508
5507
5506
5505
5504
5509
5510
5511 54
11
19
2
5253
5709
5710
5512
Ch
alk
Gro
up
Upp
er C
reta
ceou
s–D
ania
n
Bu
rial
an
om
aly
(= - -
'app
aren
t upl
ift')
C.I.
250
m
L
imit
of T
ertia
ry s
edim
ents
(ex
cl. D
ania
n)
Bu
rial
an
om
aly
in m
UT
M z
one
31
100
km
52
Max
. bur
ial
0 -
-2
50
-250
-
-50
0
-500
-
-75
0
-750
-
-100
0
>
- -1
000
Not
map
ped
Nat
. qua
d. n
o.
Wel
l dat
a
Wel
l loc
atio
n
UK
ND
K GN
L
13/2
8-2
47/2
9a-1
Cle
etho
rpes
-1
S-1
Eln
a-1
L-1
Ste
nlill
e
Bor
g-1
Fred
erik
shav
n-1
Mon
a-1
Kim
-1
Ste
vns
Nor
th S
ea B
asin
Mor
ay F
irth
Mid
-Nor
thS
ea H
ighCen
tral Graben
VikingGraben
Rin
gk.-
Fyn
-H
igh
EM
S
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 ofnegative 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 THE NORTH 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 Basinwhere Pliocene 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 Oligocene–Miocene 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 Doré and Jensen (1996). Theyfound 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 world’s 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 on Chalk Burial Anomalies
Negative chalk burial anomalies are mapped in abroad zone along the present margin of Chalkdeposits, whereas the Chalk is at maximum burial
2048 Velocity-Depth Anomalies, North Sea Chalk
Japsen 2049
Fig
ure
10
—C
orr
esp
on
din
g ar
eas
of
ove
rpre
ssu
red
Ch
alk
ou
tlin
ed f
rom
pre
ssu
re m
easu
rem
ents
an
d f
rom
Ch
alk
bu
rial
an
om
alie
s co
inci
den
t w
ith
th
ela
te C
eno
zoic
dep
oce
nte
r. (
A)
Ch
alk
fo
rmat
ion
ove
rpre
ssu
re (
Tab
le 3
). (
B)
Ch
alk
bu
rial
an
om
alie
s re
lati
ve t
o a
no
rmal
vel
oci
ty-d
epth
tre
nd
(T
able
2).
(C)
Late
Cre
tace
ou
s–C
eno
zoic
str
uct
ura
l el
emen
ts. T
he
ove
rpre
ssu
red
zo
ne
corr
esp
on
ds
to m
axim
um
th
ick
nes
s o
f th
e u
pp
er P
ost
Ch
alk
Gro
up
(T
able
3),
wh
erea
s P
aleo
cen
e sa
nd
s o
verl
yin
g th
e C
hal
k t
o t
he
no
rth
wes
t ca
use
ble
ed-o
ff o
f o
verp
ress
ure
. So
uth
of
the
Vik
ing
Gra
ben
, sh
aly
Ch
alk
cau
ses
po
siti
ve v
elo
city
an
om
alie
s ev
en w
her
e th
e C
hal
k i
s n
orm
ally
co
mp
acte
d.
On
(B
), p
oin
ts A
an
d B
in
dic
ate
the
loca
tio
n o
f th
e d
epth
pro
file
in
Fig
ure
19
. In
set
abb
revi
atio
ns
as i
n F
igu
re 9
.
45
2857
°
55°
58°
56°
54°
59°
1415
1615
1617
18
2122
78
910
3
39383023
29 37 43A E
FB
5604
5605
5506
5505
5504
2
59°
57°
55°
54°
56°
58°
0°4°
2°E
0°2°
E
6°
4°6°
44
36 42
750
100012
50
Bu
rial
an
om
aly
(m)
Ch
alk
Gro
up
Upp
er C
reta
ceou
s–D
ania
n
Bu
rial
an
om
aly
>50
0 m
C.I.
500
100
km
B
1500
-
200
0
1000
-
150
0
500
- 1
000
Bel
ow 5
00
Wel
l dat
a
Nat
. qua
d. n
o.28
A
Ove
rlap
of C
halk
Gro
upan
d S
hetla
nd G
roup
,Jo
hnso
n an
d Lo
tt,19
93
Dep
th to
mid
-Mio
cene
unco
nfor
mity
> 7
50m
C.I.
250
m,
Koc
kel,1
988b
Dis
trib
utio
n of
san
d ab
ove
Cha
lk(M
aure
en F
m.)
,K
nox
and
Hal
low
ay,1
992,
Spe
ncer
,198
7
B
28
59°
57°
55°
54°
56°
58°
0°4°
2°E
0°2°
E
57°
55°
6°
58°
56°
54°
4°6°
59°
3837
36
?
?
750
1000
1250
1415
1615
1617
18
2122
78
910
31
39
302329
28
4243
44A E
FB
5604
5605
5506
5505
5504
24
5
(217
5
(145
0
(72
5
15-2
0
10-1
5
5-10
Bel
ow 5
Ove
rpre
ssu
rein
MP
a (
psi
)
Pre
ssur
e fr
om te
st
Wel
l loc
atio
n
Pre
ssur
e fr
om m
ud w
eigh
t
Pre
ssur
e te
st in
San
dsto
ne
abo
ve C
halk
(E
lna-
1)
-290
0)
-217
5)
-145
0)
(725
)
Ch
alk
Gro
up
Upp
er C
reta
ceou
s–D
ania
n
Ove
rpre
ssu
reC
.I. 5
MP
a (7
25 p
si)
100
km
A
28N
at. q
uad.
no.
Oil
field
Dan
Siri
-1
T-1
Val
hall
30/1
C-3
16/2
8-1
16/2
-1
16/1
-3
Eko
fisk
Eln
a-1
Mon
a-1
Ove
rlap
of C
halk
Gro
upan
d S
hetla
nd G
roup
,Jo
hnso
n an
d Lo
tt,19
93
Dep
th to
mid
-Mio
cene
unco
nfor
mity
> 7
50m
C.I.
250
m,
Koc
kel,1
988b
Dis
trib
utio
n of
san
d ab
ove
Cha
lk(M
aure
en F
m.)
,K
nox
and
Hal
low
ay,1
992,
Spe
ncer
,198
7
30/6
-3 Joan
ne
UN
N
GN
LDK
Nor
th S
ea B
asin
(c)
Mor
ay F
irth
Mid
Nor
thS
ea H
ighCen
tral Graben
VikingGraben
Rin
gk.-
Fyn
-H
igh
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 57°N. The course of the burial-anomaly contours east of the study area, in theBaltic Sea, awaits further investigation.
Comparison With Other Studies and Evidence 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 reflectance) (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 withwells in common with this study is given in Table 4.Fission-track studies indicating kilometer-scale
2050 Velocity-Depth Anomalies, North Sea Chalk
Figure 11—Comparison ofVN and sonic logs from (A)pelagic carbonate depositsof Eocene–Pleistocene age 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 VN do 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
Dep
th (
km b
elow
top
sedi
men
ts)
0
Velocity (m/s)
4
3
2000 3000
2
Zm = 699 m
Japsen 2051
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 Eastern North 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 56°N, the loca-tions found in this paper and in Jensen and Schmidtare identical, just east of well DK L-1, whereas at58°N this study suggests a position 30 km fartherwest into the basin. The same relation applies for acomparison with the zero line indicated by Hansen(1996), only at 58°N 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 relativelyhigh siliciclastic content in the basal Chalk, whichhas avoided deep erosion in the area (quadrants DK5709–10) (Sorgenfrei and Buch, 1964). A glaciallyinduced reduction in velocity commonly observed inthe upper 20 m of the Chalk below the Quaternarycover in Denmark may add to this underestimation(C. Andersen, 1995, personal communication).
The area in Denmark where overburden reduc-tion may be estimated is enlarged to the south by theuse of Chalk velocities relative to previous worksbased on deeper, but less extensive, strata (Japsen,1993a; Jensen and Schmidt, 1993; Michelsen andNielsen, 1993). A minimum of 300 m is estimat-ed in southwesternmost Denmark (the Borg 1well) , whereas est imates from the easternislands are above 500 m and reach a maximumof approximately 750 m in the Stenlille wells,only 100 m lower than the estimate of Japsen(1993a) (Figure 9).
Figure 12—Chalk formationpressures vs. depth belowsea level, central North Sea(Table 3). Dashed lines A, B, and C mark geographically coherent,apparent pressure compartments in the Danish Central Grabenwith overpressures of 7 ±1,9 ±1, and 15 ±1 MPa,respectively. These apparent pressure compartments may beexplained by the small,local variations of the overburden rather than byhydraulic communication.PEkofisk shows the pressure-depth trend forNorwegian Chalk fields in the Greater Ekofisk area.Vertical pressure communication in thehydrocarbon phase mayexplain the apparent dropin overpressure with depthin the Ekofisk area. mwe =mud weight equivalent.
2.0
20 30 40 50
2.5
3.0
3.5
Chalk Formation Pressure (MPa)
Dep
th (
km b
elow
sea
leve
l)A
B
C
1.5
Valhall
Elna 1
Dan
PH = 1.02 g/cm
3 mw
e (0.44 psi/f)
16/2-1
16/1-3
16/29C-7
16/28-1
S = 2.08 g/cm 3 mwe (0.90 psi/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
2052 Velocity-Depth Anomalies, North Sea Chalk
Table 3. Chalk Group Pressure Data From Selected Wells*
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 agreeswith 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 Western North 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 Turonian–Maastrichtian 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 inthe two studies in the relevant depth interval(Appendix 2). The exclusion of the Cenomanian andDanian parts of the Chalk Group in Hillis’s (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 zones inthe southern North Sea was estimated relative tosupposedly stable nearby areas in a number ofstudies in the southern North Sea (Marie, 1975;Glennie and Boegner, 1981; Barnard and Cooper,1983; Allsop and Kirby, 1985; Cope, 1986).
In the Inner Moray Firth, moderate erosion esti-mates (<700 m in quadrant UK 13) based on Chalkvelocity data were presented by Hillis et al. (1994).Their estimates are too low because their referencewells are from an area affected by nearly 500 m oferosion (Appendix 2). Relatively high estimates(<1300 m, quadrant UK 12) based on sonic datafrom Upper Jurassic shale (Hillis et al., 1994) aremore compatible with the results of this study.
In Dutch waters, no studies of regional Ceno-zoic exhumation are available. Here, the Chalk, inparts, is buried under a thick Quaternary cover(>500 m) (Caston, 1977), and exhumation is noteasily recognized.
Timing of the Cenozoic Exhumation of theNorth Sea Basin
Timing and the existence of the regionalCenozoic exhumation of the North Sea Basin is dif-ficult to assess because of its basinwide scale, andbecause reference locations are difficult to identify.By contrast, local zones of Late Cretaceous–earlyTertiary inversion related to the Alpine orogenymay be recognized on seismic sections, and their
*See Appendix 1 for list of symbols. Norwegian and UK well pressure data courtesy Petroleum Information (Erico). Danish pressure data from in-housereports and original data; Dan/Kraka from Andersen (1995). Estimated aquifer pressures for Norwegian fields from Caillet et al. (1997). Field data: zup and dZBfrom well in field; zlow over crest of field. All Norwegian fields belong to the Greater Ekofisk area. Under Pressure Compartments: A, B, and C aregeographically coherent pressure compartments in the Danish Central Graben. A = 7 ±1 MPa, B = 9 ±1MPa, C = 15 ±1 MPa.
**Mean value of several measurements.†Overpressure relative to a hydrostatic gradient of 1.02 × 103 kg/m3 (mud weight equivalent).††Gas show in Chalk (Britze et al., 1995a).‡Shaly Chalk, overlap of Chalk Group and Shetland Group.‡‡∆zup estimated from map.§Test results from Paleocene sandstone 30 m above top Chalk.§§Chalk Group not penetrated.
2054 Velocity-Depth Anomalies, North Sea Chalk
Tab
le 4
. P
revi
ou
s E
xh
um
atio
n S
tud
ies
Co
mp
ared
wit
h T
his
Stu
dy
Wel
lsD
iffe
ren
ceLa
te C
reta
ceo
us–
Stra
tigr
aph
icC
om
par
edEs
tim
ate†
†C
eno
zoic
Stu
dy*
Met
ho
d**
Leve
lA
rea†
To
tal
(m)
Co
rrel
atio
n‡
Exh
um
atio
n
Jap
sen
ShC
Low
erD
K n
ort
hea
st19
–67
0.85
Late
Cre
tace
ou
s–ea
rly
Ter
tiar
y(1
993b
)Ju
rass
ico
nsh
ore
& o
ffsh
ore
32–
+ N
eoge
ne
Jen
sen
&
ShC
p
re-U
pp
erD
K/N
on
sho
re &
2169
0.89
Neo
gen
eSc
hm
idt
den
sity
Cre
tace
ou
s?o
ffsh
ore
35–
(199
3)(V
R)
Mic
hel
sen
ShC
Low
erD
K n
ort
hea
st3
282
–La
te C
reta
ceo
us–
earl
y T
erti
ary
& N
iels
enJu
rass
ico
nsh
ore
& o
ffsh
ore
7–
+ L
ate
Cen
ozo
ic(1
993)
Han
sen
ShC
Low
er J
ura
ssic
N s
ou
th o
f 66
°N20
–196
0.61
Late
Cen
ozo
ic(1
996)
–Ter
tiar
y64
–
Hill
is e
t al
.C
hC
Up
per
Cre
tace
ou
sU
K n
ort
hea
st8
–518
0.95
Earl
y T
erti
ary
(199
4)Sh
CU
pp
er J
ura
ssic
off
sho
re26
–
Th
om
son
VR
pre
-Up
per
UK
no
rth
east
1–6
06–
Earl
y T
erti
ary
& H
illis
Cre
tace
ou
s?o
ffsh
ore
1037
0(1
995)
Gre
enA
FTA
pre
-Lat
eU
K s
ou
thea
st1
35–
Earl
y +
mid
dle
Ter
tiar
y(1
989)
Cre
tace
ou
so
ffsh
ore
27~
1500
Bra
y et
al.
AFT
AU
K s
ou
thea
st(1
)22
5–
Earl
y +
mid
dle
Ter
tiar
y(1
992)
VR
on
sho
re &
off
sho
re2(
+5)
1280
Hill
isC
hC
Up
per
Cre
tace
ou
sU
K s
ou
thea
st38
540.
97T
erti
ary
(199
5a)
ShC
Tri
assi
cap
pro
xim
atel
y14
9–
off
sho
re
*Jap
sen
(199
3a):
28
wel
ls w
ith d
ZB
from
bot
h st
udie
s ex
clud
ing
one
wel
l with
dZ
B(s
tudy
) >
0, f
our
wel
ls d
rille
d ov
er s
alt
diap
irs (
diffe
rent
bur
ial h
isto
ry f
or t
he J
uras
sic
and
the
Late
Cre
tace
ous)
, fo
urw
ells
with
ove
rest
imat
ed d
ZB
(stu
dy)
[DK
Bø
rglu
m-1
, J-
1, F
jerr
itsle
v-1,
Fje
rrits
lev;
Pet
erse
n et
al.
(in p
ress
)].
Jens
en a
nd S
chm
idt
(199
3):
24 w
ells
with
dZ
Bfr
om b
oth
stud
ies
excl
udin
g D
K B
ørg
lum
-1,
Hal
dage
r-1,
J-1
(in
vers
ion
zone
). M
iche
lsen
and
Nie
lsen
(19
93):
Exc
ludi
ng D
K J
-1 (
inve
rsio
n zo
ne).
Exh
umat
ion
over
estim
ated
bec
ause
vel
ocity
of
over
pres
sure
d Ju
rass
ic s
hale
s in
the
Cen
tral
Gra
ben
wer
e us
ed a
s re
fere
nce
(cf.
Japs
en ,
1993
a). H
illis
et a
l. (1
994)
: Com
paris
on w
ith e
stim
ates
bas
ed o
n C
halk
dat
a (m
ean
of H
od a
nd T
or fo
rmat
ions
). T
hom
son
and
Hill
is (
1995
): E
xhum
atio
n es
timat
es fr
omve
loci
ty d
ata
are
from
Hill
is e
t al.
(199
4). C
ompa
rison
for
UK
12/
30-1
. Gre
en (
1989
): W
ells
and
out
crop
dat
a. C
ompa
rison
for
UK
Cle
etho
rpes
1. B
ray
et a
l. (1
992)
: AF
TA
dat
a fo
r 5
wel
ls fr
om G
reen
(19
89).
Wel
l UK
47/
29-1
com
pare
d w
ith U
K 4
7/18
-1. U
K C
leet
horp
es 1
als
o in
clud
ed, d
ZB
(stu
dy)
= 7
70 m
(V
R o
nly)
. Hill
is (
1995
a): C
ompa
rison
with
est
imat
es b
ased
on
Cha
lk d
ata.
**A
FT
A =
Apa
tite
fissi
on tr
ack
anal
ysis
, ChC
= C
halk
com
pact
ion,
ShC
= S
hale
com
pact
ion,
VR
= V
itrin
ite r
efle
ctan
ce.
† DK
= D
enm
ark,
N =
Nor
way
, UK
= U
nite
d K
ingd
om.
††D
iffer
ence
= -
dZB(s
tudy
) +
dZ
B(C
halk
), w
ell a
vera
ge/e
stim
ate
of d
ZB(s
tudy
) if
1 w
ell.
‡ Cor
rela
tion
betw
een
dZB(s
tudy
) an
d dZ
B(C
halk
) if
mor
e th
an 3
wel
ls.
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 Cenozoicexhumation (Japsen, 1993a). Tectonism during thelate Oligocene and Miocene was suggested bySpjeldnæs (1975) to be a major factor in the shiftfrom open-marine to terrigenous facies in theTertiary of Denmark.
Timing of Exhumation of the Western North 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 ofthe 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 suggestion isconsistent with published estimates of overburdenreduction based on studies of sediment com-paction (mainly offshore) (Bulat and Stoker, 1987;Hillis et al., 1994; Hillis, 1995a; Thomson and Hillis,1995) and of fission tracks (onshore) (Green, 1986,1989; Bray et al., 1992; Lewis et al., 1992; Green etal., 1993), as well as with the concept of two peri-ods of Tertiary exhumation (Lewis et al., 1992;Green et al., 1993).
In the southern North Sea, regional exhuma-tion was found to be of late Tertiary age by Bulatand Stoker (1987), and upper Paleocene argilla-ceous deposits on the east Midlands Shelf weresuggested to represent remnants of a more exten-sive Paleogene cover (Stewart and Bailey, 1996).The thick, shallow-marine sandstones of the mid-dle–late Miocene Utsira Formation may result fromNeogene exhumation of northern Britain and thesurrounding shelf. This formation is present in theViking Graben area between 58° and 62°N and isinterpreted to be sourced from the west (Isaksenand Tonstad, 1989).
In the Dutch sector, base Miocene, middleMiocene, and base Quaternary unconformities areprominent (van Wijhe, 1987; Zagwijn, 1989). TheChalk may have been at maximum burial depth atthe time represented by any of the unconformitiesthat separate the thin Neogene units. South of thestudy area, the Rhenish Massif has been subject touplift since the end of the Oligocene, and at anaccelerated rate since the end of the Miocene(Meyer, 1983). These observations agree with aNeogene timing of Chalk maximum burial in thesouthern Dutch sector of the North Sea.
Onset and Cessation of Neogene Exhumationof the North Sea Basin
The dating of maximum Chalk burial to theNeogene indicates a crude time interval (25–2 Ma)(Haq et al., 1987) during which the exhumation ofthe North Sea Basin took place; however, whereparts of the Neogene succession are preserved,the timing of exhumation can be specified byidentifying hiatuses in the stratigraphic recordduring which deposition followed by exhumationmay have taken place. Where most of the Tertiarysediments are removed, the timing of erosion canbe dated only indirectly by inference from lessexhumed areas.
Within the North Sea Basin, the middle Mioceneunconformity has been suggested to represent theonset of pronounced uplift by several workers(Bulat and Stoker, 1987; Jensen and Schmidt, 1993;Jordt et al., 1995; Riis, 1996; Stewart and Bailey,1996). The middle Miocene unconformity (14 Ma)(Jordt et al., 1995) is a basinwide regional downlap
Japsen 2055
2056 Velocity-Depth Anomalies, North Sea Chalk
surface (Jordt et al., 1995) that is present through-out the North Sea, apart from a narrow zone ofcontinuous 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 significant 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 Oligocene–Pliocene, and further uplift relatedto late Pliocene–Pleistocene 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 farwest as 4°E 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 thePliocene–Pleistocene 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 towhere both the upper and lower part 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 Pliocene–Pleistoceneboundary (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 bur ial anomaliesincrease toward the basin margins. At the basinmargin, where the Quaternary cover is thin, ∆zmiss= –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 13—Burial diagramsfor the Chalk Group withPaleogene or Neogeneexhumation for well UK47/29a-1 (Figure 9). Note thedifferences in the derivedburial rates. (A) Maximumburial at 60 Ma (cf. Green,1989, his figure 9; Green etal., 1993, their figure 4). (B) Maximum burial at 15Ma (cf. Hillis, 1995b, his figure 5). Well data andexhumation estimate fromBray et al. (1992). AfterJapsen (1997).
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the Quaternary is thick may well be an exaggera-tion due to a too easterly location of the zero line.The Danish L-1 well is close to the zero line, andthe Chalk is at maximum burial: dZB = +75 m(Figure 9). In this well a 37-m-thick lowerPliocene unit is overlain by lower Pleistocene sed-iments at the base of a 322-m-thick Quaternarysuccession (Laursen, 1992). If we put dZB ≈ 0, themissing section becomes the thickness of theQuaternary. About 300 m of Pliocene–lowermostPleistocene sediments are thus likely to have beeneroded at the Pliocene–Pleistocene transition atthis location.
Toward the basin center, the thickness of theremoved section is gradually reduced, and must bezero below the Quaternary depocenter, where sed-imentation was continuous throughout the lateCenozoic (Figure 15) (Gatliff et al., 1994). The cen-tral parts of the basin were thus affected byNeogene exhumation much later than the marginsin accordance with the notion of Stuevold andEldholm (1996), who suggested that increasinglygreater shelf areas would become affected by tiltingand erosion as uplift is progressing.
In the Inner Moray Firth, a Neogene timing ofexhumation means that no or only limited post-exhumational burial took place, because theQuaternary depocenter lies east of the erodedarea (Figure 14B) and, consequently, ∆zmiss ≈–dZB (>1000 m in quadrant UK 13). Hillis et al.(1994), however, assumed early Paleocene maxi-mum burial of the Chalk in that area, and that thepost-exhumational burial equaled the thicknessof the entire Cenozoic sequence (500–1000 m inquadrant UK 13). The model of Hil l is et al .(1994) thus resulted in the unlikely prediction ofremoval of about 1 km of Danian deposits beforedeposition of the Paleocene sandstones encoun-tered today. At the estimated line of zero burialanomaly (well UK 13/30-2), the missing sectionwas found to exceed 1 km, and the possibility oferosion on this order east of the zero line, acrossthe Viking Graben, was mentioned but later con-sidered unlikely (Hil l is et al . , 1994; Hil l is ,1995b). According to this study, the missing sec-tion is less than 500 m near the zero line on boththe eastern and western sides of the VikingGraben (Figure 14C), and probably zero belowthe Pliocene depocenter in the Viking Graben(Figure 15).
Near the Dutch coast, the estimated missingsection reaches 750 m due to the substantialQuaternary thickness even where the burialanomaly is moderate. This finding seems to sug-gest that at least the southern part of the Dutchterritory was affected by the Neogene exhuma-tion, as indicated by truncation of the Chalkalong 53°N, also on German territory.
Japsen 2057
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Figure 14—Estimate 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).
2058 Velocity-Depth Anomalies, North Sea Chalk
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 Pliocene–Paleozoic 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, whereasVejbæk (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 uplift of southern Norway, andfound it to be coincident with Oligocene andPliocene plate reorganizations in the NorthAtlantic. 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 15—Estimate of themissing section erodedduring the Neogene indicated above the baseQuaternary unconformity.The height of the exhumedwedge above the sea bed isthe burial anomaly, dZB,the part below the sea bedis the post-exhumationalburial, BE. Mesozoic andLate Cretaceous–Cenozoicstructural elements areindicated. Profile fromEdinburgh (Edb.) to Copenhagen (Cph.) isshown on Figure 9.
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Japsen 2059
continental uplift to represent a flexural intraplatedeformation 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 the entire North Atlantic, and rapid lateCenozoic subsidence and sedimentation have beenobserved throughout the region (compare Boldreeland Japsen, 1998). Major regional Neogeneexhumation is documented of fshore west ofScandinavia from the Barents Sea to Denmark (e.g.,Jensen et al., 1992; Japsen, 1993a; Rohrman et al.,1995; Doré and Jensen, 1996; Riis, 1996; Stuevoldand Eldholm, 1996), and an accelerated lateCenozoic sedimentation is well known in a zonewest of the same area (e.g., Nielsen et al., 1986;Kooi et al., 1991; Doré and Jensen, 1996). Neogeneuplift and erosion onshore Scandinavia has beendeduced from studies of landforms and fission tracks(e.g., Rohrman et al., 1995; Lidmar-Bergström, 1996;Riis, 1996). Regional Neogene exhumation ofBritain and the western North Sea was suggestedby Japsen (1997), and is documented for the west-ern North Sea by this study. The Faeroe-Rockallarea suffered Miocene compression, suggested tobe associated with sea-floor spreading in the NorthAtlantic (Boldreel and Andersen, 1993). JamesonLand in East Greenland experienced Cenozoicuplift and erosion at a rate that accelerated duringlate Cenozoic times (Christiansen et al., 1992;Mathiesen et al., 1995). West of Greenland, Neo-gene uplift and erosion is recognized in parts of thearea (Chalmers, 1998), while thick late Cenozoicdeposits have accumulated in other parts of thatarea (Cloetingh et al., 1990; Whittaker et al., 1997).
Around the North Atlantic, rapid late Neogene sub-sidence and sedimentation, as well as relative upliftalong basin edges, were observed by Cloetingh etal. (1990), and studies of North Atlantic passivemargin uplift were reviewed by Eyles (1996),including studies documenting late Cenozoicexhumation of eastern North America.
Explanations for these observations have com-monly been local, involving mechanisms not appli-cable to the entire area; however, vertical move-ments appear to have affected many, if not all,continental margins around the North Atlantic. Amodel explaining these large-scale, late Cenozoicphenomena must separate the effects of Paleogenesynrift plate-boundary-related uplift from theeffects of Neogene intraplate uplift (Rohrman et al.,1995; Stuevold and Eldholm, 1996; Japsen, 1997).Such a model must be constrained by global obser-vations of these intraplate events rather than bydata from a single region, and by the fact that theireffects reach beyond passive margins, as is demon-strated here in the case of the North Sea.
Conclusions Regarding Neogene Exhumation
The Chalk Group is overcompacted due to a pre-vious greater burial depth along the western andeastern margins of the North Sea Basin. The burialanomalies estimated from Chalk velocities are com-parable to estimates of overburden reduction fromother studies, but through the large areal coverageof this study I can demonstrate the link betweenthe areas covered by these previous studies using auniform data set (Figure 9). The exact magnitudeand timing of the overburden reduction and thelocation of the line of zero burial anomaly may be
Figure 16—Vertical move-ments affecting the NorthSea Basin during the Neogene/late Cenozoic.Neogene uplift and erosionalong the margins of thebasin leading to reductionof the Chalk overburden,and rapid late Cenozoicburial in the basin center.Profile is flattened at the mid-Miocene unconformity; profilefrom Edinburgh (Edb.) to Copenhagen (Cph.) isshown on Figure 9.
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disputed, but the regional scale of the erosion, andthe fact that both the western and the eastern mar-gins of the basin were affected, are documented inthis study.
The uniformity of the patterns of Chalk burialanomalies and the pre-Quaternary subcrop alsosuggests that the anomalies are measures of erosionand, moreover, that the erosion occurred duringthe Neogene (Figure 4). Only Neogene maximumburial of the Chalk leaves time for the deposition,at moderate rates, of the kilometer-thick depositsthat are now missing on top of the Chalk (Figure13). I suggest that the onset of uplift and erosion ofthe margins of North Sea Basin is reflected by themid-Miocene unconformity (14 Ma), whereas ter-mination of the erosion of the shelf is indicated bythe base Pleistocene unconformity encounteredthroughout the basin (2.4 Ma) (Zagwijn, 1989)[mid-Pliocene according to Haq et al. (1987)].Quaternary erosion may have played a role wherethe Chalk is covered by a thin layer of Quaternarysediments.
Along the present-day limit of the Chalk, the miss-ing kilometer-thick section is likely to have been ofmainly Paleogene age, whereas several hundredmeters of mainly Pliocene sediments are missingalong the line of zero burial anomaly (Figure 14).Here, the Chalk is at maximum burial today due toQuaternary reburial. The southern part of themapped Dutch sector appears to have been affectedby Neogene exhumation, but this is masked by sub-stantial Pliocene–Quaternary reburial.
Large-scale late Cenozoic vertical movementsaffected the North Atlantic region, includingScandinavia and its seaward margin from theBarents Sea to Denmark, the North Sea, Britain, theFaeroe Islands, Greenland, and eastern NorthAmerica. The Neogene exhumation of the marginsof the North Sea Basin documented here, and theaccelerated sedimentation rates in the basin centerduring the late Cenozoic, differ from the predictedsaucer-shape Cenozoic basin of a McKenzie (1978)model (Figure 16). A model explaining the lateCenozoic vertical movements must be confined byknowledge of their distribution in time and space.
OVERPRESSURING OF THE NORTH SEACHALK AQUITARD
Chalk reservoir properties depend strongly onpressure conditions in the central North Sea(Scholle, 1977). Where the Chalk is characterizedby overpressure, it contains important hydrocar-bon accumulations, such as the Ekofisk and Danfields in the Norwegian and Danish sectors (Figure10). Here, the Chalk has been considered as aregional aquifer (Megson, 1992, 1998). Where
Chalk formation pressure is close to hydrostatic,hydrocarbon accumulations are marginal or absent,as in the UK sector (Sørensen et al., 1986; D’Heur,1993). In that sector, the Chalk has been consid-ered as a regional aquitard (Cayley, 1987; Darby etal., 1996), just as the lateral f low in the GreaterEkofisk area has been considered insignificant(Caillet, 1998). The normally pressured Chalk inthe UK sector is drained by overlying Paleocenesandstones (e.g., Darby et al., 1996), whereas theoverpressure in the Chalk in the Danish sector hasbeen suggested to be governed by regional hydro-dynamic f low in the Chalk (Megson, 1992).Whether lateral or vertical f luid migration in theChalk is dominant is a fundamental question forunderstanding the petroleum system in the NorthSea Basin.
The overpressure in the North Sea Chalk is inter-esting in a wider perspective because the numberof possible causes for the overpressuring is limited.In a review of mechanisms that generate overpres-sure, Osborne and Swarbrick (1997) grouped thesecauses into three main categories: (1) ineffectivevolume reduction due to imposed stress leading todisequilibrium compaction, (2) volume expansionof pore fluid or rock matrix, and (3) hydraulic headand hydrocarbon buoyancy. In addition, transfer-ence of overpressure generated elsewhere must beconsidered. According to these workers, the princi-pal mechanisms are disequilibrium compaction andvolume expansion during gas generation. In thecase of the North Sea Chalk, volume expansionmechanisms can be ruled out. Within the Chalkthere is no source potential and only small amountsof shale that could release water during transforma-tion of clay minerals (Cayley, 1987; Kennedy,1987); furthermore, aquathermal pressuring as sug-gested by Hunt (1990) and Leonard (1993) is notfeasible in the case of the relatively permeableChalk because this mechanism requires a seal withpermeability of close to zero (Hall, 1994; Osborneand Swarbrick, 1997). Disequilibrium compactionpreviously has been suggested as the main cause ofthe overpressure in the Chalk and the overlyingshales based on qualitative arguments (Carstens,1978; D’Heur, 1993; Hall, 1993; Japsen, 1994).
In the pre-Chalk section, high pressures (up to40 MPa at 4500 m depth) (Darby et al., 1996) areencountered below a seal formed by shales andmarls (Chiarelli and Duffaud, 1980; Carstens andDypvik, 1981; Buhrig, 1989; Gaarenstroom et al.,1993; Darby et al., 1996). The main source rocks inthe area are found in these synrift sediments of themainly Late Jurassic Central and Viking grabens(Gatliff et al., 1994). In this interval, volume expan-sion processes such as gas generation may play arole in generating overpressure (Buhrig, 1989), aswell as the process of disequilibrium compaction
2060 Velocity-Depth Anomalies, North Sea Chalk
(Chiarelli and Duffaud, 1980; Carstens and Dypvik,1981; Buhrig, 1989).
Overpressured shales are observed below themid-Miocene unconformity in the central North Sea(Michelsen, 1982; Cartwright, 1994; Japsen, 1994),and this level commonly is referred to as top over-pressure. The unconformity marks the onset of pro-nounced uplift of the margins of the North SeaBasin (e.g., Jensen and Schmidt, 1993; Jordt et al.,1995), and the onset of overpressure in the Chalkprobably coincided with the increase in sedimenta-tion rates from the middle Miocene onward(Nielsen, 1979; Nielsen et al., 1986), an idea sup-ported by basin modeling (Caillet et al., 1997).
The Chalk overburden (the Post Chalk Group)(Nielsen and Japsen, 1991) may be divided into anupper and a lower part at the mid-Miocene uncon-formity (Japsen, 1994) (Figures 1, 3). This simplesubdivision is applied to help analyze the relationbetween the overpressure in the Chalk and itsdegree of undercompaction as expressed by Chalkburial anomalies relative to a normal velocity-depthtrend (equations 2, 8). I argue in the next sectionthat the load of the upper Post Chalk Group acts asthe stamp that generates the main part of the over-pressure, whereas the thickness and sealing qualityof the lower Post Chalk Group determine over-pressure retention in the Chalk.
Areal Extent of the Overpressured Chalk
The zone of overpressured Chalk in the centralNorth Sea (∆P > 5 MPa) strikes north–northwestand affects British, Danish, Dutch, German, andNorwegian territory covering an area of about 425× 125 km2 (Figure 10A). The overpressure reaches20 MPa near the center of the zone in the Hod andValhall fields. The northern limitation of the zone isin the Jæren area (58°N, quadrants N 6, N 15, UK16). The continuity of the zone between theEkofisk and the Jæren areas, however, is not wellestablished due to limited data. The southern limi-tation probably reaches south of 55°N, into theDutch quadrant F. The northwest-striking limita-tions of the zone are established on a regional scalepartly using mud weight data.
Paleocene sandstones overlie the Chalk in thenorthwestern part of the North Sea (Figure 10)(Knox and Holloway, 1992). These sandstones orig-inate from uplifted areas north and northwest ofthe area (Gatliff et al., 1994), and reach into thearea of the late Cenozoic depocenter. The shape ofthis sandstone wedge matches the crescent-shapenorthwestern limitation of the overpressured zonewith hydrostatic pressure in most of quadrant UK22, whereas high pressures are encounteredbelow the rim of the sandstone wedge. This match
suggests that the Paleocene sandstones drain theChalk to the northwest, as has previously been sug-gested (Cayley, 1987; Andersen, 1995; Darby et al.,1996; Osborne and Swarbrick, 1997). The drainagecapacity of the sandstones depends on their con-tact with the Chalk, their hydraulic transmissivity,and the frequency of shaly units. These factors sug-gest a deterioration of the drainage efficiency ofPaleocene sandstones in the distal parts of thewedge.
The region of overpressured Chalk is confined tothe area where depth to the mid-Miocene uncon-formity is greater than 1000 m and the overpres-sure is proportional to this depth (for example, the15 MPa contour matches the 1500 m depth con-tour) (Figure 10A). This agreement suggests a rela-tionship between Chalk overpressure and the loadof the upper part and the overburden. The arealextent of the overpressured Chalk also matches thearea outlined by positive Chalk burial anomalies inthe central and southern North Sea (Figures 5, 10B;equation 5), suggesting a relationship between theChalk overpressure and undercompaction reflectedin the relatively low velocities. A close correspon-dence is seen between the course of the 10 MPaoverpressure contour and the 1000 m burial-anoma-ly contour, whereas the 5 MPa overpressure contourreaches farther south than the 500 m burial-anomalycontour.
Apart from undercompaction, other factorsinf luence Chalk velocity and hence its burialanomaly (Figures 17, 18). One factor is a high shalecontent (e.g., well UK 16/28-1). A second factorinfluencing Chalk velocity is gas charge (e.g., theshaly Chalk south of the Viking Graben). Gascharge in the Chalk lowers the Chalk velocity andcauses a relatively high burial anomaly. A third fac-tor is overburden thickness that also reduces theburial anomaly (either over salt diapirs or due toregional erosion as in well N 16/2-1). South of theViking Graben, the chalky limestone facies of theChalk Group overlap with the mudstone facies ofthe Upper Cretaceous Shetland Group (Johnsonand Lott, 1993). The northern limitation of theoverpressured zone thus becomes difficult to mapfrom Chalk velocities.
Sources of Overpressure in the North Sea Chalk
Disequilibrium CompactionThe Chalk formation overpressure, ∆P, is pro-
portional to the thickness of the late Cenozoicdeposits, ∆zup, for ∆zup > 1000 m (Figure 17A). Weget σup ≈ ∆zup/100 = 15 MPa in accordance withthe contours on Figure 10 by substituting ∆zup =1500 m into equation 4. The Chalk below the late
Figure 17—Crossplots ofChalk formation overpressure, ∆P, thickness of the upperPost Chalk Group, ∆zup,and Chalk burial anomalyrelative to a normal velocity-depth trend, dZB(equations 14, 8; Tables 2,3). (A) ∆P vs. ∆zup. The overpressure is proportional to ∆zup, and in the order of theeffective load of the upperPost Chalk Group, σup(σup = ∆zup/100) (equation 4). (B) ∆P vs.dZB. The compactionalpart of the overpressure,∆Pcomp, is less than ∆Pindicating that sourcesother than disequilibrium compaction contribute tothe overpressure (∆Pcomp =dZB/100) (equation 5). (C) dZB vs. ∆zup. In generaldZB < ∆zup, indicating a net drainage of the Chalk(DC = 0%) (equation 6).
Japsen 2063
Cenozoic depocenter thus is well sealed because thepore fluids carry the full effective load of the upperoverburden; however, no significant overpressure isfound in the Chalk where it is overlain by sandstone(e.g., wells DK Elna 1, UK 16/28-1) (Figure 19).Rapid, late loading probably generates the major partof the overpressure in the Chalk where the lowerTertiary sediments are sealing; however, a range ofoverpressures is recorded for given values of ∆zup;e.g., at about 1200 m, the range is from 4 to 13 MPa.Sources other than late burial must contribute to theoverpressure, and overpressure must be preserved todifferent degrees.
The Chalk overpressure is on the order of thecompactional part of the overpressure, ∆Pcomp, aspredicted by the burial anomaly, dZB (Figure17B). We get ∆Pcomp = 10 MPa in accordance withthe contours on Figure 10 by substituting dZB =1000 m into equation 5. The line ∆P = dZB/100= ∆Pcomp in Figure 17B) marks a lower limit ofthe overpressure, and this indicates that factorsother than disequilibrium compaction contributeto the overpressure. The mean of ∆Pcomp/∆P is80% for ∆ρup = 1.03 × 103 kg/m3 for 52 datapoints obtained away from diapirs and where ∆P≥ 4 MPa.
Figure 18—Crossplot ofChalk drainage capacity vs.thickness of the lower Post Chalk Group, ∆zlow(Table 3). (A) Net drainagecapacity, DC (equation 6)vs. ∆zlow. (B) Net drainagecapacity relative to overpressure, DC∆P(equation 7) vs. ∆zlow. Both plots show that thicklower Tertiary shales prevent drainage of theChalk. Extrapolation of thetrend lines yields DC =100% and DC∆P = 70% forzero seal thickness. Theunlikely prediction of adrainage capacity, DC∆P ,that is smaller than 100%for zero seal thickness indicates that the Chalkoverpressure has been increased by noncompactional sources,whereas DC (and the burialanomaly) reflects the irreversible compaction ofthe Chalk. The straighttrend lines connect the datapoints for the Dan field andfor the well with maximumChalk formation pressure,well UK 30/1C-3. The bentlines in (B) connect datafrom the apparent pressurecompartments A, B, and C (Figure 12). Legend onFigure 17.
∆zlow, lower Post Chalk Group thickness (m)
DC
, Cha
lk n
et d
rain
age
capa
city
(%
)125
100
75
50
25
0
-25
500 1000 1500 20000
dZB = ∆Zup
dZB = 0
∆zlow, lower Post Chalk Group thickness (m)
DC
∆P, C
halk
net
dra
inag
e re
l. to
ove
rpre
ssur
e (%
)
2000150010005000
-25
0
25
50
75
100
-50
∆P = 0
AlbuskjellEkofisk
GasShale
BuoyancyTransference
DrainageOverburden reduction
(A)
16/2-1
Dan Elna 1
16/28-1Valhall
30/1C-3
(B)
Elna 1
30/1C-3
16/2-1
Dan
Valhall
16/28-1
Drainage
16/1-3
16/1-3
Albuskjell
T-1
Nora 1
30/6-3
Ruth 1
30/6-3
Ruth 1
Nora 1
T-1
Mona 1
Mona 1
A
B
C
∆P = ∆Zup/100
2064 Velocity-Depth Anomalies, North Sea Chalk
TransferenceFrom the preceding discussion it follows that the
present overpressure in the Chalk is greater thanpredicted from its degree of undercompaction asestimated from velocity data (Figure 17B). Theoverpressure thus must have increased since maxi-mum effective stress was exerted on the Chalk as itattained its present degree of compaction (see Hall,1993). This implication is in accordance with thepresence of tension fractures in the Chalk of theNorwegian Albuskjell field (Watts, 1983). Theexplanation for such unloading may be related tothe twofold acceleration of the overpressure-generating processes by the increasing burial ratesthroughout the Cenozoic (Nielsen et al., 1986).First, the loading itself leads to increasing overpres-sure with time throughout the overpressured sedi-mentary column. Second, the burial causes increas-ing hydrocarbon generation in the mainly UpperJurassic source rocks (Caillet et al., 1997; T.Bidstrup, 1997, personal communication), henceincreasing overpressure due to volume expansion(Figure 19).
Loading, however, may increase pore pressurebut not reduce the effective stress. If we consider arock volume at a time for which S = σ + P applies,after which the load is increased by δS, we get S +δS = σ + P + δS. Consequently, both σ and P areeither unchanged or increasing; that is, the effec-tive stress is increased by the part of the added loadthat is not carried by the pore fluids. Compactionfluids thus may only lower the effective stress by lat-eral fluid flow, but this is likely only over smaller dis-tances (as is shown in following sections). The effec-tive stress acting on the Chalk, however, may bereduced by transference (redistribution of overpres-sure) from the pre-Chalk section where volume-expansion processes, such as gas generation, areactive (Figure 19). The principal direction in whichthese sediments can dewater is vertical, possiblyalong pressure-induced fractures in the Chalk overaxial horsts, as evidenced by heat flow anomaliesin the UK Central Graben (Darby et al., 1996).
Decreasing effective stress with time was erro-neously suggested for a one-dimensional com-paction model of the Chalk in the NorwegianAlbuskjell field by Watts (1983), who estimatedpressure from the rate of burial; however, he didnot consider the possibility of late transference of
W E
1
4km
0
2
3
North Sea Basin
Mid North Central Ringkøbing -Sea High Graben Fyn High
?
∆P
A B
∆Zup
∆Zlow
σup ≥ ∆Pcomp
∆Ptrans
Mid-Miocene unc.
σup, effective load of the upper
Upper Post Chalk Group
Lower Post Chalk Group
Paleocene sand (where present)
Chalk Group (Upper Cretaceous–Danian)
Fluid migration
overburden (equation 1)
sedimentsUpper Jurassic–Lower Cretaceous
LEGEND
Cenozoicexcl. Danian
EW00
1
2
10
20
Cha
lk o
verp
ress
ure
(MP
a)
Cha
lk b
uria
l ano
mal
y (k
m)
A B
50 km
}}
}
+ ∆Pbuoy
∆Pcomp
+ ∆Ptrans
= ∆P
}
}
}
if Paleocenesand present
Figure 19—Sketch illustrating factors affecting Chalkoverpressure, ∆P, and possible vertical or lateral fluidflow in the North Sea Basin. Chalk overpressure andburial anomaly occur along the profile indicated on topof sketch. Overpressure due to undercompaction,∆Pcomp (equation 5); overpressure due to transference,∆Ptrans; and overpressure due to buoyancy, ∆Pbuoy. Loca-tion of profile along 56°N from A to B on Figure 10B.
overpressure from the pre-Chalk section as thecause of stress reduction.
BuoyancyFormation pressure falls along a pressure-depth
gradient of only 0.6 × 103 kg/m3 mwe (mudweight equivalent) in the Chalk fields of theGreater Ekofisk area (Figure 12), leading to a dropin overpressure with depth from 20 to 17 MPafrom the Valhall field to the Ekofisk field, wherethe latter’s reservoir is buried about 700 m deep-er. In addition, the individual hydrocarbon pres-sure gradients in the Norwegian Chalk fields lineup along the Greater Ekofisk trend (Caillet et al.,1997). Vertical pressure communication in thehydrocarbon phase may explain these observa-tions by differences in the height of a hydrocar-bon column (density about 0.6 g/cm3) on top of aregional pressure level. Such a column would gen-erate an additional overpressure due to the buoy-ancy of the hydrocarbons of about 3 MPa [= (1 –0.6) × 103 kg/m3 × 700 m × 9.81 m/s2], which cor-responds to the observed difference between theValhall and Ekofisk fields. This difference is consis-tent with the observations made by Caillet et al.(1997) that oil-water contacts in the Chalk reser-voirs are difficult to estimate, and that no free-water level may be defined in the Valhall field.
A potential for lateral flow from shallow to deep-er Chalk reservoirs in the Greater Ekofisk area wassuggested by Caillet et al. (1997) from the appar-ently higher overpressure for the more shallowfields resulting from comparison with hydrostaticpressure; however, this f low potential does notexist if the hydrocarbon phase is dominantthroughout the area because there are no major dif-ferences in overpressure between the fields relativeto the hydrocarbon gradient (Japsen, 1998).
The overpressure in the Valhall field clearly is inexcess of the effective load of the upper Post ChalkGroup (Figure 17A), as well as in excess of the pres-sure prediction based on Chalk velocities (Figure17B). Sources other than disequilibrium com-paction thus must contribute to the pressure, andthis additional pressure was induced after the timewhen the Chalk attained its present degree of com-paction. The hydrocarbons of the Greater Ekofiskarea probably were emplaced into the Chalk at alate stage in the Cenozoic.
The area of the Greater Ekofisk accumulationmay be outlined by identifying data along the trendP = 31 + 5.7 × 10–3 × z [in MPa], 2500 < z < 3350 mbelow sea level (Figure 12). The trend is drawnbetween the points of highest and lowest Chalk for-mation pressure in Caillet et al.’s (1997) figure illus-trating hydrocarbon pressure gradients. The areastrikes about 100 km from the Joanne field in the
northwest to the Valhall and Hod fields in thesoutheast, with a width up to 50 km (Figure 10).
A hydrocarbon column of the order of 700 m isfound in the Middle Jurassic sandstones of theHeather field, UK northern North Sea (Penny,1991). Here, a free-water level is not observed, andthe pressure gradient in the reservoir correspondsto a mud weight of about 0.7 g/cm3. Perhaps theheight of the hydrocarbon accumulation in theGreater Ekofisk area is less remarkable than its lat-eral extent of about 3000 km2, as opposed to the40 km2 of the Heather field.
Migration Direction for Chalk CompactionFluids
The compaction fluids expelled from the Chalkduring late Cenozoic loading may have migrated lat-erally over long distances through the Chalk itself orvertically through the Tertiary deposits (Figure 19).According to the hydrodynamic model for theDanish Central Graben, hydrocarbons and brinemoved updip in the Chalk aquifer along the regionalpressure gradient from the Ekofisk to the Dan fieldin the southeast over distances of about 100 km(Figure 10) (Damtoft et al., 1992; Megson, 1992,1998). This model provides a mechanism for movinghydrocarbons from the known mature source rocksnear the Norwegian–Danish border to the DanishChalk fields in the Dan area where good sourcepotential is difficult to identify (Damtoft et al.,1992). No direct evidence has been given for region-al fluid flow in the Chalk other than the southeast-ward dip of the free-water level in three DanishChalk fields assumed to reflect water flow in theChalk aquifer (Megson, 1992, 1998). Caillet (1998),however, suggested that dipping oil-water contactsin Chalk fields also could be caused by either reser-voir heterogeneities and variations in capillary pres-sures, or by structural tilting and low oil permeabili-ty near the oil-water contact. As pointed out byDarby et al. (1996), a given pressure gradient pre-sents only a potential for fluid flow. Whether fluidflow does occur depends on permeability and time.
The burial anomaly, dZB, generally is smallerthan the thickness of the upper Post Chalk Group,∆zup, and consequently a part of the late Cenozoicloading has led to compaction of the Chalk, and toa reduced burial anomaly (equation 5; Figure 17C).Thus, a net drainage has taken place to account forthe partial compaction. The Chalk is found todewater slowly below the Cenozoic depocenter,whereas this process is more rapid farther from thedepocenter, as expressed by the drainage capacity,DC, a measure of the ratio between dZB and ∆zup(equation 6). The drainage capacity increases fromonly 0–10% around the Ekofisk field to about 60%
Japsen 2065
for the Dan field (Table 3). This difference indrainage capacity may be explained by the decreasein seal thickness from approximately 1500 to 600 mwhen moving from the Ekofisk to the Dan field. Weget dZB = ∆zup × ∆zlow/1600 by combining the lineartrend between DC and ∆zlow, the thickness of thesealing lower Post Chalk Group, and the definitionof the drainage capacity (equation 6; Figure 18A).The Chalk burial anomaly is thus proportional to theproduct of the thickness of the upper part of theoverburden (which induces the overpressure due toundercompaction), and the thickness of the lowerpart (which determines the degree of overpressureretention).
Vertical flow through the lower Tertiary shalesmust be the dominant migration route to explainthe dependency of drainage on seal thickness and,consequently, lateral f low in the Chalk must benegligible on a regional scale. This scenario is inagreement with the conclusions for the UK sectorby Cayley (1987), who argued that the Chalkwould lose fracture permeability away from anti-clines, by Darby et al. (1996), who considered theChalk as a regional aquitard, and by Caillet (1998),who found the pressure system in the GreaterEkofisk area to be controlled by seal efficiency andthe continuing compaction due to burial. The lackof regional hydraulic communication within theChalk also may be caused by discontinuity of per-meable beds related to the diversity and the com-plex depositional history of the Chalk (e.g.,Kennedy, 1987). Vertical migration through theTertiary overburden is evidenced by gas cloudsobserved on seismic data above pronounced struc-tures (Cayley, 1987; Megson, 1992; Andersen,1995; Caillet et al., 1997). Cartwright (1994) foundevidence for episodic, basinwide fluid expulsionfrom overpressured Tertiary shales in the North SeaBasin by analyzing small, closely spaced extension-al faults from seismic sections.
Presence of thick and continuous Paleocenesand sheets overlying the Chalk determines if theChalk is normally pressured (Cayley, 1987; An-dersen, 1995; Darby et al., 1996; Osborne andSwarbrick, 1997). This condition also supports theconclusion that the Chalk drains vertically. Indeed,the hydraulic transmissivity of the Chalk must besmall to maintain the observed high lateral differ-ences in overpressure. This difference is about 10MPa over the 40 km between the Danish wellsMona-1 and Elna-1, and data from intervening wellsshow that the difference is built up continuously,unaffected by the fault boundary of the MesozoicCentral Graben (Figures 10, 19). The fault bound-aries do not act as primary pressure seals (Scholle,1977) in a pressure system determined by verticalsealing and migration. The 1995 Siri discovery inlower Tertiary sandstones 25 km from the Central
Graben proved long-distance eastward migration ofhydrocarbons from Jurassic source rocks locatedwithin the graben, similar to the well-known west-ward hydrocarbon migration (Figure 10) (Cayley,1987; Danish Energy Agency, 1996).
Are Chalk Pressure Compartments Only ofField Size?
A pressure compartment is defined by a constantlevel of overpressure according to Bradley andPowley (1994), and is seen by them as a body ofrock containing overpressured fluids that are inter-nally in free hydraulic communication. A hydrostaticgradient for a formation within an area is certainly acharacteristic of hydrostatic communication, but isan ambiguous indicator of such communication. Inthe central North Sea, the main factors determiningthe Chalk overpressure are the load of the upperoverburden and the thickness and sealing quality ofthe lower overburden. These factors change overlong distances, and may indirectly lead to uniformoverpressure in the Chalk within an area.
In the Danish Central Graben Chalk, three geo-graphically coherent pressure compartments maybe defined by having overpressures of 7 ±1, 9 ±1,and 15 ±1 MPa (compartments A, B, and C, Figure12; Table 3). The compartments with higher pres-sures are closer to the Cenozoic depocenter, andwithin each compartment the pressure is graduallychanging toward the next compartment; thesecompartments are about 40 km across, whereas theChalk fields are 2–6 km wide. In compartment A,all wells fall on the main trend of drainage vs. sealthickness, as do the wells in compartment B, apartfrom the Ruth-1 well (Figure 18B). The constantoverpressure within the compartments may beexplained by the variations of the overburden. Incompartment C, only one of five wells defining thecompartment falls on the trend. The four remainingwells in compartment C (e.g., Nora-1, T-1) and theRuth-1 wells all have relatively low drainage values(equation 7), suggesting that other than the region-al mechanisms add to the overpressure. The Ruth-1and T-1 wells are located over pronounced diapirs;therefore, hydrocarbon buoyancy or hydraulic con-nection to the nearby, more deeply buried Chalkmay have elevated the pressure in these wells.
Conclusions Regarding Chalk Overpressure
The use of Chalk velocities to outline and esti-mate overpressure, presented here for the firsttime, show that Chalk compaction is stress inducedto great depths. The causal relationship betweenoverpressure and undercompaction of the North
2066 Velocity-Depth Anomalies, North Sea Chalk
Sea Chalk and the rapid, late Cenozoic burial of theChalk are illustrated by the areal overlap of theseoccurrences (Figure 10).
Three sources contribute to the overpressure inthe North Sea Chalk: (1) disequilibrium com-paction, (2) transference of overpressure generat-ed by volume-expansion processes in the pre-Chalk section or by short-range hydrauliccommunication with deeper buried Chalk, and (3)hydrocarbon buoyancy. Disequilibrium com-paction contributes about 80% of the Chalk over-pressure in the central North Sea (Figure 17B),and the overpressure is on the order of the effec-tive load of the upper Post Chalk Group (the post-mid-Miocene sediments) (Figure 17A). The effec-tive stress exerted on the Chalk has decreasedafter its present state of compaction was attained,as evidenced by tension fractures in the Chalk.This unloading may be explained by late transfer-ence of brine sourced by processes such as gasgeneration in the pre-Chalk section and by buoy-ancy of late-emplaced hydrocarbons. Vertical pres-sure communication in the hydrocarbon phasemay explain the drop in overpressure with depthin the Greater Ekofisk area.
The degree to which overpressure generated bythe effective load of the upper Post Chalk Group isretained in the Chalk depends on the thickness andthe sealing capacity of the lower Post Chalk Group(Figure 18). This dependency suggests that lateralflow in the Chalk is negligible on a regional scale;furthermore, the almost constant overpressure inthe Chalk within areas about 40 km across may beexplained by the small, local variations of the over-burden rather than by hydraulic communication(Figure 12). Consequently, the definition of pressurecompartments by Bradley and Powley (1994)appears to be misleading when it is applied to theNorth Sea Chalk. The Chalk is consequently found toconstitute a regional aquitard within the centralNorth Sea, it has been suggested for the UK sector(Cayley, 1987; Darby et al., 1996), and the hydrody-namic model of long-distance migration within theChalk in the Danish Central Graben is consequentlyrejected (Damtoft et al., 1992; Megson, 1992). Theparadox of the Chalk acting as a local aquifer and aregional aquitard may be explained by deteriorationof permeability away from structures and by discon-tinuity of permeable beds. The pressure gradientsaway from the late Cenozoic depocenter are impor-tant for the secondary migration of hydrocarbons inthe area, but long-distance migration is possible onlywhere carrier beds such as the Tertiary sandstonesare present.
The level of overpressure in the petroleum systemof the central North Sea is a result of dynamic pro-cesses, as was pointed out by Darby et al. (1996). Ihave demonstrated how Chalk overpressure evolves
over time as the sedimentary load is increased, ascompaction fluids are expelled vertically, and asfluids are added from the pre-Chalk synrift sedi-ments (Figure 19).
CONSEQUENCES FOR DEPTH CONVERSION
Semiregional velocity-depth trends have beendetermined for the Chalk in the Danish Basin andthe Danish Central Graben (Figures 2C, 7B)(Japsen, 1994). The trends are offset, and both arecharacterized by smaller velocity-depth gradientsthan the normal trend, VN (equation 8) (1.1 and0.7, respectively, as opposed to mainly 2 m/s/m).The shallow data (z < 1500 m) are from the easternNorth Sea Basin that was affected by up to 1000 mof Neogene erosion (Japsen, 1993a) (Figure 9). Theshallowest data represent wells from the basin mar-gin where the erosion is deepest. The deep data (z> 1500 m) are from the central North Sea whereoverpressure leads to low velocities relative todepth (Figure 10). The deepest data representwells from the basin center where overpressure ismaximum. Consequently, the two semiregionaltrends deviate the most from the normal trend nearthe surface and at great depths.
The apparent depth range related to the observedvelocity range becomes enlarged in both cases. Theapparent velocity gradients (velocity range divided bydepth range) for the resulting semiregional trends thusbecome smaller than those of the normal velocity-depth trend; however, across smaller areas wherethe burial anomaly (due to regional erosion or tooverpressure) is nearly constant, differences in buri-al match the difference in effective stress exerted onthe rock. Local velocity variations thus are deter-mined by the normal velocity gradient and not bythe semiregional gradient. The normal velocity gradi-ent should be used in depth conversion as it is invelocity-anomaly depth conversion (Japsen, 1993a).In this method, a map of velocity anomalies for agiven layer is used to tie a linear velocity-depthmodel to well data by adding the velocity anomaly tothe expression V = V0 + k × z. Substitution of dV by–dZB × k (equation 2), yields V = V0 + k × z + dV =V0 + k(z – dZB) = V0 + k × zN (Figure 5). Velocityvariations are thus calculated at zN, the normalizeddepth of the layer (the depth predicted by the nor-mal velocity-depth trend for the measured velocity),which means that the local effect of either regionalerosion or overpressure is taken into account.
DISCUSSION
The North Sea Chalk is fairly homogeneous on aregional scale (e.g., Kennedy, 1987; Ziegler, 1990),
Japsen 2067
but differences in clay and flint content and localoccurrences of facies such as bryozoan mounds, aswell as resedimentation, have caused differences inthe primary rock material (e.g., D’Heur, 1986;Kennedy, 1987; Taylor and Lapre, 1987; Maliva andDickson, 1992; Surlyk, 1997). Salt diapirism andinflux of hydrocarbons are secondary agents ofstrong, but localized, alterations of the physicalproperties of the Chalk. Finally, as I have shown,variations of the burial history of the Chalk acrossthe North Sea Basin have induced major, regionaldeviations from normal compaction (cf. Scholle,1977; Bulat and Stoker, 1987; Maliva and Dickson,1992; Hillis, 1995a). A wide variation is conse-quently revealed when Chalk interval velocitiesfrom 845 North Sea wells are plotted against depth(Figure 7A). The inf luence of minor amounts ofclay on the acoustic properties of chalk is not wellestablished, but the velocity of shale in the NorthSea Basin (Hansen, 1996) is below that of purechalk. The difference is about 2000 m/s at a depthof 2500 m in accordance with the velocity anoma-lies observed for the shaly Upper Cretaceous inter-val south of the Viking Graben (Figure 8, equation8). The Chalk Group is treated as one unit in thisstudy because a subdivision was not possible dueto the lack of relevant data, but a subdivision wouldset the focus on differences within the Chalk, andcalculation of interval velocity over thinner unitswould result in a wider scatter.
The maps of velocity and burial anomalies forthe Chalk represent a twofold averaging to sup-press deviations from the mean conditions (Figures8, 9, 10B), first by calculating the mean velocity ofthe Chalk section in each well (typically over hun-dreds of meters), and second by smooth contour-ing. Many wells drilled with Chalk objectives aredrilled on structural highs on the top Chalk surfaceand are not structurally representative. Relative tothe basinwide relief of the Chalk of more than 3000 m,that of nondiapiric structures is small; for example,less than 200 m for the Dan field (Britze et al.,1995b). Data from wells drilled on or near saltdiapirs are omitted from the mapping to emphasizethe regional trends; however, other outliers maynot fit the contouring based on the kriging parame-ters. The standard deviation of Chalk burial anoma-lies may well be on the order of the 260 m foundfor shale by Hansen (1996).
Anselmetti and Eberli (1997) found little corre-lation between velocity and depth for pure car-bonate rocks based on an observed velocity inver-sion in two drill holes of about 500 m penetratingcarbonate reefs on the Great Bahama Bank; how-ever, because chalk is dominated by stable, low-magnesium calcite, a wide depth interval of theburial diagenesis is controlled by compaction(Scholle, 1977; Borre and Lind, in press). In con-
trast, the Great Bahama Bank sediments are domi-nated by metastable aragonite and dolomite(Anselmetti and Eberli, 1997) prone to earlycementation, and a compaction-controlled burialdiagenesis will not take place.
The three sonic logs in Figure 11 illustrate howmeasured chalk velocities vary relative to the sug-gested normal trend. One log is from pelagic carbon-ate deposits from the stable Ontong Java Platform,and two logs are from the Chalk Group at the east-ern margin and the central part of the North SeaBasin. The sonic logs from the Chalk Group are alsoshown shifted downward and upward, respectively,to a position of zero burial anomaly, resulting in anagreement between the logs.
CONCLUSIONS
The normal velocity-depth trend for the ChalkGroup is formulated as four linear segments todescribe how the velocity of Chalk in generalincreases slowly from about 1600 m/s at the seabed to 2700 m/s at a depth of 1100 m, and thensteeply to about 4900 m/s at a depth of 2250 m,and then more slowly to higher velocities beyondthat depth. Simple mathematical expressions fail tomatch the depth-dependent increase of Chalkvelocity as porosity approaches zero. Regional ero-sion and overpressure lead to an apparent reduc-tion of the velocity-depth gradient. The gradient ofthe normal trend, however, is a measure of thestress-dependency of velocity and should be usedin depth conversion.
The depth dependency of Chalk compactionpreviously has been approximated by single mathe-matical functions, and these were based on morerestricted databases than this study (Appendix 2)(Sclater and Christie, 1980; Bulat and Stoker, 1987;Hillis, 1995a). The upper part of Hillis’s (1995a) lin-ear transit time-depth curve differs, however, onlyslightly from the suggested trend. Sclater andChristie’s (1980) exponential porosity-depth curveprovides a general description of the porosityreduction, but fails to account for depth variationsin the compaction process. Porosities higher than40% appear to be preserved to a depth of 1 km dur-ing normal compaction. Below this depth, thesteep increase in Chalk velocity is interpreted tocorrespond to the onset of calcite cementation(Borre and Lind, in press).
Chalk interval velocities are readily availablefrom most of the North Sea Basin. A basinwidemap of the Chalk velocity anomalies relative tothe suggested trend reveals a coherent pattern ofthe positive and negative anomalies that reflectthe burial history of the Chalk dur ing theCenozoic, except south of the Viking Graben
2068 Velocity-Depth Anomalies, North Sea Chalk
where a high clastic content reduces Chalk veloc-ities. The velocity anomalies and the correspond-ing burial anomalies of ±1 km across the NorthSea relate to two physical processes affecting thevelocity-depth relation for the Chalk: (1) removalof up to 1 km of overburden along the westernand eastern margins of the basin due to Neogeneuplift and erosion and (2) overpressure withinthe Chalk exceeding 10 MPa that was induced byrapid, late Cenozoic burial where the lowerTertiary is sealing.
The agreement that I have demonstrated betweenthe Chalk burial anomalies and these physical pro-cesses provides evidence that the suggested normalvelocity-depth trend reflects normal compactionwith depth for the Chalk Group. This emphasizesthe information value of the vast well data set ofinterval velocities if the data set is combined with aconstrained normal velocity-depth trend.
APPENDIX 1: LIST OF SYMBOLS
BE Post-exhumational burial (m)DC Net drainage capacity (equation 6) (%)DC∆P Net drainage capacity relative to overpressure
(equation 7) (%)g Gravitational acceleration 9.807 (m/s2)itt Interval transit time (s/m)ittf Interval transit time of pore fluid (s/m)ittm Interval transit time of matrix (s/m)k Velocity-depth gradient (m/s/m)P Formation pressure (Pa)PH Hydrostatic pressure (Pa)∆P Formation overpressure = P – PH (Pa)∆Pbuoy Overpressure generated by buoyancy (Pa)∆Pcomp Overpressure generated by compaction disequilibrium
(equation 5) (Pa)∆Ptrans Overpressure generated by transference (redistribution
of overpressure) (Pa)S Stress exerted by the load of the overburden per unit
area (Pa)∆T Two-way traveltime thickness of a layer (s)V Instantaneous velocity (m/s)Vi Interval velocity (m/s)V0 Velocity at the surface (m/s)Vf Velocity of pore fluid (m/s)Vm Velocity of matrix (m/s)VN Normal velocity-depth trend (for the Chalk)dV Velocity anomaly relative to a normal velocity depth
trend (equation 1) (m/s)z Depth (velocity data is below sea bed; pressure data is
below sea level) (m)zm Midpoint depth of a layer (below sea bed) (m)zt Depth to top of a layer (below sea bed) (m)zb Depth to base of a layer (below sea bed) (m)zmiss Missing overburden section removed by erosion (equa-
tion 3) (m)zN Normalized depth corresponding to normal compaction
based on VNdZB Burial anomaly relative to a normal velocity depth trend
(equation 5) (m)∆z Thickness of a layer (m)∆zup Thickness of the upper Post Chalk Group (Figure 1) (m)∆zlow Thickness of the lower Post Chalk Group (Figure 1) (m)
φ Porosity (%)ρb Mean bulk density (wet) of the overburden (kg/m3)ρf Mean density of pore fluid in the overburden (kg/m3)∆ρup Mean density contrast of the upper overburden
(=[ρb – ρf]up) (kg/m3)σ Effective stress = S – P (Pa)σup Effective load of the upper overburden (equation 4) (Pa)
APPENDIX 2: COMPARISON OF COMPACTION TRENDS FOR CHALK
Scholle (1977, p. 991) demonstrated that burial depth is the“only major factor which consistently correlates with regionalporosity loss in chalks.” He presented a plot of the most typicalporosity-depth relations for chalks from 70% at sea bed to about10% at 2200 m below the sea bed based on data from the Deep-Sea Drilling Project, the Scotian Shelf, and the well NL L16-1.Sclater and Christie (1980) extended the shallow part ofScholle’s trend to pass through the lowest porosities in two nor-mally pressured wells (UK 15/16-1, UK 15/28-2), and suggestedan exponential normal porosity-depth trend, φSC:
(9)
The porosities at depth (z ≈ 3000 m) were taken as the mini-mum porosity calculated from sonic logs using the velocity-porosi-ty relations for limestone of Schlumberger (1974). VN, however,does not pass through the data point for well UK 15/28-2 (equa-tion 8, Figure 20A; Table 2) because the data point on Figure 20Aindicates mean velocity, whereas φSC is defined from maximumvelocity (Figure 8).
Bulat and Stoker (1987) estimated removed overburdenbased on interval velocities for several stratigraphic units in theUK southern North Sea by identifying data representing normalcompaction as having the lowest velocity at a given depth. A lin-ear velocity-depth trend was found to be a valid approximationfor the depths considered, and VBS for the Upper Cretaceous(approximately the Chalk Group) was estimated by Bulat andStoker (1987, their figure 4b) as
(10)
(zm < 2000 m for most wells). VBS and VN cross for V = 3090 m/s,and erosion becomes underestimated for smaller velocities due tothe lack of data representing normal compaction because theChalk is now above maximum burial in most parts of the basinwhere zm < 1500 m (Figure 20A).
Hillis (1995a) proposed a normal trend, VH, for the upperand middle Chalk in the UK southern North Sea based on theprinciples of Bulat and Stoker (1987), and furthermore assumedthe velocity of Chalk at zero depth to be comparable to thevelocity of saline water. Hillis (1995a) thus arrived at higher esti-mates of erosion than Bulat and Stoker (1987). The Cenomanianlower Chalk was not included due to lateral facies variations,whereas the Danian Chalk generally is absent in his study area.The Chalk in the wells thus selected as references (UK 38/25-1,UK 44/29-1A) is also close to normal compaction relative to VN.Based on Bulat and Stoker’s (1987) assumption of linearity ofvelocity (not transit time) with depth, Hillis (1995a) found the
VBS = 2145 + 0.75 × z
φsc = 0.7 × e-0.71 × 10-3 × z
Japsen 2069
2070 Velocity-Depth Anomalies, North Sea Chalk
following relation between interval transit time, itt (µs/ft), anddepth, z (m):
(11)
VH matches VN along segments A, B, and a part of C (V < 4250m/s) (Figure 20A). VH fails, however, to match data from thedeeply buried parts of the North Sea (segments C, in parts,and D) because VN is more than 100 m below VH for V > 4250m/s. Hillis’s (1995a) assumption of linearity between transittime and depth implies that transit time becomes zero andthen negative at depth as indicated by the accelerat ingincrease in VH with z (Figure 20A). The match between theupper parts of VH and VN means that the validity of Hillis’sargumentation is confirmed for the entire Chalk Group formoderate velocities, and with a larger data set; however,Hillis’s trend was based on wells with average depth around1000 m, and because he has few data points with V > 4250m/s, his conclusions regarding the exhumation of the UKsouthern North Sea are not disputed.
Hillis et al. (1994) established normal velocity-depth trends forthe Upper Cretaceous Hod and Tor chalk in the Inner Moray Firth
(quadrants UK 11–13). For each formation, the transit time-depthgradient was taken as the mean sonic gradient, and the well withthe lowest velocity for its burial as reference (UK 13/30-1, UK13/30-2). The problem in their study is whether “the referencewells in quadrant 13/30 themselves are above their maximumburial depth” (Hillis et al., 1994, p. 289). The estimated Hod andTor trends are well above VH (Hillis, 1995a) and, respectively, 800and 500 m above VN for V = 4300 m/s.
APPENDIX 3: VELOCITY-POROSITYCONVERSION FOR CHALK
Wyllie et al. (1956) suggested a general relation between inter-val transit time, itt, and porosity for sedimentary rocks (Figure 21):
(12)
where ittm (= 1/Vm) and ittf (= 1/Vf) are the transit times (veloci-t ies) for the matrix and the pore fluid, respectively.Schlumberger (1974) applied Wyllie’s equation for chalk with ø< 44%, substituting Vf = 1615 m/s and Vm = 6400 m/s. Raiga-Clemenceau et al. (1988) reported a poor fit between Wyllie’sequation and experimental data. They suggested the followingalternative relation:
(13)
where x is an exponent specific to the matrix characteristics.The parameters were estimated from a Chalk data set from theEkofisk oil field in the central North Sea, and yielded x = 1.76and Vm = 6403 m/s. Equation 13 matches within 2 porosity per-cent the empirical relation for l imestones given bySchlumberger (1991) in a revised edition of Schlumberger(1974). Porosities in Raiga-Clemenceau et al.’s (1988) chalk dataset were from 10 to 40%, whereas Schlumberger’s (1991) chartranges from 0 to 39%. These porosity-velocity relations are thusnot well established for φ > 40%.
φ = 1 - (V/Vm)1/x
φ = (itt - ittm) / (ittf - ittm)
ittH = 0.3048 × 106/VH = 177.5 - 57.5 × 10-3 × z
Figure 21—Porosity-velocity relations for chalk. Forcomparison of normal compaction trends for the NorthSea Chalk, the relation of Raiga-Clemenceau et al. (1988)is used for V > 2700 m/s, and for 1600 < V < 2700 m/s, alinear trend matches Ocean-Drilling Program data(Urmos et al., 1993).
4
4000 600050003000
Velocity (m/s)
Dep
th (
km)
3
2
0
2000
Dep
th (
km)
Porosity (%)50 40 30 20 10 0
4
3
2
1
0
Well data
1
(A)
(B)
Hillis (1995)Eq. 7
Eq. 4VN
Eq. 6Bulat & Stoker (1987)
Eq. 4VN
Eq. 5Sclater & Christie (1980)
Eq. 5Sclater & Christie (1980)
Hillis (1995)Eq. 7
Eq. 6Bulat & Stoker (1987)
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
60
20
0
4000 60005000
Por
osity
(%
)
Velocity (m/s)2000 3000
40
Japsen, this paperEq. 11
Schlumberger (1991)
Schlumberger (1974)Wyllie's equationEq. 8
Raiga-Clemenceauet al. (1988)Eq. 9
Urmos et al. (1993)Eq. 10
Figure 20—Comparison of normal compaction trendsfor the North Sea Chalk. VN matches the general trend ofSclater and Christie’s (1980) curve, and the trend ofHillis (1995a) for V < 4250 m/s. (A) Velocity vs. depth.(B) Porosity vs. depth. Conversion between porositiesand velocities based on equation 15 (see Appendix 3).
Urmos et al. (1993) found that log data from pelagic carbonatedeposits with 48 < φ < 74% in hole 807 (ODP Leg 130) fitted theempirical relation
(14)
Urmos et al. (1993) found that this equation did not matchdeeper water carbonate deposits. Sonic data from hole 807 wereused to support the definition of the normal velocity-depth trendfor the North Sea Chalk (equation 8).
Normal compaction trends for the North Sea Chalk may beexpressed in terms of either porosity-depth (Scholle, 1977; Sclaterand Christie, 1980) or velocity-depth relations (Bulat and Stoker,1987; Hillis, 1995a, and equation 8 in this paper). To enable compar-ison between this group of trends, a simple relation is developedhere to convert porosities to velocities. Raiga-Clemenceau et al.’s(1988) equation 13 provides a relation for the North Sea Chalkmatching that of Schlumberger (1991) for φ < 40%. Above 40%porosity, Raiga-Clemenceau et al.’s (1988) equation is not based ondata, contrary to the empirical relation of Urmos et al. (1993) (equa-tion 14). A linear trend matching Urmos et al.’s (1993) observationsis suggested here for 1600 < V < 2700 m/s defined by V(70%) = 1600m/s and V(38.9%) = 2700 m/s. The latter point corresponds to theprediction of Raiga-Clemenceau et al. (1988). We get the followingrelationships for comparison of Chalk compaction trends:
(15)
The relatively slow increase in chalk velocity at high porosi-ties (70–40%) indicates that mechanical compaction predomi-nantly takes place, leading to reorganization of the grains. Forsmaller porosities velocities, increase more rapidly due to stiffergrain contacts.
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2074 Velocity-Depth Anomalies, North Sea Chalk
ABOUT THE AUTHOR
Peter Japsen
Peter Japsen is a senior researchgeophysicist at the GeologicalSurvey of Denmark and Greenland,where he has been working with avariety of problems related toexploration and geophysical map-ping since 1980. In recent years hehas focused on the constraints onbasin development that may bedrawn from the sonic velocities ofsedimentary rocks. He currently isinvolved in research projects on the rock physics ofchalk and on Neogene uplift and tectonics around theNorth Atlantic.
