Geological Survey of Denmark and Greenland Bulletin 13, 2007
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Review of Survey activities 2006, 09-12 |
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9
The Upper Cretaceous Danian chalk may be considered to
be the economically most important rock type in Denmark.
Onshore it constitutes an important groundwater aquifer and
it is also quarried for e.g. building materials and paper pro-
duction. Offshore the chalk reservoirs contain more than
80% of the oil and gas produced in Denmark (Fig. 1).
be the economically most important rock type in Denmark.
Onshore it constitutes an important groundwater aquifer and
it is also quarried for e.g. building materials and paper pro-
duction. Offshore the chalk reservoirs contain more than
80% of the oil and gas produced in Denmark (Fig. 1).
During the last few years efforts have therefore been made
to map this important succession in the Danish and adjoin-
ing areas (Vejbæk et al . 2003). The stratigraphic interval
mapped comprises the Chalk Group of Cenomanian to
Danian ages and its stratigraphically equivalent units (Fig. 2).
The north-eastern limit of the Chalk Group is determined by
Neogene erosion. The limits of the map to the west and south
were mainly determined by the amount of available data.
ing areas (Vejbæk et al . 2003). The stratigraphic interval
mapped comprises the Chalk Group of Cenomanian to
Danian ages and its stratigraphically equivalent units (Fig. 2).
The north-eastern limit of the Chalk Group is determined by
Neogene erosion. The limits of the map to the west and south
were mainly determined by the amount of available data.
Data base
The comprehensive data base comprises high-resolution and
conventional 2-D and 3-D reflection seismic data as well as
published maps (e.g. Britze et al . 1995; Hommel 1996;
Ottesen et al . 1997; Jensen 1998; Kramarskiej 1999; Balds -
conventional 2-D and 3-D reflection seismic data as well as
published maps (e.g. Britze et al . 1995; Hommel 1996;
Ottesen et al . 1997; Jensen 1998; Kramarskiej 1999; Balds -
chuhn
et al
. 2001; Stoker 2005). More than 500 deep wells
and numerous onshore water wells have provided control for
the mapping. This is especially relevant for the mapping
where the Top Chalk is immediately overlain by the Neogene
(Fig. 3). In these areas in particular, mapping was based on
high-resolution seismic data.
and numerous onshore water wells have provided control for
the mapping. This is especially relevant for the mapping
where the Top Chalk is immediately overlain by the Neogene
(Fig. 3). In these areas in particular, mapping was based on
high-resolution seismic data.
Depth conversion
Depth conversion was undertaken by using depth-dependent
velocity functions, where the velocity V at depth z is given by:
velocity functions, where the velocity V at depth z is given by:
V="V"
0
+ dV + K×z
where
V
0
is the surface velocity,
dV
is a variation of the sur-
face velocity and
K
is the gradient of velocity increase with
depth (Table 1; e.g. Japsen 1998, 1999). The surface velocity
variation is typically mapped on the basis of well data and
may reflect lateral facies changes, burial anomalies or excess
fluid pressures.
depth (Table 1; e.g. Japsen 1998, 1999). The surface velocity
variation is typically mapped on the basis of well data and
may reflect lateral facies changes, burial anomalies or excess
fluid pressures.
Chalk depth structure maps, Central to Eastern North
Sea, Denmark
Sea, Denmark
Ole V. Vejbæk, Torben Bidstrup, Peter Britze, Mikael Erlström, Erik S. Rasmussen and Ulf Sivhed
© GEUS, 2007.
Geological Survey of Denmark and Greenland Bulletin
13, 912. Available at:
www.geus.dk/publications/bull
Fig. 1. Hydrocarbon accumulations in the North
Sea with chalk fields highlighted.
10
The Cenozoic velocity model consists of a single layer on -
shore Denmark and two layers offshore. The division be -
tween the two layers is taken at the `near Top Middle
Miocene marker' that corresponds approximately to the top
of the over-pressured section (Upper and Lower Post Chalk
Group in Table 1). The parameters for these layers were taken
from Britze et al . (1995) and Japsen (1999, 2000) who
derived a similar but segmented model for the Chalk Group.
Since the parameters are based on a large well data base from
the entire North Sea (e.g. Japsen 2000), they are applicable to
most of the North Sea.
shore Denmark and two layers offshore. The division be -
tween the two layers is taken at the `near Top Middle
Miocene marker' that corresponds approximately to the top
of the over-pressured section (Upper and Lower Post Chalk
Group in Table 1). The parameters for these layers were taken
from Britze et al . (1995) and Japsen (1999, 2000) who
derived a similar but segmented model for the Chalk Group.
Since the parameters are based on a large well data base from
the entire North Sea (e.g. Japsen 2000), they are applicable to
most of the North Sea.
Notes about the maps
In some areas where the Neogene lies
directly on the Top Chalk seismic hori-
zon, the erosional truncation of the
Chalk Group is negligible. This occurs
around Copenhagen, in northern Sjæl -
land and in south-western Scania, where
minor outliers of Selandian deposits
document the former extent of the
Chalk Group. The occurrence of
Palaeo gene sediments offshore Poland
also indicates that erosion of the Chalk
Group is generally not very deep in the
directly on the Top Chalk seismic hori-
zon, the erosional truncation of the
Chalk Group is negligible. This occurs
around Copenhagen, in northern Sjæl -
land and in south-western Scania, where
minor outliers of Selandian deposits
document the former extent of the
Chalk Group. The occurrence of
Palaeo gene sediments offshore Poland
also indicates that erosion of the Chalk
Group is generally not very deep in the
western Baltic outside the main inversion zones (Fig. 3).
In Norwegian waters, however, extensive Neogene erosion
has occurred. The erosion in these areas is sufficiently deep
for Lower Cretaceous deposits to subcrop the base of the
Neogene. Outside these areas the Chalk Group generally has
a larger areal extent than the Lower Cretaceous. (Fig. 3).
for Lower Cretaceous deposits to subcrop the base of the
Neogene. Outside these areas the Chalk Group generally has
a larger areal extent than the Lower Cretaceous. (Fig. 3).
A general increase in thickness of the Chalk Group is
found west of the SorgenfreiTornquist inversion zone. A
north-eastward increase in thickness is alsOFound in the areas
unaffected by Neogene erosion offshore southern Norway, sug-
gesting the presence of similar depocentres on the flanks of
inversion zones. Thus, inversion may also have occurred in
the south-western coastal areas of Norway.
north-eastward increase in thickness is alsOFound in the areas
unaffected by Neogene erosion offshore southern Norway, sug-
gesting the presence of similar depocentres on the flanks of
inversion zones. Thus, inversion may also have occurred in
the south-western coastal areas of Norway.
Fig. 2. Lithostratigraphic correlation for the
Upper Cretaceous Danian succession as
map ped in this paper. Based on Deegan &
Scull (1977), Isaksen & Tonstad (1989),
Johnson & Lott (1993) and Schiøler
et al
.
(2007) with additions modified from Surlyk
et
al
. (2003) and Sivhed
et al
. (1999).
Ha. Mb
, Hansa Member;
Kbh. Mb
, København Member;
Kh. Mb
, Kyrkheddinge Member;
Lk. Mb
, Landskrona Member.
Facing page:
Fig. 3. Simplified structure maps of the Chalk Group and equivalent
deposits.
A
, depth to top Chalk Group;
B
, depth to base Chalk Group
and
C
, isopach. Grey shadings in A and C indicate where the Lower
Cretaceous subcrops Quaternary sediments (i.e. where the Chalk
Group has been totally removed by erosion).
Cph
, Copenhagen.
PDF versions of the maps with more detail are available from
www.geus.dk/publications/bull/nr13/index-uk.htm
11
12
Authors' addresses
O.V.V., T.B., P.B. & E.S.R.,
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
E-mail:
ov@geus.dk
M.E. & U.S.,
Sveriges Geologiska Undersökning, Kiliansgatan 10, S-223 50, Lund, Sweden.
Hydrocarbon aspects
The Chalk Group in the Central Graben area is an important
reservoir and migration path for oil and gas. It is the most
important oil-producing interval in Denmark and is also a
major contributor to oil and gas production in Norway and
the Netherlands, while production from the Chalk Group is
still insignificant in the UK sector (Fig. 1). Traps within the
Chalk Group range from inversion-generated anticlines (e.g.
the Valhall, Roar, Tyra and South Arne fields), over salt domes
with some degree of inversion overprint (e.g. the Dan, Eko -
fisk and Svend fields) to salt diapirs (e.g. the Skjold and
Harald fields). Stratigraphic traps may also play a major role
(e.g. the Halfdan and Adda fields). These traps owe their exis-
tence to a combination of over-pressuring and early hydro-
carbon invasion to preserve the quality of their reservoirs
de spite the great depths to which they have been buried (e.g.
Anderson 1999; Vejbæk in press). Their position directly above
the main Upper Jurassic source rock also seems to be a neces-
sary condition for their existence (e.g. Anderson 1999; Sur -
lyk et al . 2003), since the generally very low permeability of
the chalk precludes long-distance migration and even keeps
accumulations in hydrodynamic dis-equilibrium (e.g. Dennis
et al . 2005; Vejbæk et al . 2005).
reservoir and migration path for oil and gas. It is the most
important oil-producing interval in Denmark and is also a
major contributor to oil and gas production in Norway and
the Netherlands, while production from the Chalk Group is
still insignificant in the UK sector (Fig. 1). Traps within the
Chalk Group range from inversion-generated anticlines (e.g.
the Valhall, Roar, Tyra and South Arne fields), over salt domes
with some degree of inversion overprint (e.g. the Dan, Eko -
fisk and Svend fields) to salt diapirs (e.g. the Skjold and
Harald fields). Stratigraphic traps may also play a major role
(e.g. the Halfdan and Adda fields). These traps owe their exis-
tence to a combination of over-pressuring and early hydro-
carbon invasion to preserve the quality of their reservoirs
de spite the great depths to which they have been buried (e.g.
Anderson 1999; Vejbæk in press). Their position directly above
the main Upper Jurassic source rock also seems to be a neces-
sary condition for their existence (e.g. Anderson 1999; Sur -
lyk et al . 2003), since the generally very low permeability of
the chalk precludes long-distance migration and even keeps
accumulations in hydrodynamic dis-equilibrium (e.g. Dennis
et al . 2005; Vejbæk et al . 2005).
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