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Figure Captions
 Figure
1. Distribution and thickness of the Lower Cretaceous reservoir
succession in the Danish Central Graben. The
location of the Valdemar Development and the Adda Field is shown.
 Figure
2.  Log panel with wells from the Valdemar area showing the sequence
stratigraphic subdivision of the Cromer Knoll Group.
 Figure
3. Petrophysical and sedimentological parameters for the Lower
Cretaceous reservoir units.
 Figure
4. Porosity versus insoluble residue relationship for the various
reservoir units. Different trend lines for the Sola and Tuxen Formations
display the variation between the two stratigraphic formations.
 Figure
5. Comparison of Upper and Lower Cretaceous porosity-permeability data.
 Figure
6. Porosity/depth trend for the upper part of the Tuxen Formation. Based
on well data and interpretation of wireline log data.
Sequence Stratigraphy
The sequence
stratigraphy of the Lower Cretaceous deep water succession of the Cromer
Knoll Group is based on integration of core-based facies descriptions,
log stratigraphy and biostratigraphic data. Refinement and modification
of the sequence stratigraphy based on detailed palynology and
nannofossil work on core data from the North Jens-1 well at the Valdemar
Field resulted in the recognition of eight depositional sequences. They
have subsequently been traced regionally, and the sequence stratigraphy
provides a detailed framework for correlation within which it is
possible to evaluate lateral and depth dependent variations in reservoir
properties both within the Valdemar area (Figures
1 and 2) and regionally.
The Lower Cretaceous
reservoir units are limited to sequences CK 3 B CK 6. The sequences have
been subdivided further based on lithological variations, and facies-dependent
lateral variations in reservoir properties have been evaluated based on
the recognition of sequence boundaries as time-specific surfaces (Figure
3).
The reservoir zonation
was initially based on well logs and core data from the vertical North
Jens-1 well that allowed 14 reservoir units to be distinguished. The
zonation was then extended to include all (11) conventional wells in the
Valdemar Field area and subsequently the 34 wells penetrating the Lower
Cretaceous succession within the Danish Central Graben. In the Valdemar
area, individual units range in thickness from 0B59 feet (0B18 m), with
an average from 6B35 feet (2B11 m) and the log-based units are
accordingly below seismic resolution.
The reservoir interval
consists of deep-water chalk and argillaceous chalk interbedded at a
variety of scales with marlstone. Seven depositional facies have been
distinguished in the North Jens-1 well using a modified JCR (Joint Chalk
Research) classification: 1) Chalk; 2) Marly chalk (a. Low argillaceous
chalk; b. High argillaceous chalk); 3) Marlstone (a. Chalky marlstone;
b. Marlstone); 4) Claystone; and 5) Pebbly chalk.
The individual
reservoir zones are internally heterogeneous, and quantification of
average reservoir properties are based on core data. The reservoir units
are a composite of two or more of these facies and show variable degrees
of heterogeneity in the form of lamination, bedding or flasers (Figure
3). The best reservoir properties are recorded in units dominated by
chalk and low argillaceous chalk whereas units dominated of marlstone,
high argillaceous chalk and claystone form poor reservoirs and may act
as internal barriers.
The reservoir units
have been ranked into four categories: poor, fair, good and very good,
based on their characteristics combined with estimates of net/gross
ratios. Due to the heterogeneity of the reservoir units, the
classification is based on average values, with a bias towards data from
conventional wells as the horizontal wells do not provide information in
the vertical direction.
The reservoir is
divided into two stratigraphic compartments separated by a low porosity
and clay-rich limestone interval corresponding to the Upper Tuxen-2 and
Lower Sola-1 units. The lower compartment is internally heterogeneous
and consists of two zones (the Lower Tuxen-1, -2 and -3 units and the
Middle Tuxen-1, -2 and Upper Tuxen-1 units), separated by the Munk Marl
unit. The upper compartment is also internally heterogeneous, the Lower
Sola-2 and -3 reservoir zone is separated from the Upper Sola-1 and -2
reservoir zone by the Fischschiefer unit.
Porosity was found to
have a direct relationship to clay content, and the porosity/clay
relationship has been estimated for each reservoir unit by plotting core
porosity versus insoluble residue (IR) (Figure 4). The data plot on two
parallel trend lines with an offset of approximately 10 p.u. indicating
different relationships between the absolute values for IR (clay
content) and porosity in the two stratigraphic compartments, but a
similar reduction of porosity of 5-7 p.u. per 10 % increase in IR
(clay).
The difference in
clay/porosity relationship between the two stratigraphic compartments is
possibly facies controlled; the lower compartment is more intensely
bioturbated and there is therefore a higher risk of destruction of
coccosphere plates into single crystals and higher degree of clay-chalk
mixing than in the upper compartment.
Plots of porosity
versus permeability for Maastrichtian, Danian and Lower Cretaceous
samples show a distribution of data following 3 different trend lines
(Figure 5). The trends are related to different matrix textures which
again are controlled by the composition and pore-geometry of the rocks.
The size of the pore throats in the Lower Cretaceous is approximately
1/3 of what is found in the Upper Cretaceous Chalk so the capillary
entry pressure and irreducible water saturation are considerably higher
in the Lower Cretaceous than in the Upper Cretaceous.
The
analytical data indicate a linear relationship between porosity and clay
content along with an exponential relationship between porosity and
permeability. The porosity was found to be dependent on pressure as
well, and the carbonates in the Valdemar Field are undercompacted due to
high pore pressure. Based on corrected log data, it is evident that
porosity also is depth-dependent (Figure 6) and that reservoir quality limestones (porosity >20%) are restricted to depths less than 9000 feet
(3 km).
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