In carbonates where the fractures are mostly open and
forming a three-dimensional fracture network, the
effective permeability can be very good. However,
fractures are often formed during several phases of
tectonic deformation and then only some of the
fractures may remain open. The overall drainage during production is then more difficult to predict.
Fractured carbonate reservoirs often consist of
dolomite. Well-cemented massive dolomite is rather
brittle forming large fracture networks when subjected
to tectonic forces.
Fractures in dolomite may be preserved longer than
in limestones. This is because dolomite is both
mechanically stronger and less soluble than calcite so
that it will take longer for the fractures to be cemented
up. For this reason we may find dolomite reservoirs at
great depth (5–6 km) and also in uplifted basins. Fractured reservoirs may have very high permeability so
that the reservoirs can be produced even if the average
porosity is low.
5.8.2.3 Chalk
Chalk is a pelagic sediment where calcareous algae
(coccolithophores) are dominant. Coccolithophores consist of low-Mg calcite and are therefore mineralogically
stable during early diagenesis. The presence of
aragonitic skeletal material is fairly meagre, so these
mineralogically unstable fossils are not an important
calcite source for early cementation. During periods
with little pelagic sedimentation seafloor cementation
may form hardgrounds. These are often encrusted by
bivalves, boring organisms, bryozoa and sponges and
may be mineralised by glauconite and phosphate.
For the most part the chalk is rather pure CaCO 3 but
more clay-rich intervals like the Plenus Marl occur in
the North Sea reservoirs, forming a tight lowpermeability zone. Thin clay laminae enhance pressure solution and often develop into stylolites. The
dissolution of calcite by pressure solution is the most
important source of carbonate cement.
During diapirism of the underlying Zechstein salt
(Upper Permian) the chalk became unstable and was
reworked by gravity flows (turbidites and debris
flows). The eroded sediments were already somewhat
cemented and the debris flow units have proved to be
better reservoir rocks than the primary chalk deposits.
The early cementation may have strengthened the
grains or aggregates of grains so that compaction
during burial was reduced. In addition, the salt doming
has produced fracturing in the chalk and these
fractures have been essential in increasing the overall
reservoir permeability.
Early filling of oil and gas in carbonate rocks like
the chalk will retard calcite cementation and tend to
preserved porosity compared to water-saturated chalk.
5.8.2.4 Carbonate diagenesis and reservoir
properties
The initial mineralogical and textural composition
determines the loss of porosity by mechanical and
chemical compaction during burial. Since most carbonate sediments have been precipitated biologically
both the mineralogy and the textural properties are
primarily controlled by the organisms and later
reworking. The content of unstable minerals, mainly
aragonite and Mg calcite, and also the grain size ranging from mud to very coarse material, reflects the
organisms that precipitated the carbonate.
In carbonate mud, aragonite will in most cases be
replaced by calcite, producing a dense microcrystalline limestone. High contents of Sr are evidence of
primary aragonite content.
Sediments containing almost only calcite, like the
coccolliths making up chalk, will be rather stable
chemically and also mechanically because the overburden stress in these fine-grained sediments is
distributed on so many grain contacts. This is why
the chalk in the Ekofisk Field may preserve 30%
porosity at 2.5–3.0 km depth (Fig. 5.64).
Carbonate sand representing a coastal facies is likely
to be flushed by freshwater, dissolving aragonite and
precipitating calcite. This process may create karstic
porosity and also enhanced porosity in sandstones.
At greater depth however the bulk chemical composition of carbonates does not change significantly
except when influenced by hydrothermal activity
(Bjørlykke and Jahren 2012, Ehrenberg et al. 2012).
Below the reach of meteoric water flow the porewater
flow is very low, limited by the rate of compaction
(See Chap. 4).
In marine sedimentary basins there is nearly always
some calcite, also in sandstones and shales and the
porewater is therefore in equilibrium with calcite. This
reduces the potential for dissolution, transport of
solids in solution and precipitation both by advective
flow of porewater and by diffusion, because the concentration gradients are so small.
212
N.-M. Hanken et al.
forming a three-dimensional fracture network, the
effective permeability can be very good. However,
fractures are often formed during several phases of
tectonic deformation and then only some of the
fractures may remain open. The overall drainage during production is then more difficult to predict.
Fractured carbonate reservoirs often consist of
dolomite. Well-cemented massive dolomite is rather
brittle forming large fracture networks when subjected
to tectonic forces.
Fractures in dolomite may be preserved longer than
in limestones. This is because dolomite is both
mechanically stronger and less soluble than calcite so
that it will take longer for the fractures to be cemented
up. For this reason we may find dolomite reservoirs at
great depth (5–6 km) and also in uplifted basins. Fractured reservoirs may have very high permeability so
that the reservoirs can be produced even if the average
porosity is low.
5.8.2.3 Chalk
Chalk is a pelagic sediment where calcareous algae
(coccolithophores) are dominant. Coccolithophores consist of low-Mg calcite and are therefore mineralogically
stable during early diagenesis. The presence of
aragonitic skeletal material is fairly meagre, so these
mineralogically unstable fossils are not an important
calcite source for early cementation. During periods
with little pelagic sedimentation seafloor cementation
may form hardgrounds. These are often encrusted by
bivalves, boring organisms, bryozoa and sponges and
may be mineralised by glauconite and phosphate.
For the most part the chalk is rather pure CaCO 3 but
more clay-rich intervals like the Plenus Marl occur in
the North Sea reservoirs, forming a tight lowpermeability zone. Thin clay laminae enhance pressure solution and often develop into stylolites. The
dissolution of calcite by pressure solution is the most
important source of carbonate cement.
During diapirism of the underlying Zechstein salt
(Upper Permian) the chalk became unstable and was
reworked by gravity flows (turbidites and debris
flows). The eroded sediments were already somewhat
cemented and the debris flow units have proved to be
better reservoir rocks than the primary chalk deposits.
The early cementation may have strengthened the
grains or aggregates of grains so that compaction
during burial was reduced. In addition, the salt doming
has produced fracturing in the chalk and these
fractures have been essential in increasing the overall
reservoir permeability.
Early filling of oil and gas in carbonate rocks like
the chalk will retard calcite cementation and tend to
preserved porosity compared to water-saturated chalk.
5.8.2.4 Carbonate diagenesis and reservoir
properties
The initial mineralogical and textural composition
determines the loss of porosity by mechanical and
chemical compaction during burial. Since most carbonate sediments have been precipitated biologically
both the mineralogy and the textural properties are
primarily controlled by the organisms and later
reworking. The content of unstable minerals, mainly
aragonite and Mg calcite, and also the grain size ranging from mud to very coarse material, reflects the
organisms that precipitated the carbonate.
In carbonate mud, aragonite will in most cases be
replaced by calcite, producing a dense microcrystalline limestone. High contents of Sr are evidence of
primary aragonite content.
Sediments containing almost only calcite, like the
coccolliths making up chalk, will be rather stable
chemically and also mechanically because the overburden stress in these fine-grained sediments is
distributed on so many grain contacts. This is why
the chalk in the Ekofisk Field may preserve 30%
porosity at 2.5–3.0 km depth (Fig. 5.64).
Carbonate sand representing a coastal facies is likely
to be flushed by freshwater, dissolving aragonite and
precipitating calcite. This process may create karstic
porosity and also enhanced porosity in sandstones.
At greater depth however the bulk chemical composition of carbonates does not change significantly
except when influenced by hydrothermal activity
(Bjørlykke and Jahren 2012, Ehrenberg et al. 2012).
Below the reach of meteoric water flow the porewater
flow is very low, limited by the rate of compaction
(See Chap. 4).
In marine sedimentary basins there is nearly always
some calcite, also in sandstones and shales and the
porewater is therefore in equilibrium with calcite. This
reduces the potential for dissolution, transport of
solids in solution and precipitation both by advective
flow of porewater and by diffusion, because the concentration gradients are so small.
212
N.-M. Hanken et al.