1. The sediment particles are in most cases produced
locally within the basin by biological production
(fossils) or by chemical precipitation.
2. The mineralogy and the textures depend very much
on the organisms precipitating the carbonates.
3. Carbonate minerals have higher solubility than silicate minerals in porewater, and also higher reaction
rates at low temperature. Dissolution of grains and
precipitation of cements may therefore be important also at shallow depth (<2 km).
4. The reservoir quality is highly dependent on the
porosity, which may be of different types. The
porosity in carbonate rocks may be pore space
between grains (intergranular porosity) or porosity
within grains (intragranular porosity), commonly
fossils. These primary porosity types are usually
strongly modified even at shallow depth during
burial. Well-cemented carbonate rocks may be brittle, particularly during uplift, and tectonic fracturing may produce important fracture porosity and
permeability.
5. Fractured carbonate reservoirs are very important
both in limestones and dolostones (a rock of
dolomite).
The most important porosity types in carbonate
rocks are the following:
A. Primary porosity is the pore space that existed in
the sediment just after deposition prior to diagenetic alteration. During burial the primary porosity
is reduced by compaction and cementation.
1. Primary intergranular porosity (pore space
between carbonate grains)
Intergranular porosity (inter-particle porosity) is
the preserved primary pore space between
grains. Pore shapes may vary considerably and
are homogeneous only when the enclosing
particles are of uniform size and shape (e.g.
well-sorted ooids). The porosity increases with
better sorting and more irregular grain shapes,
attaining values up to approximately 60%. Also
carbonate mud, just after deposition, has high
intergranular porosity (70–80%) between the
small mud particles, but the permeability is low
and in most cases this porosity is rapidly reduced
by mechanical compaction during early burial.
2. Intragranular porosity (porosity inside grains)
Intragranular porosity (intra-particle porosity)
consists of pore space that occurs within grains.
It is commonly formed by the decay of organic
material within carbonate skeletons (e.g. foraminifera, corals and bryozoans).
3. Growth-framework porosity (porosity inside a
rigid framework produced by fossils)
Growth-framework porosity is developed
between and within organisms during the inplace growth of a carbonate framework. Typical
examples are reef corals with uneven and patchily developed pores between the branches or
between different colonial organisms.
4. Shelter porosity
Shelter porosity is often found below large
plate-like grains which have acted as umbrellas
protecting the pore space beneath from being
filled with finer material. Typically porosity
may be preserved below the convex side of
molluscs and brachiopod shells. Organic material which is later decomposed may have produced shelter porosity which can be preserved if
there is early cementation.
B. Secondary porosity is pore space produced by dissolution of grains or cement after deposition. Secondary porosity may therefore be regarded as an
addition to the primary porosity. This requires a net
transport of carbonate in solution out of the rock. If
aragonite dissolves and the same amount of calcite
precipitates inside the rock, no porosity is gained,
though the distribution of pore space and the permeability may change drastically. Meteoric water
is capable of dissolving both aragonite and calcite
and transporting the dissolved material out from a
limestone so that the net porosity is increased.
At greater burial depth however the porewater
is nearly always in equilibrium with calcite which
is present in most marine sediments. The potential
for significant net dissolution and formation of
secondary porosity is therefore very limited.
1. Mouldic porosity
Mouldic porosity is formed by the selective
dissolution of grains, particularly skeletal material with a primary aragonitic composition.
Whole fossils or grains like ooids may dissolve
after the precipitation of cement in the primary
pores. These secondary pores thus become
moulds of the dissolved structures. Mouldic
porosity may also form by dissolution of fossils
from a matrix of carbonate mud which has been
210
N.-M. Hanken et al.
locally within the basin by biological production
(fossils) or by chemical precipitation.
2. The mineralogy and the textures depend very much
on the organisms precipitating the carbonates.
3. Carbonate minerals have higher solubility than silicate minerals in porewater, and also higher reaction
rates at low temperature. Dissolution of grains and
precipitation of cements may therefore be important also at shallow depth (<2 km).
4. The reservoir quality is highly dependent on the
porosity, which may be of different types. The
porosity in carbonate rocks may be pore space
between grains (intergranular porosity) or porosity
within grains (intragranular porosity), commonly
fossils. These primary porosity types are usually
strongly modified even at shallow depth during
burial. Well-cemented carbonate rocks may be brittle, particularly during uplift, and tectonic fracturing may produce important fracture porosity and
permeability.
5. Fractured carbonate reservoirs are very important
both in limestones and dolostones (a rock of
dolomite).
The most important porosity types in carbonate
rocks are the following:
A. Primary porosity is the pore space that existed in
the sediment just after deposition prior to diagenetic alteration. During burial the primary porosity
is reduced by compaction and cementation.
1. Primary intergranular porosity (pore space
between carbonate grains)
Intergranular porosity (inter-particle porosity) is
the preserved primary pore space between
grains. Pore shapes may vary considerably and
are homogeneous only when the enclosing
particles are of uniform size and shape (e.g.
well-sorted ooids). The porosity increases with
better sorting and more irregular grain shapes,
attaining values up to approximately 60%. Also
carbonate mud, just after deposition, has high
intergranular porosity (70–80%) between the
small mud particles, but the permeability is low
and in most cases this porosity is rapidly reduced
by mechanical compaction during early burial.
2. Intragranular porosity (porosity inside grains)
Intragranular porosity (intra-particle porosity)
consists of pore space that occurs within grains.
It is commonly formed by the decay of organic
material within carbonate skeletons (e.g. foraminifera, corals and bryozoans).
3. Growth-framework porosity (porosity inside a
rigid framework produced by fossils)
Growth-framework porosity is developed
between and within organisms during the inplace growth of a carbonate framework. Typical
examples are reef corals with uneven and patchily developed pores between the branches or
between different colonial organisms.
4. Shelter porosity
Shelter porosity is often found below large
plate-like grains which have acted as umbrellas
protecting the pore space beneath from being
filled with finer material. Typically porosity
may be preserved below the convex side of
molluscs and brachiopod shells. Organic material which is later decomposed may have produced shelter porosity which can be preserved if
there is early cementation.
B. Secondary porosity is pore space produced by dissolution of grains or cement after deposition. Secondary porosity may therefore be regarded as an
addition to the primary porosity. This requires a net
transport of carbonate in solution out of the rock. If
aragonite dissolves and the same amount of calcite
precipitates inside the rock, no porosity is gained,
though the distribution of pore space and the permeability may change drastically. Meteoric water
is capable of dissolving both aragonite and calcite
and transporting the dissolved material out from a
limestone so that the net porosity is increased.
At greater burial depth however the porewater
is nearly always in equilibrium with calcite which
is present in most marine sediments. The potential
for significant net dissolution and formation of
secondary porosity is therefore very limited.
1. Mouldic porosity
Mouldic porosity is formed by the selective
dissolution of grains, particularly skeletal material with a primary aragonitic composition.
Whole fossils or grains like ooids may dissolve
after the precipitation of cement in the primary
pores. These secondary pores thus become
moulds of the dissolved structures. Mouldic
porosity may also form by dissolution of fossils
from a matrix of carbonate mud which has been
210
N.-M. Hanken et al.