lithified. In the Jurassic Smackover Formation
of Arkansas, USA, oomouldic porosity was created by the dissolution of ooids during freshwater flushing (More 1989).
2. Fenestral porosity
Fenestral porosity consists generally of small
elongate to equant pores which are typically
1–10 mm in diameter. These characteristic pores
are often arranged in layers within the sediment
and are most commonly encountered in algal mats
facies where they have been produced by
decaying organic matter or by desiccation.
3. Breccia porosity
Breccia porosity may form by the collapse of a
rock due to dissolution (e.g. dissolution of
evaporites or limestone dissolution during
karst weathering) or tectonic deformation.
4. Fracture porosity
Fracture porosity commonly forms due to folding, faulting, salt doming, differential compaction, salt dissolution or fluid overpressure.
Carbonate sediments often become cemented
at shallow depth and may fracture and dilate
so that fracture porosity is produced. Open
fractures will gradually be filled with cement
but fractures in dolomite may remain open longer than those in limestones.
Shrinkage porosity is a special type of fracture
porosity formed during early diagenesis when
sediments may shrink i.e. due to dewatering.
Septarian fractures in limestone concretions
are also a kind of shrinkage fracture.
5. Vuggy, channel and cavern porosity
Vuggy, channel and cavern (karstic) porosity
are not fabric selective, i.e. they cut across
grains and/or cement boundaries. The pores
are of irregular size and shape and may or may
not be interconnected. Many vugs are solutionenlarged moulds where evidence of the precursor grain has been destroyed by dissolution of
the neighbouring matrix. Vuggy pores are commonly 1 mm–1 m in diameter. Cavern porosity
commonly relates to meteoric (karstic) leaching
and is differentiated from vuggy porosity by the
larger pore size (man-sized or larger). Karst is
formed by dissolution related to underground
river systems and can therefore be very extensive and form large oil reservoirs.
5.8.2 Some Examples of Carbonate
Reservoirs
5.8.2.1 Grainstones
Well-sorted carbonate sands (grainstones) are good
reservoirs if the porosity is not reduced too much by
cementation. The sand grains are usually ooids or
fossil fragments. If the grains are mainly of low-Mg
calcite, most of the porosity tends to be primary
porosity which gradually fills with cement during
burial. The source of most of the cement is then
normally pressure solution along stylolites or graingrain contacts.
Precipitation of early carbonate cement reduces the
stresses at grain contacts and strengthens the grain
framework. The mechanical compaction may therefore
be reduced. When many of the grains consist of aragonite they may dissolve, producing secondary porosity
while the dissolved carbonate fills the primary porosity.
Dissolution of aragonitic ooids and fossils produces
moulds which have very little communication between
them and such mouldic reservoirs are characterized by
rather high porosity but low permeability.
5.8.2.2 Fractured Reservoirs
The term fracture is used for any break in a rock and
includes cracks, joints and faults. In many cases the
fracture is less permeable than the matrix, but some
may also be partly or totally open. Open fractures
formed by extension are common during uplift and
folding. Well-cemented limestones and dolomites
tend to have brittle properties and faulting may produce a breccia with good permeability.
The rock fragments produced by the brecciation
take the stress and prevent the fault plane from closing.
Movements (off-set) along the fault plane have a similar effect. The porosity and permeability produced in
carbonates is, however, temporary because it may
relatively rapidly be filled with carbonate cement.
Filling of open fractures by oil or gas will retard or
inhibit calcite cementation and thus contribute to the
conservation of the porosity. This is particularly true if
the rocks are relatively oil-wet.
Fractures in limestones are commonly cemented by
diffusion of carbonate from the adjacent matrix. Flow
of water upward along the fault plane will cause dissolution of calcite rather than precipitation because of
the retrograde solubility with respect to temperature.
5 Carbonate Sediments
211
of Arkansas, USA, oomouldic porosity was created by the dissolution of ooids during freshwater flushing (More 1989).
2. Fenestral porosity
Fenestral porosity consists generally of small
elongate to equant pores which are typically
1–10 mm in diameter. These characteristic pores
are often arranged in layers within the sediment
and are most commonly encountered in algal mats
facies where they have been produced by
decaying organic matter or by desiccation.
3. Breccia porosity
Breccia porosity may form by the collapse of a
rock due to dissolution (e.g. dissolution of
evaporites or limestone dissolution during
karst weathering) or tectonic deformation.
4. Fracture porosity
Fracture porosity commonly forms due to folding, faulting, salt doming, differential compaction, salt dissolution or fluid overpressure.
Carbonate sediments often become cemented
at shallow depth and may fracture and dilate
so that fracture porosity is produced. Open
fractures will gradually be filled with cement
but fractures in dolomite may remain open longer than those in limestones.
Shrinkage porosity is a special type of fracture
porosity formed during early diagenesis when
sediments may shrink i.e. due to dewatering.
Septarian fractures in limestone concretions
are also a kind of shrinkage fracture.
5. Vuggy, channel and cavern porosity
Vuggy, channel and cavern (karstic) porosity
are not fabric selective, i.e. they cut across
grains and/or cement boundaries. The pores
are of irregular size and shape and may or may
not be interconnected. Many vugs are solutionenlarged moulds where evidence of the precursor grain has been destroyed by dissolution of
the neighbouring matrix. Vuggy pores are commonly 1 mm–1 m in diameter. Cavern porosity
commonly relates to meteoric (karstic) leaching
and is differentiated from vuggy porosity by the
larger pore size (man-sized or larger). Karst is
formed by dissolution related to underground
river systems and can therefore be very extensive and form large oil reservoirs.
5.8.2 Some Examples of Carbonate
Reservoirs
5.8.2.1 Grainstones
Well-sorted carbonate sands (grainstones) are good
reservoirs if the porosity is not reduced too much by
cementation. The sand grains are usually ooids or
fossil fragments. If the grains are mainly of low-Mg
calcite, most of the porosity tends to be primary
porosity which gradually fills with cement during
burial. The source of most of the cement is then
normally pressure solution along stylolites or graingrain contacts.
Precipitation of early carbonate cement reduces the
stresses at grain contacts and strengthens the grain
framework. The mechanical compaction may therefore
be reduced. When many of the grains consist of aragonite they may dissolve, producing secondary porosity
while the dissolved carbonate fills the primary porosity.
Dissolution of aragonitic ooids and fossils produces
moulds which have very little communication between
them and such mouldic reservoirs are characterized by
rather high porosity but low permeability.
5.8.2.2 Fractured Reservoirs
The term fracture is used for any break in a rock and
includes cracks, joints and faults. In many cases the
fracture is less permeable than the matrix, but some
may also be partly or totally open. Open fractures
formed by extension are common during uplift and
folding. Well-cemented limestones and dolomites
tend to have brittle properties and faulting may produce a breccia with good permeability.
The rock fragments produced by the brecciation
take the stress and prevent the fault plane from closing.
Movements (off-set) along the fault plane have a similar effect. The porosity and permeability produced in
carbonates is, however, temporary because it may
relatively rapidly be filled with carbonate cement.
Filling of open fractures by oil or gas will retard or
inhibit calcite cementation and thus contribute to the
conservation of the porosity. This is particularly true if
the rocks are relatively oil-wet.
Fractures in limestones are commonly cemented by
diffusion of carbonate from the adjacent matrix. Flow
of water upward along the fault plane will cause dissolution of calcite rather than precipitation because of
the retrograde solubility with respect to temperature.
5 Carbonate Sediments
211