4.14 Different Types of Sandstone
It is important to distinguish between sandstones of
different primary composition:
Volcanoclastic sandstones may vary greatly in
composition depending on the volcanic source and
the depositional environments. Basic volcanic rocks
in particular have a very low content of stable grains
like quartz, but a high content of basic feldspar and
pyroxenes which break down rapidly, both mechanically and chemically. Matrix-rich sandstones like
greywackes may have had a higher sand content at
the time of deposition because many of the grains were
unstable during diagenesis and became effectively part
of the matrix. What were deposited as grains of volcanic rock fragments may be squeezed so that they
become a chlorite-rich matrix.
Volcanoclastic sandstones lose most of their porosity at rather shallow depth (<1–2 km) and therefore
make poor reservoir rocks. However, the geothermal
gradients in volcanic regions may be high, causing
source rocks to mature at shallow burial depth and
thus increase the potential for migration into shallow
structures.
Lithic sandstones have a high content (>10%) of
rock fragments. Normal and coarse-grained granites
and gneisses produce grains that mostly consist of a
single mineral while sandstones derived from finergrained igneous and metamorphic rocks are mostly
comprised of rock fragments. Rock fragments are
generally weaker than quartz and feldspar grains, as
has been demonstrated experimentally (Pittman and
Larese 1991).
Arkoses contain more than 25% feldspar and such
sandstones are typical of tectonically active basins like
rift basins where the erosion, transport and deposition
of basement derived rocks is fast, leaving little time
for feldspar to weather. Temperature and rainfall also
play a role here. Arkoses compact more mechanically
than quartzitic sandstones, leaving a smaller intergranular volume to be cemented with quartz at greater
depth. The area available for quartz cementation is
also reduced since quartz does not grow on feldspar.
Feldspathic sandstones and quartzites are the most
common sandstone reservoir rocks. The feldspar content is usually a function of the source climate and the
relief in the drainage area. On tectonically stable
cratons sediments are repeatedly eroded and deposited
and some feldspar and mica is dissolved during each
cycle.
Palaeozoic quartzites typically occur as transgressive sheet sands on cratons. On the North American
craton there are good examples of this in the Lower
Palaeozoic sequence. Such clean shallow marine
sandstones have extremely good reservoir properties
at shallow to moderate burial depth. This is likewise
the case with aeolian sandstones. Fluvial sandstones
are also normally well-sorted in such environments
because they are often reworked aeolian sands.
Carbonate cement in shallow marine sandstones is
mostly derived by recrystallisation of calcareous
organisms. Meteoric water will dissolve aragonite
and precipitate calcite in sandstones, producing early
cement.
In modern environments, particularly in beach and
shoreface settings, fragments of crushed calcareous
organisms are quite common. We find less carbonate
cement in fluvial sandstones because of the lower
biogenic carbonate production in freshwater. Carbonate cement has a local source in most cases, but may be
redistributed and concentrated by diffusion. The range
of effective diffusion is generally small (<1 m)
because the porewater is in equilibrium with calcite
and there are small concentration gradients. Advective
flow will transport dissolved carbonate but can not
precipitate tight carbonate cement because the permeability decreases as precipitation proceeds. The advective flow will then tend to bypass the volume where
carbonate cementation has started. In the case of
compaction-driven upwards-directed (cooling) porewater, the solubility of calcite will increase, causing
dissolution rather than precipitation.
Aeolian sandstones and other desert sandstones generally show less evidence of meteoric water flushing
than fluvial and shallow marine sandstones. Sandstones
like the Permian Rotliegend from the southern North
Sea have relatively low amounts of kaolinite and more
smectite or illite as pore-filling cement. However, even
deserts have groundwater so some leaching occurs.
Fluvial sediments will normally be flushed by groundwater after deposition and in most cases show ample
evidence of feldspar leaching. Reworking of such
sediments will bring authigenic kaolinite into the clastic clay fraction. Continental sandstones often have
haematite or manganese oxide coatings on quartz
grains and this may inhibit quartz overgrowth.
4 Sandstones and Sandstone Reservoirs
139
It is important to distinguish between sandstones of
different primary composition:
Volcanoclastic sandstones may vary greatly in
composition depending on the volcanic source and
the depositional environments. Basic volcanic rocks
in particular have a very low content of stable grains
like quartz, but a high content of basic feldspar and
pyroxenes which break down rapidly, both mechanically and chemically. Matrix-rich sandstones like
greywackes may have had a higher sand content at
the time of deposition because many of the grains were
unstable during diagenesis and became effectively part
of the matrix. What were deposited as grains of volcanic rock fragments may be squeezed so that they
become a chlorite-rich matrix.
Volcanoclastic sandstones lose most of their porosity at rather shallow depth (<1–2 km) and therefore
make poor reservoir rocks. However, the geothermal
gradients in volcanic regions may be high, causing
source rocks to mature at shallow burial depth and
thus increase the potential for migration into shallow
structures.
Lithic sandstones have a high content (>10%) of
rock fragments. Normal and coarse-grained granites
and gneisses produce grains that mostly consist of a
single mineral while sandstones derived from finergrained igneous and metamorphic rocks are mostly
comprised of rock fragments. Rock fragments are
generally weaker than quartz and feldspar grains, as
has been demonstrated experimentally (Pittman and
Larese 1991).
Arkoses contain more than 25% feldspar and such
sandstones are typical of tectonically active basins like
rift basins where the erosion, transport and deposition
of basement derived rocks is fast, leaving little time
for feldspar to weather. Temperature and rainfall also
play a role here. Arkoses compact more mechanically
than quartzitic sandstones, leaving a smaller intergranular volume to be cemented with quartz at greater
depth. The area available for quartz cementation is
also reduced since quartz does not grow on feldspar.
Feldspathic sandstones and quartzites are the most
common sandstone reservoir rocks. The feldspar content is usually a function of the source climate and the
relief in the drainage area. On tectonically stable
cratons sediments are repeatedly eroded and deposited
and some feldspar and mica is dissolved during each
cycle.
Palaeozoic quartzites typically occur as transgressive sheet sands on cratons. On the North American
craton there are good examples of this in the Lower
Palaeozoic sequence. Such clean shallow marine
sandstones have extremely good reservoir properties
at shallow to moderate burial depth. This is likewise
the case with aeolian sandstones. Fluvial sandstones
are also normally well-sorted in such environments
because they are often reworked aeolian sands.
Carbonate cement in shallow marine sandstones is
mostly derived by recrystallisation of calcareous
organisms. Meteoric water will dissolve aragonite
and precipitate calcite in sandstones, producing early
cement.
In modern environments, particularly in beach and
shoreface settings, fragments of crushed calcareous
organisms are quite common. We find less carbonate
cement in fluvial sandstones because of the lower
biogenic carbonate production in freshwater. Carbonate cement has a local source in most cases, but may be
redistributed and concentrated by diffusion. The range
of effective diffusion is generally small (<1 m)
because the porewater is in equilibrium with calcite
and there are small concentration gradients. Advective
flow will transport dissolved carbonate but can not
precipitate tight carbonate cement because the permeability decreases as precipitation proceeds. The advective flow will then tend to bypass the volume where
carbonate cementation has started. In the case of
compaction-driven upwards-directed (cooling) porewater, the solubility of calcite will increase, causing
dissolution rather than precipitation.
Aeolian sandstones and other desert sandstones generally show less evidence of meteoric water flushing
than fluvial and shallow marine sandstones. Sandstones
like the Permian Rotliegend from the southern North
Sea have relatively low amounts of kaolinite and more
smectite or illite as pore-filling cement. However, even
deserts have groundwater so some leaching occurs.
Fluvial sediments will normally be flushed by groundwater after deposition and in most cases show ample
evidence of feldspar leaching. Reworking of such
sediments will bring authigenic kaolinite into the clastic clay fraction. Continental sandstones often have
haematite or manganese oxide coatings on quartz
grains and this may inhibit quartz overgrowth.
4 Sandstones and Sandstone Reservoirs
139