Turbidites form important reservoirs in many
basins and although the reservoir quality may be less
favourable than within shallow marine sandstones,
they often form extensive vertically-stacked reservoir
sequences and this may compensate for the lower
porosity.
Turbiditic sandstones generally have a higher clay
content than shallow marine sandstones. There is however a wide range of clay contents in turbidites from
rather clean, usually proximal and channel facies, to
more clay-rich distal and overbank facies.
Sandstones with clay contents higher than 10–15%
lose their porosity rapidly with mechanical compaction, because the detrital clay acts as a lubricant in the
compaction of the quartz grains. Turbiditic sandstones
of Paleocene and Eocene age form very important
reservoirs in the North Sea. The Frigg sandstone in
the Frigg Field in the Norwegian Sector is an example,
where the reservoir quality is rather good despite
representing a distal facies relative to the Shetland
Platform where the sand originated.
Because turbidites are deposited further away from
land, they are less exposed to flushing by meteoric
water. Turbidites normally contain less evidence of
feldspar dissolution and authigenic kaolinite precipitation than shallow marine and fluvial sandstones. In
proximal turbiditic facies, however, the sandstones
may have good contact with the meteoric water lens.
In the North Sea, Tertiary turbidites generally have a
relatively low content of authigenic kaolinite, but it
may be higher in proximal Cretaceous turbidities
which formed around islands produced by local uplift.
The total flow of meteoric water through
sandstones is a function of meteoric water flux and
the sedimentation rate. At low sedimentation rates a
given volume of sand spends more time in the shallow
zone of meteoric water flushing.
Cretaceous and Tertiary turbidite sandstones are
often very tight due to pervasive carbonate cement.
This may be due to pelagic carbonate organisms being
mixed in with the turbidite sand and recrystallising
into carbonate cement. This makes many Tertiary
sandstones hard and indurated even if they have not
been buried very deeply.
Many of the planktonic carbonate organisms developed during the Jurassic, so before then the sources of
carbonate cement in deep sea sandstones were more
restricted.
4.15 Predictions of Reservoir Quality
The oil industry has a practical need to be able to
predict the properties of reservoir rocks ahead of drilling. When planning petroleum production the rock
properties between the wells must also be estimated.
Particularly in the deeper reservoirs, porosity is the
most important factor determining the economic viability of a prospect. The main diagenetic processes
with quartz cementation are summarised in Fig. 4.15.
In a relatively mature basin the porosity/depth
functions of the different reservoir rocks can be treated
statistically so that the uncertainty of the estimates can
be expressed. The estimates based on the statistical
averages can also be adjusted up or down as a function
of temperature, stress etc., depending on what the interpreter considers most significant. The Middle Jurassic
Brent sandstone in the North Sea has been intensively
studied and a relatively linear trend found between
burial depth and porosity (Giles et al. 1992, Bjørlykke
et al. 1992, Ramm et al. 1992, Wilson 1994). Porosity
predictions will depend on the primary sediment composition and the subsequent compaction processes.
Above we have discussed some of the processes
that cause reductions in porosity and permeability. All
these processes are driven towards denser packing of
grains and thermodynamically more stable mineral
assemblages as the stress and temperature increases
during burial. The kinetics of mineral reactions
determines the rate of thermodynamic equilibriation,
which increases as an exponential function of
temperature.
The rate of compaction as a function of stress can
be measured experimentally using rock mechanics
testing procedures. However, the reactions involved
in chemical compaction are so slow, particularly in
silicate rocks, that it is difficult to reproduce them in
the laboratory although in some cases this is now
becoming possible.
In the field of clastic diagenesis, petrographic
observations about mineralogy and textural
relationships are used to interpret the sequence of
dissolution and mineral precipitation and its relationship to changes in porosity and permeability. It is
however important to consider the geochemical
constraints on diagenetic reactions. During burial the
reaction must add up so the dissolution is balanced by
precipitation because there are strong limitations with
140
K. Bjørlykke and J. Jahren
basins and although the reservoir quality may be less
favourable than within shallow marine sandstones,
they often form extensive vertically-stacked reservoir
sequences and this may compensate for the lower
porosity.
Turbiditic sandstones generally have a higher clay
content than shallow marine sandstones. There is however a wide range of clay contents in turbidites from
rather clean, usually proximal and channel facies, to
more clay-rich distal and overbank facies.
Sandstones with clay contents higher than 10–15%
lose their porosity rapidly with mechanical compaction, because the detrital clay acts as a lubricant in the
compaction of the quartz grains. Turbiditic sandstones
of Paleocene and Eocene age form very important
reservoirs in the North Sea. The Frigg sandstone in
the Frigg Field in the Norwegian Sector is an example,
where the reservoir quality is rather good despite
representing a distal facies relative to the Shetland
Platform where the sand originated.
Because turbidites are deposited further away from
land, they are less exposed to flushing by meteoric
water. Turbidites normally contain less evidence of
feldspar dissolution and authigenic kaolinite precipitation than shallow marine and fluvial sandstones. In
proximal turbiditic facies, however, the sandstones
may have good contact with the meteoric water lens.
In the North Sea, Tertiary turbidites generally have a
relatively low content of authigenic kaolinite, but it
may be higher in proximal Cretaceous turbidities
which formed around islands produced by local uplift.
The total flow of meteoric water through
sandstones is a function of meteoric water flux and
the sedimentation rate. At low sedimentation rates a
given volume of sand spends more time in the shallow
zone of meteoric water flushing.
Cretaceous and Tertiary turbidite sandstones are
often very tight due to pervasive carbonate cement.
This may be due to pelagic carbonate organisms being
mixed in with the turbidite sand and recrystallising
into carbonate cement. This makes many Tertiary
sandstones hard and indurated even if they have not
been buried very deeply.
Many of the planktonic carbonate organisms developed during the Jurassic, so before then the sources of
carbonate cement in deep sea sandstones were more
restricted.
4.15 Predictions of Reservoir Quality
The oil industry has a practical need to be able to
predict the properties of reservoir rocks ahead of drilling. When planning petroleum production the rock
properties between the wells must also be estimated.
Particularly in the deeper reservoirs, porosity is the
most important factor determining the economic viability of a prospect. The main diagenetic processes
with quartz cementation are summarised in Fig. 4.15.
In a relatively mature basin the porosity/depth
functions of the different reservoir rocks can be treated
statistically so that the uncertainty of the estimates can
be expressed. The estimates based on the statistical
averages can also be adjusted up or down as a function
of temperature, stress etc., depending on what the interpreter considers most significant. The Middle Jurassic
Brent sandstone in the North Sea has been intensively
studied and a relatively linear trend found between
burial depth and porosity (Giles et al. 1992, Bjørlykke
et al. 1992, Ramm et al. 1992, Wilson 1994). Porosity
predictions will depend on the primary sediment composition and the subsequent compaction processes.
Above we have discussed some of the processes
that cause reductions in porosity and permeability. All
these processes are driven towards denser packing of
grains and thermodynamically more stable mineral
assemblages as the stress and temperature increases
during burial. The kinetics of mineral reactions
determines the rate of thermodynamic equilibriation,
which increases as an exponential function of
temperature.
The rate of compaction as a function of stress can
be measured experimentally using rock mechanics
testing procedures. However, the reactions involved
in chemical compaction are so slow, particularly in
silicate rocks, that it is difficult to reproduce them in
the laboratory although in some cases this is now
becoming possible.
In the field of clastic diagenesis, petrographic
observations about mineralogy and textural
relationships are used to interpret the sequence of
dissolution and mineral precipitation and its relationship to changes in porosity and permeability. It is
however important to consider the geochemical
constraints on diagenetic reactions. During burial the
reaction must add up so the dissolution is balanced by
precipitation because there are strong limitations with
140
K. Bjørlykke and J. Jahren