carbonate reservoirs may account for almost 40%.
Many of the reservoirs in the Middle East are carbonate rocks but in the rest of the world the percentage of
carbonate reservoirs is lower.
The most important aspects of reservoir rocks
include:
(1) The external geometry such as the thickness and
extent of the reservoir rock in all directions.
(2) The average porosity, pore size and pore
geometry.
(3) The distribution of permeability in the reservoirs,
particularly high permeability conduits and low
permeability barriers to fluid flow.
(4) Mineralogy and wettability of the pore network.
The properties of sandstone and carbonate
reservoirs are primarily linked to the depositional
environment, the textural and mineralogical composition and the burial history. A good background in
general sedimentology, facies analysis and sequence
stratigraphy is therefore important.
Nevertheless, many of the important properties of
reservoir rocks linked to changes in facies and smaller
faults are below the vertical resolution of exploration
seismic (15–30 m) and it is important to establish
relationships between facies models, diagenetic processes and reservoir properties. The properties of
faults are also very important factors determining oil
flow during production.
1.11 External Geometry of Reservoir
Rocks
The external geometry of reservoir rocks is largely
determined by the depositional environments, but
faulting and diagenesis may define the lateral or vertical extent of a reservoir.
Fluvial sandstones typically represent point bar
sequences in a meandering river system. The lateral
accretion of the point bar will deposit a sandstone
layer extending to the width of the meander belt in
the valley. The thicknesses of channel sandstones are
limited by the depth of the river. The primary thickness at the time of deposition is, however, reduced by
10–30% or more by compaction.
The overbank muds will become tight shales which
will reduce the vertical permeability. Fluvial channels
are characterised by fining-upwards sequences with
the highest permeability near the base. This makes it
more likely that water will break through along the
basal part and the oil will be by-passed in the finergrained upper part during production.
Braided stream facies will tend to have higher sand/
shale ratios and will normally have better lateral and
vertical permeabilitites on a larger scale.
The ratio between the intervals with high enough
porosity and permeability to be produced (net or pay),
and the total sequence (gross), will be mostly determined by the primary facies relationships. The net/
gross ratio is often taken to be approximately equal
to the sand/shale ratio but even at moderate burial
depths many sandstones are not reservoir rocks, due
to poor sorting or carbonate cement.
Aeolian dunes also have specific external
geometries and there are many different types. Here
the net/gross will be very high. Aeolian sand is often
reworked by transgressions, accumulating as marine
sediments in drowned topographic depressions
(valleys).
Marine sandstones deposited as delta mouth bars,
shoreface accretion and barrier islands have
thicknesses controlled by the wave energy (wave
base depth). In protected environments, particularly
inter-distributary bays, the shoreface sandstones may
be very thin. Each shallow marine unit has a limited
thickness controlled by fairweather wave base. Local
subsidence or transgressions can increase the thickness of these sand deposits.
The tidal range is very important in determining the
thickness and the length of tidal channel sandstones.
Tidal channels and also fluvial channels in deltas tend
to be oriented perpendicular to coastlines.
Drilling into shallow marine sandstones, it would
be very important to determine whether it was a barrier
island which would represent an elongated reservoir
parallel to the coastline, or a tidal sandstone which
tends to be oriented perpendicular to the coastline. In
some cases dipmeter logs could help to determine the
orientation of cross-beddding and progradation direction of sand bars.
Turbidites may be laterally very extensive, but may
also be confined to narrow submarine channels. In
either case they may form very thick sequences
because there is ample accommodation space. We
may have very thick sequences of stacked sandstone
reservoir rocks in slope and deepwater facies and this
may compensate for the lower porosity and permeability compared to beach deposits.
20
K. Bjørlykke
Many of the reservoirs in the Middle East are carbonate rocks but in the rest of the world the percentage of
carbonate reservoirs is lower.
The most important aspects of reservoir rocks
include:
(1) The external geometry such as the thickness and
extent of the reservoir rock in all directions.
(2) The average porosity, pore size and pore
geometry.
(3) The distribution of permeability in the reservoirs,
particularly high permeability conduits and low
permeability barriers to fluid flow.
(4) Mineralogy and wettability of the pore network.
The properties of sandstone and carbonate
reservoirs are primarily linked to the depositional
environment, the textural and mineralogical composition and the burial history. A good background in
general sedimentology, facies analysis and sequence
stratigraphy is therefore important.
Nevertheless, many of the important properties of
reservoir rocks linked to changes in facies and smaller
faults are below the vertical resolution of exploration
seismic (15–30 m) and it is important to establish
relationships between facies models, diagenetic processes and reservoir properties. The properties of
faults are also very important factors determining oil
flow during production.
1.11 External Geometry of Reservoir
Rocks
The external geometry of reservoir rocks is largely
determined by the depositional environments, but
faulting and diagenesis may define the lateral or vertical extent of a reservoir.
Fluvial sandstones typically represent point bar
sequences in a meandering river system. The lateral
accretion of the point bar will deposit a sandstone
layer extending to the width of the meander belt in
the valley. The thicknesses of channel sandstones are
limited by the depth of the river. The primary thickness at the time of deposition is, however, reduced by
10–30% or more by compaction.
The overbank muds will become tight shales which
will reduce the vertical permeability. Fluvial channels
are characterised by fining-upwards sequences with
the highest permeability near the base. This makes it
more likely that water will break through along the
basal part and the oil will be by-passed in the finergrained upper part during production.
Braided stream facies will tend to have higher sand/
shale ratios and will normally have better lateral and
vertical permeabilitites on a larger scale.
The ratio between the intervals with high enough
porosity and permeability to be produced (net or pay),
and the total sequence (gross), will be mostly determined by the primary facies relationships. The net/
gross ratio is often taken to be approximately equal
to the sand/shale ratio but even at moderate burial
depths many sandstones are not reservoir rocks, due
to poor sorting or carbonate cement.
Aeolian dunes also have specific external
geometries and there are many different types. Here
the net/gross will be very high. Aeolian sand is often
reworked by transgressions, accumulating as marine
sediments in drowned topographic depressions
(valleys).
Marine sandstones deposited as delta mouth bars,
shoreface accretion and barrier islands have
thicknesses controlled by the wave energy (wave
base depth). In protected environments, particularly
inter-distributary bays, the shoreface sandstones may
be very thin. Each shallow marine unit has a limited
thickness controlled by fairweather wave base. Local
subsidence or transgressions can increase the thickness of these sand deposits.
The tidal range is very important in determining the
thickness and the length of tidal channel sandstones.
Tidal channels and also fluvial channels in deltas tend
to be oriented perpendicular to coastlines.
Drilling into shallow marine sandstones, it would
be very important to determine whether it was a barrier
island which would represent an elongated reservoir
parallel to the coastline, or a tidal sandstone which
tends to be oriented perpendicular to the coastline. In
some cases dipmeter logs could help to determine the
orientation of cross-beddding and progradation direction of sand bars.
Turbidites may be laterally very extensive, but may
also be confined to narrow submarine channels. In
either case they may form very thick sequences
because there is ample accommodation space. We
may have very thick sequences of stacked sandstone
reservoir rocks in slope and deepwater facies and this
may compensate for the lower porosity and permeability compared to beach deposits.
20
K. Bjørlykke