Turbidites and fluvial sandstones form finingupwards units while marine shoreface and mouth bar
sandstones are coarsening-upwards. This becomes
very significant during production because oil and
gas will be concentrated in the upper part.
Coarsening-upwards sandstones therefore have the
best properties for flow of oil and gas during
production.
1.12 Changes in Rock Properties During
Burial and Uplift (Diagenesis)
The changes in properties are due to increased burial
and also to uplift. Both sandstone and carbonate
reservoirs undergo diagenesis, which will cause a
reduction in porosity and permeability as a function
of increasing burial.
The reduction in porosity (compaction) may be
mechanical in response to increased effective stress
from the overburden, or chemical as a result of the
dissolution and precipitation of minerals. The porosity
of reservoir sandstones or carbonates may increase
with depth in certain intervals, but this is because of
the changes in the primary sediment composition.
Each lithology has a different porosity depth curve.
In a uniform primary lithology the porosity and the
density will be reduced as a function of burial depth
(temperature and stress). Overpressure causes reduced
effective stress resulting in less mechanical compaction. Near the surface, meteoric flow may cause dissolution and a net increase in the porosity in carbonates
(karst) and even, to a certain extent, in sandstones.
In continuously subsiding basins, open faults and
fractures will be rare because of the progressive compaction processes. Shales may however fracture at
high overpressures (fracture pressure) near the top of
structural highs, but that may be separate from the
fault planes.
During uplift and erosion (exhumation, unloading)
the rocks will be subjected to extension, and extensional fractures will be produced. The porosity will not
increase significantly but the extensional fractures will
increase the permeability and thus improve the reservoir properties. Unfortunately the cap rock may also
fracture during unloading, causing leakage from the
reservoir.
To understand the properties of reservoir rock we
need to integrate what we know about the
sedimentology (depositional environments) of reservoir rocks, sediment composition (provenance), diagenesis and the structural geology.
Prior to drilling exploration wells, nearly all our
knowledge about a reservoir is based on geophysical
data. Even after data has been acquired from exploration wells, and also from production wells, prediction
of the reservoir properties continues to be mostly
based on geophysical methods and extrapolation
between wells.
Geophysical methods including 3D and 4D seismic
now provide a much more detailed picture of the
reservoir than only 10–20 years ago. The methods
for detecting fluid contacts, not only gas/water
contacts but also oil/water contacts, from seismic
data have improved greatly.
1.13 Carbonate Reservoirs
Reefs stand up as positive structures and may be
draped by mud and form a stratigraphic trap determined by the size of the reef structure.
Carbonate reefs, and other carbonate deposits
which can be reservoirs, form in a wide range of
environments but all require clear water without
much clay sedimentation. Reefs are deposited in high
energy environments along coastlines exposed to high
wave energy. The Bahamas carbonate platform has
well-developed reefs on the exposed eastern side but
not on the more protected western side. Coral reefs
also require warm water (>20
C) and do not form
where cold water is upwelling, e.g. along the coast of
West Africa.
Reefs build up on the seafloor and may be buried
beneath mud during transgressions. The reef then
becomes a perfect stratigraphic trap, often with good
permeability both vertically and horizontally.
Reefs can form long continuous barriers as in
Australia (Great Barrier Reef). In the US much oil
was found by following Jurassic and Cretaceous reef
trends around the Gulf of Mexico.
High energy beach deposits on carbonate banks
may consist of well-sorted carbonate sand
(grainstones). Ooid sands (ooliths) are formed as
beach and shoreface deposits and may have limited
vertical thickness, reflecting the wave base. They may
however stack up and form thicker sequences of such
rocks. Ooids may also be transported from the shelf
1 Introduction to Petroleum Geology
21
sandstones are coarsening-upwards. This becomes
very significant during production because oil and
gas will be concentrated in the upper part.
Coarsening-upwards sandstones therefore have the
best properties for flow of oil and gas during
production.
1.12 Changes in Rock Properties During
Burial and Uplift (Diagenesis)
The changes in properties are due to increased burial
and also to uplift. Both sandstone and carbonate
reservoirs undergo diagenesis, which will cause a
reduction in porosity and permeability as a function
of increasing burial.
The reduction in porosity (compaction) may be
mechanical in response to increased effective stress
from the overburden, or chemical as a result of the
dissolution and precipitation of minerals. The porosity
of reservoir sandstones or carbonates may increase
with depth in certain intervals, but this is because of
the changes in the primary sediment composition.
Each lithology has a different porosity depth curve.
In a uniform primary lithology the porosity and the
density will be reduced as a function of burial depth
(temperature and stress). Overpressure causes reduced
effective stress resulting in less mechanical compaction. Near the surface, meteoric flow may cause dissolution and a net increase in the porosity in carbonates
(karst) and even, to a certain extent, in sandstones.
In continuously subsiding basins, open faults and
fractures will be rare because of the progressive compaction processes. Shales may however fracture at
high overpressures (fracture pressure) near the top of
structural highs, but that may be separate from the
fault planes.
During uplift and erosion (exhumation, unloading)
the rocks will be subjected to extension, and extensional fractures will be produced. The porosity will not
increase significantly but the extensional fractures will
increase the permeability and thus improve the reservoir properties. Unfortunately the cap rock may also
fracture during unloading, causing leakage from the
reservoir.
To understand the properties of reservoir rock we
need to integrate what we know about the
sedimentology (depositional environments) of reservoir rocks, sediment composition (provenance), diagenesis and the structural geology.
Prior to drilling exploration wells, nearly all our
knowledge about a reservoir is based on geophysical
data. Even after data has been acquired from exploration wells, and also from production wells, prediction
of the reservoir properties continues to be mostly
based on geophysical methods and extrapolation
between wells.
Geophysical methods including 3D and 4D seismic
now provide a much more detailed picture of the
reservoir than only 10–20 years ago. The methods
for detecting fluid contacts, not only gas/water
contacts but also oil/water contacts, from seismic
data have improved greatly.
1.13 Carbonate Reservoirs
Reefs stand up as positive structures and may be
draped by mud and form a stratigraphic trap determined by the size of the reef structure.
Carbonate reefs, and other carbonate deposits
which can be reservoirs, form in a wide range of
environments but all require clear water without
much clay sedimentation. Reefs are deposited in high
energy environments along coastlines exposed to high
wave energy. The Bahamas carbonate platform has
well-developed reefs on the exposed eastern side but
not on the more protected western side. Coral reefs
also require warm water (>20
C) and do not form
where cold water is upwelling, e.g. along the coast of
West Africa.
Reefs build up on the seafloor and may be buried
beneath mud during transgressions. The reef then
becomes a perfect stratigraphic trap, often with good
permeability both vertically and horizontally.
Reefs can form long continuous barriers as in
Australia (Great Barrier Reef). In the US much oil
was found by following Jurassic and Cretaceous reef
trends around the Gulf of Mexico.
High energy beach deposits on carbonate banks
may consist of well-sorted carbonate sand
(grainstones). Ooid sands (ooliths) are formed as
beach and shoreface deposits and may have limited
vertical thickness, reflecting the wave base. They may
however stack up and form thicker sequences of such
rocks. Ooids may also be transported from the shelf
1 Introduction to Petroleum Geology
21