Chapter 8
Seismic Stratigraphy, Sequence Stratigraphy
and Basin Analysis
Knut Bjørlykke
8.1
Seismic Stratigraphy
Seismic records are based on measurements of the time
sound waves (seismic waves) take to travel through
rock. The sound or signal is produced by explosives or
compressed air (air guns). Rock is an elastic medium
and the velocity of sound conveys a lot of information
about the properties of the rock. Normal sound waves
(P-waves) travel through both the solid phase, which for
the most part consists of minerals or rock fragments, and
the liquid or gas in the pores. Shear waves (S-waves) on
the other hand can only go through the solid phase.
The most important parameters influencing the
velocity of sound are: porosity, mineral composition,
and the degree of cementation. These factors determine
the stiffness of the rock (Bulk modulus, see Chap. 11).
The velocity of sound waves in water is about 1,500 m/s,
but depends on temperature and salt concentration.
Sound passes through unconsolidated sediments at
velocities which are only slightly higher than the velocity in water (1,500–2,000 m/s, and sometimes even
lower) because they have high water content and
because the framework on which the sediment grains
are based does not offer any real strength (stiffness) as a
medium for the seismic waves.
Cementation of sand with carbonate or siliceous
cement will bind the grains together in a framework
which will increase the stiffness and velocity considerably even if the porosity is relatively high. Compaction
due to overlying sediments which causes water to be
expelled will also give higher velocities, not only
because the water content decreases, but because more
numerous and larger contacts are formed between the
clastic grains. Velocities in moderately consolidated
sediments, such as the Tertiary sediments of the North
Sea, are 2–3 km/s. In more consolidated (compacted and
cemented) sedimentary rocks which have not been
subjected to metamorphosis, velocities are mostly
between 3 and 5 km/s. This is the case for many of the
Mesozoic sediments in the North Sea. Metamorphic and
eruptive rocks will have velocities of about 5–6 km/s.
Limestones will often have higher velocities than
sandstones at the same depth because they often are
more cemented and because carbonate cement has a
high degree of rigidity and low compressibility. Carbonate reefs may be strongly cemented and have high
velocities at shallow depth. Sandstone in turn provides
a more rigid medium for sound waves than shale at the
same depth, because of its grain-supported structure.
If the rocks do not contain oil and gas we can assume
that their porosity is identical with the water content in
the rock. Velocity will then be a function of porosity (φ),
and if we know the velocity of sound in the rock matrix,
we can calculate the porosity using Wyllie’s equation:
1=V r ¼ 1 À φ
ð
Þ=V m þ φ=V f
where
V r ¼ velocity in rock when saturated with liquid, i.e.
the measured velocity
V f ¼ velocity in the fluid
V m ¼ velocity in the rock matrix.
The inverse values of the velocities are expressions
of the time the signals take to travel through a layer of
certain thickness.
K. Bjørlykke (*)
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_8, # Springer-Verlag Berlin Heidelberg 2015
255
Seismic Stratigraphy, Sequence Stratigraphy
and Basin Analysis
Knut Bjørlykke
8.1
Seismic Stratigraphy
Seismic records are based on measurements of the time
sound waves (seismic waves) take to travel through
rock. The sound or signal is produced by explosives or
compressed air (air guns). Rock is an elastic medium
and the velocity of sound conveys a lot of information
about the properties of the rock. Normal sound waves
(P-waves) travel through both the solid phase, which for
the most part consists of minerals or rock fragments, and
the liquid or gas in the pores. Shear waves (S-waves) on
the other hand can only go through the solid phase.
The most important parameters influencing the
velocity of sound are: porosity, mineral composition,
and the degree of cementation. These factors determine
the stiffness of the rock (Bulk modulus, see Chap. 11).
The velocity of sound waves in water is about 1,500 m/s,
but depends on temperature and salt concentration.
Sound passes through unconsolidated sediments at
velocities which are only slightly higher than the velocity in water (1,500–2,000 m/s, and sometimes even
lower) because they have high water content and
because the framework on which the sediment grains
are based does not offer any real strength (stiffness) as a
medium for the seismic waves.
Cementation of sand with carbonate or siliceous
cement will bind the grains together in a framework
which will increase the stiffness and velocity considerably even if the porosity is relatively high. Compaction
due to overlying sediments which causes water to be
expelled will also give higher velocities, not only
because the water content decreases, but because more
numerous and larger contacts are formed between the
clastic grains. Velocities in moderately consolidated
sediments, such as the Tertiary sediments of the North
Sea, are 2–3 km/s. In more consolidated (compacted and
cemented) sedimentary rocks which have not been
subjected to metamorphosis, velocities are mostly
between 3 and 5 km/s. This is the case for many of the
Mesozoic sediments in the North Sea. Metamorphic and
eruptive rocks will have velocities of about 5–6 km/s.
Limestones will often have higher velocities than
sandstones at the same depth because they often are
more cemented and because carbonate cement has a
high degree of rigidity and low compressibility. Carbonate reefs may be strongly cemented and have high
velocities at shallow depth. Sandstone in turn provides
a more rigid medium for sound waves than shale at the
same depth, because of its grain-supported structure.
If the rocks do not contain oil and gas we can assume
that their porosity is identical with the water content in
the rock. Velocity will then be a function of porosity (φ),
and if we know the velocity of sound in the rock matrix,
we can calculate the porosity using Wyllie’s equation:
1=V r ¼ 1 À φ
ð
Þ=V m þ φ=V f
where
V r ¼ velocity in rock when saturated with liquid, i.e.
the measured velocity
V f ¼ velocity in the fluid
V m ¼ velocity in the rock matrix.
The inverse values of the velocities are expressions
of the time the signals take to travel through a layer of
certain thickness.
K. Bjørlykke (*)
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_8, # Springer-Verlag Berlin Heidelberg 2015
255