to the land and the shoreline moves toward higher
ground, resulting in flooding) and regressions are not
always directly related to sea level changes. When a
delta builds out into the sea, there is a local regression
on the delta even if the sea level has not fallen. If
sedimentation is sufficiently rapid, we can have a
local regression on a delta even with a rising sea
level. Along a coastline we can in fact have
transgressions in some areas and regressions in others,
at the same time, depending on the rate of sedimentation or erosion with respect to sea level change. A
regression caused by a fall in sea level is called a
forced regression. Changes in sea level can be due to:
• Local tectonic movements, for example uplift of a
horst or subsidence of a graben structure.
• Plate-tectonic movements which can be of great
extent, but are not global.
• Sea level changes. These are global and are called
eustatic sea level changes.
During the Quaternary, cyclic changes in sea level
of up to 120 m accompanied the growth and decay of
continental ice sheets. These changes were rapid and
of large magnitude and can be traced throughout much
of the world. In the areas which had supported ice
sheets, such as Scandinavia, the melting of the ice
led to isostatic uplift due to unloading, and this
exceeded the rise in sea level so that there was a
regression. When seismic stratigraphy was established
(Vail et al. 1977), it was assumed that most of the
variations in sea level that could be interpreted from
seismic records were attributable to eustatic changes
and thus could be employed for global correlation.
However accumulation of ice on the continents is the
only known process capable of producing large and
rapid global sea level changes. Such ice ages are
known from the Quaternary, the CarboniferousPermian, late Ordovician and the end of the
Precambrian.
Nevertheless, transgressions and regressions can be
correlated over greater or lesser distances, depending
on the type of tectonic displacement. It is often difficult to distinguish between eustatic sea level changes
and ones caused by more local or regional conditions
and we therefore prefer to use the term relative sea
level change. On a delta, one delta lobe may prograde
and produce a regressive sequence while a transgression occurs with carbonate sedimentation on an adjacent lobe.
17.8 Integration and Visualisation of
Seismic Data
In order to establish a reservoir model, different
geophysical measurements such as seismic, well
logs and rock physics need to be integrated. Since
these measurements are carried out employing
very different frequency regimes, there is a problem
of scaling. Rock physics models of laboratory
measurements use ultrasound frequencies (kHz to
MHz) whereas sonic tools (well logging) operate
with frequencies in the kHz range. Seismic data fall
within the range of hertz (Hz). Assuming the velocity
of a sedimentary layer is 3,000 m/s, a seismic
frequency of 30 Hz will give 100 m wavelength
whereas sonic log frequency of 30 kHz will give
10 cm wavelength and the ultrasound frequency of
300 kHz will give 1 cm wavelength. Therefore, the
vertical resolutions of seismic, sonic and ultrasonic
frequencies are 25 m, 2.5 cm and 2.5 mm, respectively (Fig. 17.35). The major problem is then how to
best incorporate all geophysical data into a reservoir
model. Rock physics investigations define how porescale variations in properties like mineralogy, fluid
content and grain geometry affect the acoustic
response of a core sample. As the scale of investigations increases from laboratory to well log to seismic, low frequency measurements (seismic or well
logs) average over different rock types and sediments
in addition to local pore-scale variations. As a result,
spatial heterogeneity and preferential sampling at
well log or seismic scales can cause a shift in the
rock physics relationship away from that determined
in the lab. Therefore, critical understanding and
strategies are necessary to incorporate seismic, well
logs and rock physics to find links between qualitative geological parameters and quantitative geophysical measurements. Integration strategies and
utilisation of different geophysical data are discussed
thoroughly in Chap. 18.
As the use of 3D seismic increasingly becomes an
integral part of hydrocarbon exploration, visualisation
techniques also continue to evolve as software and
hardware improves. Visualisation of target datasets
of various sizes and formats in a powerful graphical
environment is the answer to efficiently understanding
data quality problems, anomalies and trends. Such an
environment responds dynamically to manipulation
452
N.H. Mondol
ground, resulting in flooding) and regressions are not
always directly related to sea level changes. When a
delta builds out into the sea, there is a local regression
on the delta even if the sea level has not fallen. If
sedimentation is sufficiently rapid, we can have a
local regression on a delta even with a rising sea
level. Along a coastline we can in fact have
transgressions in some areas and regressions in others,
at the same time, depending on the rate of sedimentation or erosion with respect to sea level change. A
regression caused by a fall in sea level is called a
forced regression. Changes in sea level can be due to:
• Local tectonic movements, for example uplift of a
horst or subsidence of a graben structure.
• Plate-tectonic movements which can be of great
extent, but are not global.
• Sea level changes. These are global and are called
eustatic sea level changes.
During the Quaternary, cyclic changes in sea level
of up to 120 m accompanied the growth and decay of
continental ice sheets. These changes were rapid and
of large magnitude and can be traced throughout much
of the world. In the areas which had supported ice
sheets, such as Scandinavia, the melting of the ice
led to isostatic uplift due to unloading, and this
exceeded the rise in sea level so that there was a
regression. When seismic stratigraphy was established
(Vail et al. 1977), it was assumed that most of the
variations in sea level that could be interpreted from
seismic records were attributable to eustatic changes
and thus could be employed for global correlation.
However accumulation of ice on the continents is the
only known process capable of producing large and
rapid global sea level changes. Such ice ages are
known from the Quaternary, the CarboniferousPermian, late Ordovician and the end of the
Precambrian.
Nevertheless, transgressions and regressions can be
correlated over greater or lesser distances, depending
on the type of tectonic displacement. It is often difficult to distinguish between eustatic sea level changes
and ones caused by more local or regional conditions
and we therefore prefer to use the term relative sea
level change. On a delta, one delta lobe may prograde
and produce a regressive sequence while a transgression occurs with carbonate sedimentation on an adjacent lobe.
17.8 Integration and Visualisation of
Seismic Data
In order to establish a reservoir model, different
geophysical measurements such as seismic, well
logs and rock physics need to be integrated. Since
these measurements are carried out employing
very different frequency regimes, there is a problem
of scaling. Rock physics models of laboratory
measurements use ultrasound frequencies (kHz to
MHz) whereas sonic tools (well logging) operate
with frequencies in the kHz range. Seismic data fall
within the range of hertz (Hz). Assuming the velocity
of a sedimentary layer is 3,000 m/s, a seismic
frequency of 30 Hz will give 100 m wavelength
whereas sonic log frequency of 30 kHz will give
10 cm wavelength and the ultrasound frequency of
300 kHz will give 1 cm wavelength. Therefore, the
vertical resolutions of seismic, sonic and ultrasonic
frequencies are 25 m, 2.5 cm and 2.5 mm, respectively (Fig. 17.35). The major problem is then how to
best incorporate all geophysical data into a reservoir
model. Rock physics investigations define how porescale variations in properties like mineralogy, fluid
content and grain geometry affect the acoustic
response of a core sample. As the scale of investigations increases from laboratory to well log to seismic, low frequency measurements (seismic or well
logs) average over different rock types and sediments
in addition to local pore-scale variations. As a result,
spatial heterogeneity and preferential sampling at
well log or seismic scales can cause a shift in the
rock physics relationship away from that determined
in the lab. Therefore, critical understanding and
strategies are necessary to incorporate seismic, well
logs and rock physics to find links between qualitative geological parameters and quantitative geophysical measurements. Integration strategies and
utilisation of different geophysical data are discussed
thoroughly in Chap. 18.
As the use of 3D seismic increasingly becomes an
integral part of hydrocarbon exploration, visualisation
techniques also continue to evolve as software and
hardware improves. Visualisation of target datasets
of various sizes and formats in a powerful graphical
environment is the answer to efficiently understanding
data quality problems, anomalies and trends. Such an
environment responds dynamically to manipulation
452
N.H. Mondol