Geology Reference
In-Depth Information
3 30'W
3 00'W
Scotland
Moray
Firth
900
800
700
58 00'N
200
200
400
300
200
Fig. 4.47
Time-structure map of
reflector at the base of the Lower
Cretaceous in the Moray Firth off
northeast Scotland, UK. Contour values
represent two-way travel times of
reflection event in milliseconds. (Courtesy
British Geological Survey, Edinburgh,
UK.)
Sealed outcrops
Faults
0
20 km
environments represent potential reservoir rocks; and
coastal mud and evaporite sequences represent potential
seals (Fig. 4.50); the identification of these components
in seismic sequences can thus help to focus an explo-
ration programme by identifying areas of high potential.
An example of seismic stratigraphy based on three-
dimensional data is illustrated in Plate 4.4. The seiscrop
of Plate 4.4(a) shows a meandering stream channel pre-
served in a Neogene sedimentary sequence in the Gulf
of Thailand. The channel geometry and the distinctive
lithofacies of the channel fill lead to its clear identifica-
tion as a distinctive seismic facies. Use of such seiscrops
over a wider area enables the regional mapping of a
Neogene deltaic environment (Plate 4.4(b)).
Major seismic sequences can often be correlated
across broad regions of continental margins and clearly
give evidence of being associated with major sea-level
changes. The application of seismic stratigraphy in areas
of good chronostratigraphical control has led to the
development of a model of global cycles of major sea-
level change and associated transgressive and regressive
depositional sequences throughout the Mesozoic and
Cenozoic (Payton 1977). Application of the methods of
seismic stratigraphy in offshore sedimentary basins with
Upper boundary
Erosional
Toplap
Concordant
Lower boundary
Onlap
Downlap
Concordant
Fig. 4.48
Different types of geological boundary defining seismic
sequences. (After Sheriff 1980.)
for example, parallel reflections characterize some
shallow-water shelf environments whilst the deeper-
water shelf edge and slope environments are often
marked by the development of major sigmoidal or
oblique cross-bedded units. The ability to identify par-
ticular sedimentary environments and predict lithofacies
from analysis of seismic sections can be of great value to
exploration programmes, providing a pointer to the
location of potential source, reservoir and/or seal rocks.
Thus, organic-rich basinal muds represent potential
source rocks; discrete sand bodies developed in shelf
