Geology Reference
In-Depth Information
Fig. 4.61
Interpreted seismic section across the NorthViking gas field, North Sea. (Courtesy Conoco UK Ltd.)
fault block constitute the main reservoirs, the oil and
gas being trapped beneath a capping of uncon-
formably overlying shales of
continental crust typically has a rather transparent seis-
mic character. Within this, localized bands of dipping
reflectors, interpreted as fault zones, pass down into the
lower crust (see e.g. Barazangi & Brown (1986) and the
special issue of
Tectonophysics
173
(1990) for a wide range
of relevant papers). By contrast, the lower crust is often
found to be highly reflective with discontinuous hori-
zontal or gently dipping events giving an overall layered
appearance (Fig. 4.63).The origin of this layering is un-
certain, but the main possibilities appear to be primary
igneous layering, horizontal shear zones and zones of
fluid concentration (e.g. Klemperer
et al
. 1987). All may
contribute in some measure to the observed reflectivity.
Where refraction and reflection data both exist, the base
of the zone of reflectivity is found to coincide with the
Mohorovicic discontinuity as defined by refraction in-
terpretation of head wave arrivals from the uppermost
mantle (Barton 1986).
The use of reflection seismics for high-resolution
studies of shallow geology is a field of growing im-
portance in which developments are linked directly
to recent technical advances. Highly portable digital
multichannel data acquisition systems, backed up by
PC-based processing packages, make it possible to
Late Jurassic and
Cretaceous age.
Reflection profiling at crustal and lithospheric scale is
now being carried out by many developed countries.
Following on from the extensive use of multichannel
reflection profiling to investigate the crustal structure of
oceanic areas, national programmes such as the US
COCORP project (Consortium for Continental Re-
flection Profiling; Brewer & Oliver 1980) and the British
BIRPS project (British Institutions Reflection Profiling
Syndicate; Brewer 1983) have produced seismic sections
through the entire continental crust and the uppermost
part of the underlying mantle. These national pro-
grammes utilize essentially the same data acquisition
systems and processing techniques as the oil industry,
whilst increasing the size of source arrays and detector
spread lengths; recording times of 15 s are commonly
employed, as compared with a standard oil industry
recording time of about 4 s. A typical BIRPS section is
illustrated in Fig. 4.63.
Crustal reflection profiling results from several differ-
ent continental areas reveal that the upper part of the
