STRIPE SLIP FAULT IN SOUTHERN NORTH SEA.
The objective of the practical is to study the changes in the in the structural style across a
southern North Sea strike slip fault system by interpreting seismic sections from 5 dip
lines which runs NE-SW(Dip lines F,G,H,I.and N) Lines N and G are about 7km apart . A
strike line, Line A was used to correlate the horizons from a borehole, well A and between
seismic lines.
PROCEDURE AND REPORT
A synthetic seismogram from well A was used to locate the different horizons present on
the seismic sections from line A and N, which were then used to correlate between the
other seismic lines. By doing this, the different horizons were picked on all the seismic
lines and the faults which make up the strike slip fault system were identified on the all
the seismic lines, giving some insight into the deformation that has affected the different
horizons. A time structure map was later produced by contouring up the result of the
interpretation, from which the sense of motion of the strike slip fault motion of the strike
slip fault was deduced.
The colour code and the two way travel-time for the different horizons as obtained from
the correlation of the horizons from the synthetic seismogram with line N are given
below.
BASE PLEISTOCENE
TWT(s)
COLOUR
Top Pleistocene
~0.30
Blue
Top Miocene
~0.40
Yellow
Top upper Cretaceous (chalk)
~0.77
Light green
Near Top Lower Cretaceous
~1.18
Dark green
Late Cimmerian Unconformity
~1.30
Brown
Top Bunter Shale
~1.47
Pink
Top Salt (Zechstein)
~1.63
Red
Rotliegendes Sandstone
Top Carboniferous
~1.74
~ 1.85
Orange
Purple
Discussion.
From the different seismic sections interpreted it can be seen that numerous faults which
have displaced the different horizons are present within the study area. These faults form a
strike slip fault system. This can be seen from the structural characteristics of the dip
lines N and G (as seen on the seismic section that corresponds to both lines), where both
compressional and extensional fault patterns as well as flower structure usually
associated with strike slip fault zones can be seen associated.
The salt which directly overlies the Rotliegendes sandstone can be seen in some zones to
have moved upwards along some of the faults, seismic evidence suggesting these can be
seen on the seismic section from some of the dip lines where such zones (along or
adjacent to the fault) appear as zones lacking internal reflections and the pull up observed
beneath these zones.
The different structural styles above and below the salt include faulting and flexure
(“swells and lows”) of the overlying strata has been , which is as a result of salt
movement. In places where the salt has pierced through the overlying sediments, rim
synclines formed as a result of the strain imposed on the overlying strata by the upward
movement of salt, can be seen adjacent to the upward moving salt. In some parts the salt
can be seen to have slighty decoupled faults from the overburden.
A prominent unconformity (late Cimmerian Unconformity coloured brown) marked by an
erosional surface occurs in the study area. As seen on line G the Bunter shale appears to
have disappeared in the North eastern part (NE of CDP 340) , a possible reason for this is
that the Bunter shale in this region was eroded during the erosion that occurred in the late
Cimmerian ( which Marked the Cimmerian unconformity).
The dipping reflectors seen on just below the Cimmerian unconformity on line A are
clinoforms top lapping against the Cimmerian unconformity.
The study area has a complex structure as a result of the different structural characteristics
seen on the dip lines.
The earliest faults are the normal faults formed during rifting) which can be seen to have
displaced the Rotliegendes sandstone (orange Horizon) and top carboniferous (purple),
these faults were active until the deposition of the Anhydrite. The next sets of faults are
the reverse faults (which formed as a result of compressional tectonics). These faults
formed during the cretaceous and displaced all the horizons that pre-dates the chalk and
some displaced of these faults displaced the chalk as well. This was followed by reversal
of tectonic movement ( a change from compression movement to extensional movement)
that led to the formation of younger normal faults and caused some of the reverse faults to
be reactivated as normal faults in giving rise to inversion structure in the Miocene. The
complex pattern of faulting in the study area formed a strike-slip fault system in the study
area.
Geologic History
A summary of the geologic history of the area is given below:
Deposition of top carboniferous and Rotliedgendes sandstone occurred before rifting.
Rifting commenced during the deposition of salt and stopped prior to the deposition of the
anhydrite within the salt. This was later followed the deposition of the bunter shale and
some other sediments (unidentified). After this there was a period of non deposition,
during which erosion took place (within this period, the Bunter shale was completely
eroded in the eastern part), this marked the Late Cimmerian unconformity. This was then
followed by the deposition of chalk, at a time when the area was in compression and
reverse faults were formed. Reversal from compressional tectonics to extensional tectonic
resulted in formation of younger normal fault and reactivation some of the reverse faults
(causing inversion). Salt movement later occurred, during which the salt migrated
upwards along the faults.