Fractures, faults and earthquakes - Introducing Tectonics, Rock Structures and Mountain Belts - page 35

would have resulted if no bending
had taken place is smoothed out.
In some cases, normal faults, as
they descend into lower levels of the
crust, may become curved; such faults
are termed listric faults and may even
become sub-horizontal, allowing the
fault blocks above them to rotate and
slide over the basal fault, which acts as a
detachment horizon (Figure 5.4C). This
allows much larger extensions to take
place. Figure 5.4D shows how stretch-
ing of the crust by means of a low-angle
normal fault may ultimately lead to the
rise of the mantle to the surface and the
creation of an ocean basin. As the struc-
ture evolves, the original detachment
fault becomes inoperative in the foot-
wall and further extension takes place
along steeper 'successor' normal faults.

A

hangingwall s ynform

5

tilted block

34 35

B C

Extensional faulting in petroleum
exploration
Many of the world's oil and gas fields
are hosted in structural traps created
by extensional faulting (e.g. Figure
5.5B), and much of the present-day
petroleum exploration has focused
on the mapping by remote means
of under-sea extensional structures.
Such structures are typical of exten-
sional basins and passive continental
margins (e.g. the North Sea and the
Atlantic margin west of Shetland,
where exploration activity in the
UK is currently concentrated). Our
detailed knowledge of extensional
faulting is largely due to interpreta-
tive seismic sections produced in the
course of petroleum exploration.
Figure 5.5A shows a typical example
from the western North Sea basin of the
structural and stratigraphic information
that can be obtained by this method.

petroleum
deposit

petroleum
deposit

impermeable
layers

cap
rock

cap
rocks

reservoir
rock

reservoir
rock

unconformity

source
rock

source
rock

f ault

Figure 5.5 A. Seismic section from the inner Moray Firth: an example to demonstrate the use of
reflection seismic prospecting to investigate submarine structure. The reflecting horizons (shown
in pink) are identified from borehole and other geological data and show an extensional fault array
affecting Jurassic strata (coloured in blue) overlain unconformably by basal Upper Cretaceous
(green); the faults were initiated during the Jurassic but activity continued into the Cretaceous; note
that the extension has resulted in the rotation and development of a synform in the hangingwall
and that the basal Cretaceous strata onlap the pre-Cretaceous surface, indicating a contemporary
slope up to the right. The vertical scale is in two-way travel time (TWT) for the seismic waves. Image
courtesy of Virtual Seismic Atlas, Rob Butler. B, C. Types of structural petroleum traps

The North Sea basin is dominated by
a series of grabens developed during
the Triassic to Jurassic period and the
petroleum deposits, which mostly
originate in Jurassic shales, are trapped
in the overlying sandstone formations

and capped by impervious rocks such
as the Upper Cretaceous Chalk Forma-
tion (Figure 5.5B, C). Determining the
detailed geometry of the structural
traps is therefore critical in finding
and exploiting petroleum reserves.