south England and in many parts of the North Sea
Basin.
1.8
Hydrocarbon Traps
Traps consist of porous reservoir rocks overlain by
tight (low permeability) rocks which do not allow oil
or gas to pass. These must form structures closed at the
top such that they collect oil and gas, which is lighter
than water. We can think of an oil trap as a barrel or
bucket upside down (Fig. 1.9) which can then be filled
with petroleum which rises through the water until it is
full. The point where the petroleum can leak from this
structure is called the spill point. The closure is the
maximum oil column that the structure can hold
before leaking through the spill point (Fig. 1.10).
Hydrocarbons may also leak through the cap rock.
The cap rock may not be 100% effective in
preventing the upward flow of hydrocarbons, but
these will still accumulate if the rate of leakage is
less than the rate of supply up to the trap. Cap rocks
are not totally impermeable with respect to water, but
may be impermeable to oil and gas due to capillary
resistance in the small pores.
Traps can be classified according to the type of
structure that produces them. We distinguish between:
(1) Structural traps that are formed by structural
deformation (folding, doming or faulting) of rocks.
(2) Stratigraphic traps which are related to primary
features in the sedimentary sequences and do not
require structural deformation like faulting or
folding. This may be sandstones pinching out in
shales due to primary changes in facies (Fig. 1.11).
Carbonate reefs tend to form primary structures
which function as stratigraphic traps.
There are also combinations between stratigraphic
and structural traps (Fig. 1.12).
It is important to establish when structural traps
were formed in relation to the migration of the petroleum. Structures formed after the main phase of source
rock maturation and associated migration will not be
effective traps. In some cases traps formed late can
collect gas which normally is generated and migrates
later than oil.
Stratigraphic traps, by contrast, have been there all
the time, and the timing of the migration is not so
important. They may however depend on slight tilting
of the strata involved.
Fig. 1.8 Kimmeridge Clay (Upper Jurassic) unconformably
overlain by the lower Chalk (Upper Cretaceous). This is a very
good source rock and equivalent shales are the main source
rocks for oil and also much of the gas in the North Sea basin
and also further north in mid Norway (Haltenbanken) and the
Barents Sea. A c.50 m.y. hiatus separates the Albian age Red
Chalk carbonate facies from the underling black shale facies.
From South Ferriby, Yorkshire, England
Oil/gas
Shale
Sandstone
Oil/gas
Fig. 1.9 Examples of structural traps. Simple anticlinal trap
and a fault-controlled trap
14
K. Bjørlykke
Basin.
1.8
Hydrocarbon Traps
Traps consist of porous reservoir rocks overlain by
tight (low permeability) rocks which do not allow oil
or gas to pass. These must form structures closed at the
top such that they collect oil and gas, which is lighter
than water. We can think of an oil trap as a barrel or
bucket upside down (Fig. 1.9) which can then be filled
with petroleum which rises through the water until it is
full. The point where the petroleum can leak from this
structure is called the spill point. The closure is the
maximum oil column that the structure can hold
before leaking through the spill point (Fig. 1.10).
Hydrocarbons may also leak through the cap rock.
The cap rock may not be 100% effective in
preventing the upward flow of hydrocarbons, but
these will still accumulate if the rate of leakage is
less than the rate of supply up to the trap. Cap rocks
are not totally impermeable with respect to water, but
may be impermeable to oil and gas due to capillary
resistance in the small pores.
Traps can be classified according to the type of
structure that produces them. We distinguish between:
(1) Structural traps that are formed by structural
deformation (folding, doming or faulting) of rocks.
(2) Stratigraphic traps which are related to primary
features in the sedimentary sequences and do not
require structural deformation like faulting or
folding. This may be sandstones pinching out in
shales due to primary changes in facies (Fig. 1.11).
Carbonate reefs tend to form primary structures
which function as stratigraphic traps.
There are also combinations between stratigraphic
and structural traps (Fig. 1.12).
It is important to establish when structural traps
were formed in relation to the migration of the petroleum. Structures formed after the main phase of source
rock maturation and associated migration will not be
effective traps. In some cases traps formed late can
collect gas which normally is generated and migrates
later than oil.
Stratigraphic traps, by contrast, have been there all
the time, and the timing of the migration is not so
important. They may however depend on slight tilting
of the strata involved.
Fig. 1.8 Kimmeridge Clay (Upper Jurassic) unconformably
overlain by the lower Chalk (Upper Cretaceous). This is a very
good source rock and equivalent shales are the main source
rocks for oil and also much of the gas in the North Sea basin
and also further north in mid Norway (Haltenbanken) and the
Barents Sea. A c.50 m.y. hiatus separates the Albian age Red
Chalk carbonate facies from the underling black shale facies.
From South Ferriby, Yorkshire, England
Oil/gas
Shale
Sandstone
Oil/gas
Fig. 1.9 Examples of structural traps. Simple anticlinal trap
and a fault-controlled trap
14
K. Bjørlykke