because warm surface water is less dense than the
colder bottom water. Black mud deposited at the bottom of lakes may produce good source rocks. In cold
climates, however, the water in the lakes overturns in
the winter because the maximum water density is at
4
C, preventing the stable stratification required to
form source rocks.
1.4
Formation of Source Rocks
All marine organic material is formed near the surface
of the ocean, in the photic zone, through photosynthesis. For the most part this is algae. Some phytoplankton are broken down chemically and oxidised and
some are eaten by zooplankton. Both types of plankton
are eaten by higher organisms which concentrate the
indigestible part of the organic matter into fecal pellets
which may be incorporated into sediments. Plankton is
made up of very small organisms which sink so slowly
that they are in most cases almost entirely degraded
(oxidised) before they reach the bottom. Pellets, on the
other hand, are the size of sand grains and sink more
rapidly, and this organic matter is more likely to be
preserved in the sediments.
On the bottom, organic matter will be subjected to
breakdown by micro-organisms (bacteria). It will also
be eaten by burrowing organisms which live in the top
portion of the sediments. The activity of these
organisms contributes to reducing the organic content
of the sediments because most of the organic matter is
digested when the sediment is eaten. Bioturbation also
stirs up the sediments, exposing them more to the
oxygen-bearing bottom water. However, if the bottom
water is stagnant, the lack of oxygen and the toxicity
of H 2 S will exclude most life forms. The resultant lack
of bioturbation will thus preserve more organic matter
in the sediment together with perfect, undisturbed,
lamination. Stagnant, or anoxic, conditions are defined
by an oxygen content of <0.5 ml/l water. Sulphatereducing bacteria, however, can use a good deal of
organic matter and precipitate sulphides (e.g. FeS 2 ). If
the sediments contain insufficient soluble iron or other
metals which could precipitate sulphides, more sulphur will be incorporated in the organic matter and
will eventually be enriched in the oil derived from
such source beds.
Except where the water is completely stagnant,
slow sedimentation rates will result in each sediment
layer spending longer in the bioturbation and
microbiological breakdown zones, and consequently
less organic matter will be preserved in the sediment.
Rapid sedimentation leads to more of the deposited
organic matter being preserved but from the outset it
will be highly diluted with mineral grains. Consequently an intermediate sedimentation rate in relation
to organic production (10–100 mm/1,000 years)
results in the best source rocks.
As we have seen, the net accumulation of organic
matter in sediments is not so much a function of the
total productivity, but rather of the relationship
between productivity and biogenic breakdown and
oxidation. In areas with powerful traction currents,
most organic matter will be oxidised. An important
source of oxygen-rich water in the deep ocean is the
cold surface water which sinks to the bottom of the
ocean in polar regions and flows along the ocean floor
towards equatorial regions. This flow balances the
surface flow to higher latitudes like the Gulf Stream
in the Atlantic.
These bottom flows are of considerable magnitude
during glacial periods, when large amounts of cold
water are sinking near ice sheet peripheries. In warm
periods, for example during the Cretaceous, the poles
were probably ice-free and there was much less cold
surface water available to sink down and drive the
ocean conveyor system. The deeper parts of the Atlantic experienced stagnant bottom conditions during
such periods.
Limited water circulation in semi-enclosed marine
basins due to restricted outflow over a shallow threshold is a common cause of stagnant water bodies
(Fig. 1.2a). The Black Sea is a good example. In
response to an abundant freshwater supply from rivers
and a relatively low evaporation rate, a low salinity
surface layer leads to density stratification in the water
column and a consequent reduction in circulation. In
basins with little precipitation and where there is net
evaporation, the surface water will have higher salinity
and density than the water below it, and will sink
down. This circulation brings with it oxygen from
the surface and can give oxidising bottom conditions
with little chance for organic matter to survive to form
source rocks.
Lakes or semi-enclosed marine basins often have a
temperature- or salinity-induced density stratification
so that oxygenated surface water does not mix with
water in the deeper part of the basin. This leads to
1 Introduction to Petroleum Geology
9
colder bottom water. Black mud deposited at the bottom of lakes may produce good source rocks. In cold
climates, however, the water in the lakes overturns in
the winter because the maximum water density is at
4
C, preventing the stable stratification required to
form source rocks.
1.4
Formation of Source Rocks
All marine organic material is formed near the surface
of the ocean, in the photic zone, through photosynthesis. For the most part this is algae. Some phytoplankton are broken down chemically and oxidised and
some are eaten by zooplankton. Both types of plankton
are eaten by higher organisms which concentrate the
indigestible part of the organic matter into fecal pellets
which may be incorporated into sediments. Plankton is
made up of very small organisms which sink so slowly
that they are in most cases almost entirely degraded
(oxidised) before they reach the bottom. Pellets, on the
other hand, are the size of sand grains and sink more
rapidly, and this organic matter is more likely to be
preserved in the sediments.
On the bottom, organic matter will be subjected to
breakdown by micro-organisms (bacteria). It will also
be eaten by burrowing organisms which live in the top
portion of the sediments. The activity of these
organisms contributes to reducing the organic content
of the sediments because most of the organic matter is
digested when the sediment is eaten. Bioturbation also
stirs up the sediments, exposing them more to the
oxygen-bearing bottom water. However, if the bottom
water is stagnant, the lack of oxygen and the toxicity
of H 2 S will exclude most life forms. The resultant lack
of bioturbation will thus preserve more organic matter
in the sediment together with perfect, undisturbed,
lamination. Stagnant, or anoxic, conditions are defined
by an oxygen content of <0.5 ml/l water. Sulphatereducing bacteria, however, can use a good deal of
organic matter and precipitate sulphides (e.g. FeS 2 ). If
the sediments contain insufficient soluble iron or other
metals which could precipitate sulphides, more sulphur will be incorporated in the organic matter and
will eventually be enriched in the oil derived from
such source beds.
Except where the water is completely stagnant,
slow sedimentation rates will result in each sediment
layer spending longer in the bioturbation and
microbiological breakdown zones, and consequently
less organic matter will be preserved in the sediment.
Rapid sedimentation leads to more of the deposited
organic matter being preserved but from the outset it
will be highly diluted with mineral grains. Consequently an intermediate sedimentation rate in relation
to organic production (10–100 mm/1,000 years)
results in the best source rocks.
As we have seen, the net accumulation of organic
matter in sediments is not so much a function of the
total productivity, but rather of the relationship
between productivity and biogenic breakdown and
oxidation. In areas with powerful traction currents,
most organic matter will be oxidised. An important
source of oxygen-rich water in the deep ocean is the
cold surface water which sinks to the bottom of the
ocean in polar regions and flows along the ocean floor
towards equatorial regions. This flow balances the
surface flow to higher latitudes like the Gulf Stream
in the Atlantic.
These bottom flows are of considerable magnitude
during glacial periods, when large amounts of cold
water are sinking near ice sheet peripheries. In warm
periods, for example during the Cretaceous, the poles
were probably ice-free and there was much less cold
surface water available to sink down and drive the
ocean conveyor system. The deeper parts of the Atlantic experienced stagnant bottom conditions during
such periods.
Limited water circulation in semi-enclosed marine
basins due to restricted outflow over a shallow threshold is a common cause of stagnant water bodies
(Fig. 1.2a). The Black Sea is a good example. In
response to an abundant freshwater supply from rivers
and a relatively low evaporation rate, a low salinity
surface layer leads to density stratification in the water
column and a consequent reduction in circulation. In
basins with little precipitation and where there is net
evaporation, the surface water will have higher salinity
and density than the water below it, and will sink
down. This circulation brings with it oxygen from
the surface and can give oxidising bottom conditions
with little chance for organic matter to survive to form
source rocks.
Lakes or semi-enclosed marine basins often have a
temperature- or salinity-induced density stratification
so that oxygenated surface water does not mix with
water in the deeper part of the basin. This leads to
1 Introduction to Petroleum Geology
9