Oil shale is a source rock exposed near the surface.
If the source rock (shale) is mature it will have a
characteristic smell of hydrocarbons, but it may not
be mature so that hydrocarbons have not been
generated. If the oil shale is mature much of the oil
has escaped by primary migration. Since the
hydrocarbons are thoroughly disseminated in the
fine-grained sediment, oil cannot be produced in the
same way as from sandstone or carbonate reservoirs.
The hydrocarbons can only be obtained by breaking
and crushing the shale and heating to distill off the
interspersed hydrocarbons. Shales can however contain gas which can be produced when there is a network of small fractures. Gas shale is expected to be an
important source of petroleum in the years to come,
particularly in the US. Very large amounts of fossil
fuels are stored in organic-rich mudstones or shales
that have not been buried deeply enough for the
organic matter to be converted to petroleum. In this
case very little hydrocarbon has excaped but these
deposits must be mined and heated to 400–500
C in
ovens to generate petroleum (pyrolysis).
The Tertiary Green River Shale in Colorado, Utah
and Wyoming represents one of the largest petroleum
reservoirs in the world. This is a lake deposit, and the
organic matter consisted mainly of algae.
Although very large quantities of petroleum can be
produced from oil shale, production costs are at present too high compared to conventional oil. There are
also serious environmental problems involved in production from oil shale, and the process requires very
large quantities of water, a resource which is not
always plentiful.
The oil reserves in such deposits exceed conventional oil reserves, but the expense and environmental
issues involved with production from these types of
reservoirs clearly limit their exploitation. This is particularly true of production from oil shale.
1.2
Accumulations of Organic Matter
It is well documented that oil accumulations are of
organic origin and formed from organic matter in
sediments. Methane can be formed inorganically and
is found in the atmosphere of several other planets, but
inorganic methane from the interior of the Earth is
likely to be well dispersed and thus not form major
gas accumulations in the Earth’s crust.
The organic matter from which petroleum is
derived originated through photosynthesis, i.e. storage
of solar energy (Fig. 1.1).
Sunlight is continuously transformed into such
energy on Earth but only a very small proportion of
the solar energy is preserved as organic matter and
petroleum. The oil and gas which forms in sedimentary basins each year is thus minute in comparison
with the rate of exploitation (production) and consumption. In practice petroleum must therefore be
regarded as a non-renewable resource even though
some petroleum is being formed all the time.
Most of the organic materials which occur in source
rocks for petroleum are algae, formed by photosynthesis. The zooplankton and higher organisms that are
also represented grazed the algae and were thus indirectly dependent on photosynthesis too. The energy
which we release when burning petroleum is therefore
stored solar energy. Since petroleum is derived from
organic matter, it is important to understand how and
where sediments with a high content of organic matter
are deposited.
The total production of organic material in the
world’s oceans is now 5Â10
10 tonnes/year. Nutrients
for this organic production are supplied by erosion of
rocks on land and transported into the ocean. The
supply of nutrients is therefore greatest in coastal
areas, particularly where sediment-laden rivers discharge into the sea. Plant debris is also supplied
directly from the land in coastal areas.
Biological production is greatest in the uppermost
20–30 m of the ocean and most of the phytoplankton
growth takes place in this zone. In clear water, sunlight
penetrates much deeper than in turbid water, but in
clear water there is usually little nutrient supply. At
about 100–150 m depth, sunlight is too weak for
photosynthesis even in very clear water.
Phytoplankton provides nutrition for all other
marine life in the oceans. Zooplankton feed on phytoplankton and therefore proliferate only where there is
vigorous phytoplankton production. Organisms sink
after they have died, and may decay so that nutrients
are released and recycled at greater depths.
Basins with restricted water circulation will preserve more organic matter and produce good source
rocks which may mature to generate oil and gas
(Fig. 1.2a, b).
In polar regions, cold dense water sinks to great
depths and flows along the bottom of the deep oceans
4
K. Bjørlykke
If the source rock (shale) is mature it will have a
characteristic smell of hydrocarbons, but it may not
be mature so that hydrocarbons have not been
generated. If the oil shale is mature much of the oil
has escaped by primary migration. Since the
hydrocarbons are thoroughly disseminated in the
fine-grained sediment, oil cannot be produced in the
same way as from sandstone or carbonate reservoirs.
The hydrocarbons can only be obtained by breaking
and crushing the shale and heating to distill off the
interspersed hydrocarbons. Shales can however contain gas which can be produced when there is a network of small fractures. Gas shale is expected to be an
important source of petroleum in the years to come,
particularly in the US. Very large amounts of fossil
fuels are stored in organic-rich mudstones or shales
that have not been buried deeply enough for the
organic matter to be converted to petroleum. In this
case very little hydrocarbon has excaped but these
deposits must be mined and heated to 400–500
C in
ovens to generate petroleum (pyrolysis).
The Tertiary Green River Shale in Colorado, Utah
and Wyoming represents one of the largest petroleum
reservoirs in the world. This is a lake deposit, and the
organic matter consisted mainly of algae.
Although very large quantities of petroleum can be
produced from oil shale, production costs are at present too high compared to conventional oil. There are
also serious environmental problems involved in production from oil shale, and the process requires very
large quantities of water, a resource which is not
always plentiful.
The oil reserves in such deposits exceed conventional oil reserves, but the expense and environmental
issues involved with production from these types of
reservoirs clearly limit their exploitation. This is particularly true of production from oil shale.
1.2
Accumulations of Organic Matter
It is well documented that oil accumulations are of
organic origin and formed from organic matter in
sediments. Methane can be formed inorganically and
is found in the atmosphere of several other planets, but
inorganic methane from the interior of the Earth is
likely to be well dispersed and thus not form major
gas accumulations in the Earth’s crust.
The organic matter from which petroleum is
derived originated through photosynthesis, i.e. storage
of solar energy (Fig. 1.1).
Sunlight is continuously transformed into such
energy on Earth but only a very small proportion of
the solar energy is preserved as organic matter and
petroleum. The oil and gas which forms in sedimentary basins each year is thus minute in comparison
with the rate of exploitation (production) and consumption. In practice petroleum must therefore be
regarded as a non-renewable resource even though
some petroleum is being formed all the time.
Most of the organic materials which occur in source
rocks for petroleum are algae, formed by photosynthesis. The zooplankton and higher organisms that are
also represented grazed the algae and were thus indirectly dependent on photosynthesis too. The energy
which we release when burning petroleum is therefore
stored solar energy. Since petroleum is derived from
organic matter, it is important to understand how and
where sediments with a high content of organic matter
are deposited.
The total production of organic material in the
world’s oceans is now 5Â10
10 tonnes/year. Nutrients
for this organic production are supplied by erosion of
rocks on land and transported into the ocean. The
supply of nutrients is therefore greatest in coastal
areas, particularly where sediment-laden rivers discharge into the sea. Plant debris is also supplied
directly from the land in coastal areas.
Biological production is greatest in the uppermost
20–30 m of the ocean and most of the phytoplankton
growth takes place in this zone. In clear water, sunlight
penetrates much deeper than in turbid water, but in
clear water there is usually little nutrient supply. At
about 100–150 m depth, sunlight is too weak for
photosynthesis even in very clear water.
Phytoplankton provides nutrition for all other
marine life in the oceans. Zooplankton feed on phytoplankton and therefore proliferate only where there is
vigorous phytoplankton production. Organisms sink
after they have died, and may decay so that nutrients
are released and recycled at greater depths.
Basins with restricted water circulation will preserve more organic matter and produce good source
rocks which may mature to generate oil and gas
(Fig. 1.2a, b).
In polar regions, cold dense water sinks to great
depths and flows along the bottom of the deep oceans
4
K. Bjørlykke