The occurrence of gas in onshore shales has been
well known in many European countries but very little
has been produced so far.
In the UK Lower Carboniferous shales and coal
were buried deeply in front of the Variscian metamorphic front in the south of England and adjacent parts of
the North Sea basin. The Carboniferous Bowland
Shale in NW England has been regarded as a potential
target for shale gas production in Britain. Also in parts
of Northern England and Scotland have Upper
Palaeozoic sediments been buried deep enough to
generate significant gas. At 4–5 km burial depth gas
is generated from primary oil. The shales are then so
hard and indurated that they can sustaina fracture
network produced by natural fracturing during uplift
and during fracking.
In the Liassic and Kimmeridge clays (shales) there
are potential source rocks for both thermal gas and
biogenic gas. They are immature in many areas and
may not be hard enough for effective fracturing. When
developed as carbonate-cemented shales they become
brittle but the fractures may then become cemented up.
The total reserves of shale gas is very difficult to
estimate due to uncertainty about gas prices and the
energy policy of each country.
There are considerable reserves of shale gas which
can be produced in Europe. Most countries have however introduced strong restrictions on the development
of shale gas. In many countries such as Germany and
France many of the potential areas for such production
are rather densely populated and there are strong environmental concerns about pollution of groundwater
etc. In Germany, Poland and France there are shales
of Jurassic, Permian and Carboniferous age which
could be produced, but it is uncertain to what extent
they will be produced. Much of these resources are in
densely populated areas with strict environmental
restrictions, and France has banned fracking. Also in
Spain, Romania and Bulgaria there are large reserves
in terms of shale gas.
Shale gas represents very large reserves of
hydrocarbons which can be used directly as gas, but
can also be converted to diesel fuels for cars and
trucks. This gas can also be mixed with heavy oil
and tar sand to make regular petrol. Production of
gas from shales requires much water for fracturing,
and produced water may also create environmental
problems.
Shale Oil is produced from source rocks, mostly
shales, which have not been buried as deeply as those
with mostly gas and have only reached the oil window.
Oil is then produced by fracturing in the same way as
for gas. Shale oil may be found in the same formations
and sedimentary basins as shale gas but where the
maximum burial depth (temperature) has been lower.
Until about 10 years ago it was not considered
possible to produce oils from shales. In recent years
oil production from shales has increased rapidly particularly in the US, increasing their domestic production and reducing their import.
The fact that it is now possible to produce oil from
shale has changed petroleum geology and the distribution of reserves. Conventional oil is limited to
reservoirs rocks with sufficient quality and a structural
trap. Shale oil may be found in very extensive layers of
shale which have had a maximum burial temperature
corresponding to the oil window.
The late Cretaceous Eagle Ford Formation of
South Texas has since 2008 developed into a major
oil producer. This is a 100 m thick transgressive
organic-rich black shale with some carbonate layers
and siltstones. This makes it a good producer in many
areas. The Eagle Ford shale was deposited on top of
the Woodbine sandstone and overlain by the Austin
Chalk.
The Bakken Formation (late Devonian) in the
Willistone basin is found subcropping in much of
North Dakota, Montana and Saskatchewan but is not
exposed. It is black shale deposited as organic-rich
mud with layers of dolomite. The average porosity is
about 5% and the permeability is very low (0.04 mD)
but due to new fracturing technology production rates
have increased very significantly and the reserves have
recently been estimated to about 24 billion bbl.
23.5 Gas Hydrates (Clathrates)
Gas hydrates are crystalline solids almost like ice,
consisting of gas (mostly methane) surrounded by
water. Most common is methane clathrate which is
water molecules bonded by hydrogen and with gas
588
K. Bjørlykke
well known in many European countries but very little
has been produced so far.
In the UK Lower Carboniferous shales and coal
were buried deeply in front of the Variscian metamorphic front in the south of England and adjacent parts of
the North Sea basin. The Carboniferous Bowland
Shale in NW England has been regarded as a potential
target for shale gas production in Britain. Also in parts
of Northern England and Scotland have Upper
Palaeozoic sediments been buried deep enough to
generate significant gas. At 4–5 km burial depth gas
is generated from primary oil. The shales are then so
hard and indurated that they can sustaina fracture
network produced by natural fracturing during uplift
and during fracking.
In the Liassic and Kimmeridge clays (shales) there
are potential source rocks for both thermal gas and
biogenic gas. They are immature in many areas and
may not be hard enough for effective fracturing. When
developed as carbonate-cemented shales they become
brittle but the fractures may then become cemented up.
The total reserves of shale gas is very difficult to
estimate due to uncertainty about gas prices and the
energy policy of each country.
There are considerable reserves of shale gas which
can be produced in Europe. Most countries have however introduced strong restrictions on the development
of shale gas. In many countries such as Germany and
France many of the potential areas for such production
are rather densely populated and there are strong environmental concerns about pollution of groundwater
etc. In Germany, Poland and France there are shales
of Jurassic, Permian and Carboniferous age which
could be produced, but it is uncertain to what extent
they will be produced. Much of these resources are in
densely populated areas with strict environmental
restrictions, and France has banned fracking. Also in
Spain, Romania and Bulgaria there are large reserves
in terms of shale gas.
Shale gas represents very large reserves of
hydrocarbons which can be used directly as gas, but
can also be converted to diesel fuels for cars and
trucks. This gas can also be mixed with heavy oil
and tar sand to make regular petrol. Production of
gas from shales requires much water for fracturing,
and produced water may also create environmental
problems.
Shale Oil is produced from source rocks, mostly
shales, which have not been buried as deeply as those
with mostly gas and have only reached the oil window.
Oil is then produced by fracturing in the same way as
for gas. Shale oil may be found in the same formations
and sedimentary basins as shale gas but where the
maximum burial depth (temperature) has been lower.
Until about 10 years ago it was not considered
possible to produce oils from shales. In recent years
oil production from shales has increased rapidly particularly in the US, increasing their domestic production and reducing their import.
The fact that it is now possible to produce oil from
shale has changed petroleum geology and the distribution of reserves. Conventional oil is limited to
reservoirs rocks with sufficient quality and a structural
trap. Shale oil may be found in very extensive layers of
shale which have had a maximum burial temperature
corresponding to the oil window.
The late Cretaceous Eagle Ford Formation of
South Texas has since 2008 developed into a major
oil producer. This is a 100 m thick transgressive
organic-rich black shale with some carbonate layers
and siltstones. This makes it a good producer in many
areas. The Eagle Ford shale was deposited on top of
the Woodbine sandstone and overlain by the Austin
Chalk.
The Bakken Formation (late Devonian) in the
Willistone basin is found subcropping in much of
North Dakota, Montana and Saskatchewan but is not
exposed. It is black shale deposited as organic-rich
mud with layers of dolomite. The average porosity is
about 5% and the permeability is very low (0.04 mD)
but due to new fracturing technology production rates
have increased very significantly and the reserves have
recently been estimated to about 24 billion bbl.
23.5 Gas Hydrates (Clathrates)
Gas hydrates are crystalline solids almost like ice,
consisting of gas (mostly methane) surrounded by
water. Most common is methane clathrate which is
water molecules bonded by hydrogen and with gas
588
K. Bjørlykke