oil shales. The release of CO 2 is therefore high also
during production. Oil shales may only contain 5–10%
organic matter and the volume of waste will then be
10–20 times the oil produced. The waste consists of
coke and also smectite formed in the heating process
and is very difficult to store. It is also rich in heavy
metals, including vanadium and uranium which are
typical of black shales.
Production of oil from oil shales requires very large
amounts of water, so in dry areas the water supply may
be a limiting factor. There are therefore considerable
environmental problems linked to the exploitation of
oil shales as a major source of oil.
Attempts has also been made to heat the shale in the
ground so that the environmental problems related to
water supply and storage of waste is reduced.
By establishing an underground barrier by freezing
the ground using refrigerated fluid a “freeze wall”
is established. The purpose is to prevent the
hydrocarbons from escaping and groundwater from
entering into the area which is heated.
Some source rocks may have enough permeability to
serve as reservoir rocks too. The Miocene Monterey
Formation in California is an organic-rich diatomaceous
source rock, which is also a reservoir rock. Oil can in this
case be produced because of tectonic fracturing which
has enhanced the permeability. Source rocks may also
be interbedded with thin sandstones or limestones and
then only a very short migration is required.
23.3 Coal Bed Methane (CBM)
Coal and kerogen rock with a high content of plant
material (Type III kerogen) are the main source rocks
for gas which will migrate to a reservoir rock or up to
the surface. Relatively large quantities of gas (methane) will, however, be retained in the coal because its
micro-porous structure provides a very large surface
area for adsorbed gas. This can be exploited by drilling
into the coal. Commercial production of methane from
coal is common in the US. Near the surface gas can
leak off from coal but where coal is buried a few
hundred metres much of the gas is still retained in
the coals and can be produced by drilling. Coal has
very low permeability and the flow of gas to the wells
depends on thin fractures (cleats) developed during
uplift. The gas produced from coal is normally very
pure methane with low sulphur content and is referred
to as sweet gas. Wyoming has very large reserves of
coal bed methane in the Powder River, Bighorn, Wind
River and Green River basins. These are Cretaceous
and Tertiary coals which have been buried more
deeply and then uplifted. Gas may be produced from
these coals down to depths of 1.5–2 km (5,000 ft). The
reserves of CBM are large in the USA (20Â10
12 m
3 )
and Canada also has major reserves.
Methane is constantly being formed by bacteria at
shallow depth and it may be argued that this is at least
to some extent a renewable resource but the rate of
accumulation is slow compared to our consumption.
Artificial growth of algae may produce some oil and
gas and consume CO 2 .
23.4 Shale Gas and Shale Oil
Organic-rich shales which have been buried to depths
where most of the oil and gas has been generated and
expelled may nevertheless contain considerable
amounts of gas. The gas remaining in these shales is
present in very small pores and may also be partly
adsorbed on remaining organic matter or its residue
(coke) and on clay minerals. The shales have been
uplifted and may therefore have small extensional
fractures, but they must be hydrofractured by water
injection to increase the permeability.
Barnett Shale is tight shale of Mississippian age in
Texas, containing at least 2.5 trillion cubic feet of gas.
It is referred to as a tight gas reservoir. Much of the gas
is in urban areas such as the Dallas-Forth Worth area.
The permeability of the shale matrix is generally
very low but there may be thin silty layers and also
fractures that increase the effective permeability.
Hydraulic fracturing can be carried out to further
increase the permeability, and horizontal drilling also
helps to produce more gas. The Woodford Shale
(Devonian) in Oklahoma can be almost 100 m thick.
Devonian tight gas shales include the Middle Devonian Marcellus Shales in the Appalachians. These are
now mostly at 1–2 km depth but have previously been
buried much deeper (5–6 km or more).
Shale gas is estimated to account for 50% of the gas
produced in North America by 2020.
Higher gas prices will also result in increased interest in shale gas in other parts of the world.
23 Unconventional Hydrocarbons: Oil Shales, Heavy Oil. . .
587
during production. Oil shales may only contain 5–10%
organic matter and the volume of waste will then be
10–20 times the oil produced. The waste consists of
coke and also smectite formed in the heating process
and is very difficult to store. It is also rich in heavy
metals, including vanadium and uranium which are
typical of black shales.
Production of oil from oil shales requires very large
amounts of water, so in dry areas the water supply may
be a limiting factor. There are therefore considerable
environmental problems linked to the exploitation of
oil shales as a major source of oil.
Attempts has also been made to heat the shale in the
ground so that the environmental problems related to
water supply and storage of waste is reduced.
By establishing an underground barrier by freezing
the ground using refrigerated fluid a “freeze wall”
is established. The purpose is to prevent the
hydrocarbons from escaping and groundwater from
entering into the area which is heated.
Some source rocks may have enough permeability to
serve as reservoir rocks too. The Miocene Monterey
Formation in California is an organic-rich diatomaceous
source rock, which is also a reservoir rock. Oil can in this
case be produced because of tectonic fracturing which
has enhanced the permeability. Source rocks may also
be interbedded with thin sandstones or limestones and
then only a very short migration is required.
23.3 Coal Bed Methane (CBM)
Coal and kerogen rock with a high content of plant
material (Type III kerogen) are the main source rocks
for gas which will migrate to a reservoir rock or up to
the surface. Relatively large quantities of gas (methane) will, however, be retained in the coal because its
micro-porous structure provides a very large surface
area for adsorbed gas. This can be exploited by drilling
into the coal. Commercial production of methane from
coal is common in the US. Near the surface gas can
leak off from coal but where coal is buried a few
hundred metres much of the gas is still retained in
the coals and can be produced by drilling. Coal has
very low permeability and the flow of gas to the wells
depends on thin fractures (cleats) developed during
uplift. The gas produced from coal is normally very
pure methane with low sulphur content and is referred
to as sweet gas. Wyoming has very large reserves of
coal bed methane in the Powder River, Bighorn, Wind
River and Green River basins. These are Cretaceous
and Tertiary coals which have been buried more
deeply and then uplifted. Gas may be produced from
these coals down to depths of 1.5–2 km (5,000 ft). The
reserves of CBM are large in the USA (20Â10
12 m
3 )
and Canada also has major reserves.
Methane is constantly being formed by bacteria at
shallow depth and it may be argued that this is at least
to some extent a renewable resource but the rate of
accumulation is slow compared to our consumption.
Artificial growth of algae may produce some oil and
gas and consume CO 2 .
23.4 Shale Gas and Shale Oil
Organic-rich shales which have been buried to depths
where most of the oil and gas has been generated and
expelled may nevertheless contain considerable
amounts of gas. The gas remaining in these shales is
present in very small pores and may also be partly
adsorbed on remaining organic matter or its residue
(coke) and on clay minerals. The shales have been
uplifted and may therefore have small extensional
fractures, but they must be hydrofractured by water
injection to increase the permeability.
Barnett Shale is tight shale of Mississippian age in
Texas, containing at least 2.5 trillion cubic feet of gas.
It is referred to as a tight gas reservoir. Much of the gas
is in urban areas such as the Dallas-Forth Worth area.
The permeability of the shale matrix is generally
very low but there may be thin silty layers and also
fractures that increase the effective permeability.
Hydraulic fracturing can be carried out to further
increase the permeability, and horizontal drilling also
helps to produce more gas. The Woodford Shale
(Devonian) in Oklahoma can be almost 100 m thick.
Devonian tight gas shales include the Middle Devonian Marcellus Shales in the Appalachians. These are
now mostly at 1–2 km depth but have previously been
buried much deeper (5–6 km or more).
Shale gas is estimated to account for 50% of the gas
produced in North America by 2020.
Higher gas prices will also result in increased interest in shale gas in other parts of the world.
23 Unconventional Hydrocarbons: Oil Shales, Heavy Oil. . .
587