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Coal is the world’s most abundant fossil fuel by far. While accessible
stocks of gas and oil will last 50 years or so at the rates we now exploit
them, there is enough coal available for three centuries at least. But that
doesn’t mean that we have nothing to worry about.

Stocks of raw materials are assessed in two ways: as resources and as
reserves. Broadly, resources are defined as the amount of raw material known
to exist; reserves are the fraction of those resources that can be exploited
with existing technology. Most definitions of reserves also include an economic
factor, limiting them to deposits that are worth exploiting.

The world has coal resources equivalent to at least 10 000 billion tonnes
of oil, the traditional unit for comparing stocks of fossil fuels. This
would last a few millennia if it became exploitable. More deposits are being
discovered as the coal industry develops in Southeast Asia, South America
and especially China, which, together with Russia, Ukraine and North America,
have the world’s richest coal deposits.

British Coal puts Britain’s coal resources at 130 billion tonnes. This
amount, based on an update of surveys of mines made at the turn of the century,
includes all coal seams more than 60 centimetres thick and lying less than
1200 metres underground. It is a conservative estimate; the coalfields certainly
extend to greater depths.

Oil and gas resources are less accessible and assessments of them more
uncertain. The broad picture is clear, however: they are overwhelmingly
concentrated in the Middle East, but reserves may grow in the former Soviet
Union, the US and Southeast Asia, as more deposits are discovered.

The world’s coal reserves are equivalent to just 694 billion tonnes
of oil, less than one-tenth of coal resources. North America, the former
Soviet Union and Asia, including Australasia, each have reserves of more
than 200 billion tonnes of oil, while the bulk of the rest is in Europe
and Africa. Britain’s reserves are small: estimates vary, from more than
2 billion tonnes to less than 1 billion tonnes.

Oil and gas reserves worldwide amount to 135 and 112 billion tonnes
respectively – together, less than one-third of those of coal. The Middle
East has most of the oil (89 billion tonnes), followed a long way behind
by South America (17 billion tonnes). The Middle East also has plenty of
gas (34 billion tonnes), although not as much as the former Soviet Union
(45 billion tonnes). Nearly all of Britain’s oil and gas supplies come from
the North Sea: reserves of oil total 500 million tonnes, while those of
gas amount to 450 million tonnes.

GAS UNLOCKED

Natural gas reserves will be augmented by the development of novel gas
sources. Enormous volumes of natural gas are held as gas hydrates within
the crystal lattice of ice in sea-floor sediments in areas such as the Arctic
Ocean and the Gulf of Mexico. At present, however, gas hydrates are a menace
to drillers; when a drill bit cuts through them, the change in pressure
can release the gas abruptly in an explosion.

One unconventional gas source is already paying dividends. Many coalfields
around the world are plagued by the problem of gas in the seams, which is
released – sometimes explosively – during mining. Researchers in the US
and Australia are developing ways of draining the gas from the seams before
mining. This technology is now essential if Australia is to develop its
underground coal resources, according to Basil Beamish, director of the
Coalseam Gas Research Institute at the James Cook University of North Queensland
in Townsville. ‘We have no choice – if we want the coal, we have to drain
the gas.’

Coal-seam gas could double Australia’s gas reserves: the Australian
Gas Association estimates that it could yield the equivalent of 2 billion
tonnes of oil. ‘Once we began to find huge amounts of gas in the Bowen Basin
(in Queensland and New South Wales), we realised that perhaps it could be
a resource in its own right,’ says Beamish. ‘But it’s still early days.’

How long world reserves will last depends on how quickly they are depleted.
Based on the amount used in 1991, we have enough oil for 43 years, enough
gas for 61 years and enough coal for more than 300 years. Regionally, however,
there is great disparity. The Middle East has more than 100 years of oil
left at its current rate of exploitation, whereas North America has only
10 years and Britain 5 years. The main producers of gas in the Middle East
– Iran, Iraq, Qatar and Saudi Arabia – each have a supply of more than 100
years, but Britain’s is likely to last only 10 years. Russia and Ukraine
have more than 500 years of coal, but Western Europe has around 30 years.
In Britain, the dramatic fall in the demand for coal – from 60 million tonnes
per year in 1991 to an expected 20 million tonnes next year if the ‘dash
for gas’ persists – may extend the life of the reserves from 33 to 100 years,
but only if it remains feasible to exploit those reserves from ‘mothballed’
mines.

However, these figures do not take account of unexplored deposits, particularly
those of gas. Gas prices are low, so producers are reluctant to put money
into exploring and developing new fields, which are likely to be small and
increasingly remote from the existing network of pipelines and terminals.
Only when people are prepared to pay more for gas will there be an increase
in investment in exploration, production and especially transport infrastructure.

It is also a lean time for the oil business, with prices relatively
low. But oil is in more demand than gas, and the big international companies
are anxious to ensure that they control a share of the market in the long
term. This means finding big reservoirs in new oilfields, to supply the
US and Western Europe.

FROM SURPLUS TO DEFICIT?

Given the uneven distribution of these natural resources around the
world, how fast a country consumes its reserves will determine how much
it can export, what it needs to import and when. At present, the US is the
world’s greatest consumer of energy, though mostly from its own reserves.
Oil is the exception. In 1991, the US produced more than 500 million tonnes
of oil, but consumed almost 800 million tonnes. Though it also consumed
more gas than it produced – 507 million tonnes against 455 million tonnes
– the deficit was small. The country had a surplus of coal, producing 539
million tonnes and consuming 477 million tonnes.

Europe too will have to face a future of importing its fuels. In 1991,
Western Europe used three times as much oil as it produced, with Germany,
France and Italy accounting for nearly 1 billion tonnes each of the total
6 billion tonnes consumed. Britain used about the same amount, but from
its own reserves.

Oil supplies will continue to come from the Middle East and the former
Soviet Union, but Southeast Asia, with its growing reserves, may also contribute.
Indonesia, for example, produced twice as much oil as it consumed in 1991.
Currently, South America is exporting increasing amounts of oil to the US,
Europe and Japan.

Most of the exploration for coal worldwide is a search for the massive
Cretaceous and Tertiary deposits of brown coal. Compared with the black
coal deposits of the older Carboniferous period, brown coals are relatively
easy to mine and, despite their lower calorific values, are prized in the
international market for their low levels of sulphur . The exploration
strategy is to identify sedimentary rocks that formed in conditions where
peat could accumulate steadily for many millennia in a suitable climate.
Through a chance combination of climate and shifting of the continents over
the past 150 million years, these conditions have developed in Southeast
Asia, China in particular, North America, the former Soviet Union and Australia.
And in many of these areas, the coals have not been deeply buried, nor have
they been strongly folded and faulted, for example by mountain ranges forming
nearby. The result is thick seams of coal close enough to the surface to
be mined as opencast pits – by far the most efficient and cheapest method
of recovering coal. Even where coals have to be mined underground, continuous
seams are easier and cheaper to work than folded, faulted seams. The older
coals, such as Britain’s Carboniferous seams, are not only buried deeper,
but are also extensively folded and faulted, making mining costly and unpredictable.

Opencast mines exist in Britain, but their output is falling. The reason
for this decline is not a lack of suitable sites – there are proven reserves
of 300 million tonnes, and 25 million tonnes more are found each year. The
problem is growing opposition from people who do not want opencast site
nearby.

The pressure on land is increasing, so the future for Britain’s coal
industry will lie underground. Barry Whittaker, professor of mining in the
Department of Mining and Mineral Exploration at the University of Leeds,
is optimistic about future prospects for deep-mined coal. ‘The trouble with
a lot of existing mines is that they were constructed at a time when technology
was rather primitive, and miners have had to adapt them since,’ he says.
‘They are not geared up like modern mines to producing coal like a factory.
In new coalfields, new mines with modern technology could be even more efficient,
make profits and maybe even compete with coal from overseas.’

Environmental pressures are already changing the value of coal. In the
US, the choice is between black coals that have high calorific values but
also high levels of sulphur and are found in the relatively thin seams
of the Appalachian coalfield, and the younger coals found farther west that
have lower calorific values but contain less sulphur and are found in relatively
thick seams. The demands of coal users for low-ash, low-sulphur coal for
power stations have already made a difference, according to Nick Fedorko,
head of coal projects at the West Virginia Geological and Economic Survey:
‘There’s been pressure on the electric power industry to find the cleanest
fuel. A lot of mining and production has moved to the western coalfields,
where the coal is lower rank, but also lower sulphur. It’s even making inroads
into traditional markets here in the east.’ To compensate, the coal producers
of the eastern US are eyeing likely export markets, including Europe.

Export competition is already hotting up in the Pacific, where Japan
wants to import top-rank, low-ash coals to fuel its metals industry, and
Australia, Indonesia and a growing host of nearby coal producers want to
supply it. As heavy industries such as steel-making grow in countries such
as North Korea, these industrialising nations are moving into the market
for coal that only a few countries can produce and export competitively.

Australia’s need to keep its share of the export market is a factor
pushing its coal industry in a direction different from that taken in the
US, according to Ian Smith, of the CSIRO Coal Division in North Ryde, New
South Wales. Traditionally, Australia’s coal has come primarily from very
profitable opencast pits. Smith says this balance is already shifting. ‘The
industry is moving more to underground mining to get the high quality hard
(black) coals that we need for export.’

* * *

So where do fossil fuels come from?

Most coal is fossil peat, the remains of land plants that grew in many
different types of swamps and wetlands, which are collectively called mires.
For peat to form, the plants must provide a generous supply of decaying
debris and the mire must stay waterlogged – if it dries out, the plants
die and their carbon-rich components disperse. These special conditions
occur in only a few places worldwide. In the present climate, the thickest
peat mires are in regions such as Indonesia, where the climate favours warm,
soggy coastal plains.

Mires have to be thick if their peat is to become coal. It takes between
4000 and 100 000 years for 1 metre of peat to accumulate, and each metre
of coal comes from about 10 metres of peat. But huge layers of peat can
build up if the mire is subsiding at about the same rate as the vegetation
is accumulating, so that the surface of the mire stays wet, but not completely
submerged. This can happen in large deltas, where the weight of sediment
dumped by the river leads to a steady lowering of the land surface. Larger-scale
subsidence, such as that occurring ahead of growing mountain belts, can
lead to peat accumulating for millions of years. The local environment in
which the peat forms plays the biggest role in controlling how thick and
continuous the seams are.

Coal seams in Carboniferous rocks, which are around 300 million years
old, tend to be less than 5 metres thick because glaciation in Carboniferous
times prevented peat mires from building up. ‘The sea level was going up
and down like a yo-yo,’ says Peter McCabe, a coal geologist with the US
Geological Survey in Denver, Colorado. Plants died when the mires flooded
and when they dried out. Each fluctuation in the sea level stopped peat
from forming and another thin, discontinuous coal seam was born. British
coal, like that in the eastern US, Western Europe, northern China and the
former Soviet Union, occurs in Carboniferous rocks. As a result, coal seams
mined in Britain are between 1 and 3 metres thick, with most around 1 metre
thick.

Seams in younger Cretaceous rocks, dating back 150 million years, can
be much thicker and spread over more of the globe. Seams as thick as 30
metres are common in Australia and in the Midwest of the US, and they can
be mined much more efficiently than seams a few metres thick. One Cretaceous
seam in the Fuxin Basin of northeast China is 125 metres thick. The Cretaceous
is known for coal that formed at middle and high latitudes (more than 30
degrees north or south), unlike the Carboniferous deposits, which were mainly
equatorial. ‘The big difference is that there was no polar ice in Cretaceous
times, so we have coal forming in the Cretaceous at very high latitudes,’
says McCabe. The warm, equable climate of Cretaceous times meant that plants
could form peat in most parts of the world, including the north and the
south. ‘There are coals on the north slope of Alaska that formed at around
70 degrees north.’

Coal is mostly carbon; it also contains sulphur, nitrogen and chlorine
compounds, and trace amounts of heavy metals such as lead. There are also
a range of components such as tiny grains of silicates in the form of mud
or sand, which form the residue, or ash, left after the coal is burnt.
The chemical make-up of coal, increasingly the subject of environmental
concern, is again determined in the mires where it formed. When coal burns,
sulphur, nitrogen and chlorine are released as ‘acid rain’. The ash is a
problem too. Not only must it be disposed of, but the heavy metals it contains
may leach out of waste dumps and pollute ground water.

‘Appalachian and European coals in Carboniferous rocks formed at low
latitudes and tend to be fairly high in sulphur,’ says McCabe. ‘The high-latitude
Cretaceous coals on the North Slope of Alaska have very low sulphur.’ Researchers
are examining modern mires to see which conditions determine the levels
of sulphur, nitrogen and chlorine. They aim to meet the demand for clean
coal by being able to predict where coals low in these impurities can be
found.

Apart from its composition, the usefulness of coal depends on the heat
that different types yield, described as the coal’s rank, which broadly
describes its calorific value. This depends on what proportion of the organic
compounds in the peat has been converted to carbon by the heat it experienced
as it was buried – a process known as coalification. Coalification also
produces volatile components such as methane, and reduces the moisture content
of the coal. High-rank coals, such as anthracite and bituminous coals (black
and hard coals), have a high calorific value, and low-rank coals, generally
graded as sub-bituminous or brown coal and lignite, have low values. Lignite
is the lowest-rank coal, not much better than the dried peat that is burnt
instead of firewood where peat is accumulating today.

Peats generally subside and undergo coalification to some degree. The
length of time they have been underground is the main factor that determines
their rank; older coals tend to be higher rank. So Carboniferous coalfields,
such as those in Britain, tend to contain the more energy-packed black coal.
For the same amount of heat, black coal produces less carbon dioxide and
other pollutants, notably sulphur. When lignite and brown coal are burnt,
the organic components of the plant matter that have not been converted
to coal also burn and give off carbon dioxide, but do not generate as much
heat as black coal.

Dividing coal reserves into high-rank and low-rank coals alters the
picture of worldwide assets. More than half the coal reserves of North American
and the countries of the former Soviet Union are brown coal. Asia and Australia
have the biggest reserves of black coal. And brown coal and lignite make
up two-thirds of Europe’s total reserves.

Oil and gas also originate from living things, from creatures whose
carbon experienced a different history after the creatures died. Most oil
comes from sediments that formed underwater. Microscopic creatures that
died and fell to the sea floor provided carbon in the form of organic molecules.
The subsequent burial of the sediment as the continental margin subsided
provided the right conditions – collectively termed the oil kitchen – for
a suite of chemical reactions that rearranged these organic molecules into
the various types of short-chain hydrocarbon molecules that make up the
oil found in reservoirs. If the source rocks, as the carbon-rich sediments
are called, get too hot, or the process continues for too long, all the
oil is broken down into simple volatile components such as methane – the
main constituent of natural gas. Few source rocks are also reservoirs; the
oil has to migrate to a suitably porous and permeable rock and be trapped
there, otherwise it oozes out at the surface as an oil seep.

Finding the right combination of source, subsidence and reservoir is
the art of successful oil exploration. Good sites are rifted continental
margins, where shallow seas teeming with life lie next to deeper water containing
sandy sediments that can be good reservoirs. As the sea becomes deeper,
the sands are topped by impermeable mud, which acts as a trap. Exploration
is currently focusing on areas such as the east coast of Africa, South America,
the South China Sea and much of Eastern Europe and Asia.

There is usually gas where there is oil, because it is a by-product
of chemical reactions in the oil kitchen, but it can be found alone. The
process of coalification also produces methane. This gas can escape the
coal and accumulate in a suitable reservoir rock, as is believed to have
happened in the southern North Sea, where the continuation of England’s
coalfields beneath the seabed may be the source for Britain’s major gas
fields.