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Left: An oil rig - pumps oil
from deep underground. Right:
A coal mine - large quarry diggers carve out the resource.
What is a fossil fuel?
Fossil fuels are hydrocarbons such as coal, oil and natural gas, sourced
from the organic remains of prehistoric organisms.
From left: a lump of coal, liquid
petroleum and a gas flame.
When these fuels are burnt, the energy released can be harnessed
to produce electricity, power vehicles, heat homes, cook food and
much more. They are also used in the production of important
materials such as plastics.
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How do fossil fuels form?
In order to answer this question, we must first distinguish oil
and gas from coal. Generally speaking, oil and gas are formed from
the organic remains of marine organisms which become entrained
within sea-floor sediments. Coal, by contrast, is typically formed
in non-marine settings from the remains of land vegetation.
Oil and gas formation begins with the accumulation of
organics on the sea-floor; these are the dead remains of organisms
living in the water column, such as microscopic plankton, which rain
down on the sea floor below. This will only occur in rather unusual
settings, where the sea floor is stagnant such that there is no
oxygen present to break the organic remains down and no sea-floor
dwelling organisms present that might feed on the organics. A high
sediment accumulation rate of may also help to bury the organics
before the action of decay can break them down. As the sediment pile
becomes deeper the organics within it are subjected to heat and
pressure which leads to formation of oil and then gas. For oil and
gas extraction, it is important that the source rock is not
'over-cooked' or the hydrocarbons will be destroyed. There must be
suitable reservoir-rock, such as a porous sandstone, into which the
hydrocarbons can migrate and accumulate. This must be overlain by an
impervious cap-rock, such as a clay, which prevents the hydrocarbons
from escaping to the surface. Finally, the geometry of the reservoir
and cap-rock bodies must be such that the hydrocarbons become
trapped; usually folding will suffice.
explaining the generation and entrapment of oil and gas.
Fossil ferns within a sheet of coal.
Coal typically forms on land from vegetation in lowland, swampy,
mire environments. Stagnant waterlogged soil prevents the
accumulated plant debris from breaking down. The recognisable
remains of plants are often visible within coals and associated
shales, confirming their plant-origin. The picture above shows a
piece of coal containing a network of fossilised fern leaves - clear
evidence that it was formed from vegetable remains.
More recent peat deposits.
The accumulated plant debris initially forms a material known as
peat. The geological processes of burial beneath later sediment and
alteration by heat and pressure convert the peat to coal; a process
known as coalification.
For the peat to become coal, it must be buried by sediment.
Burial compacts the peat and, consequently, much water is squeezed
out during the first stages of burial. Continued burial and the
addition of heat and time cause the complex hydrocarbon compounds in
the peat to break down and alter in a variety of ways. The gaseous
alteration products (methane is one) are typically expelled from the
deposit, and the deposit becomes more and more carbon-rich as the
other elements disperse. The stages of this trend proceed from plant
debris through peat, lignite, sub-bituminous coal, bituminous coal,
anthracite coal to graphite (a pure carbon mineral). Source:
From left: Oil is refined to produce
petrol and diesel, power stations burn fuel to produce energy, gas
is used for everyday cooking.
Fossil fuels are burnt to release energy in the form of heat.
This energy can be used to power cars, produce electricity, heat
homes, cook food to name but a few.
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What are the advantages of fossil fuels?
Straightforward combustion process
What are the disadvantages of fossil fuels?
Thought to be a major cause of global warming
Cause of acid rain
Not sustainable in the long-term
Politics and Economics can cause major price
Extraction can be very damaging to the landscape
Will fossil fuels run out?
Fossil fuels are essentially a non-renewable energy source. The
geological processes which create them take millions of years, so
they cannot be replaced within human timescales once they have gone.
It is impossible to estimate when fossil fuels will disappear, but
within the next 100 years it is widely believed that the cost of
finding and extracting new deposits will render them too expensive
for everyday use.
What are the alternative renewable energy sources?
From left: Wind turbines generate
electricity, solar panels convert sunlight into electricity, geothermal
plant utilise underground heat.
Renewable energy sources can provide comparable energy production and
heating. At present some technologies are cost inhibitive, but the rate of
technological development is making them more affordable with time. Other
examples of renewable energy sources include: water powered turbines, tidal
generators, nuclear, compost and rape seed oil.
Join us on a fossil hunt
Left: A birthday party with
a twist - fossil hunting at
Right: A family hold their prized ammonite at Beachy Head.
Discovering Fossils guided fossil hunts reveal evidence of life that existed
millions of years ago. Whether it's your first time fossil hunting or you're
looking to expand your subject knowledge, our fossil hunts provide an
enjoyable and educational experience for all. To find out more