Originally they were derived from coal-tar, the black viscous waste product left over from distilling coal for gas.

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Crude oil, or its substitutes, will be produced for decades, but at an increasingly expensive tariff, both in terms of cost and energy, as it is sourced in the form of a heavy, sour (high-sulphur) material, or in synthetic form from tar-sands, shale and coal.

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Unconventional sources of oil and gas that are set for further exploitation include Canada's tar sands, shale and coal-bed methane deposits around the world.

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For their calculations, the two assumed that where there was a gap between demand and conventional supply it would be filled with synthetic fuels, first with tar-sands oil and later with oil from coal and shale. (According to high-end estimates, coal and oil shale could together yield some ten trillion barrels of unconventional crude.) They then calculated what the impact would be on global carbon-dioxide levels.

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The greens may not be able to stop the Canadians from extracting oil from the Alberta tar sands, but they would succeed in wiping out American coal-fired power.

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