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.

FORBES: Dr Richard Pike (1950-2011) and Peak Oil

By then many refiners will have expanded and upgraded to take not only heavy crude but also synthetic crude from Canada's oil sands.

FORBES: Wild Frontier

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.

NEWYORKER: Unconventional Crude

For every barrel of synthetic crude that Suncor eventually produces, forty-five hundred pounds of tar sands have to be dug up and separated.

NEWYORKER: Unconventional Crude