For millions of years, oil has been a fuel on the move. It is discovered not where it was originally formed from organic sediments beneath shallow prehistoric seas, but where its upward or lateral movement through strata of porous rock formed from such sediments was trapped or sealed in the remote past under some impervious layer of ‘caprock’. It is produced when it moves through the pores of those underlying strata to the points where drilling has pierced and reduced pressure at the bottom of a well to allow the oil, driven by water or gas interfaced with it, to rise to the top. Once it has reached the surface, it can be moved more cheaply than any other fuel over the long distances that it often has to travel to where it can be used. Whether or not ‘the economics of a liquid’ makes the oil industry unique, its readiness to flow naturally remains the dominant physical factor in all the myriad actions of this industry's technology.

Oil moves throughout the world mainly as crude, in huge physical volumes. Nowadays crude is bought and sold in yet far larger contractual volumes of ‘paper barrels’. But it is almost never consumed as crude. Final consumers use the petroleum products refined from it, in patterns that have often varied widely between and within different groups of countries, and that alter everywhere over time.

‘Crude’ signifies unrefined mineral oil. In any other sense, the adjective is a misnomer for intense complexity. There are about 40,000 separate oilfields in the world, all in principle producing individual crudes.