The share of petroleum liquids in world energy production grew rapidly during the 1950s and 1960s, reaching half of all energy production in 1973. (rstb.royalsocietypublishing.org)
In each, the relative share of petroleum liquids declines and the relative share of coal and natural gas increases. (rstb.royalsocietypublishing.org)
For convenience, petroleum liquids as defined here will be subsequently referred to as «oil». (rstb.royalsocietypublishing.org)