In the past few years, as countries such as Venezuela and Russia have made it increasingly difficult and expensive for international oil companies to explore for, and produce oil, the ExxonMobils and Royal Dutch Shells of the world have sought refuge in the politically stable oil sands of Canada.

Here there are vast pools of oil trapped deep in sand, from which it is expensive and time-consuming to extract. But there is so much of it. The tar sands represent the world’s biggest proven reserve outside Saudi Arabia.