US politicians first came up with the idea of an oil stockpile in the early 1970s, after an oil embargo by Middle East nations caused prices to skyrocket around the world. Members of the Organization of Arab Petroleum Exporting Countries - including Iran, Iraq, Kuwait, Qatar and Saudi Arabia - refused to export oil to the US because it supported Israel in the 1973 Arab-Israeli War.

The war lasted just three weeks in October that year. But the embargo - which also targeted other countries - lasted until March 1974, causing prices to quadruple worldwide from about $3 to nearly $12 per barrel.

Pictures of cars queuing up at petrol pumps in affected nations became some of the defining images of the crisis.

The US Congress passed the Energy Policy and Conservation Act in 1975. It established the Strategic Petroleum Reserve in the event of another major supply problem.
What is the reserve?

At present, there are four sites where oil is stored: near Freeport and Winnie in Texas, and outside Lake Charles and Baton Rouge in Louisiana.

Each site has several man-made salt caverns up to a kilometre (3,300ft) underground where the oil is stored. This is far cheaper than keeping it in tanks above ground, and safer - the chemical composition of the salt and the geological pressure prevents any oil from leaking out.

The largest site at Bryan Mound near Freeport has a storage capacity equivalent to 254 million barrels of oil.

The reserve's website says that on 13 September there were 644.8 million barrels of oil held in these caves

According to the US Energy Information Administration, Americans used 20.5 million barrels of petroleum a day on average in 2018 - meaning there's enough oil to keep the country going for about 31 days.