Strategic Petroleum Reserve and countries that maintain the maximum energy security

Strategic Petroleum Reserve (SPR) is a volume of crude oil held by the government of a particular country, as well as private industry, placed in underground storage (salt caverns) and excluded from a country’s proven oil reserves, to safeguard the economy and help maintain national security during an energy crisis. 

During wartimes, countries indulge in various forms of embargoes, oil embargo being one of them and the one that causes the maximum economic disruption. The availability of strategic petroleum stocks during such times helps mitigate the magnitude of the market's reaction to the crisis. In the event of a supply interruption or a total oil embargo, introducing oil into the market from the SPR offsets the lost supply and in doing so helps calm markets, mitigate sharp price spikes, and reduce economic disruptions (i.e., gross domestic product loss). The SPR also helps buy time for the crisis to sort itself out or for diplomacy to seek some resolution.

United States maintains the largest SPR inventory of all countries, with its inventory totalling roughly 725 million barrels of crude oil. America's SPR is sufficient enough to last for about 40 days if used as per usual daily crude oil consumption rate and when consumed at a wartime rationing rate, America's SPR could last anywhere between 75 and 90 days.

Japan has the second largest SPR inventory in the world with stocks totalling roughly 325 million barrels. When used as per usual, non-rationed daily consumption rate, Japan's SPR inventory could easily last anywhere between 80 and 100 days. However when consumed at a wartime rationing rate, Japan's SPR could last roughly five months. Thus Japan maintains the maximum energy security of all major countries in the world even though its total SPR inventory is roughly half that of America's.

South Korea maintains the third largest SPR inventory in the world with crude oil stocks totalling roughly 280 million barrels, sufficient to last 75-90 days when used up at non-wartime daily consumption rate.