This, the US dollar is a global currency due to its reserve currency status, also for the fact that oil is traded in dollars and finally because you can go to some countries and pay with it.
yes, the currency used for buying petroleum is called petrodollar which ensures trade taking place as it is essential and as dollar is used in trade almost everytime such a transaction takes place, its value is increased making it more secure for people. Along with that, other markets and countries also have a role to play which result in US becoming the most widely used currency, making it in a way, the global currency.
If I'm not mistaken, Iran under sanctions had been selling oil to China for euros. In their hatred for America, they went so far as to have set up a bourse on the island of Kish in the Persian Gulf. The bourse had been intended to operate in various currencies other than the US dollar (primarily the euro and Iranian rial)...
Don't know what's become of this enterprise, though
So basically this means that Iran competed with Russia to sell oil to China. What's the point competing anyway because Iran's money goes to the Russian reserves in return of missiles
I was wondering how China got that extra cash to invest in the oil from Iran, figured out its from trade surplus with the US itself 
World trade, nice concept.