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, Iran now competes with Russia in sell oil to China. But Iran's cash goes to Russia anyway to buy missile systems (under the latest agreement, for "domestic" use only). But where is China getting excess cash to buy Iranian oil? From it's trade surplus with the US. Obama's world trade policies are working well.