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.