1.) Trading surpluses of oil producing nations, originally called petrodollars.
2.) Currencies of oil producing nations which tend to rise in value against other currencies When the price of oil rises (and fall when it falls).
3.) Currencies used as a unit of account to price oil in the international market.
The U.S originally had their agreement with Saudia Arabia that stated the Saudis price their oil in $-USD-$ and offer the U.S a "Lower Cost Price On Each Drum Of Oil" purchased from them.
In December 2013: Obama stabbed the Saudi's in the back on their agreement by annointing IRAN regional power with the "Nuclear Deal" which left Saudi Arabia weak and vulnerable to Irans threats.
It is then that Saudia Arabia realized the U.S renigged on their agreement national security agreement, and refused to sell Oil to the U.S. Saudi Arabia is now selling their oil to Russia and China; and no longer pricing it in U.S Dollars.
"WAKE UP CALL" Saudia Arabia is now backing away from the "U.S Dollar" and no longer selling their oil to the U.S because the U.S and Iran are now playing footsies with one another and renigged on their National Security Agreement.
NOTE: U.S no longer getting oil from Suaida Arabia.. so where are they getting it now? And why is it cheaper?
This all resorts back to ISIS, taking over Iraq/Syria, and controlling the oil fields in Syria with a supply route to Turkey. The U.S is now getting their oil from ISIS.
Remember hearing about Russia bombing ISIS's oil convoys in route to Turkey?.... Yeah.. This is why (Go : ) to learn more about it.