dollars backed by gold became petrodollars, keeping their global importance b/c oil was paid for in dollars by us and other countries.
“In 1971 Richard Nixon was forced to close the gold window taking the U.S. off the gold standard and setting into motion a massive devaluation of the U.S. dollar. In an effort to prop up the value of the dollar Nixon negotiated a deal with Saudi Arabia that in exchange for arms and protection they would denominate all future oil sales in U.S. dollars. Subsequently, the other OPEC countries agreed to similar deals thus ensuring a global demand for U.S. dollars and allowing the U.S. to export some of its inflation.
Since these dollars did not circulate within the country they were not part of the normal money supply, economists felt another term was necessary to describe the dollars received by petroleum exporting countries (OPEC) in exchange for oil, so the term petrodollar was coined by Georgetown University economics professor, Ibrahim Oweiss.”
https://en.wikipedia.org/wiki/Petrodollar