Duff:
There are many good articles on-line as well as books written about this issue. Those will explain it much better than I can.
I'm in my peak veggie/planting season - lot of work and little sleep now for several weeks.
You can look at Iraq War II - there are thoughts, perhaps conspiracy theories, out there that the entire WMD argument was manufactured so that the US would invade Iraq to protect the use of USD in Iraqi exchange for oil as Saddam said he going to abandon the use of the USD in favor of the Iraqi dinar and gold as well as Iran creating an Iranian oil bourse.
Here is one article about the Iraqi War and the petrodollar.
http://www.resilience.org/stories/2005-08-08/petrodollar-warfare-dollars-euros-and-upcoming-iranian-oil-bourse
In my honest and humble opinion, a currency remains a reserve currency for as long as the country's military can keep it so. The past reserve currencies were replaced as their empires failed - the most recent being the transition from the British Empire to the US Empire.
I do not think the Yuan is positioned to be the next reserve currency. There are WAY too many internal issues there and I do not think the PBOC WANTS the Yuan to be the reserve currency.
For another little fun research project --> Look up which countries in the world DO NOT HAVE A Central Bank......and which ones most recently created one.