SINGAPORE/LONDON/NEW YORK/TOKYO (Reuters) – A year on from the start of one of the biggest oil price crashes in history, the driving force behind the slide remains intact: there is still too much crude. While output continues to grow, the economic outlook has darkened in top energy consumer China, where oil demand has been one of the few bright spots in the market. Add to the mix record output by the Organization of the Petroleum Exporting Countries (OPEC) and the possibility of a return of Iranian crude exports, and further price turbulence looks almost certain.