When U.S. investment bank Goldman Sachs said last year that oil could fall as low as $20 per barrel, it assigned a fairly low probability to that scenario. Fast-forward five months and in some parts of the world the forecast has already proved correct. Canadian physical crude has been selling this week at below $20 per barrel, less than it costs to extract and transport. Traders in the options market, meanwhile, are taking protection against prices falling below $25. The developments reflect growing concerns that a market already awash in too much oil is now suffering the double-whammy of a sharp slowdown in U.S. and Chinese demand.