What's behind the Oil Slump? Supply and Demand issues

This via Bloomberg

Oil was trading at $86 a few weeks ago, but is now at 66.55 (Brent Crude). What is behind the fall? Bloomberg piece gives the following reasons

1. Weak Demand. OPEC and the International Energy Agency lowered their forecasts for growth in oil demand next year

2. Supply factors led to a 15% decline in price. They cite the US decision to grant waivers to 8 countries importing oil from Iran as easing some of the supply concern. Expectations of non-OPEC supply (especially the US), also a factor

3. Reduced energy demand is due to slowing global GDP

Interesting graphic on the change of oil prices in relation to demand vs supply. See below