1. Oil prices have fallen by more than a fifth since hitting 2014 highs in June.

North Sea Brent, the benchmark for more than half the world's oil, has slid by more than US$30 a barrel, or over 28 per cent, from its June high. Brent for November delivery was down 81 cents at US$82.97 a barrel by 12:50pm on Thursday, its lowest mark since November 2010.

The US oil benchmark, West Texas Intermediate (WTI), fell US$1.49 to US$80.29. It slid to as low as US$80.01 on Wednesday, its weakest since June 2012.

2. Economic reports this week and Ebola fears have accelerated the slide in oil prices.

Data from inflation to manufacturing to retail sales from the United States, China, Germany and Japan have raised fears the world's biggest economies are in danger of stalling.

Concern about the spread of Ebola are rattling investors' nerves, particularly in the United States.

3. Prices are sliding because of a global supply glut.

The International Energy Agency said this week that demand for oil in 2015 will grow far slower than previously forecast as global economies remain weak. It cut its 2015 estimate for oil demand growth by 300,000 barrels per day (bpd) from its previous forecast and now expects demand growth of 1.1 million bpd to 93.5 million. It cut its 2014 estimate by 200,000 bpd to 0.7 million bpd.

Even as demand has slowed, supply has grown mainly because of a jump in the US production of shale gas. The shale revolution in the United States has seen hundreds of US companies drilling thousands of new wells sending te country's oil output surging to the most in almost 30 years.

To illustrate, U.S. crude inventories rose 10 million barrels in the week to Oct. 10 to 370.7 million barrels compared to market expectations for an increase of 2.8 million barrels.

To make things worse for oil, OPEC, which supplies about 40 per cent of the world's crude, has responded to the US threat by signalling it will not slash output to rescue prices - effectively starting a price war.

OPEC pumped 30.94 million barrels a day in September, the most since August 2013, according to a Bloomberg survey.

Saudi Arabia, the largest of the Organization of the Petroleum Exporting Countries, privately told the oil market this month that it is willing to allow prices to slide as low as US$80 for a year or two. Led by Saudi Arabia, Kuwait, Iran and Iraq have deepened the discount for their crude to Asian customers, sources said.

4. Will oil prices keep falling?

Some of the world's biggest banks say the collapse in oil is just about over, according to a Bloomberg News report on Thursday.

They are counting on OPEC cutting output as quickly as next month after ministers from the 12-member cartel meet in Vienna on Nov. 27 to discuss production and price levels.

But analysts are generally split on whether OPEC members will agree on production cuts. Saudi Arabia, the world's largest oil exporter, looks determined to protect its market share by keeping its production steady even as prices tumble. Smaller producers - like Russia, Iran and Venezuela - who stand to lose the most from the drop in prices are keen to cut supply.

If OPEC cannot agree, oil may break below its US$80 floor.

Even if OPEC takes no action, industry experts say they expect oil prices to start recovering in 2016, or possibly 2015 as the expensive drilling for shale oil by small US producers becomes unprofitable with prices spiraling downwards.

Most US producers, though, have hedged next year's production at much higher prices and are racing to lock in 2016, so US output could continue surging for another year or two even if the free-fall in oil prices continues.

5. Falling oil prices is a "double-edged sword for Asia".

This is what Frederic Neumann, an economist with HSBC in Hong Kong, told Wall Street Journal this week.

Asia is the world's largest oil-importing region so falling prices reduce the region's massive import bill, cutting costs for businesses and consumers and giving central banks room to lower interest rates at a time of slowing global growth.

But the very fact that oil is falling so steeply is not a good sign. Crude's weakness signals slowing demand from China and Europe, a development that risks hurting Asia's big exporters like South Korea.

For Japan, falling oil prices puts more money in consumers' pockets, in itself a plus for the economy. But it also risks stoking the deflation threatening the economy by reducing pressure on consumer prices.