Oil begins the last quarter of 2011 at its lowest level in more than a year as fears of another recession spread.
Investors are concerned about a pair of recent announcements that point to weaker demand and even lower energy prices this year.
Greece, at the centre of the European debt crisis, said over the weekend that it would miss spending targets it set, despite severe cost-cutting. And China's manufacturing sector appeared to cool off in September.
"We're also at a lull in the market" after the US summer driving season, independent analyst Stephen Schork said. "This is when you tend to see weakness" in oil prices.
Benchmark crude on Monday fell US59 cents to $US78.64 per barrel in early afternoon trading in New York.
Prices were higher after tumbling as low as $US76.85 earlier in the day. Oil hasn't been that low since September 2010.
In London, Brent crude dropped US36 cents to $US102.40 a barrel.
An ongoing worry for investors in recent months has been Greece's debt problems and their impact on the rest of Europe.
Greece has been relying on international aid to pay its bills, but further loan instalments may be in jeopardy.
Without more help, Greece will start running out of money in two weeks. A Greek default could spread to neighbouring countries and possibly trigger widespread banking problems. That would hamper world energy demand as lending slows down and businesses cut spending.
Meanwhile, manufacturing surveys out of China pointed to weaker activity in September as prices for raw materials rose. One survey suggested manufacturing was stagnant, while another showed slight improvement. Both were disappointments for oil analysts and traders.
China and other developing nations are expected to drive global energy growth in coming years. Sluggish manufacturing activity suggests softer demand for energy.
In the US a trade group reported that manufacturing activity grew last month, although the Institute for Supply Management index showed that the pace of growth remained weak.
Since peaking near $US114 per barrel in May, benchmark oil has dropped about 30 per cent.
In other energy trading, heating oil and petrol futures were both down about one cent at $US2.7676 and $US2.5228 per gallon, respectively. Natural gas lost about two cents at $US3.648 per 1,000 cubic feet.
