Oil prices are far from the peak of $147 a barrel seen last July
The price of oil has fallen below $60 a barrel, as concerns persist about the state of the world economy.
Crude oil for August delivery slipped 54 cents to $59.87 a barrel, having fallen as low as $58.72. In London Brent crude fell 60 cents to $60.50.
Persistent concerns about the state of the world economy, following poor economic data, have sent oil prices lower recently.
The International Energy Agency (IEA) predicts demand to fall 2.9% in 2009.
The IEA said as developing nations recover economically, this will help boost the price of oil but it predicts demand will not rise until 2010.
Oil prices had started to recover recently, from a low of around $30 a barrel, but have slipped again amid signs that demand for oil will remain low for some time.
Last July, oil peaked at around $147 a barrel.
"All the focus is on demand," said Christopher Moltke-Leth, head of sales trading at Saxo Capital Markets.
North Americans have reduced their overall spending, including how much they spend on petrol, as they drive less.
Shares in energy firms fell on Friday after Chevron said its refining margins dropped in the second quarter, which will cause its earnings for the three-month period to fall sharply below the same period a year ago.
By close of trade in London, BP was 0.8% lower, Royal Dutch Shell declined 1%, BG Group shed 1.5% and Tullow Oil declined 1.2%
In addition to hopes about an imminent recovery being dashed, fears about the pending second quarter earnings have also added to market jitters.
"The second quarter earnings season is going to be very important for crude [oil]," said Mr Moltke-Leth.