Oil prices slid further below the $50 a barrel mark yesterday to a near six-year low amid concerns of a bigger oil surplus in the coming months.

ICE February Brent, the international oil marker, dropped $2.93 to $47.18 a barrel in afternoon trading, while Nymex February West Texas Intermediate, the US benchmark, fell $2.46 to $45.90 a barrel. Both hovered at levels last seen in April 2009.

"The market is still in the process of pricing-in what is shaping up to be an ugly first half [of the year] from a fundamentals' perspective," said Michael Wittner, oil analyst at Société Générale.

Opec's November decision to maintain output at 30 million barrels a day, rather than cut production to shore up prices, has lubricated a plunge that started in June. Relentless production from the US and output from countries such as Libya and Iraq that surpassed estimates coincided with a demand slowdown in Europe and Asia amid economic weakness, all putting pressure on prices.

David Hufton, chief executive at London-based broker PVM, said there seemed to be "no sign of a strategy rethink by Saudi Arabia and its production allies. They appear to be as committed as ever to price finding its own level and seeing what happens.

“They are in a commercial battle on two fronts,” he added. “A battle against the fall in oil consumption and a battle against the rise in competing production, and their only weapon is price.”

The spread between the two oil markers stood at $1.30 a barrel in afternoon trading, after narrowing in earlier trading to 92 cents, in a reflection of a weak Brent crude market. The last time the spread was this small was three months ago.

With pressure on oil prices likely to exist, Goldman Sachs lowered its average 2015 forecast for Brent to $50.40 a barrel from $83.75. It cut its WTI outlook for this year to $47.15 a barrel from $73.75.

“We expect that the global market imbalance will be larger [in the first half of 2015] than we had previously expected,” Goldman analysts said.

Prices would need to stay lower for a while yet to meaningfully choke off production and reduce the oversupply in the oil market, said the bank, which also slashed 2016 estimates.

Goldman, which said prices could fall into the $30-$40 a barrel range, followed a string of market analysts that have downwardly revised their price estimates. Société Générale last week cut its 2015 forecast for Brent crude to $55 a barrel and WTI to $51.

Oil services group Baker Hughes said on Friday that the US oil rig count fell by 61 last week, the sharpest weekly fall since 1991. Although the number of rigs has decreased in 10 out of the last 13 weeks, the size and speed of any production response remains uncertain.

Carsten Fritsch at Commerzbank said market observers would have to wait until the second half of the year before US oil output slowed.

The surplus of more than one million barrels expected by analysts in the next six months has only helped widen the so-called contango structure in the oil market – jargon for when prices for future delivery exceed current levels.

In turn, the world’s biggest oil traders are eyeing deals to store crude and refined products on tankers, hoping to profit from their future sale at higher prices. – Copyright The Financial Times Limited 2015