By Claire Milhench
LONDON (Reuters) - Some commodity fund managers believe oil prices could slide to $60 per barrel if OPEC does not agree a significant output cut when it meets in Vienna this week.
Brent crude futures <LCOc1> have fallen by a third since June, touching a four-year low of $76.76 a barrel on Nov. 14.
They could tumble further if OPEC does not agree to cut at least one million barrels per day (bpd), according to some commodity fund managers' forecasts.
"The market would question the credibility of OPEC and its influence on global oil markets if there was no cut," said Daniel Bathe, of Lupus Alpha Commodity Invest Fund.
That could send Brent down to around $60, Bathe said.
"Herding behaviour and a shift to net negative speculative positions should accelerate the price plunge," he said.
Yet fund managers and brokerage analysts are divided over whether OPEC will reach an agreement on cutting output.
Bathe put the likelihood at no more than 50 percent.
Oil prices have been falling since the summer due to abundant supply, partly from U.S. shale oil, and because of low demand growth, particularly in Europe and Asia.
As a result, some investors believe a small cut of around 500,000 bpd would not be enough to calm the markets.
Doug King, chief investment officer of RCMA Capital, sees Brent falling to $70 per barrel even with a cut of one million bpd.
"With this, I would expect lower prices in the first half of 2015," he said.
If OPEC fails to agree a cut, prices will drop "further and quite quickly", with U.S. crude <CLc1> possibly sliding to $60, he said.
U.S. crude closed at $76.51 on Friday, with Brent just above $80.
With member states struggling to balance budgets, many OPEC countries will be pushing for an output cut when OPEC meets in Vienna on Nov. 27.
"Prices below $80 are putting significant strain on the cartel's weakest members such as Venezuela," said Nicolas Robin, a commodities fund manager at Threadneedle.
He said a bigger cut, of one million bpd or more, was an "outlier scenario" but said such a move could rapidly push prices above $85.
"A move higher would likely be accelerated by the lack of liquidity owing to the U.S. (Thanksgiving) holiday next week," Robin added.
Doug Hepworth of Gresham Investment Management said bigger cut was needed to lift prices.
"A surprise significant cut, say of 2 million bpd, is needed to push prices back up to $80. And that would have to be accompanied by some newfound discipline in the non-Saudi members," Hepworth said.
The market has been awash with conspiracy theories as to why Saudi Arabia has not already intervened. New York Times columnist Thomas Friedman hinted at "a global oil war under way pitting the United States and Saudi Arabia on one side against Russia and Iran on the other".
Hepworth argued that Saudi Arabia appeared pretty happy with current pricing levels and suggested they were waiting to see where the cut-off point for U.S. production was.
"Time is on their side, they can afford to wait," he said, stressing he was talking months, not years, but added if oil fell below $70 that waiting time "shrinks to weeks".
Tom Nelson, of Investec Global Energy Fund, said he believed Saudi Arabia had allowed the price to fall to incentivise smaller OPEC producers, which often rely on the biggest producer to intervene, to join Riyadh in cutting output.
"They (the Saudis) want to cut but they don't want to cut alone," Nelson said, adding that a cut of between one million and 1.5 million bpd should be sufficient to balance the market.
"The market really wants to see that OPEC is still functioning ... if there is a small cut, with an accompanying statement of coherence from OPEC that presents a united front, and talks about seeing demand recovery, and some moderation of supply growth, then Brent could move up to $80-$90."
(Additional reporting by Eric Onstad; editing by Robin Pomeroy and Jason Neely)