NEW YORK (Reuters) - A debate is emerging over how curbs on energy market speculation may impact oil prices, with at least one major bank boldly expecting the new rules will trigger a 30-percent price plunge.

The outcome holds wide-ranging implications for G20 developed nations collectively spending as much as $4.8 trillion to stimulate their economies through the worst global recession in decades.

“Regulators don’t and shouldn’t talk about trying to influence prices,” said John Brodman, a former Deputy Assistant Secretary at the U.S. Department of Energy. “But there’s a growing political imperative out there. An oil price rise of $30 a barrel would offset 40 percent of the stimulus spending. That’s not what these countries are looking for.”

The U.S. Commodity Futures Trading Commission is moving to tighten regulation, working with more than a dozen overseas regulators to apply rules on exchanges where commodities trade. The CFTC is expected to impose federal position limits in oil and natural gas markets soon.

The new rules may end exemptions that allow banks, funds and retail investor groups to build large positions unchecked.

So far, the CFTC push hasn’t kept most analysts from forecasting oil prices will rise, spurred by higher fuel demand as the world’s biggest economies regain steam.

“Overall, we think that it is way premature to call a market collapse due to potential limits in energy markets,” said Olivier Jakob at consultants Petromatrix in Switzerland.

But some believe the new rules could hit oil prices hard.

“It’s a bold view,” said Eugen Weinberg, head of commodities at Germany’s Commerzbank. “But we’re convinced that regulation is coming and markets will react.”

Commerzbank reduced its oil price forecast 27 percent, predicting oil may fall from more than $70 a barrel now, to $50 by the end of the year, and an average $55 in 2010.

Commerzbank’s outlook is uncommon. A Reuters poll of 30 analysts this week showed the consensus oil price forecast rose for a fifth straight month, with barrels expected to fetch $73 in 2010.

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U.S. bank Goldman Sachs GS.N, a giant oil trader itself, sees oil rising unfettered to $92 a barrel over the next 12 months.

Analysts aren’t sure how regulation will affect trading volume, or whether lower volumes would affect prices at all. But some believe prices are bound to fall when speculators, or noncommercial traders, get squeezed by the new rules.

“The writing is on the wall now. If real position limits are put in place, the price of commodities will fall,” said Michael Greenberger, a University of Maryland law professor and former director at the CFTC.

Noncommercial traders have come to dominate oil trading on New York Mercantile Exchange, or NYMEX CME.O. As of April 2008, three months before oil rose to a record $147 a barrel, they accounted for 71 percent of buying, up from 34 percent in 2000, according to a 2008 CFTC report.

About 50 percent of traders in the U.S. oil futures market today are speculators, up from around 20 percent before 2002, according to a Rice University study published on Thursday.

Exemptions have let operators like the U.S. Natural Gas Fund UNG.P -- an exchange-traded fund (ETF) that allows retail investors bet on rising natural gas prices -- to control the majority of buying in some key NYMEX gas contracts.

But UNG said recently it would scale back its position in natural gas contracts to comply with looming regulation. UNG’s announcement came days ahead of a rout in natural gas prices, which fell to new seven-year lows.

“New, tighter volume limits have a far greater chance of pushing prices sharply to the downside,” said one veteran oil market consultant in Connecticut, who requested anonymity.

LCM Commodities managing director Ed Morse said if CFTC rules do limit liquidity, less trading volume could make bearish fundamentals harder to ignore. A recent rise in Saudi Arabia’s oil output capacity may help keep oil prices in the $40 to $75 a barrel range, Morse wrote recently.

Most analysts warn it’s impossible to draw clear oil price implications from CFTC’s moves so far.

“Obviously if the (CFTC) actions are large and misguided, you can have a knee-jerk reaction, but one really needs to wait for the details,” said Amrita Sen of Barclays Capital.

Policymakers have made clear the push to oversee oil trading isn’t going away, after the G20 this year approved a combined $4.8 trillion in stimulus spending through 2010.

Leaders of the G8 economies asked international bodies to study ways of intervening in oil markets when they met in Italy in July. Finance ministers and central banks may revisit the issue when they gather in London September 4-5.