LONDON » As oil prices continue to plummet, the once-dominant international cartel of producers is losing its sway over the global energy markets.

At its meeting Thursday in Vienna, the Organization of the Petroleum Exporting Countries, the group of 12 oil producing nations, is under pressure to at least announce modest cuts in an effort to shore up the markets. But any cuts would be modest and would probably do little to prop up oil prices, assuming the countries even stick to an agreement.

OPEC is caught in a swirl of forces that are depressing prices, down more than 25 percent since June to around $80 a barrel. And the cartel’s muscle, namely its control over one-third of the world’s oil production, is not enough to fight them.

Asian and European economies are slowing, which has cut demand. And supply is booming, with shale drilling in the United States adding as much as a million barrels a day this year.

OPEC’s cuts, provided the member nations come to an agreement, are likely to amount to only a million barrels a day. At that level, they are not likely to make an appreciable difference in a market that produces more than 90 million barrels daily.

If anything, a move by OPEC would only complicate the economic calculus for its member nations, by reducing their revenues. Venezuela, Nigeria and Iran have come to rely on oil money to pay for social programs vital to their stability.

"OPEC has been caught by surprise by the market," said Badr H. Jafar, president of Crescent Petroleum, an oil and gas company based in the United Arab Emirates. "This may be the meeting where they try to regain control."

Founded more than a half century ago, OPEC wrenched supremacy over the oil market from the giant multinational energy companies and used its production heft to drive prices. Some Arab members of OPEC staged an oil embargo against the United States and other countries that supported Israel during the war in 1973, a move that increased prices.

In the 1980s, an oil glut emerged as new production came onto the market from Britain, Norway, Alaska and elsewhere. Saudi Arabia slashed production sharply to try to balance the markets. When other producers failed to follow, the Saudis ramped up output, contributing to a price collapse.

Unity among OPEC members has been sporadic ever since, and the group’s options at the moment are not appealing.

If it does cut production significantly and higher prices result, shale drillers in the United States will have an incentive to invest more and increase output. That would only further crowd out OPEC production from world markets.

A long period of low prices might eventually curb shale oil production. But it is far from clear just how bad it would have to become to affect output in the United States.

Even with the recent weakness, U.S. production continues to soar. The Bakken and Eagle Ford shale fields of North Dakota and Texas — two principal drivers of the country’s energy renaissance — increased output by 90,000 barrels a day in October, up 3.4 percent from the previous month, according to Bentek Energy, a consulting firm.

Low prices also crimp OPEC countries.

Some countries like Saudi Arabia, Kuwait and the United Arab Emirates have built up an estimated $2 trillion in rainy-day funds and could withstand a long period of low prices. Others, like Venezuela and Iran, would feel the squeeze. The two countries have called for reductions in supplies, although neither is in the economic position to cut production by itself.

Officials in Saudi Arabia have been largely quiet of late, which is further unnerving markets. Saudi Arabia is the linchpin of the cartel. And analysts say the Saudis and OPEC may be struggling to devise a coordinated strategy to prop up the markets.

"OPEC is watching and not acting because there is literally very little it can do," says Sadad al-Husseini, a former executive vice president for exploration and production at Saudi Aramco, the national oil company.

OPEC has made difficult decisions before. In 2008, the organization agreed to 4.2 million barrels per day in cuts to stem falling prices during the global financial crisis. It worked, with oil bottoming out around $37 a barrel before steadily recovering.

This time, however, analysts say the organization looks more divided.

"OPEC is split: The majority don’t like the current price levels, the minority don’t mind," said Kamel al-Harami, former president of Kuwait Petroleum International, a state oil company.

Three Persian Gulf producers in the minority view, Saudi Arabia, Kuwait and the United Arab Emirates, account for about half of the organization’s output. They are only moderately hurt by the low prices, and may even be helped in the long run if investment in competing high-cost oil fields in Canada and the United States dries up. All three countries are rivals of Iran, which is more vulnerable to lower prices.

But sanctions, internal strife and political turmoil have curbed output in Iran and Venezuela and left production in Libya unreliable. Fereidun Fesharaki, chairman of the market research firm FGE, estimates that as much as 3.5 million barrels per day, more than 10 percent of OPEC’s output, may be off the market for such reasons.

"You have been eating my lunch," is the general sentiment expressed by such members to their more fortunate brethren, he said.

Energy analysts say that OPEC’s biggest problem is the shale oil revolution in the United States. Railroad tracks connecting North Dakota oil fields to the East Coast have bolstered domestic supplies, forcing Nigerian producers and others in Africa to export more oil to China and the rest of Asia. Saudi Arabia, in turn, has lowered prices to protect its share of the Asian markets.

While there are still restrictions on shipping crude from the United States, there has been a boom in exporting refined products to Europe and elsewhere, further cutting into OPEC markets. As domestic crude production grows, political pressures to remove the export ban altogether are beginning to mount.

Adding to the difficulties, OPEC has been operating without quotas since the end of 2011, when it agreed to a group production target instead of individual country ceilings. Reimposing quotas might be a long and fraught exercise since targets have political and economic significance in each country.

Iraq has been building up its production and may further increase it now that an interim agreement has been reached with the autonomous Kurdistan area for sharing the proceeds from Kurdish oil. Iraq also is unlikely to agree to production curbs as it struggles to resist Islamic State insurgents.

"The group has headed toward Vienna with the aim and hope of achieving some form of cuts, but the timing and extent of the cut remains up in the air," says Richard Mallinson, an analyst at Energy Aspects, a London-based market research firm.

Analysts say that without quotas, any cuts may end up being meaningless. But it may be particularly difficult for the Saudis to persuade other OPEC members to agree to share in the cuts, as the country is likely to insist.

"The Saudis have not forgotten the searing experience of the 1980s," when the country cut production on its own and lost significant share, said Bhushan Bahree, a longtime analyst of OPEC who is now at the market research firm IHS. "They do not want a repeat."

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