As economic growth outside the U.S. has flagged, producers and traders are beginning to worry that demand for crude will also decline. In export-dependent Germany, a purchasing managers index hit a four-year low, well below the level economists were expecting.The steepness of the drop has prompted Saudi Arabia and the Organization of the Petroleum Exporting Countries to consider a plan to quietly cut production to bolster prices, according to people familiar with the matter.

The idea would see the cartel retain the official output targets it set in 2016. But, because Saudi Arabia is overshooting those targets by nearly 1 million barrels a day, it would effectively be a cut. Such a move may help support prices without raising the ire of President Trump, who has been calling on OPEC to keep prices lower.

Investors remain skeptical that the OPEC meeting in Vienna on Dec. 6 will be able to turn the tide on oil supply enough to support prices.

A big reason why: the emergence of the U.S. oil industry as one of the world’s most important players. Ballooning shale production—American output has nearly doubled since the start of 2012—has made the U.S. a key supplier and exacerbated worries about a global glut of crude.

“I never thought I would hear these kinds of numbers coming out of the U.S.,” said Bob Yawger, director of the futures division at Mizuho Securities USA. “This is going to force OPEC’s hand.”

Power Play

The U.S. now exports more energy than it imports, by one measure.

U.S. energy trade balance, monthly

Natural gas

This summer, the U.S. surpassed Saudi Arabia and Russia as the largest crude-oil producer—a title it hadn’t held since 1973, according to the International Energy Agency. Monthly output in the U.S. was a record 11.65 million barrels a day in September and nearly the same amount in October, according to energy consulting firm Wood Mackenzie, while Saudi Arabia’s supply was nearly 11 million barrels a day last month and Russian production stood at 11.4 million a day.

“It used to be the world was divided into OPEC and non-OPEC,” said Daniel Yergin, vice chairman of IHS Markit, which projects the U.S. will be a net exporter of petroleum in the early 2020s. “Now it’s the world of the big three.”

In recent weeks, that has been reflected in a bumper amount of oil in storage. U.S. crude stockpiles have climbed for nine consecutive weeks. Inventories advanced by 4.9 million barrels in the week ended Nov. 16, and rose more than 10 million barrels the week before, the largest one-week increase since February 2017.

Bottlenecks in getting oil out of the prolific Permian basin in Texas have led to a big divergence in the benchmark prices of oil. The global benchmark, Brent crude, trades for roughly $9 more than West Texas Intermediate, which is harder to get to global markets.

However, the U.S. has continued to pump oil and many expect those hurdles to be cleared next year as new pipelines are built, unleashing even more crude on the rest of the world.

Uncertainty on the geopolitical front has also contributed to worries about oversupply.

Mr. Trump has signaled a willingness to look past the killing of a prominent U.S.-based journalist in his relations with Saudi Arabia. And the U.S., after months touting strict enforcement of sanctions on Iran, granted more generous waivers than expected for eight governments to buy Iranian oil. This could lead to higher-than-expected supply from the Islamic Republic.

Saudi officials said Mr. Trump pressured their country into ramping up oil production to record levels ahead of the sanctions on Iran’s petroleum industry, the Journal has reported.

“They are trying to be as cooperative with us as possible,“ said Douglas Hepworth, chief operating officer at Gresham Investment Management LLC, a $7 billion commodities firm with about one-third of its assets in energy. “What triggered the whole thing was everybody in the world moving to full capacity to try and make the world safe for Iranian sanctions.”

The strength of global production now threatens to overwhelm demand. This could pressure OPEC and its allies such as Russia to cut back when the group meets next month in hopes of regaining more direct control of global supply.

Such a combination helped rein in the last oil price rout two years ago—defying skeptics who previously warned OPEC’s grip on world markets had slipped thanks to U.S. shale. In 2016, the cartel teamed up with Russia and a group of like-minded, non-OPEC oil producers. They throttled back hard and stayed disciplined, slowly draining the world of the buildup of inventory that now is starting to slosh around the world again. The big question is whether they can pull off the same sort of deal now, and how long it might take to drain supply again.

Adding to the pressure on oil is a stronger U.S. dollar. Since crude is priced in dollars, it becomes more expensive for foreign buyers when the U.S. currency rises. On Nov. 12, the dollar jumped to its highest level since March 2017, bolstered by expectations of higher interest rates. This could start to hinder global demand, one of the initial drivers that underpinned the recovery in crude.

If the price of oil drops too far, too fast, that could also hurt U.S. producers, especially in the shale patch. Most shale drillers now maintain they can break even at $50 or lower. But the falling prices have begun to eat into their profitability, and some may be forced to curtail spending next year and reduce ambitious growth plans if prices decline much more.

Mr. Trump has expressed hope that oil prices will fall lower in tweets and comments this week. His remarks have upset some shale drillers, who say continued drops could hurt the U.S. fracking industry, and the Trump administration’s stated goals of American “energy dominance,” at a time when the country’s oil output is at all-time highs.

The U.S. broadly is reaping benefits. Consumers are enjoying lower prices at the gasoline pump. Higher oil production helped the U.S. lower its merchandise trade deficit by nearly $250 billion in 2017 from a decade earlier, according to a recent report from IHS Markit.

As the U.S. has exported more oil and natural gas, the country’s energy trade balance swung into surplus in October, according to data from Bank of America Merrill Lynch.

The fact that the U.S. is exporting more than it imports helps insulate the country from global price swings and raises the stakes for OPEC in its decision whether or not to cut production, analysts said.

“We’re rewriting the rules about how we think about global trade competition and market share,” said Michael Tran, energy strategist at RBC Capital Markets. “There’s no playbook or context given how quick growth has been in the U.S.”

—Benoit Faucon and Summer Said contributed to this article.