LONDON (Bloomberg) -- For the oil market, it looks like the real OPEC meeting will come a week ahead of schedule.

The cartel is set to meet on Dec. 6 in Vienna, but days earlier the key decision makers are set to gather on the sidelines of the G20 summit in Buenos Aires in a meeting that may well decide the direction of oil prices in 2019.

Saudi Crown Prince Mohammed bin Salman and Russian President Vladimir Putin, who lead the world’s two largest oil exporters and have been working together to manage the oil market for the past two years, both plan to be in the Argentinian capital at the end of next week. Just as important will be U.S. President Donald Trump, who’s made his opposition to OPEC a regular theme in his Twitter diplomacy.

"I expect President Trump will be discussing the optimal price range with Crown Prince Mohamed bin Salman and President Putin at the G20," said Bob McNally, president of Washington consultant Rapidan Energy Advisors LLC and a former White House energy official.

The oil market is abuzz with talk that MBS, as Prince Mohammed is known, may not be able to defy Trump’s desire for lower oil prices after the White House supported him following the killing of Washington Post columnist Jamal Khashoggi.

"The market is assuming the Saudis won’t be able to cut," said Amrita Sen, chief oil analyst at Energy Aspects Ltd. in London.

Khalid Al-Falih and Alexander Novak, the Saudi and Russian energy ministers, are also scheduled to travel to Buenos Aires together with their principals, according to people familiar with their plans, asking not to be named because their agendas haven’t been disclosed yet. Their presence reinforces the impression that Saudi Arabia and Russia will try to reach a deal ahead of the OPEC meeting a few days later.

It wouldn’t the first time the two energy superpowers used a G20 to decide on oil policy.

At a summit in Hangzhou, China, in early Sept. 2016 Putin sat down with MBS to discuss how to revive oil prices. Hours after their private meeting, the Saudi and Russian oil ministers appeared in a joint press conference. The message was clear: Riyadh and Moscow were working together and a few days later,they announced that OPEC and a number of non-OPEC nations would cut production.

The gathering in Buenos Aires comes after a week of near-panic in the oil market. Brent crude, the global benchmark, plunged 6.1% to a one-year low of $58.80/bbl on Friday, down 22% this month on growing concerns the world is oversupplied. West Texas Intermediate, the U.S. benchmark, fell close to $50/bbl.

Trump had already celebrated the plunge on Wednesday, tweeting: "Oil prices getting lower. Great!" Yet, he wants more: "Thank you to Saudi Arabia, but let’s go lower!"

But the petro-diplomacy is complicated by the fallout from Khashoggi’s murder. Trump on Thursday confirmed that the Central Intelligence Agency told him that MBS " might have done it," in reference to who was responsible for the assassination. But he insisted that the CIA “didn’t conclude” that the prince gave the order.

"You can conclude maybe he did or maybe he didn’t," Trump said about the CIA report. "Whether he did or whether he didn’t, he denies it vehemently.”

Trump has vowed that the Khashoggi killing won’t upend the White House’s relations with the prince.

"We want low oil prices and Saudi Arabia’s really done a good job," he said. For the U.S. president, cheap energy equates to a tax cut for the middle class -- key to maintaining a tired-looking economic expansion.

For MBS, Trump’s support is key to avoiding more aggressive American action. Congressional leaders have been more skeptical about his denials. Senator Lindsey Graham, a senior Republican who was a long-time supporter of Saudi Arabia, is pushing for "serious sanctions, including appropriate members of the royal family."

So far, the 33-year old prince appears to be doing his best to keep oil prices low. Saudi crude production has reached an all-time high in November, surging to 10.8 MMbbl to 10.9 MMbpd, up from 10.65 million in October, according to industry executives who track Saudi output.

Yet, not everyone in the oil market is convinced that MBS will keep the taps open to please Trump. As much as the U.S. president wants low oil prices, the Saudi prince needs higher oil prices to to finance social and military spending, as well as the lavish lifestyles of hundreds of princes.

"It remains our view that the Kingdom will adopt a ‘Saudi First’ policy and prioritize its own economic and social welfare above pleasing the American president," said Helima Croft, chief commodities strategist at RBC Capital Mark