U.S. antitrust enforcers are investigating ExxonMobil's plan to acquire Pioneer Natural Resources, which would be the largest oil-and-gas deal in two decades, according to securities filings.

The Federal Trade Commission has sought additional information from the companies about the deal, a step it takes when reviewing whether a merger could be anticompetitive under U.S. law, Pioneer disclosed in a filing Tuesday. Merger investigations on average take about 10 months to complete, according to data compiled by law firm Dechert.

The FTC, which shares antitrust authority with the Justice Department, can sue in court to block a merger or decline to take action, effectively clearing the deal. Companies sometimes cancel deals when they learn an antitrust agency plans to sue to stop the transaction. The agencies annually investigate about 2% to 3% of the deals that are large enough to require reporting to the government.

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Exxon in October proposed to buy Pioneer in a $59.5 billion all-stock deal, a takeover that would make Exxon the largest oil producer in the Permian Basin of West Texas and New Mexico, the most active U.S. oil field. It would be Exxon’s largest deal since its $75 billion merger with Mobil in the late 1990s.

"We’re working to provide requested information," Exxon spokeswoman Michelle Gray said in a written statement. "From an anti-competitive perspective, it’s critical to remember the combined companies represent approximately 5% of the total U.S. oil and gas production."

Oil companies’ shares fell as U.S. crude prices dropped Tuesday. Exxon shares fell almost 2% Tuesday to $100.44. Pioneer shares also fell 2% to $225.78.

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Some oil executives and deal makers had anticipated a recent series of acquisitions would draw federal antitrust scrutiny. But antitrust regulators have rarely taken measures to thwart deals involving oil producers, often taking the view that their products compete globally.

Investors expect larger oil companies to continue scooping up smaller rivals. As top-tier drilling spots have become harder to find in the Permian, executives have realized they can extract more value out of their land by selling it than by drilling it.

Two weeks after Exxon announced its plans, Chevron said it would buy Hess in a $53 billion all-stock deal. Occidental Petroleum is in talks to purchase CrownRock, a large private oil company in the Permian, for more than $10 billion, The Wall Street Journal reported last week.

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Some analysts have argued Exxon and Pioneer together make up a negligible portion of global oil production, and that the deal couldn’t affect prices for oil or fuel. Exxon said it could save billions of dollars in value via synergies by bringing its operations to Pioneer’s land, and played down the need to make cuts to head count. 

Exxon Chief Executive Darren Woods said the deal would increase U.S. energy security and benefit consumers by bringing the oil company’s technical prowess and financial wherewithal to bear on shale resources. 

More than half of U.S. shale oil production comes from the Permian Basin. Pioneer’s acreage is some of the region’s best, with some 16 years of drilling locations left at its current pace of operations, according to energy analytics firm Enverus.

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Exxon has said the companies could pump 2 million barrels of oil a day by 2027 in the Permian. The region currently produces nearly 6 million barrels a day, according to the Energy Information Administration.

The FTC issued the investigative request to the companies on Monday. Pioneer said the companies are cooperating and still expect the deal to be completed in the first half of 2024.

In 2016, oil-field service companies Halliburton and Baker Hughes canceled a nearly $35 billion tie-up after the Justice Department and regulators in other countries pushed back. Those companies provide equipment and services to oil producers like Exxon.

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The FTC during the Biden administration has taken a tough stance on mergers, although its court record is mixed, particularly when it tried to block large technology deals.

The commission under Chair Lina Khan has taken action in court to challenge 11 deals, including acquisitions by Meta Platforms and Microsoft. Nineteen firms have abandoned deals after coming under FTC investigation, including five that were called off after the FTC sued to block them.