The Securities and Exchange Commission launched an investigation of Exxon Mobil Corp. XOM -4.81% after an employee filed a whistleblower complaint last fall alleging that the energy giant overvalued one of its most important oil and gas properties, according to people familiar with the matter. Several people involved in valuing a key asset in the Permian Basin, currently the highest-producing U.S. oil field, complained during an internal assessment in 2019 that employees were being forced to use unrealistic assumptions about how quickly the company could drill wells there to arrive at a higher value, according to a copy of the complaint, which was reviewed by The Wall Street Journal.

At least one of the employees who complained was fired last year, according to a person familiar with the matter. The Journal previously reported that there had been internal disagreements over the valuation.

The SEC began investigating the claims after receiving the complaint, people familiar with the matter said. The current status of the investigation is unknown. A SEC spokesman declined to comment. Exxon spokesman Casey Norton declined to comment on the existence of an investigation. He said that if asked by authorities about the 2019 assessment, Exxon would provide information that shows Exxon’s actual performance exceeded the drilling estimates. Mr. Norton also said Exxon doesn’t comment on employees’ performance.

“We cannot comment on the reasons for an unnamed employee’s separation from the company,” Mr. Norton said. Under the SEC whistleblower program, individuals who provide information that results in a penalty can receive a share of the monetary penalty.

The investigation might now fall to an appointee of the incoming Biden administration, which is expected to choose Gary Gensler, a former financial regulator and Goldman Sachs Group Inc. executive, to head the SEC, the Journal reported Tuesday.

Exxon endured one of its worst-ever financial performances in 2020, posting billions of dollars in losses following an unprecedented decline in fossil-fuel demand as the coronavirus spread around the globe.

In November, Exxon pulled back from an ambitious plan by Chief Executive Darren Woods to boost its overall oil and gas production by one million barrels a day by 2025. The company said it would cut billions of dollars from its capital expenditures over the next five years and invest only in its best assets. Still, Exxon has said the Permian Basin, in West Texas and New Mexico, remains essential to its plans.