The Siberian driller increased its cash pile 6 percent in dollar terms last year, according to a statement Saturday. Surgutneftegas didn’t include a breakdown by currency in its 2017 report to international standards. For at least the past six years, it has held most of the cash the U.S. greenback.

Investors are watching Surgut’s accounts closely after the U.S. Treasury earlier this month included Chief Executive Officer Vladimir Bogdanov in a sanctions list. That has raised concern in Russia that Surgut could also come under sanctions. The penalties could mean the company is left stranded with dollars it can’t easily use as banks, contractors and customers likely to steer clear.

Similar penalties against Russian billionaire Oleg Deripaska and his United Co. Rusal have already caused market convulsions as the company’s contractors and banks have frozen dealings with the aluminum giant.

Should Surgut faces sanctions similar to Rusal, the company could have problems using the dollars as all such transactions are monitored by the U.S., said Alexei Panich, a partner at an international law firm Herbert Smith Freehills. Russian banks may continue to hold dollar deposits under existing contracts but Surgut may face difficulties in extending tenures and putting more cash in.

The lion’s share of the company’s cash is in long-term deposits in state-run Sberbank PJSC, according to a person close to the oil company. The bank held 44 percent of Surgut’s deposits in 2012, the last time it disclosed the information. Most of the remainder was with VTB PJSC, Gazprombank JSC and the local unit of Italy’s UniCredit SpA.

Surgutneftegas, Sberbank and UniCredit declined to comment. Others didn’t respond.

Surgut was included in U.S. sanctions imposed in 2014 over Russia’s role in the deadly Ukraine conflict. Those curbs targeted some technology supply and access to foreign capital markets, which aren’t vital for the company and haven’t affected its operations.

Surgut doesn’t borrow as it spends just enough money to keep output flat and avoids acquisitions. The company’s owners are a mystery. Run by Bogdanov, one of the last so-called red directors, or former Soviet managers, still in charge, it is owned mostly by its employees, according to public statements from government officials.

Bogdanov, 66, cemented his hold over Surgutneftegas in the mid-1990s, when the cash-strapped government emerging after the collapse of the Soviet Union auctioned off Russia’s industrial jewels. Surgut’s pension fund acquired 40 percent of the company, which a few years later was divided among two dozen legal entities with interlocking ownerships, all registered in the city of Surgut, according to bankers working with the producer and its managers.

These entities and their successors had about 1 trillion rubles in long-term investments in the company at the end of 2016, according to Russia’s Spark-Interfax legal database, giving them more than 75 percent. None has more than 5 percent at any given time, meaning the company isn’t obliged to disclose their names.

Bogdanov, who officially owns less than 1 percent, has said the cash pile would help the company in a crisis.

“He knows he can’t use this money personally,” Alexander Ryazanov, a former deputy head of gas exporter Gazprom PJSC, who has known Bogdanov for more than 20 years, said in an interview three years ago.