Iran plans to sign a preliminary $6 billion deal with France’s Total SA on Tuesday to help develop an offshore gas field, an agreement that would mark the first Western energy investment there since international sanctions were lifted this year.

Under the deal, Total, China National Petroleum Corp. and Iran’s state-owned Petropars would develop part of a giant gas field in the Persian Gulf known as South Pars, a press official at Iran’s oil ministry said.

It wasn’t clear how much of the $6 billion investment would come from Total, or how the deal would be structured for Total to steer clear of U.S. restrictions still in effect.

The agreement with the French oil giant could be a harbinger for the return of more Western companies to Iran’s vast energy industry and represents a step forward for the Islamic Republic’s goals of ramping up production of oil and gas over the next several years.

European oil companies have been slow to return to Iran since the Persian Gulf country secured an end to sanctions on its energy industry by agreeing to curbs on its national nuclear program in January. American sanctions related to terrorism and weapons remain in effect on Iran. They forbid U.S. banks from dealing directly with Iran, and prevent all business deals with the Iranian Revolutionary Guard Corps, which is deeply involved in the country’s oil and gas sector.