Chevron Phillips Chemical offered to acquire Nova Chemicals for more than $15 billion including debt, Reuters reported Thursday, citing people familiar with the matter.

Chevron Phillips is a joint venture of Chevron (CVX) - Get Report and Phillips 66 (PSX) - Get Report , the multinational energy companies based respectively in San Ramon, California, and Houston. Nova is a unit of Mubadala Investment of Abu Dhabi.

Mubadala this year has been looking into selling Nova, the Calgary plastics producer, Reuters reported.

The sources told Reuters that Mubadala isn't certain to accept the bid from Chevron Phillips and that another bidder might yet emerge.

In Thursday trading on the New York Stock Exchange, Chevron closed 1.1% higher at $124.28, while Phillips 66 shares climbed 3.7% to $90.83.