Nexen said late Tuesday that it had refilled its so-called joint voluntary notice with CFIUS "by mutual agreement." It gave no reason for the move.
Cnooc, a subsidiary of state-owned China National Offshore Oil Corp., in July bid $27.50 per share for Calgary, Alberta-based Nexen, its partner in Canada's Long Lake oil sands project. The offer values Nexen's equity at about $15.1 billion and includes about $4.3 billion of debt.
The U.S. review will have focused on Nexen's assets in the Gulf of Mexico, and had been tipped by some analysts as a potential stumbling block for the deal.
Under the terms of the original timetable CFIUS had until the end of this week to deliver its judgment. That placed it in the delicate position of pre-empting Canadian regulators, where either approval or rejection of the deal would mark a political and investment watershed. Canada is scheduled to deliver its verdict on Dec. 10.
Nexen owns leases accounting for about 5% of Canada's oil sands reserves, though the majority of its assets are outside of Canada.
The deal also needs approval from regulators in the EU and China. Cnooc's chairman Wang Yilin said in November that he was confident of getting all the necessary approvals by the end of the year.
The refiling of the papers with CFIUS means that the regulator now has up to 75 days to rule on the proposed acquisition, including an initial 30-day period and the option to extend for a further 45 days.
The acquisition has already been cleared by U.S. antitrust regulators and by Nexen's shareholders.
Investors remain wary of the prospects of the deal being blocked by regulators. Nexen shares closed on Tuesday at C$24.10 ($24.27), down C$0.55, or 2.2%, near their lowest price since Cnooc announced its offer.
French mergers and acquisitions lawyer Laurent Faugerolas joined Dechert LLP. For other updates launch today's Movers & shakers slideshow.
After announcing in July that it was exploring strategic alternatives, TeleCommunication Systems Inc. has agreed to sell to Comtech Telecommunications Corp. in a transaction with an enterprise value of $430.8 million. More video