CNOOC offered $15 billion for Nexen Inc. in July. The Canadian government has looked to Asian markets as a potential export venue for the country's vast oil and natural gas assets.
Nexen announced that roughly 90 percent of the shareholders eligible to do so voted in favor of the CNOOC deal. The company said the terms of the arrangement with CNOOC are subject to the approval of the provincial government of Alberta.
CNOOC recently submitted a request to the U.S. government to review the takeover bid. The U.S. Committee on Foreign Investment needs to review the deal because Nexen has operations in the U.S. waters of the Gulf of Mexico.
Some U.S. lawmakers have objected to the CNOOC bid, saying it would give foreign companies an advantage in the U.S oil and gas market.
Nexen serves as the operator in so-called oil sands and shale natural gas reserves in western Canada. Its exploration portfolio includes assets in the British waters of the North Sea as well as the Gulf of Mexico and western Africa.