Canada on Friday approved two major takeovers of energy firms by the Chinese state-owned giant CNOOC and Malaysia\'s national oil company Petronas -- deals worth an estimated more than US$20 billion in total. Prime Minister Stephen Harper announced regulatory approval for CNOOC\'s $15.1 billion takeover of oil and gas company Nexen, and Petronas\' offer for Canadian gas producer Progress Energy Resources, estimated at $5.5 billion. However, he cautioned that Ottawa would henceforth block attempts by foreign state-owned companies to buy assets in Canada\'s lucrative oil sands. The CNOOC takeover of Nexen is China\'s largest foreign investment and its largest energy deal, according to data firm Dealogic. An earlier acquisition proposal by Petronas for Progress Energy, aimed at securing stable supplies of liquefied natural gas (LNG) from North America, was rejected by the Canadian industry minister.