That puts those Canadian companies under Communist China’s laws.
Nexen had holdings in the tar sands and nat gas wells in western Canada. Also holdings in the North Sea and the Gulf of Mexico.
To avoid any problems with the US over the Gulf of Mexico, CNOOC announced that they would not take the role of operating partner on any of the wells in the Gulf.
You can find more info by googling Nexen or Nexen FIPA.