Exhibit 8.1





Subsidiaries



As of December 31, 2015, we owned, directly
or indirectly, the following subsidiaries.



|Name
of entity                                       | Our interest | Jurisdiction of
incorporation          
|---------------------------------------------------- | ------------ | ---------------------------------------
|CNOOC China Limited                                  | 100%         | Tianjin, PRC                           
|CNOOC International Limited                          | 100%         | British Virgin Islands                 
|China Offshore Oil (Singapore) International Pte Ltd | 100%         | Singapore                              
|CNOOC Finance (2003) Limited                         | 100%         | British Virgin Islands                 
|Malacca Petroleum Limited                            | 100%         | Bermuda                                
|OOGC America LLC                                     | 100%         | Delaware, USA                          
|OOGC Malacca Limited                                 | 100%         | Bermuda                                
|CNOOC Southeast Asia Limited                         | 100%         | Bermuda                                
|CNOOC Africa Holding Ltd.                            | 100%         | British Virgin Islands                 
|CNOOC Africa Ltd.                                    | 100%         | British Virgin Islands                 
|CNOOC Africa (UK) Limited                            | 100%         | London, U.K.                           
|CNOOC SES Ltd.                                       | 100%         | Labuan, F.T.,
        
        Malaysia
|CNOOC Poleng Ltd.                                    | 100%         | Labuan, F.T.,
        
        Malaysia
|CNOOC Madura Ltd.                                    | 100%         | Labuan, F.T.,
        
        Malaysia
|CNOOC NWS Private Limited                            | 100%         | Singapore                              
|CNOOC Australia E&P Pty Ltd                          | 100%         | Australia                              
|CNOOC Wiriagar Holding Limited                       | 100%         | British Virgin Islands                 
|CNOOC Muturi Holding Limited                         | 100%         | British Virgin Islands                 
|CNOOC Muturi Limited                                 | 100%         | The Isle of Man                        
|CNOOC Belgium BVBA                                   | 100%         | Belgium                                
|CNOOC Australia Limited                              | 100%         | British Virgin Islands                 
|CNOOC Exploration & Production Nigeria
Limited       | 100%         | Nigeria                                
|CNOOC Hong Kong Holding Limited                      | 100%         | Hong Kong                              







|CNOOC Myanmar Holding Ltd.                             | 100% | British Virgin Islands                 
|------------------------------------------------------ | ---- | ---------------------------------------
|CNOOC Myanmar Ltd.                                     | 100% | British Virgin Islands                 
|CNOOC Middle East (Qatar) Limited                      | 100% | British Virgin Islands                 
|CNOOC Congo SA                                         | 100% | Republic of Congo                      
|CNOOC Caspian (Kazakhstan) Ltd.                        | 100% | Cayman Islands                         
|CNOOC Australia International Holdings Private Limited | 100% | Singapore                              
|CNOOC Australia Energy Capital Management Pty Ltd      | 100% | Australia                              
|CNOOC Batanghari Ltd.                                  | 100% | Labuan, F.T.,
        
        Malaysia
|CNOOC Palung Aru Ltd.                                  | 100% | Labuan, F.T.,
        
        Malaysia
|CNOOC FPSO Holding Limited                             | 100% | Liberia                                
|CNOOC Liberia Limited                                  | 100% | Liberia                                
|CNOOC Uganda (BVI) Ltd                                 | 100% | British Virgin Islands                 
|De coöperatieve vereniging CNOOC Netherlands U.A.      | 100% | The Netherlands                        
|CNOOC Netherlands B.V.                                 | 100% | The Netherlands                        
|CNOOC Uganda Ltd                                       | 100% | Uganda                                 
|CNOOC Deepwater Development Limited                    | 100% | Zhuhai, PRC                            
|CNOOC Hainan Dock Limited                              | 100% | Hainan, PRC                            
|CNOOC Iraq Limited                                     | 100% | British Virgin Islands                 
|CNOOC Finance (2011) Limited                           | 100% | British Virgin Islands                 
|CNOOC Algeria (BVI) Limited                            | 100% | British Virgin Islands                 
|CNOOC Reserves Fund I, Ltd.                            | 100% | Cayman Islands                         
|CNOOC Luxembourg S.à r.l.                              | 100% | Luxembourg                             
|                                                       |      |                                        







|CNOOC Finance (2012) Limited                                                          | 100%                       | British Virgin Islands                
|------------------------------------------------------------------------------------- | -------------------------- | --------------------------------------
|CNOOC Finance (2013) Limited                                                          | 100%                       | British Virgin Islands                
|Tainan-Chaoshan Petroleum Operating Company Limited*                                  | 50%                        | British Virgin Islands                
|Husky - CNOOC Madura Limited *                                                        | 40%                        | British Virgin Islands                
|Chaoyang Petroleum (BVI) Limited*                                                     | 50%                        | British Virgin Islands                
|Chaoyang Petroleum (Trinidad) Block 3A Limited*                                       | 50%                        | Barbados                              
|Chaoyang Petroleum (Trinidad) Block 2C Limited*                                       | 50%                        | Barbados                              
|Bridas Corporation*                                                                   | 50%                        | British Virgin Islands                
|Axion Energy Holding Ltd.*                                                            | 50%                        | British Virgin Islands                
|PNG Energy Limited                                                                    | 70%                        | British Virgin Islands                
|GINI Energy Ltd                                                                       | 70%                        | Papua New Guinea                      
|CNOOC Iceland ehf.                                                                    | 100%                       | Iceland                               
|CNOOC Luxembourg Holding
        
        S.à r.l.                                    | 100%                       | Luxembourg                            
|CNOOC PETROLEUM BRASIL LTDA                                                           | 100%                       | Brazil                                
|CNOOC International Energy Services (Beijing) Limited                                 | 100%                       | Beijing, PRC                          
|China Offshore Oil Corporation E&P Mexico, S.A.P.I. de C.V.                           | 100%                       | Mexico                                
|China Offshore Oil Corporation Petroleum Mexico, S.A.P.I. de C.V.                     | 100%                       | Mexico                                
|China Offshore Oil Corporation E&P Services Mexico, S.A.P.I. de C.V.                  | 100%                       | Mexico                                
|CNOOC New Zealand E&P Limited
        
        CNOOC Finance (2015) Australia Pty Ltd | 100%
        
        100% | New Zealand
        
        Australia
|Aurora Liquefied Natural Gas Ltd.                                                     | 60%                        | Alberta, Canada                       







|Canadian Nexen Petroleum East
        
        Al Hajr Ltd.                                                                                                                                                                                            | 100%                                                                                                                             | Alberta, Canada                                                                                                                                          
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | -------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------
|Canadian Petroleum Yemen Limited                                                                                                                                                                                                                       | 100%                                                                                                                             | Jersey                                                                                                                                                   
|CanadianOxy Offshore
        
        Production Co.
        
        CNOOC Canada Energy Ltd.CNOOC Nexen Finance (2014)
        
        ULC
        
        CNOOC Nexen Petroleum
        
        Guyana Limited
        
        CNOOC UK Limited | 100%
        
        100%
        
        100%
        
        
        
        100%
        
        
        
        100% | Delaware, USA
        
        British Columbia, Canada
        
        Nova Scotia, Canada
        
        Barbados
        
        England and Wales
|ICM Assurance Ltd.                                                                                                                                                                                                                                     | 100%                                                                                                                             | Barbados                                                                                                                                                 
|International Capital
        
        Holdings Limited                                                                                                                                                                                                | 100%                                                                                                                             | Malta                                                                                                                                                    
|Nexen Brasil Exploração
        
        Petróleo Ltda.                                                                                                                                                                                                | 100%                                                                                                                             | Brazil                                                                                                                                                   
|Nexen E & P Services
        
        Nigeria Limited                                                                                                                                                                                                  | 100%                                                                                                                             | Nigeria                                                                                                                                                  
|Nexen Energy Acquisitions
        
        Holdings Limited                                                                                                                                                                                            | 100%                                                                                                                             | Jersey                                                                                                                                                   
|Nexen Energy Capital
        
        Management U.S.A. Inc.                                                                                                                                                                                           | 100%                                                                                                                             | Delaware, USA                                                                                                                                            
|Nexen Energy Holdings
        
        International Limited                                                                                                                                                                                           | 100%                                                                                                                             | Jersey                                                                                                                                                   
|Nexen Energy Holdings
        
        U.S.A. Inc.                                                                                                                                                                                                     | 100%                                                                                                                             | Delaware, USA                                                                                                                                            
|Nexen Energy Marketing
        
        Europe Limited                                                                                                                                                                                                 | 100%                                                                                                                             | England and Wales                                                                                                                                        
|Nexen Energy Marketing
        
        Nigeria Limited                                                                                                                                                                                                | 100%                                                                                                                             | Nigeria                                                                                                                                                  
|Nexen Energy Marketing
        
        U.S.A. Inc.                                                                                                                                                                                                    | 100%                                                                                                                             | Delaware, USA                                                                                                                                            
|Nexen Energy Services
        
        International Ltd.                                                                                                                                                                                              | 100%                                                                                                                             | England and Wales                                                                                                                                        
|Nexen Energy Services U.S.A. Inc.                                                                                                                                                                                                                      | 100%                                                                                                                             | Delaware, USA                                                                                                                                            







|Nexen Energy ULC                                           | 100% | British Columbia, Canada
|---------------------------------------------------------- | ---- | ------------------------
|Nexen Ettrick U.K. Limited                                 | 100% | England and Wales       
|Nexen Field Services
        
        Nigeria Limited      | 100% | Nigeria                 
|Nexen Holdings U.S.A. Inc.                                 | 100% | Delaware, USA           
|Nexen Holdings West
        
        Africa Limited        | 100% | Jersey                  
|Nexen Marketing
        
        Singapore Pte Ltd.        | 100% | Singapore               
|Nexen New Venture Jersey
        
        Limited          | 100% | Jersey                  
|Nexen No. 8 Ltd.                                           | 100% | Alberta, Canada         
|Nexen Oil & Gas Holdings
        
        U.S.A. Inc.      | 100% | Delaware, USA           
|Nexen Oilfield Services
        
        Nigeria Limited   | 100% | Jersey                  
|Nexen Petroleum Colombia
        
        Limited          | 100% | Jersey                  
|Nexen Petroleum Deepwater
        
        Nigeria Limited | 100% | Nigeria                 
|Nexen Petroleum Dragon U.K.
        
        Limited       | 100% | England and Wales       
|Nexen Petroleum Exploration and Production Nigeria Limited | 100% | Nigeria                 
|Nexen Petroleum Nigeria Limited                            | 100% | Nigeria                 
|Nexen Petroleum Offshore U.S.A. Inc.                       | 100% | Delaware, USA           
|Nexen Petroleum Operations
        
        Yemen Limited  | 100% | Jersey                  
|Nexen Petroleum Poland Sp.
        
        zo.o.          | 100% | Poland                  
|Nexen Petroleum Sales U.S.A.
        
        Inc.         | 100% | Texas, USA              
|Nexen Petroleum U.K. Holdings
        
        Limited     | 100% | England and Wales       







|Nexen Petroleum U.K. Limited                             | 100% | England and Wales   
|-------------------------------------------------------- | ---- | --------------------
|Nexen Petroleum U.S.A. Inc.                              | 100% | Delaware, USA       
|Nexen Resource Holdings U.K.
        
        Limited    | 100% | England and Wales   
|Nexen Services Jersey Limited                            | 100% | Jersey              
|Wascana Energy 2001 Ltd.                                 | 100% | Saskatchewan, Canada
|Canadian Nexen Petroleum Yemen                           | 100% | Alberta, Canada     
|Nexen Crossfield Partnership                             | 100% | Alberta, Canada     
|Nexen Marketing                                          | 100% | Alberta, Canada     
|Nexen Med Hat-Hatton Partnership                         | 100% | Alberta, Canada     
|Nexen Nigerian Holdings
        
        Coöperatief U.A | 100% | The Netherlands     
|Nexen Oil Sands Partnership                              | 100% | Alberta, Canada     
|CNOOC Finance (2015) U.S.A. LLC                          | 100% | Delaware, USA       





*We and our partners jointly control these
entities.