[FORM 20-F (Mark One) ¨ OR ý For the fiscal year ended December 31, 2012 OR ¨ For the transition period from _________________ to _______________ OR ¨ Date of event requiring this shell company report CNOOC LIMITED N/A Hong Kong (Jurisdiction of incorporation or organization) 65th Floor, Bank of China Tower One Garden Road, Central Hong Kong Hua Zhong th] [EXECUTION COPY CNOOC LIMITED and CNOOC CANADA HOLDING LTD. and NEXEN INC. ARRANGEMENT AGREEMENT July 23, 2012 TABLE OF CONTENTS ARTICLE 1 INTERPRETATION Section 1.1 Defined Terms 1 Section 1.2 Certain Rules of Interpretation 16 ARTICLE 2 THE ARRANGEMENT Section 2.1 Arrangement] [Subsidiaries As of December 31, 2012, we owned, directly or indirectly, the following subsidiaries. All of these entities are private limited liability companies and they do business in their corporate names. 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%] [CNOOC Limited (incorporated under laws of Hong Kong with limited liability) Code of Ethics for Directors and Senior Officers 21 August 2012 I. INTRODUCTION Each of the directors and Senior Officers has the responsibility to obey the law and act honestly and ethically. To that end, this Code of Ethics is a guide intended to assist each of the directors] [CERTIFICATIONS I, Fanrong Li, certify that: 1. I have reviewed this annual report on Form 20-F of CNOOC Limited; 2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made,] [CERTIFICATIONS I, Hua Zhong, certify that: 1. I have reviewed this annual report on Form 20-F of CNOOC Limited; 2. Based on my knowledge, this report does not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements made, in light of the circumstances under which such statements were made,] [CERTIFICATIONS 1. the Report fully complies with the requirements of Section 13(a) or 15(d) of the Exchange Act; and 2. the information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of CNOOC Limited. Fanrong Li Chief Executive Officer and President Hua Zhong Chief Financial Officer] [CNOOC LIMITED Estimated Future Reserves and Income Attributable to Certain Leasehold Interests And Derived Through Certain Production Sharing Contracts SEC Parameters As of December 31, 2012 Herman G. Acuna, P.E. Martin J. Cocco, P.E. TBPE License No. 92254 TBPE License No. 104589 Managing Senior Vice President – International Vice President – International [SEAL] [SEAL] RYDER SCOTT COMPANY, L.P. TBPE Firm] [CNOOC LIMITED Estimated Future Reserves and Income CNOOC’s 20% Ownership of Pan American Energy LLC SEC Parameters As of December 31, 2012 Guale Ramírez, P.E. Martín J. Cocco, P.E. TBPE License No. 48318 TBPE License No. 104589 Managing Senior Vice President – International Vice President – International [SEAL] [SEAL] RYDER SCOTT COMPANY, L.P. TBPE Firm Registration No. F-1580 RYDER SCOTT] [INDEPENDENT LETTER THE GREATER ANGOSTURA FIELDS, BLOCK 2C, TRINIDAD & TOBAGO ESTIMATED PROVED RESERVES AND FINANCIAL DATA, BASED ON SEC RULES Prepared for CNOOC LIMITED s t As at 31 March, 2013 GAFFNEY, CLINE & ASSOCIATES (CONSULTANTS) PTE LTD This summary letter is based on Gaffney, Cline & Associates’ official report and has been provided at the request of CNOOC] [INDEPENDENT LETTER THE MISSAN OIL FIELDS IN EASTERN IRAQ ESTIMATED PROVED RESERVES AND FINANCIAL DATA, BASED ON SEC RULES Prepared for CNOOC LIMITED As at 31 st December, 2012 March, 2013 GAFFNEY, CLINE & ASSOCIATES (CONSULTANTS) PTE LTD This summary letter is based on Gaffney, Cline & Associates’ official report and has been provided at the request of CNOOC Limited.] [Janua r y 2 5 , 20 13 CNOOC Limited 1600, 555 - 4 th 1 Av enue SW C a l gary , A l berta T2 P 3E7 e n t i o n : Att R eserves R e v i ew C o mmit t ee of t h e Board o f Director s of] [Hydrocarbon Liquids & Natural Gas Reserves Audit Report at Year End 2012. Cerro Dragon, Anticlinal Funes, Piedra Clavada, Koluel Kaike and Acambuco in Argentina and Caipipendi in Bolivia CNOOC Limited Submitted B y : Date Submitte d : Project Number 411 North Sam Houston Parkway STE 400, Houston TX 77060 rpsgroup.com Hydrocarbon Liquids & Natural Gas Reserves Audit Report at] [April 24, 2013 100 F Street, N.E. Dear Sirs/Madams, Yours faithfully, Hong Kong] [CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM Hong Kong April 24, 2013] [Consent of Independent Consultant Houston, Texas April 22, 2013] [Gaffney, Cline & Associates (Consultants) Pte. Ltd. 80 Anson Road #31-01C Fuji Xerox Towers Singapore 079907 Telephone: +65 6225 6951 www.gaffney-cline.com YDH/dh/PS-12-2146&PS-12-2147/2013/L0187 nd 22 CNOOC Limited No. 25, Chaoyangmenbei Dajie Dongcheng District Beijing 100010, P.R. China Dear Sire, Consent of Independent Consultant Yours sincerely, GAFFNEY, CLINE & ASSOCIATES (CONSULTANTS) PTE LTD Stephen A. Sakowski Regional QA Director - Asia Pacific] [Consent of Independent Consultant Sincerely, McDANIEL & ASSOCIATES CONSULTANTS LTD. 2200, Bow Valley Square 3, 255 - 5 Avenue SW Calgary, Alberta T2P 3G6 Dated: April 19, 2013 2200, Bow Valley Square 3, 255 - 5 Avenue SW, Calgary, AB T2P 3G6 Tel: (403) 262-5506 Fax: (403) 233-2744 www.mcdan.com] [Consent of Independent Consultant RPS Debbie Perkins Vice President [Place] Houston, Tx. [Date] April 19, 2013] [Lee Keeling and Associates, Inc. Petroleum Consultants First Place Tower ● 15 East Fifth Street Tulsa, Oklahoma 74103-4350 ● (918) 587-5521 www.lkaengineers.com Consent of Independent Consultant 15 East Fifth Street, Suite 3500, Tulsa, Oklahoma 74103-4350 USA April 19, 2013]