[CNOOC Limited Hua Zhong Joint Company Secretary Dated: April 9, 2013 Description 99.1 “ ”. Announcement dated April 8, 2013, entitled 99.2 “ ”. Announcement dated April 8, 2013, entitled 99.3 “ ”. Announcement dated April 8, 2013, entitled 99.4 “ ”. Announcement dated April 8, 2013, entitled 99.5 “ ”. Announcement dated April 8, 2013, entitled]

[CNOOC Limited Hua Zhong Joint Company Secretary Dated: March 22, 2013 Description 99.1 “2012 Annual Results Announcement ” Announcement dated March 22, 2013, entitled 99.2 “Lay A Solid Foundation for Future Growth ” Press Release dated March 22, 2013, entitled 99.3 “Announcement in Respect of Proposed Change of Independent Auditors ” Announcement dated March 22, 2013, entitled]

[NEXEN INC. and DEUTSCHE BANK TRUST COMPANY AMERICAS and CNOOC LIMITED FOURTH SUPPLEMENTAL INDENTURE to the SENIOR DEBT INDENTURE Dated as of May 4, 2007 Fourth Supplemental Indenture CBCA Nexen DEUTSCHE BANK TRUST COMPANY AMERICAS Trustee Guarantor FOURTH SUPPLEMENTAL INDENTURE (this “ RECITALS WHEREAS, Original Indenture First Supplemental Indenture Second Supplemental Indenture Third Supplemental Indenture Indenture WHEREAS, Arrangement WHEREAS, Proposed] [Dated as of March 22, 2013 NEXEN INC. -and- CIBC MELLON TRUST COMPANY -and- THE BANK OF NEW YORK MELLON -and- CNOOC LIMITED SEVENTH SUPPLEMENTAL INDENTURE to the TRUST INDENTURE Dated as of April 28, 1998 TABLE OF CONTENTS 1. INTERPRETATIONS AND AMENDEMENTS 2 1.1 Supplemental Indenture 2 1.2 Definitions in Indenture 2 1.3 Interpretation not Affected by Headings 2] [New York Menlo Park Washington DC São Paulo London Paris Madrid Tokyo Beijing Hong Kong LLP Davis Polk & Wardwell 450 Lexington Avenue New York, NY 10017 212 450 4000 tel 212 450 5800 fax March 22, 2013 CNOOC Limited 65th Floor, Bank of China Tower One Garden Road, Central Hong Kong Ladies and Gentlemen: . We, as your counsel,] [March 22, 2013 File: 80545/311 65th Floor, Bank of China Tower One Garden Road, Central Hong Kong Seventh Supplemental Indenture Canadian Trustee U.S. Trustee Trustee Original Indenture First Supplemental Indenture Second Supplemental Indenture Third Supplemental Indenture Fourth Supplemental Indenture Fifth Supplemental Indenture Sixth Supplemental Indenture Original Supplemental Indentures Indenture We have acted as Canadian counsel to Nexen Inc. in connection] [New York Menlo Park Washington DC São Paulo London Paris Madrid Tokyo Beijing Hong Kong Resident Hong Kong Partners Davis Polk & Wardwell Hong Kong Solicitors The Hong Kong Club Building 3A Chater Road Hong Kong 852 2533 3321 tel 852 2533 1721 fax lillian.lian@davispolk.com * William F. Barron * Bonnie Chan * † Paul K. Y. Chow † Antony]