Nexen Responds to a Pipeline Failure Near its Long Lake Facility in Alberta

Calgary, Alberta, July 16, 2015 – Nexen Energy ULC (Nexen) is responding to an incident at its Long Lake oil sands facility south of Fort McMurray.

In the afternoon of July 15, 2015, an emulsion leak from a pipeline was discovered within Nexen’s Long Lake operations. Our emergency response plan has been activated and our response personnel are onsite. The leak has been stabilized and the site is under control.

The size of the spill is estimated to be 5,000 m3 of emulsion, over an area of approximately 16,000 m2 mostly within a compacted pipeline corridor. Emulsion is a mixture of bitumen, produced water and sand.

All necessary steps and precautions have been taken, and Nexen will continue to utilize all its resources to protect the health and safety of our employees, contractors, the public and the environment, and to contain and clean up the spill. There were no injuries due to this incident and the pipeline has been isolated. Nexen continues to investigate.

Nexen has informed the Alberta Energy Regulator and will continue to work with them throughout our response.

Nexen is a wholly-owned subsidiary of CNOOC Limited.

This release includes “forward-looking statements” within the meaning of the United States Private Securities Litigation Reform Act of 1995, including statements regarding expected future events, business prospectus or financial results. The words “expect”, “anticipate”, “continue”, “estimate”, “objective”, “ongoing”, “may”, “will”, “project”, “should”, “believe”, “plans”, “intends” and similar expressions are intended to identify such forward-looking statements. These statements are based on assumptions and analyses made by the Company in light of its experience and its perception of historical trends, current conditions and expected future developments, as well as other factors the Company believes are appropriate under the circumstances. However, whether actual results and developments will meet the expectations and predictions of the Company depends on a number of risks and uncertainties which could cause the actual results, performance and financial condition to differ materially from the Company’s expectations, including but not limited to those associated with fluctuations in crude oil and natural gas prices, the exploration or development activities, the capital expenditure requirements, the business strategy, whether the transactions entered into by the Group can complete on schedule pursuant to their terms and timetable or at all, the highly competitive nature of the oil and natural gas industries, the foreign operations, environmental liabilities and compliance requirements, and economic and political conditions in the People’s Republic of China. For a description of these and other risks and uncertainties, please see the documents the Company files from time to time with the United States Securities and Exchange Commission, including the 2014 Annual Report on Form 20-F filed on 22 April 2015.

Consequently, all of the forward-looking statements made in this press release are qualified by these cautionary statements. The Company cannot assure that the results or developments anticipated will be realised or, even if substantially realised, that they will have the expected effect on the Company, its business or operations.