Exhibit 12.1







CERTIFICATIONS



I, Guangyu Yuan, certify that:



| | 1. | I have reviewed this annual
                                         report on Form 20-F of CNOOC Limited;
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| | 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, not misleading with respect to the period covered by this
                                         report;
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| | 3. | Based on my knowledge, the financial
                                         statements, and other financial information included in this report, fairly present in
                                         all material respects the financial condition, results of operations and cash flows of
                                         the company as of, and for, the periods presented in this report;
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| | 4. | The company’s other certifying
                                         officer(s) and I are responsible for establishing and maintaining disclosure controls
                                         and procedures (as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal
                                         control over financial reporting (as defined in Exchange Act Rules 13a-15(f) and 15d-15(f))
                                         for the company and have:
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| | (a) | Designed such disclosure controls
                                         and procedures, or caused such disclosure controls and procedures to be designed under
                                         our supervision, to ensure that material information relating to the company, including
                                         its consolidated subsidiaries, is made known to us by others within those entities, particularly
                                         during the period in which this report is being prepared;
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| | (b) | Designed such internal control
                                         over financial reporting, or caused such internal control over financial reporting to
                                         be designed under our supervision, to provide reasonable assurance regarding the reliability
                                         of financial reporting and the preparation of financial statements for external purposes
                                         in accordance with generally accepted accounting principles;
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| | (c) | Evaluated the effectiveness
                                         of the company’s disclosure controls and procedures and presented in this report
                                         our conclusions about the effectiveness of the disclosure controls and procedures, as
                                         of the end of the period covered by this report based on such evaluation; and
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| | (d) | Disclosed in this report any
                                         change in the company’s internal control over financial reporting that occurred
                                         during the period covered by the annual report that has materially affected, or is reasonably
                                         likely to materially affect, the company’s internal control over financial reporting;
                                         and
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| | 5. | The company’s other certifying
                                         officer(s) and I have disclosed, based on our most recent evaluation of internal control
                                         over financial reporting, to the company’s auditors and the audit committee of
                                         the company’s board of directors (or persons performing the equivalent functions):
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| | (a) | All significant deficiencies
                                         and material weaknesses in the design or operation of internal control over financial
                                         reporting which are reasonably likely to adversely affect the company’s ability
                                         to record, process, summarize and report financial information; and
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| | (b) | Any fraud, whether or not material,
                                         that involves management or other employees who have a significant role in the company’s
                                         internal control over financial reporting.
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Date:
April21,
2017



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