Exhibit
12.1



CERTIFICATIONS



I, Guangyu Yuan, certify that:



| | 1. | I
                                         have reviewed this annual report on Form 20-F of CNOOC Limited as amended by Amendment
                                         No.1 thereto;
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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: April
19, 2018



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