SHANGHAI—The Chinese energy company China National Offshore Oil Corp. said it sanctioned one of its top lawyers for conflicts of interest with Baker & McKenzie, a U.S. law firm that has often represented it and that in Beijing is led by the woman’s husband.

In an email sent Monday to at least two major international law firms amid widespread industry talk about the relationship, Cnooc said it found that Karen Kang Xin violated unspecified national regulations and its corporate policies by accepting trips to Europe and...