PetroChina, with some 550,000 employees, is Asia's biggest oil producer by volume and the world's second-most-valuable energy company by market capitalization, behind Exxon Mobil Corp.
On Monday, PetroChina denied a news report that one of its vice presidents and its chief accountant had been taken in for questioning by anti-corruption investigators.
Political analysts say the investigation appears to be part of efforts by China's new leadership under President Xi Jinping to tighten control over state-owned energy companies.
PetroChina's former chairman, Jiang Jiemin, was fired last week as head of the Cabinet body that oversees China's biggest state-owned companies, the State-owned Assets Supervision and Administration Commission.
The commission's Communist Party secretary, Zhang Yi, visited rank-and-file PetroChina employees at two oilfields in China's northeast last week to affirm the ruling party's faith in their work, according to a SASAC statement.
The newspaper 21st Century Business Herald said Zhang's visit was an effort to "stabilize morale."