Shares of PetroChina, the country's largest oil and gas producer, fell 4.4 per cent after the firm said three of its top managers were under investigation by Beijing for alleged violations of discipline.
The stock of Kunlun Energy, its gas distribution unit, tumbled 13.5 per cent.
Analysts said the lack of clarity about the allegations and whether they involved the firms' assets troubled investors, but they expected the share prices to recover, as replacements had been identified for the managers, who resigned for "personal reasons".
Li Hualin, PetroChina's board secretary and vice-president; Ran Xinquan, vice-president and general manager of operations at its largest gas field; and chief geologist Wang Daofu were being investigated, the State Council's State-owned Assets Supervision and Administration Commission (Sasac) said on Tuesday.
Li was also chairman of Kunlun, which benefited from asset injections from PetroChina at cheap valuations.
"[All the] positions have been filled with new candidates, and the change is unlikely to result in an adjustment in … strategy," a Standard Chartered research note said.
But Jefferies Securities analysts cautioned that "this may be the beginning of a much-needed house cleaning". They said: "We have always believed that PetroChina's management sat on some very good assets but did not run a very tight ship."
Caixin Online quoted unnamed sources at parent China National Petroleum Corp as saying the National Audit Office and other disciplinary bodies have begun investigations of PetroChina. A PetroChina spokesman said he is not aware of any probe and it is normal for some units of the firm to be audited at any time.