1. Job transfer: Shi Daohan may have left Changqing Oilfield due to normal job transfer or job adjustment. In this case, resignation is usually normal, in order to give full play to his professional and management skills in a wider range and better serve the company's development.

2. Organizational restructuring: China Petroleum or its management may decide to make personnel changes for various reasons, such as business development and organizational restructuring. In this case, Shi Daohan's resignation may be part of helping the company achieve better development in the new strategic direction.

The above reasons are only some possible situations, and the actual reasons may vary according to the specific circumstances of individuals and organizations. For specific reasons, it is suggested to contact PetroChina or Shi Daohan directly to understand.