Objective The Yanchang Formation Chang 8 reservoir in the Jiyuan region of Ordos Basin possesses abundant tight oil resources. Quantitative characterization and analysis of the present−day in−situ stress field is a key component to ensure profitable development. This study aims to reveal the present−day in−situ stress characteristics of the Chang 8 reservoir and clarify their significance for hydraulic fracturing treatments.

Methods This study uses methods of imaging logging interpretation, in−situ stress experimental testing and conventional logging calculations to conduct a comprehensive study on the present−day dominant orientation, vertical variation patterns and mechanisms of in−situ stress in the Chang 8 reservoir.

Results The results show that: (1) The horizontal maximum principal stress in the Yanchang Formation Chang 8 reservoir of Jiyuan region is mainly in the ENE−WSW direction; (2) The horizontal maximum and minimum principal stress magnitude of the Chang 8 reservoir is in the range of 49.17~65.68 MPa and 41.87~56.31 MPa, respectively. The horizontal principal stress difference is between 2.78~13.39 MPa; (3) Differences in lithology and rock mechanical properties, burial depth, and discontinuous structural planes are the primary factors controlling the vertical variation and orientation of the present−day in−situ stress. Conlcusions The Chang 8 reservoir in the Jiyuan area exhibits a distinct dominant stress orientation and a normal faulting stress regime, providing favorable geomechanical conditions for the formation of complex hydraulic fracture networks. Vertically, hydraulic fractures are prone to extend both upward and downward; laterally, they may be captured by natural fractures. In intervals or zones with relatively high horizontal stress differences, hydraulic fractures are likely to bypass or penetrate natural fractures to continue propagating. The findings can provide in-situ stress field parameters and a foundation for optimizing fracturing for the development of Chang 8 tight oil reservoirs in Jiyuan area.