Characteristics of Lower Paleozoic strike-slip faults and their significance for oil and gas exploration in Xunyi-Yijun area, Ordos Basin

In recent years, the Ordos Basin has undergone a reevaluation, transitioning from a perception of a single block to a focus on understanding the influence of multiple stages and fault systems on hydrocarbon accumulation. This study utilizes the latest 3D seismic and drilling data alongside a theoretical model of strike-slip fault to analyze fault geometry and kinematics in the Xunyi-Yijun area of the basin's southern region. The mechanisms of fault formation are predicted and the related impacts on the formation and accumulation of deep carbonate rocks are also discussed. The results indicate the presence of three fault systems in the Xunyi-Yijun area, with decoupling between deep and shallow layers. The Lower Paleozoic strike-slip faults exhibit distinct characteristics, including high vertical structure, dip swing, and a "flower-shaped" structure in the section. The faults display "compression shear in the northwest, tension shear in the center, and segmental deformation on trunk fractures" on their planes.Moreover, the Lower Paleozoic strike-slip faults experienced two stages of tectonic activities with greater intensity observed from the Late Ordovician to the Silurian, marking the primary period of fault development. The activity intensity during the Middle to Late Permian was relatively weaker, with faults exhibiting inherited strike-slip characteristics. These faults formed two groups, one with a NE strike and the other with a NW strike. Additionally, during the second stage of the Caledonian period, ocean subduction and closure lead to stress being transferred to the basin interior. It is believed that the NE and NW strike basement weak zones existed in the southern Ordos Basin. As a result, the Lower Paleozoic strike-slip fault system likely formed due to the activation of oblique compression. This fault system has the potential to enhance the physical properties of deep carbonate reservoirs and create high-quality karst or tectonic fracture reservoirs in the Xunyi-Yijun area. Moreover, The formation of the "upper source and lower reservoir-lateral source and lateral reservoir" combination along the strike-slip fault belt is an area of interest for deep oil and gas exploration in the Xunyi-Yijun region.