Meso-Cenozoic structural characteristics and their reservoir controls of structural transition area in China Craton: A case study of Yanchang Formation in Zhenjing area of southwestern Ordos Basin

Characteristics and formation mechanisms of Mesozoic faults in Zhenjing area were analyzed according to 3D seismic data, logging curves and mineral testing, etc. Combining thermal evolution of source rock and reservoir physical property inversion, the dynamic evolution process of hydrocarbon accumulation in Chang 8 Member was restored, and the control effects of tectonic activities since Mesozoic on the formation of Chang 8 hydrocarbon accumulation was discussed. It is considered that the structural deformation of Zhenjing structural transition area in Ordos Basin is significantly affected by the basin margin, through which the complex fault system is developed. The structural characteristics and their evolution play important roles in controlling the formation and distribution of oil and gas reservoirs. The following conclusions are drawn finally: (1) Three groups of Mesozoic faults are developed in NW, NEE and nearly EW directions, with belt slip of en echelon type on the plane, short distance dislocation and high-steep output on the profile. Therein, NW-trending main faults were strike slip during Indosinian; The activity of NE-trending faults strengthened in the Mid-Late Yanshanian; and the density of NE-trending secondary faults increased and a large number of shear fractures were derived in Himalayan Period. (2) The Chang 8 reservoir experienced four stages: its primitive sediments were formed in Late Triassic; its lithologic reservoirs were formed by near-source charging of low-mature oil during Early Cretaceous; the rapid transportation of mature oil happened in Late Cretaceous to form structural-lithologic reservoirs controlled by faults and fractures; and the existing reservoirs have been adjusted since Late Cretaceous. (3) The Indosinian Movement controlled the distribution of source rocks and reservoirs, and the Type I NW-trending strike-slip faults controlled the Mesozoic tectonic framework in Zhenjing area; The Mid-Late Yanshanian accelerated the thermal evolution of source rocks, affected the process of reservoir densification, and the Type II faults provided vertical transport; The Himalayan Movement activated the pre-existing Mesozoic faults, controlled the adjustment range and the differential enrichment of oil reservoirs. Among them, the type II tension-torsion faults in the NEE-trending have good conductivity, which are the most effective oil-rich faults.