Abstract: Low-grade faults are relatively developed in the Gufengzhuang area on the western margin of the Ordos Basin. Their characteristics and sealing capabilities are closely related to hydrocarbon accumulation. It is significant to analyze the characteristics and sealing capacities of the faults for petroleum exploration under the complex tectonic conditions. The characteristics of the low-grade faults was analyzed based on seismic and well data, and the sealing capabilities of the faults were evaluated using the displacement pressure difference between reservoirs and faults, according to the matching relationship between faults and hydrocarbon accumulation and the lithology of the Yanchang Formation. The result indicates that the faults are mainly reversed Late Jurassic to Early Cretaceous faults characterized by small fault throw, large dip and short extension, and in echelon distribution from NW to SE. The Yanchang Formation in the study area is thin interbeds of sandstone and mudstone, which are easy to develop fault rocks, so the sealing capability depends on the displacement pressure difference between the fault rock and related reservoir. The mudstone in the fault rock accounts for 20% to 60%, and the displacement pressure is generally less than 1.5MPa, which is generally smaller than the displacement pressure in the reservoir. Most faults are open, and less are close. During the early time of the Early Cretaceous, the open faults acted as transport systems for hydrocarbon migration, and hydrocarbon accumulated in low-relief traps and high-permeability reservoirs near the faults, controlled by the surrounding rock and sealing faults.

Key words: western margin of the Ordos Basin, low-grade fault, quantitative evaluation of fault sealing capability, fault rock, displacement pressure