Abstract: Based on the analysis of thin section, X-ray diffraction, formation water data, the characteristics and genesis of the low-resistivity reservoir in upper submember of Es3 are systematically studied in Huimin sag of Baohai Bay basin. In the study area, the resistivity of the low-resistivity reservoir is less than 10 Ω·m, the water saturation is higher than 50% and the resistivity index is lower than 2. The reservoir is dominated by siltstone with relatively high mud content. The clay minerals are mainly composed of kaolinite and illite/montmorillonite mixing layer, with the secondary of Illite. The reservoir space is dominated by primary pores. The porosity is 3.70%~31.60%, averaging 20.63% and the permeability is 0.6~55.4 mD. The samples with the permeability less than 10.0 mD account for 79%. Therefore, the reservoir is classified as mid-high porosity and low-permeability reservoir. The salinity of formation water in the low-resistivity reservoir is relatively high with the average of 31.43 g/L. Multi stages of volcanism and faulting resulted in the upward invasion of high-salinity formation water and then the typical low-resistivity reservoirs formed in the study area