Abstract: Water curtain system effectiveness is important for the safe and stable operation of underground crude oil storage caverns; however, current methods on evaluating the water curtain systems effectiveness are not efficient and convenient. Based on the first large water sealing petroleum storage caverns in China, this study analyzed the hydrogeochemical feature and the mineral composition characteristic of rock mass by water quality tests, SEM analysis, and statistical analysis. The groundwater curtaining system effectiveness were then assessed based on the hydraulic connection analysis. The results show that groundwater is dominated by HCO₃ – Na•Ca type water in the early stage of oil storage running. With the interaction between groundwater and mineral components in rock, such as potassium feldspar, albite, and anorthite, the concentration of K⁺, Na⁺, Ca²⁺, Mg²⁺, and \mathrmHCO_3^- increases, and pH decreases. The concentration of Cl⁻ maintained in the allowable range, indicating no groundwater corrosion on the support system. The similarity of hydrogeochemical characteristics between water in the water curtaining system and background groundwater shows that water from the system has a hydraulic connection with the background groundwater, forming a good containment for the storage. This study provides an important basis for safe and effective operation of oil storage and a scientific method for judging the water curtain system effectiveness.