Wells are always drilled in areas with a relatively great probability of good reservoir quality. The data from these well sources are distributed asymmetrically in the interesting area. Usually, some data are spatially clustered. It is necessary to correct this preferential clustering before making statistics. This paper introduces the Thiessen polygon method to determine spatial affecting sphere of each well and provides a new idea to solve the problem of determining polygon for the polygonal declustering method. A case study in the Shinan oil field is presented. The research result shows that this method is feasible.