Abstract: For the districts with large trap areas and less well data, probability method is the most effective way to estimate entrapped petroleum resources. Firstly, the rationality of each geological parameter distribution model is checked with A-D test method, K-S test method and Chi-square (χ2) test for oilfield A in Pearl River Mouth basin. Meanwhile, the validity of reserves parameter distribution is tested with their geological meanings and related rules stipulated by SPE and SEC, based on which, a parameter probability distribution model suitable for oilfield A is established. Taking Z7 reservoir of oilfield A as an example, using the above mentioned methods and processes and considering the saddle-like trap configuration and low degree of well control in Z7 reservoir, shape factors are introduced to reduce the influences of irregular structural shape on reserves estimation. The case study shows that quantitative probability distribution of each reserves parameter can be obtained by using the probability method, which is helpful for uncertainty analysis of factors influencing reserves estimation; and more objective and comprehensive evaluations on petroleum resource potential and risks could be made. The calculated reserves obtained from probability method and deterministic method are close to each other in Z7 reservoir, with the error less than 2%. The study shows that the reserves estimation method presented in this paper has certain guidance significance for resource evaluation in the districts with large trap areas and less well data.