In order to find the development position of high quality reservoir and determine the direction of later exploration and development in Weixinan Sag of Beibuwan Basin, this paper studied the location and type of abnormal clay mineral transformation in Weizhou Formation, Weixinan Sag, and identified the cause of abnormally high porosity and permeability zones based on the method of combining experimental analysis and thermodynamic calculation. The results show that, under the influence of thermal fluid activity, abnormal transformation of clay minerals is developed in the study area at the burial depth of 2 300~2 900 m, which can be divided into two types: chloritization and kaolinization. Thermodynamic calculation results also confirm that the chloritization of kaolinite and kaolinization of illite have thermal mechanical priority in Weixinan Sag, which is the important cause of high quality reservoir of the Weizhou Formation in Weixinan Sag. The abnormally high porosity and permeability zones are concentrated in two layers, where the the proportion of chlorite mass is more than 35% (the proportion of kaolinite mass is between 30% and 35%) and the proportion of kaolinite mass is more than 45% (the proportion of chlorite mass is between 10% and 20%). Among them, abnormal kaolinization is mainly developed within the depth range of 2 300~2 400 m and 2 500~2 700 m, abnormal chloritization is mainly developed within the depth range of 2 400~2 500 m and 2 700~2 900 m.