A new method for the calculation of secondary porosity originating from the dissolution of feldspars in deeply buried formations and its application: A case study of the Chang 8₁ Formation in Longdong area, Ordos Basin

Secondary porosity is an important accumulation space in clastic reservoirs. Among secondary pores, those pores originating from the dissolution of feldspars are dominant. A new method to calculate the volumes of secondary porosity from the dissolution of feldspars in deeply buried formations based on thermodynamic principle is proposed. The calculation method is as follows. The volume of secondary porosity originating from the dissolution of potassium feldspars=0.28×kaolinite content or 0.36×illite content. The volume of secondary porosity originating from the dissolution of albite feldspars=0.10×kaolinite content or 0.17×illite content. The volume of secondary porosity originating from the dissolution of anorthite feldspars=0.014×kaolinite content or 0.08×illite content. After a thorough investigation of petrologic characteristics, the new calculation method was applied to the Chang 8₁ Formation in Longdong area, Ordos Basin. The volumes of secondary porosity originating from the dissolution of feldspars in deeply buried formations in the Chang 8₁ Formation in the study area were calculated with the new method, and were compared with measured and simulated porosities. The average value of calculated secondary porosity from the dissolution of feldspars of 51 core samples from the Chang 8₁ Formation is 1.32%, which is close to the average measured value (1.44%) for those samples. The comparison between calculated and simulated results also confirms the reliability of this new method.