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The quality of a caprock is closely related to its thickness. Statistical data show that there is an obvious positive correlation between caprock thickness and hydrocarbon column height. This report established that the starting pressure gradient is the key factor in the relationship between caprock thickness and hydrocarbon column height by discussing the concepts and principles of breakthrough pressure, capillary pressure, and starting pressure. Analysis of the physical–chemical properties of the surfaces of mineral particles revealed that the structure of the diffused electric double layer stably develops in the pore throat at the micro- to nanoscale. This is a microcosm of the starting pressure generated in the argillaceous caprock. Based on force analysis, this report establishes that in the process by which fluid breaks through the argillaceous caprock, hydrocarbons slowly displace the formation water in the pore throats and mainly overcome the capillary pressure and adsorption resistance. The quantitative relationship between the caprock thickness and the maximum hydrocarbon column height is determined by considering the starting pressure gradient, and a method is proposed to calculate the maximum hydrocarbon column height of the caprock based on the formation overpressure. This method is successfully applied, and verification via gas testing is performed for offshore oil and gas fields by considering the X gas reservoir. The proposed method has good application prospects in the evaluation of argillaceous caprocks and reservoir risk prediction.