Overpressure determination is essential for sweet spot identification, resource evaluation as well as drilling in organic-rich shale, a typical self-contained source-reservoir system. Traditional pressure prediction methods designed for conventional source rocks (e.g., the Eaton’s method) were typically established based on mechanical compaction, which did not consider the increase in organic porosity. In this study, a practical approach was introduced to investigate the impact of organic pores on overpressure estimation in organic-rich shale, where overpressure primarily results from gas generation, through introducing organic pores into the expression of sonic transit time and determining overpressure with organic-pore–corrected sonic readings. The proposed organic porosity and overpressure estimation models were applied to the Duvernay Shale of the Western Canada Sedimentary Basin to investigate the impact of organic porosity on prediction of overpressure due to fluid expansion in gas generation windows. A comparison of formation pressure predictions from the application example in the Duvernay Shale demonstrates that the proposed method can significantly improve prediction of the pore fluid pressure in mature source rock. The revised method provides better estimates of overpressure with an improved coefficient of determination of R² = 0.7. In contrast, without considering the impact from organic pores in the shale, the traditional Eaton’s method overestimates the pressure by up to 42.83% and the correlation between the predicted and the observed results is poor (R² = 0.15).