The Kangan and Dalan formations are two of the main reservoirs of the South Pars gas field, which is situated in the Zagros sedimentary basin in southwest Iran. Precise assessment of shale volume is essential for the accurate petrophysical characterization of these reservoirs, especially concerning porosity, fluid distribution, and lithology. The influence of shale on reservoir quality needs to be carefully examined because of its intricate organic and mineral compositions, as well as its tiny pore structures. In this study, we examine how shale volume affects petrophysical characteristics in the Kangan and Dalan formations, particularly in gas well P19, which has a thickness of 375 m, ranging from 3363 to 3738 m. We evaluate the impact of shale on important petrophysical parameters using probabilistic techniques. Though the shale content in Kangan and Dalan is small, its impact on petrophysical evaluation is great. This is because shale has bound water and higher total porosity, which, if not correctly handled, results in an overestimation of porosity and water saturation, influencing hydrocarbon volume calculations. Specifically, we proved by comparison that the neglect of shale effects in the porosity model resulted in an overestimation of the effective porosity by 1.1%. The reason for the discrepancy is the impact of shale on petrophysical properties, as evidenced by comparative calculations using the Archie and Indonesia methods. The results show how important shale is for accurate petrophysical characterization and how neglecting its effects can result in serious errors. Shale volume was estimated using a probabilistic method, and a new zonation system (X1, X2, X3) was created to more clearly differentiate reservoir quality within the formations.