biostudies-literatureLiu XNatural Science Foundation of Chongqing, ChinaScience and Technology Special Project of PetroChina Company LimitedScience and Technology Research Program of Chongqing Education Municipal Commissione0328726https://www.ebi.ac.uk/biostudies/studies/S-EPMC12410822biostudies-literatureUnknown20(9)Accurately determining initial water saturation is essential for assessing reservoir resources and optimizing development, yet conventional J-function methods face challenges such as variability in curve fitting due to sample heterogeneity, less-than-optimal grouping schemes, and limited core sample availability. This study addressed these issues by analyzing 55 J-function curves from the N reservoir in Iraq's Zagros Basin, identifying permeability as the primary factor influencing curve shape. A novel classification method was developed, categorizing curves into power function and exponential function types based on permeability thresholds. Improved J-function models were established by correlating undetermined coefficients with the reservoir quality index (RQI), enabling continuous, tailPloS oneA delicate calculation method for reservoir initial water saturation based on a novel approach to J-function construction.PMC12410822KJQN202301249KJZD-K2022012042024DJ1001CSTB2022NSCQ-MSX1135Liu XYang HHong LKang HDong LQu YfalseA delicate calculation method for reservoir initial water saturation based on a novel approach to J-function construction.Accurately determining initial water saturation is essential for assessing reservoir resources and optimizing development, yet conventional J-function methods face challenges such as variability in curve fitting due to sample heterogeneity, less-than-optimal grouping schemes, and limited core sample availability. This study addressed these issues by analyzing 55 J-function curves from the N reservoir in Iraq's Zagros Basin, identifying permeability as the primary factor influencing curve shape. A novel classification method was developed, categorizing curves into power function and exponential function types based on permeability thresholds. Improved J-function models were established by correlating undetermined coefficients with the reservoir quality index (RQI), enabling continuous, tail2025-01-01T00:00:00Z20252026-05-29T19:43:07.379Z2026-04-08T05:50:01.384ZS-EPMC124108224090668210.1371/journal.pone.0328726