Abstract: Total organic carbon (TOC) content is the most important parameter in determining the quality of source rocks. In a case study of a transitional shale gas reservoir in the Longtan Formation at Well C1 in area Q, the Sichuan Basin, our efforts focus on how to improve TOC calculation for coal-measure shale gas reservoirs. Based on the geological characteristics of the study area and natural gamma ray spectral log data, the Δlog R method is modified to formulate a dual Δlog R method based on natural gamma ray spectroscopy logging. The reservoir is lithologically divided into two types: coal seam and non-coal seam, according to core TOC data. Th/K and Th/U correlations with TOC contents in coal seams and non-coal seams are analyzed to select the data with the highest correlations for TOC calculation using Δlog R at coal seams and non-coal seams. The results show improved correlations up to 0.78 and 0.85, decreased absolute errors to 0.01% and 0.01%, and decreased relative errors to 14.93% and 12.53% for TOC calculation at coal seams and non-coal seams, Well C1, respectively compared with the traditional method. The dual Δlog R method based on natural gamma ray spectroscopy logging is also successfully applied to the Y Formation at Well C2 in area S, the Sichuan Basin. The improved method is feasible in the context of high clay content, strong heterogeneity, and black coal seams in the study area; the problem of the low correlation between TOC content and U content could also be solved. This method could be applied to similar prospects and offer technical support to shale gas exploration and production.