Re-Evaluation of the Source Rocks of the Upper Triassic Xujiahe Formation in the Sichuan Basin

The Upper Triassic Xujiahe Formation (T3x) represents a critical terrestrial source rock system in the Sichuan Basin, exhibiting pronounced vertical and lateral heterogeneity. Previous stratigraphic subdivisions relied primarily on lithological correlations rather than a systematic sequence stratigraphic framework. This approach has led to significant inconsistencies in source rock evaluation. Furthermore, recent discoveries of large gas fields, coupled with data from newly drilled wells, necessitate a comprehensive reassessment of this system. In this study, we re-evaluate the geochemical characteristics and spatial distribution of these source rocks within a newly established sequence stratigraphic framework. This assessment utilizes a robust dataset comprising total organic carbon (TOC) content, Rock-Eval pyrolysis, and vitrinite reflectance (Ro) measurements. The results indicate that the source rocks of the New Member 5 (T3x5) in the slope belt of Central Sichuan exhibit the highest hydrocarbon generative potential. These rocks are characterized by high organic abundance (with 40% of samples showing TOC ≥ 2.0 wt.%), are dominated by Type III and II2 kerogen (humic–sapropelic), and have reached the mature to high-maturity stage (Ro ranging from 1.0% to 1.7%). Notably, the cumulative thickness of these high-quality source rocks reaches 100~150 m. Specifically, the T3x5 intervals in the Qiulin and Tianfu areas are identified as the most favorable hydrocarbon-generating centers. This reassessment under the new stratigraphic division provides a refined theoretical basis for future exploration targeting the Xujiahe Formation in the Sichuan Basin.