Due to the complex lithology of fine-grained sedimentary rocks and the difficulty to determine the “sweet spot” of the fifth member of Xujiahe Formation（short for Xu-5 Member） of western Sichuan Depression

Sichuan Basin

a comprehensive geological analysis was conducted. This analysis encompassed various geological data sources

including X-ray diffraction

thin sections

scanning electron microscopy

physical properties

organic carbon content

and elemental analysis. Additionally

core observations were incorporated into the study. Upon scrutinizing this array of data

a classification of 14 distinct lithofacies types was established. This categorization was formulated based on the analysis of lithofacies and the evolution of sedimentary paleoenvironments. Furthermore

through a comparative examination of shale gas evaluation parameters

the primary lithofacies types constituting the “sweet spots” were identified. This not only sheds light on the composition of Xu-5 Member but also provides valuable guidance for the exploration of continental shale gas within this formation. The results indicate that: ①The main lithofacies of Xu-5 Member in western Sichuan Depression are massive clayey siltstone and massive mixed sedimentary rock

followed by laminated mixed sedimentary rock

laminated clayey siltstone and massive silty clay rock; ②The sedimentary paleoenvironment of Xu-5 Member undergoes the process from dry heat sedimentary with shallow brackish water to wet sedimentary with deep fresh water

then back to dry heat sedimentary with shallow brackish water. The land source input decreases first and then increases. The middle part of Xu-5 Member is the “sweet spot” for the enrichment of organic matter. ③Laminated clayey siltstone and laminated mixed sedimentary rock are favorable lithofacies types for “sweet spot” development due to the high organic matter content

debris particle dissolution pore

high content of brittle minerals and good fracturing performance.