Diagenesis and reservoir quality in tight gas sandstones: The fourth member of the Upper Triassic Xujiahe Formation, Central Sichuan Basin, Southwest China

Abstract

Mineralogical, petrographic, and geochemical analyses were performed to investigate the sandstone composition and texture, pore system, and diagenetic minerals of Upper Triassic sandstones in the Central Sichuan Basin, Southwest China. The results show that authigenic quartz, clay minerals, and carbonates are the main pore‐filling constituents. The pore systems consist of intergranular porosity, secondary porosity, microporosity, and microfractures. Loss of depositional porosity is greater due to compaction than to cementation. Eodiagenesis includes mechanical compaction, precipitation of early calcite cements, kaolinite, and smectite, and leaching of feldspars by meteoric water. Mesodiagenesis consists of compaction, framework grain dissolution, and subsequent precipitation of illite, quartz, and late carbonate cements. Six lithofacies were identified on the basis of petrographic analyses, namely, (a) argillaceous sandstones; (b) poorly sorted sandstones; (c) quartz‐cemented sandstones; (d) carbonate‐cemented sandstones; (e) clay‐mineral‐cemented sandstones, and (f) clean sandstones with abundant secondary porosity. The best reservoir‐quality rocks have high percentages of detrital quartz but low percentages of matrix, quartz, and carbonate cement. Differences in textural and compositional attributes of various lithofacies significantly affect porosity‐depth trends. The diagenetic evolution pathways and reservoir‐quality prediction models of various lithofacies are reconstructed by integrated petrographic data. This work provides insights into describing the different lithofacies by petrographic analysis and helps to investigate how detrital composition and texture influence the diagenesis and reservoir quality evolution in tight gas sandstones.