Diagenesis and implication for the reservoir quality of the tight sandstone reservoir in the depression zone of foreland basin, Upper Triassic Xujiahe Formation, Western Sichuan Foreland Basin, China: Evidence from petrography and geochemistry

Abstract

Tight sandstones are products of complex and various diagenetic events that significantly affect reservoir quality and heterogeneity. In this study, tight sandstones of the Upper Triassic Xu3 Member in the Western Sichuan Foreland Basin were examined as a case study. Petrographic and geochemical analyses were combined to investigate diagenetic variability and its effects on reservoir quality. The sandstone in this zone consisted of coarse-grained to siltstone-grained poorly to moderately sorted sublitharenite and litharenite. And the sandstone exhibits poor physical properties. The diagenesis during the eodiagenetic stage included compaction, dissolution, early-stage quartz cementation, early-stage carbonate cementation (calcite and dolomite), and kaolinite formation. The mesodiagenetic stage encompassed compaction, dissolution, late-stage quartz cementation, intermediate- and late-stage carbonate cementation (ferro-calcite, ankerite, and calcite in fractures), and clay cementation (kaolinite, illite, and chlorite). Compaction destroyed the tight sandstone reservoir to a greater degree than cementation. Specifically, compaction and cementation reduced the initial porosity by 80.70% and 15.50%, respectively. Dissolution emerges as primary constructive diagenesis, promoting the formation of authigenic minerals. The vertical variability in dissolution and cementation within the cycle contributed to increased reservoir heterogeneity. Relatively high-quality reservoirs in the sandstone overlaying sandstone style occurred at the base of the cycle. In the mudstone overlaying sandstone style, relatively high-quality reservoirs developed at both the base and the top of the sandstone layers. These findings enhance our understanding of diagenetic influences on reservoir properties and the distribution of high-quality reservoirs. Consequently, they offer valuable insights for advancing unconventional oil and gas exploration in depression zones of foreland basins.