Diagenesis and pore evolution of tight sandstone reservoir: a case study from the Upper Triassic reservoir of the southwest Sichuan Basin, China

Diagenesis of tight sandstone reservoir dominates pore evolution, and further controls the distribution of relatively high-quality reservoirs. The Upper Triassic sandstone reservoir in the southwest Sichuan Basin, with overall porosity less than 6% and permeability less than 0.1×10-3 μm2, is a typical tight sandstone reservoir. In this study, a comprehensive analysis of cores, cast slices and fluid inclusion shows that the tight sandstone reservoir-dominated by intragranular dissolved pore, intergranular dissolved pore and moldic pore-experienced compaction, cementation, dissolution and metasomatism. Paragenetic relationship and fluid inclusion homogenization temperatures reveal the succession of the Upper Triassic sandstone: compaction→first phase (sequentially) fracture, illite cementation, quartz overgrowth and calcite cementation→second phase quartz overgrowth and siliceous infill→dissolution of feldspar and debris→chlorite cementation or illite cementation→second phase calcite cementation→dissolution of feldspar and debris→second phase fracture→third phase quartz overgrowth and siliceous infill followed by carbonate cementation→carbonate replacement of feldspar and quartz→third phase fracture→late stage calcite and quartz cementation. The measured vitrinite reflectivity (Ro) value and homogenization temperature of fluid inclusions indicate the sandstone is currently in the A2 to B diagenetic stage, when compaction, the main factor of reservoir densification, causes roughly 27.5% porosity loss; while siliceous and carbonate cementation are responsible for only 5.3% porosity loss. © 2018, Editorial Office of Earth Science Frontiers. All right reserved.