Diagenetic constraints on the heterogeneity of tight sandstone reservoirs: A case study on the Upper Triassic Xujiahe Formation in the Sichuan Basin, southwest China

Abstract Heterogeneity is an essential feature of tight sandstones because it is directly related to reservoir quality. The Upper Triassic Xujiahe Formation in the Sichuan Basin was used as a case study to analyse the diagenetic constraints on the heterogeneity of tight sandstone reservoirs. The three main diagenetic processes of interest are mechanical compaction, cementation, and dissolution. A dissolution facies, quartz cement facies, and carbonate cement facies present sequentially from the centre to the margin of sand bodies. Abundant feldspars and rock fragments were dissolved to form secondary porosity or filled by carbonate cement. Authigenic chlorite coatings, generated from eogenesis to the deep burial stage, are thickest in high-porosity-samples. The δ 13 C of carbonate cement ranges from −10.56‰ to 1.15‰ (PDB), suggesting multiple sources of carbon, including carbonate fragments, early inorganic carbonate cement, and organically derived CO 2 . Based on δ 18 O, carbonate cementation occurred from 33 °C to 110 °C. Microthermometric data of aqueous inclusions indicate that quartz cement precipitated at a temperature range of 120 °C–140 °C. A summary model of the diagenetic constraints on the heterogeneity of sandstones was constructed. The spatial distribution of digenetic facies is controlled by differences in fluid flow velocity and pH. Fluids, especially hydrocarbons, migrated preferentially within the centre of sand bodies, and caused dissolution. Dissolution generated secondary porosity and provided materials for authigenic mineral precipitation. Carbonate fragments were better preserved and superimposed with later carbonate cement along the sandstone-mudstone boundary. Quartz overgrowths formed after carbonate cementation, and developed in the transition zone between the intervals of carbonate cementation and dissolution. Consequently, the reservoir quality of the Xujiahe sandstones decreases gradually moving from the centre to the margin of the sand bodies.