Reservoir quality of the Lower-Middle Permian Shan 2 and He 1 members in the Ordos Basin, China: Implications for depositional and diagenetic processes and the role of volcanic tuffaceous sediment in tight sandstones

The Permian Shan 2 and He 1 members (Shan 2-He 1 Mbr) are the main tight gas-producing intervals in the Ordos Basin. This reservoir mainly comprises low-permeability and ultra-low-permeability fluvial-deltaic sandstones. However, the formation of tight sandstone reservoirs, particularly those associated with volcanic tuffaceous sediment, remains poorly understood. Therefore, in this study, we aimed to identify the most favourable mechanism for reservoir formation using petrological, mineralogical, scanning electron microscopy with energy dispersive spectroscopy, X-ray diffraction, and porosity and permeability analyses to investigate the characteristics and porosity changes of the Shan 2-He 1 Mbr. The reservoir consists predominantly of lithic and lithic quartz sandstones, with an average porosity of 6.07% and average permeability of 0.434 mD. Depositional processes were crucial in controlling reservoir formation. Different sedimentary microfacies exhibited varying physical properties, of which coarse-grained braided bars and channel fill that formed in high-energy environments exhibited superior petrophysical properties. Rapid early compaction was the main cause of reservoir densification. Additionally, illite cement, secondary quartz grain enlargement, and late-stage carbonate cement negatively affected the reservoir quality. Secondary pores formed through the dissolution of terrigenous clasts, volcanic tuffaceous sediment (VTS), and carbonate cement. Diagenetic processes introduced heterogeneity and modified the reservoir, resulting in an effective reservoir space that comprises dissolution and intercrystalline pores, with limited primary pores. The reservoir physical properties were influenced by the sedimentary facies, siliceous mineral content, diagenetic processes, and the presence of VTS. Therefore, a comprehensive understanding of these factors is vital for identifying favourable reservoir formation conditions in the Shan 2-He 1 Mbr.