The diagenetic controls of the reservoir heterogeneity in the tight sand gas reservoirs of the Zizhou Area in China's east Ordos Basin: Implications for reservoir quality predictions

The Shan 2(3) sandstone formations are important exploratory targets for tight gas resources in the Zizhou Area of the Ordos Basin. In this study, we investigated the effects of diagenesis on the heterogeneity of tight sandstone deposits in similar sedimentary facies and identified the main controlling factors of reservoir quality. A total of 169 samples were collected from 50 wells for a series of analyses. The heterogeneity of the sandstones is mainly controlled by cement and pore-throat characteristics, which showed regional differences in the study area consistent with the regional variations in the detrital components. These results indicate that the diagenesis influenced by the sediment composition is the main controlling factor of the reservoir heterogeneity. The compaction dominated by ductile grains is the major cause of the primary porosity losses in the sandstones, followed by the quartz and clay (kaolinite and illite) cementation. Residual intergranular pores rarely occur in the sandstones with a ductile lithic grain content greater than 2%. Altered kaolinite and recrystallized illite commonly occur in the sandstones with high ductile grain content. Residual intergranular pores, quartz over-growths, and authigenic kaolinite are the most common phases in the sandstones rich in detrital quartz. The dissolution formed secondary pores and provided a minor contribution to reservoirs permeability. Secondary pores predominantly occur in the quartz arenite due to the preservation of the primary pores. The main controlling factor for the quality of the sandstones is the pore type, which has pronounced effects on both the pore structure and the pore-throat size distribution. The residual intergranular pores preserved in sandstones will lead to larger radii and better sorting characteristics of the pore-throats, which subsequently contribute to the higher permeability. Therefore, the sandstones with the higher content of primary intergranular pores are considered high-quality reservoirs. They are characterized by quartz richness and mainly located in the Zizhou-Mizhi Area. The inter-crystalline micropores occur in the clay mineral, resulting in small sized and poor-sorted pore-throats. They give subordinate contributions to the reservoir quality and are the key factor of the extremely low permeability in sandstones. The dry layers of the study area are mainly composed of sandstones dominated by inter-crystalline micropores. These sandstones are characterized by the high content of ductile grains and are mainly located in the Qingjian Area.