Diagenesis and its influence on reservoir quality of Chang 8 ¹ sandstones from Upper Triassic Yanchang Formation in Zhenjing area, Ordos Basin

Chang 8 ¹ reservoir sandstones of upper Triassic Yanchang Formation in Zhenjing area of Ordos Basin mainly consist of feldspathic litharenite and lithic arkose, with low mineralogical maturity and median-high textural maturity. They belong to extra-low porosity and permeability reservoirs, with dissolved intergranular pores as the chief accumulation space. The Chang 8 ¹ reservoir sandstones are now in the A stage of mesodiagenesis, and the main diagenesis of which including compaction, cementation, dissolution and replacement. The mechanical compaction, which principally took place in the early time of burial history, is the predominant reason for reservoir quality deterioration of Chang 8 ¹ sandstones with burial depth above 2000 m. Accompanied by the different cementation and dissolution taking place progressively with the burial depth increasing, the destruction degree resulted by mechanical compaction was weakened. The destruction impact of cementation is characterized by precipitation of different cement in primary and secondary pores, in which the cementation of authigenic clay minerals and late stage carbonate have exerted a dominant influence on reservoir quality, making the porosity and permeability decreased quickly. However, the pore-lining chlorite, authigenic kaolinite and early stage carbonate cement have dual effects on reservoir quality, and the constructive influence of pore-lining chlorite cementation over the destructive one. The pore-lining chlorite has preserved the reservoir quality by weakening the degree of mechanical compaction, inhibiting the subsequent generation of quartz cement, and providing the passageways for the entrance of acid fluid of the A stage of mesodiagenesis and flowout of dissolved resultant products, making a large number of secondary pores produced. Dissolution is the key factor for the formation of secondary pores and improvement of pore throat structures of Chang 8 ¹ reservoir sandstones. There are four dissolution zones in the Chang 8 ¹ sandstones, located in the vicinity of 1800 m, 2000 m, 2100 m, and 2300 m perspectively, with the feldspar as the main dissolved grain type. The result shows that the sandstone intervals with dissolution and pore-lining chlorite cementation occurring together are the optimum zones for the oil and gas accumulation.