Influence of diagenetic evolution on tight sandstone reservoir flow capacity: Chang 8 reservoir of Yanchang Formation in southern Maling, Ordos Basin

The difference in diagenetic evolution is the main reason for the difference in microscopic pore-throat structure affecting storage and flow capacity of reservoirs. The diagenesis and porosity evolution of the Chang 8 reservoirs in the southern Maling area of the Ordos Basin were studied quantitatively via many types of experiments, including thin sections, scanning electron microscopy, cathodoluminescence, X-ray diffraction, high pressure mercury intrusion and nuclear magnetic resonance. The Chang 8 sandstones have similar rock types, with lower compositional and higher structural maturities. The stage of diagenetic evolution is from the late stage A to the early stage B. The difference in diagenetic evolution is mainly reflected in the difference between the contents of rigid and plastic particles on compaction. For sandstones with relatively high compaction, dissolution is the main factor controlling reservoir quality. For sandstones that have undergone a certain compaction, in which residual intergranular pores are still developing, the filling degree of chlorite films and calcareous cement is the key factor affecting reservoir quality. Quantitative calculation of pore evolution reveals that the pore reduction rates due to compaction and cementation are 49.51% and 30.6%, respectively. The amount of dissolution is 7.0%. Compaction is the main factor that causes reservoir densification in this area. Different diagenetic transformation types have different effects on the development characteristics of reservoir pore structure. Reservoirs with chlorite film shows the best quality. The feldspar dissolution-based reservoir has better reservoir capacity, but poorer permeability. Reservoirs with strong compaction and calcium cementation are relatively poorer quality. The degree of compaction, chlorite cementation, dissolution and calcium filling are the main factors affecting the pore size, reservoir capacity and permeability.