Selective dissolution of eodiagenesis cements and its impact on the quality evolution of reservoirs in the Xing'anling Group, Suderte Oil Field, Hailar Basin, China

Reservoirs in the Xing'anling Group in the Suderte Oil Field, Hailar Basin exhibit ultra-low to low permeability and high tuffaceous material content. This study comprehensively analyzed diagenesis and quality evolution of these low-permeability reservoirs using thin sections, SEM samples, rock physical properties, pore water data, as well as geochemical numerical simulations. Calcite and analcite are the two main types of cements precipitated in the eodiagenetic stage at shallow burial depths in the reservoirs. These two cements occupied significant primary intergranular pores and effectively retarded deep burial compaction. Petrography textures suggest selective dissolution of massive analcite and little dissolution of calcite in the mesodiagenetic stage. Chemical calculations utilizing the Geochemist's Workbench 9.0 indicated that the equilibrium constant of the calcite leaching reaction is significantly smaller than that of the analcite leaching reaction, resulting in extensive dissolution of analcite rather than calcite in the geochemical system with both minerals present. Numerical simulations with constraints of kinetics and pore water chemistry demonstrated that the pore water in the Xing'anling group is saturated with respect to calcite, but undersaturated with analcite, leading to dissolution of large amounts of analcite and no dissolution of calcite. Significant secondary intergranular pores have formed in analcite-cemented reservoirs from selective dissolution of analcite in the mesodiagenetic stage; the analcite dissolution formed preferential flow paths in the reservoirs, which promoted feldspar dissolution; and dissolution of such minerals led to the present reservoirs with medium porosity and low permeability. Calcite-cemented tight reservoirs have not experienced extensive dissolution of cements, so they exhibit ultra-low porosity and permeability.