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

Reservoirs in the Xing’anling Group in theSuderte Oil Field, Hailar Basin exhibit ultra-low to lowpermeability and high tuffaceous material content. Thisstudy comprehensively analyzed diagenesis and qualityevolution of these low-permeability reservoirs using thinsections, SEM samples, rock physical properties, porewater data, as well as geochemical numerical simulations.Calcite and analcite are the two main types of cementsprecipitated in the eodiagenetic stage at shallow burialdepths in the reservoirs. These two cements occupied significantprimary intergranular pores and effectively retardeddeep burial compaction. Petrography textures suggestselective dissolution of massive analcite and little dissolutionof calcite in the mesodiagenetic stage. Chemicalcalculations utilizing the Geochemist’s Workbench 9.0indicated that the equilibrium constant of the calciteleaching reaction is significantly smaller than that of theanalcite leaching reaction, resulting in extensive dissolutionof analcite rather than calcite in the geochemicalsystem with both minerals present. Numerical simulationswith constraints of kinetics and pore water chemistrydemonstrated that the pore water in the Xing’anling groupis saturated with respect to calcite, but undersaturated withanalcite, leading to dissolution of large amounts of analciteand no dissolution of calcite. Significant secondary intergranularpores have formed in analcite-cemented reservoirsfrom selective dissolution of analcite in the mesodiageneticstage; the analcite dissolution formed preferential flowpaths in the reservoirs, which promoted feldspar dissolution;and dissolution of such minerals led to the presentreservoirs with medium porosity and low permeability.Calcite-cemented tight reservoirs have not experiencedextensive dissolution of cements, so they exhibit ultra-lowporosity and permeability.