Optimal selection of favorable areas for CO₂ geological storage in the Majiagou Formation in the Ordos Basin

Carbon dioxide (CO₂) geological storage (CGS) is considered as one of the feasible choices for large-scale CO₂ emission reduction worldwide, and the primary issue in CGS is how to select areas favorable for storing CO₂. The research conducted by Chinese scholars on CO₂ geological storage in the Ordos Basin has mainly focused on numerical simulations of CO₂ storage, the estimation of the CO₂ storage capacity, and the CO₂ storage mechanism. Few studies have been conducted on the selection of CO₂ geological storage area, or even on a methodology for ranking CO₂ geological storage sites in carbonate reservoirs. This study provides a reference for the ranking of CGS areas in carbonate reservoirs. First, from the geological view of CO₂ storage reservoir, logging data and core petrographic analyses with cast thin sections, scanning electron microscopy and formation water salinity tests were used to systematically investigate the reservoir type, lithology, porosity, permeability, burial depth, thickness, temperature, and pressure conditions, and water salinity of the Majiagou formation. Second, from the perspective of CO₂ storage safety, literature research, and seismic interpretation data were used to investigate the sealing performance of the strata overlying the Majiagou Formation and its related fault system in detail. Third, from the economic perspective of CO₂ storage, considering the distance from the source of the CO₂ emissions, the extent of the regional exploration, and the degree of the influence of CO₂ storage on the exploitation of other mineral resources, the CO₂ storage areas in the Majiagou Formation in the Ordos Basin were prioritized. The results show that (1) the Wushenqi-Jingbian-Yan'an karst slope area (I₁) is the best CO2 geological storage area, (2) the Yulin-Mizhi karst basin area (I₂) is a favorable area for CGS in the Ordos Basin. (3) A site ranking methodology for CGS is also proposed, providing a reference for the selection of CO₂ geological storage areas in carbonate reservoirs based on the technical, safety, and economic feasibility of the CGS.