Saline aquifer distribution and CO₂ geological storage potential evaluation of the Zhifang Formation, Northeastern Ordos Basin

CO₂ geological storage in saline aquifers is an effective carbon neutrality technology. The Zhifang Formation has a moderate burial depth, relatively low drilling costs, and does not conflict with oil and gas development. Research on the development and distribution patterns of its saline aquifers and the potential for CO₂ geological storage in these aquifers has practical significance for implement CO₂ saline aquifer geological storage projects in the study area. The main rock types of the Zhifang Formation reservoir are medium-grained lithic feldspar sandstone and feldspathic lithic sandstone, with pore fillings primarily composed of cement, mainly montmorillonite, illite/montmorillonite mixed layers, and chlorite/montmorillonite mixed layers. The Zhifang Formation has an average porosity of 12.55% and permeability of 4.54 mD, classifying it as a low-porosity and ultra-low permeability reservoir. The main pore types are dissolution intergranular pores, followed by dissolution intragranular pores. The average maximum pore throat radius of the Zhifang Formation is 7.04 μm, with an average median radius of 0.25 μm. The pore distribution concentration in the 5th member of the Qian Formation is higher than that of the Zhifang Formation. The average single-layer thickness of the saline aquifer in the 3rd member of the Zhifang Formation is 6.7 m, with an average cumulative thickness of 15.6 m. The average single-layer thickness of the saline aquifer in the 4th member is 5.1 m, with an average cumulative thickness of 11.7 m. The high-value areas of the Zhifang Formation saline aquifer thickness are controlled by distributary channel sand bodies, showing a north-northeast trending strip distribution. The total storage capacity of the Zhifang Formation in the study area is 13461×10⁴ tons. Due to the greater burial depth in the western part of the study area, resulting in higher reservoir temperature and pressure, the CO₂ density is increased compared to the eastern part, leading to greater storage potential.