Comparative Analysis of CO₂ Sequestration Potential in Shale Reservoirs Insights from the Longmaxi and Qiongzhusi Formations

Shale reservoirs offer significant potential for CO2 geological sequestration due to their extensive nanopore networks and heterogeneous pore systems. This study comparatively assessed the CO2 storage potential of the Lower Silurian Longmaxi and Lower Cambrian Qiongzhusi shales through an integrated approach involving organic geochemical analysis, mineralogical characterization through X-ray diffraction (XRD), mercury intrusion capillary pressure (MICP), low-pressure nitrogen and carbon dioxide physisorption, field-emission scanning electron microscopy (FE-SEM), stochastic 3D microstructure reconstruction, multifractal analysis, and three-dimensional succolarity computation. The results demonstrate that mineral assemblages and diagenetic history govern pore preservation: Longmaxi shales, with moderate maturity and shallower burial, retain abundant organic-hosted mesopores, whereas overmature and deeply buried Qiongzhusi shales are strongly compacted and mineralized, reducing pore availability. Multifractal spectra and 3D reconstructions reveal that Longmaxi develops broader singularity spectra and higher succolarity values, reflecting more isotropic meso-/macropore connectivity at the SEM scale, while Qiongzhusi exhibits narrower spectra and lower succolarity, indicating micropore-dominated and anisotropic networks. Longmaxi has nanometer-scale throats (D50 ≈ 10–25 nm) with high CO2 breakthrough pressures (P10 ≈ 0.57 MPa) and ultra-low RGPZ permeability (mean ≈ 1.5 × 10−2 nD); Qiongzhusi has micrometer-scale throats (D50 ≈ 1–3 μm), very low breakthrough pressures (P10 ≈ 0.018 MPa), and much higher permeability (mean ≈ 4.63 × 103 nD). Storage partitioning further differs: Longmaxi’s median total capacity is ≈15.6 kg m−3 with adsorption ≈ 93%, whereas Qiongzhusi’s median is ≈12.8 kg m−3 with adsorption ≈ 70%. We infer Longmaxi favors secure adsorption-dominated retention but suffers from injectivity limits; Qiongzhusi favors injectivity but requires reliable seals.