Evolution of pore-fracture system across different maturity levels and its implications for carbon dioxide sequestration in lacustrine shale

The geometry and topology of shale pore-fracture systems govern hydrocarbon migration and control the feasibility of geological carbon dioxide storage in shale reservoirs. This study examines lacustrine shale across a range of maturities by integrating (ultra) small-angle neutron scattering, repeated mercury intrusion capillary pressure, field-emission scanning electron microscopy, and computed tomography following Wood’s metal impregnation. The pore system is divided into four pore-size classes, and their volumes and connectivity are tracked with increasing thermal maturity. At low maturity, mechanical compaction and early cementation reduce the total pore volume and concentrate connected porosity in fractures. As maturity increases, newly formed organic-matter pores lead to a modest increase in total pore volume, while liquid hydrocarbons generated within the oil window occupy part of the pore space and weaken pore-fracture connectivity. At high maturity, the secondary cracking of liquid hydrocarbons to gas raises pore pressure, partially reopens previously sealed pores and fractures, and enhances both total pore volume and pore-fracture connectivity. These results indicate that mature to high-mature lacustrine shales provide more pore surface area, storage space, and connected pathways for the long-term storage of carbon dioxide than low-maturity shales.