Effects of Inorganic CO2 Intrusion on Diagenesis and Reservoir Quality of Lacustrine Conglomerate Sandstones: Implications for Geological Carbon Sequestration

Mantle-sourced fluids are common in rift basins and often migrate into the reservoir along deep faults. They have a significant role in hydrocarbon evolution and reservoir quality modifications. In this study, we selected Chagan Sag in China as an example and applied reactive transport modeling to explore the influence of mantle-derived CO2 on diagenesis and reservoir quality evolution in the Lower Cretaceous Bayingebi conglomerate reservoirs. The geochemical modeling constrained by natural CO2 reservoirs provides beneficial information on the underground physical processes and chemical reactions during CO2 injection. This is also an efficient means of validating geological carbon sequestration models. Petrographic, geochemical, and modeling data reveal that CO2 intrusion could reduce the reservoir porosity and permeability via significant CO2-water-mineral interactions. Ankerite and illite are the dominant diagenetic minerals responsible for reservoir quality reduction. Among the different lithologies, the higher reservoir quality conglomerate is mainly caused by its high initial porosity and small quantities of clay minerals, with relatively weak carbonate cementation. The lower reservoir quality finer-grained conglomeratic siltstone is caused by intensive albitization, carbonation, and illitization. In the long-term CO2-water-rock interaction process, CO2 can be sequestered through the precipitation of secondary carbonate minerals such as ankerite and dolomite.