Maoyun Wang ^1,2* Jianhui Zeng ^1,2* Chuanming Li ^1,2 Wenfei Wei ^1,2* Huanle Zhang ^1,2* Huwang Cui ^1,2*

• ¹ College of Geosciences, China University of Petroleum, Beijing, China
• ² State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, Beijing, China

This study takes a typical mantle-derived CO2 gas reservoir in the Dongying Sag as an example. The thin section observation, scanning electron microscopy, X-ray diffraction, C and O isotope analysis, and porosity and permeability data were used to establish how CO2 affects the reservoir in various fluid environments. The results show that carbonate minerals in mantle-derived CO2 reservoirs are products of the interaction between CO2, water, and rocks in low-temperature hydrothermal and high-temperature magmatic-hydrothermal environments. CO 2 has dual effects on sandstone reservoirs. The dissolution of minerals by CO2 significantly enhances the storage and seepage capacity of the reservoirs, while the carbonate cement generated by CO2-water-rock interaction can also lead to a serious deterioration in reservoir quality. In the open fluid environments near faults, by-products of CO2 and feldspar dissolution are discharged from the reservoir through fluid flow, and the acidic environment formed by CO2 inhibits carbonate cementation, significantly improving the physical properties of reservoirs. In relatively closed fluid environments far from faults, with increasing depth, the concentration of CO2 decreases, resulting in weakened dissolution, while carbonate cementation strengthens. This leads to a gradual decrease in reservoir porosity and permeability. This study reveals the significant roles of fluid environments in the CO2-water-rock interaction. The additional information provided by this work is expected to contribute to oil and gas exploration in CO2-rich regions.