STRATEGIC AND GEODYNAMIC ANALYSES OF GEO-SEQUESTRATION OF CO2 IN CHINA OFFSHORE SEDIMENTARY BASINS

Focusing on the national strategic goal of "Carbon Peaking and Carbon Neutrality", this paper comprehensively analyzes the strategic conditions and targets suitable for large-scale CO2 geo-storage in the China offshore basins, from the perspectives of fault activity, basin pressure, tectonic subsidence, seismicity, and geothermal gradient. It is considered that the East China Sea Shelf Basin, Pearl River Mouth Basin, eastern Qiongdongnan Basin, and the central South China Sea basin are the best geological storage areas for CO2, although this does not exclude suitable targets in other unfavorable sedimentary basins since a specific geo-sequestration target is small in area. The suitable CO2 storage strata in the East China Sea Shelf, Pearl River Mouth, and Qiongdongnan Basins include the bottom salt-water layer of the late rapid subsidence sediments in the open-sea environment and the hydrocarbon-bearing units in the thermal subsidence sedimentary sequences. Between 800 and 4000 m depths beneath the seafloor, the porosity is greater than 10%, and the hydrostatic and lithostatic pressures vary from ~ 8 to ~ 40 MPa and from ~ 13 to ~ 83 MPa, respectively. In this pressure and suitable geothermal gradient ranges, CO2 exists in a supercritical state, and its density is relatively stable with temperature and pressure changes, which is beneficial to the flow and permeation of CO2. The scale and number of mafic magmatic rock formations in the basins also provide good conditions for CO2 geological sequestration and permanent mineralization. Although operationally difficult and expensive, CO2 storage in the central South China Sea basin is very safe. CO2 injected deep into the oceanic basalt can undergo basalt mineralization, but if CO2 is escaped as the mineralization process is relatively slow, escaped CO2 can be further trapped by multiple other storage processes, including pyroclastic rock mineralization, seafloor sediment sequestration, seabed sediment CO2 hydrate storage, carbonate neutralization reaction, seabed carbon lake, ocean dissolution, etc. The existing six International Oceanic Discovery Program (IODP) boreholes that have encountered basement basalt in the central basin of the South China Sea can provide a good scientific and engineering foundation for the pilot CO2 storage experiment in the South China Sea basin. © 2023 Chinese Journal of Theoretical and Applied Mechanics Press. All rights reserved.