A natural analogue for carbon capture and storage: Petrographic and geochemical changes in sandstone after CO2 emplacement in the Yinggehai Basin, South China Sea

Sandstone reservoirs in the Yinggehai Basin, South China Sea, con-tain high CO2 concentrations and provide an ideal natural site to study carbon capture and storage. Here, we conducted petro-graphic observations, carbon-oxygen isotopic analyses, and num-erical modeling to constrain the petrographic and geochemical evolutionary characteristics related to CO2 emplacement. This study showed that dawsonite, siderite, microcrystalline quartz, dolomite, kaolinite, and ankerite can precipitate in reservoir sand-stone after CO2 influx, which is supported by our numerical simu-lation results. Furthermore, compared with CO2-poor sandstone formations, most CO2-enriched reservoirs are significantly en-riched in kaolinite. The d13C values of ankerite and dawsonite range from-4.7%o to-0.6%o and from-4.3%o to-0.8%o, respectively, indicating that the carbon sources are primarily re-lated to inorganic CO2. The dolomite d13C values range from -6.8%o to-3.3%o, implying the addition of 13C depleted organic carbon. The siderite d13C values are in a range of-6.3%o to -0.5%o, suggesting mixed carbon sources. Based on this evidence, we propose that kaolinite formation can be attributed to the disso-lution of precursor minerals under high CO2 concentrations, and that metal ions bonded in aluminosilicate minerals will be released into formation waters and then will promote the formation of authigenic minerals. The enrichment of kaolinite and ankerite can be employed as an alternative indicator of CO2-enriched reservoirs if dawsonite is absent, and carbonate minerals can effectively sequester CO2 and be beneficial to carbon capture and storage.