Role of CO2-water-rock interactions and implications for CO2 sequestration in Eocene deeply buried sandstones in the Bonan Sag, eastern Bohai Bay Basin, China

This study explores CO2-water-rock interactions in Eocene hydrocarbon sandstone reservoirs in eastern Bohai Bay Basin, China, to document mineral alterations and their implications for CO2 geological sequestration. Petrographic, geochemical and reactive transport modeling evidences reveal that various inorganic mineral alterations occur during organic CO2 intrusion from adjacent source rocks into sandstone reservoirs including: (1) pervasive dissolution of calcite and chlorite minerals that are re-precipitated as secondary, more stable dolomite and ankerite cements; (2) precipitation of low-temperature calcite cements (27–49 °C) and relatively high-temperature dolomite (88–111 °C) and ankerite (103–136 °C) cements based on their δ18OPDB values (−11.91‰ to −7.86‰ for calcite; −15.16‰ to −12.48‰ for dolomite and −17.71‰ to −14.32‰ for ankerite); (3) Carbon sources for dolomite and ankerite cements (δ13CPDB from −7.59‰ to −3.58‰) are a mixture of carbon derived from early-formed calcite precursors (δ13CPDB from–0.26‰ to +3.75‰) with significant proportions of organic carbon (δ13CPDB from −25‰ to −10‰); (4) plagioclase dissolution spatially associated with precipitation of quartz and kaolinite. Reactive transport modeling results illustrate that mineral dissolution is coupled temporally and spatially with precipitation of secondary mineral phases. It is inferred that advective and diffusive mass transport of dissolved species in the extremely low pore-fluids is greatly restricted and mineral kinetics exerts more influences on CO2–water–rock interactions in the deep burial environments. The overall mineral alterations in the CO2-mediated geochemical system are the consequences of complicated interconnected reactions in the simulated model. Kinetically slow dissolution reactions of aluminosilicate minerals (e.g. chlorite) are regarded as the rate-limiting step for precipitation of more stable carbonate phases and CO2 sequestration. Therefore, the occurrences, abundances and distribution patterns of the aluminosilicate minerals coupled with their kinetic reaction rates are likely to affect the long-term fate of the safe CO2 storage in the deeply buried siliciclastic reservoirs.