--> --> Evidences of Localized CO2-induced Diagenesis in the Cretaceous Quantou Formation, Southern Songliao Basin, China

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Evidences of Localized CO2-induced Diagenesis in the Cretaceous Quantou Formation, Southern Songliao Basin, China


Mineral carbonatization (MC) is the formation of solid carbonate minerals from aqueous CO2 or directly from a CO2 fluid phase. In underground CO2 storage settings, MC immobilizes CO2 and thereby reduces the risk for leakages into shallow groundwater aquifers containing potable water, or into the ocean-atmosphere system. Mineral reactions are normally regarded slow compared to the timeframes of CO2 injection, but some studies indicate that some of the mineral reactions may be important on timescales even down to tens of years. Evidences of CO2 induced diagenesis in the Quantou Formation, Southern Songliao Basin, have been investigated using petrographic analyses, X-ray diffraction, stable isotopes, fluid inclusions and kinetic-thermodynamic modeling. The studied sandstones have two distinct zones based on their mineralogy, one “pristine” being composed of quartz, plagioclase, chlorite, calcite and dolomite, and one “altered” mainly consisting of quartz, illite and illite/smectite, ankerite, and dawsonite. The ankerite/dawsonite and secondary clay-bearing zones are related to the major deep rooted faults in this area. The δ13C value of carbonate cements in the altered zone (mainly centered on −0.9‰ to −8.0‰) is higher than that in the “pristine” sediments (mainly centered on −8.0‰ to −12.9‰). We therefore proposed that these zones have experienced CO2 charged hydrothermal (~120–140 °C) alteration and with a similar precursor mineral assemblage as in the plagioclase/chlorite rich zones. Kinetic modeling furthermore suggests that the observed alteration must have been under high CO2 partial pressures, with the closest match between simulations and XRD data found at the highest attempted pressure of 100 bar, and the source for the dawsonite was calcic plagioclase dissolution with a corresponding external CO2. A good match between model and observations was found using 50,000 years simulation with a pore surface-area normalization. The modeling also suggests that using kinetic data from the literature together with reactive surface areas normalized to the surface area of the pores provides good estimates for the alteration of tight sandstones such as the Quantou Formation (~8% porosity).