Geochemical controls on coupled glauconite–feldspar diagenesis and its implications for fluid–rock interactions in glauconitic-rich sandstones

Abstract

Glauconite and K-feldspar, two abundant aluminosilicate minerals in natural glauconitic sandstones, undergo complex diagenetic dissolution in deep subsurface environments, influencing fluid chemistry and mineral reactivity. Their coupled dissolution forms a dynamic feedback system that influences fluid storage capacity and flow pathways in glauconitic sandstones, yet the mechanisms governing their interaction remain insufficiently understood. This study revisits the diagenetic processes controlling their coupled dissolution and its impact on secondary porosity evolution in deeply buried glauconitic sandstones of the Oligocene Songnan-Baodao Sag, Qiongdongnan Basin. By integrating petrography, geochemical modelling, and microstructural analysis of glauconite–feldspar interactions in varying partial pressure of CO₂ (pCO₂) and mineralogical conditions, we systematically investigate the dissolution characteristics and controlling factors of the coupled dissolution process. Geochemical modelling demonstrates that microfracture development in glauconite markedly increases its effective surface area, accelerating dissolution and enhancing accumulation of

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in pore waters, which in turn modulates K-feldspar dissolution and precipitation dynamics. The pCO₂ critically controls mineral solubility with high pCO₂ sustaining continuous K-feldspar dissolution, whereas low pCO₂ leads to rapid solution saturation, suppressing further dissolution and potentially inducing re-precipitation that may occlude porosity. Furthermore, the glauconite-to-K-feldspar mass ratio governs their coupled behaviour, low glauconite content promotes feldspar dissolution, while high glauconite abundance inhibits it. This relationship is reflected in two distinct patterns of porosity development, one characterized by dominant glauconite dissolution with limited alteration of K-feldspar, and another where K-feldspar dissolution is more pronounced while glauconite remains relatively unaltered. These findings clarify key controls on glauconite–feldspar diagenesis and provide theoretical insight for predicting porosity evolution in deeply buried glauconitic sandstone reservoirs. They also highlight the broader implications for CO₂ storage and geothermal energy systems, where mineralogical variability exerts a strong influence on fluid–rock interactions in sedimentary basins.