Hydrothermal Development of Authigenic Smectite and Its Subsequent Illitization: Experimental Insights from Glauconitic Greensand

Glauconite-rich sands can generate authigenic clays during burial, as glauconite breaks down and supplies material for smectite that may subsequently transform into illite. The smectite-to-illite transformation is a key burial diagenetic reaction that strongly influences sandstone reservoir quality; however, the reaction pathways and resulting textures in glauconite-rich sands remain insufficiently documented. To better constrain illitization in glauconitic systems, we conducted four hydrothermal batch experiments using glauconitic greensand from the Arnager Greensand Formation (Bornholm Island, Denmark), reacted with Red Sea water at 80 °C, 150 °C, 200 °C, and 250 °C for 21 days. Reaction products were characterized using bulk and clay-fraction XRD, XRF, and SEM–EDS, together with pre- and post-reaction fluid chemistry. At 80 °C, early dissolution of glauconite and other detrital components (K-feldspar, muscovite and calcite) was observed, resulting in increased concentrations of dissolved ions in the fluid but no authigenic clay formation. Authigenic smectite first developed at 150 °C, occurring primarily as grain-coating clay. With further heating to 200 °C, smectite began to transform into mixed-layer illite–smectite, accompanied by the first clear development of illite textures. At 250 °C, illitization advanced significantly, and the reacted products were dominated by wavy to fibrous illite. The resulting clay minerals and their grain-coating morphologies are comparable to coatings reported from buried sandstone reservoirs. These findings suggest that glauconite alteration can generate grain-coating clays that may help limit quartz cement growth and preserve porosity. However, the development of wavy/fibrous illite may also obstruct pore spaces and reduce permeability. Overall, glauconite-derived clay evolution may preserve porosity while still degrading permeability, and the net reservoir effect depends on the morphology and distribution (thickness and coverage) of the newly developed clay minerals.