Kaolinite Illitization Under Hydrothermal Conditions: Experimental Insight into Transformation Pathways

Illite plays a critical role in diagenetic processes of sedimentary rocks, influencing geochemical evolution, reservoir quality, and fluid flow pathways. This study experimentally investigated the hydrothermal transformation of kaolinite to illite using sandstones from the Upper Ordovician Quwarah Member of Qasim Formation, northwest Saudi Arabia. Experiments were performed to simulate burial diagenesis involving illitization using three fluid systems: a synthetic solution (0.2 M KCl and 0.5 M MgCl₂), natural Red Sea water, and modified Red Sea water (0.2 M KCl and 0.5 M MgCl₂ + Red Sea water) at different temperatures (80 °C, 150 °C, 200 °C, 250 °C). Analysis included thin-section petrography, scanning electron microscopy, and X-ray diffraction, whereas the elemental compositions of the experimental solutions were analyzed using ICP-MS and Ion Chromatography. At 80 °C and 150 °C, kaolinite underwent dissolution without significant mineralogical changes. At 200 °C, continued kaolinite disaggregation and dissolution produced smectite and mixed smectite–chlorite/illite layers, indicating early transformation pathways. At 250 °C, fluid chemistry exerted strong control on clay minerals. The synthetic solution formed smectite with minor chlorite and illite; Red Sea water favored well developed smectite; the modified Red Sea water promoted well-developed illite due to increased potassium availability. The experiments show illitization is strongly temperature dependent and primarily controlled by potassium activity in fluids. The study provides insights into clay mineral evolution with different diagenetic conditions, which can be useful to evaluate diagenetic impact on reservoir qualities.