ABSTRACT
Chlorite is an important clay mineral in sandstone reservoirs across many petroliferous basins worldwide, originating from a wide range of mineral precursors. Among the predominant chlorite precursors (e.g., berthierine, smectite, odinite, etc.), the kaolinite precursor and its conversion mechanism to chlorite remain the least understood. In this study, we conducted a series of hydrothermal-reactor experiments at 50, 100, 150, 200, and 250°C, each with a duration of 336 hours. The experiments were conducted in a solution containing 0.1 M NaCl, KCl, CaCl2·2H2O, and MgCl2·6H2O, with an additional experiment utilizing Red Sea water to investigate and verify the transformation of the clay phases. To synthesize chlorite from kaolinite, we used a quartz-rich sandstone from the Upper Ordovician Qasim Formation (Quwarah Sandstone Member, NW Saudi Arabia) as the starting material. This sandstone is characterized by authigenic kaolinite and hematite cements. Results of the experiments indicate that at lower temperatures (50 and 100°C), no new clay phases were formed. However, at 150°C, kaolinite transformed into dioctahedral smectite (montmorillonite), accompanied by minor precipitation of quartz cement. Authigenic smectite, chlorite, and illite were formed at 200 and 250°C in both the synthetic solution and the Red Sea water. The formation of chlorite occurred via two main pathways: 1) direct dissolution of kaolinite and crystallization of chlorite and 2) transformation of kaolinite to an intermediate smectite phase, which subsequently converted into chlorite through a combination of dissolution–crystallization and solid-state transformation. Before smectite conversion into chlorite, the smectites incorporated Fe and Mg into their structure, resulting in the formation of trioctahedral and tri-dioctahedral smectites with compositional gaps. The conversion of kaolinite into chlorite resulted in a significant increase in intercrystalline porosity owing to smaller crystal size of chlorite. Illitized kaolinite and smectite formed as an additional clay phase, with the required K+ supplied from the solutions. The study further revealed that the Red Sea water produced more Fe-rich smectite and chlorite, compared to the synthetic solution, due to the relatively lower Mg content in the seawater. The dissolution of kaolinite and the conversion of smectite into chlorite released abundant silica, which precipitated as quartz overgrowths where clay coatings were absent. These findings provide detailed insights into the kaolinite-to-chlorite conversion process and can aid in modeling chlorite and quartz cement nucleation in deeply buried reservoirs.
