Effect of Firing on Clay Content and Mineral Transformations in Bandera Gray Sandstone: Implications for the Clay-CO₂Interaction

Understanding the impact of rock firing on clay minerals and mineralogical transformations of chemically unstable framework components of sandstone reservoirs is crucial for the effective evaluation of the geologic CO2 storage potentials of clastic reservoirs. The Bandera Gray (BG) sandstone has been the focus of many researchers, primarily owing to its high clay content (0.1 to up to 10.7%). However, the effect of firing on the clay content and mineralogical transformations in the BG sandstone remains enigmatic. In this study, we employed X-ray diffraction (XRD), scanning electron microscopy (SEM), and SEM-energy-dispersive X-ray spectroscopy (EDX) techniques to investigate the role of firing on clays in the BG sandstone. Three BG sandstone samples were heated for 6 h at three different temperatures (25, 700, and 1100 °C). The results indicate that at 25 °C, no transformation has occurred, and the main framework grains consist of quartz and albite, whereas chlorite, dolomite, illite, and kaolinite are the main cements in the sandstones. At 700 °C, however, kaolinite has largely transformed into amorphous metakaolinite, whereas illite content has increased significantly from 4.2 to 11.6%, principally owing to the illitization of kaolinized mica grains and the in situ supply of K+ from the altered mica. The illitization of kaolinite and the decomposition of kaolinite and chlorite at this temperature have released silica, which precipitated as microquartz, thereby increasing the overall quartz content in the sandstone. At 1100 °C, both illite and chlorite have completely disappeared and decomposed into silica, which reacted with dolomite and calcite (from dolomite decarbonation) to form various minerals such as diopside, gehlenite, monticellite, and wollastonite. Consequently, the findings of the study suggest that temperatures above 700 °C are required to transform and degrade all clay types for the BG sandstone in order to avoid any reaction between clays and CO2 during core flooding under acidic environments.