Impact of grain-coating illite in preventing quartz cementation: Example from permo-carboniferous sandstone, Central Saudi Arabia

The origin and role of clay coatings (e.g., chlorite) in inhibiting quartz cementation in sandstones have been an area of research during the last three decades. However, the role of grain-coating illite in porosity preservation has received little attention. An integrated petrographic-mineralogical study is conducted to investigate the mode of occurrence of illite coatings, their coverage, thickness, and role in inhibiting the development of quartz overgrowth in a Permo-Carboniferous sandstone reservoir in Central Saudi Arabia. Results of the study reveal that illite coatings occur in two forms: tangential and radial to grain surfaces, both of which are effective in arresting quartz cementation. Illite-coating coverage exhibits and inverse relationship with quartz cement volume, and the coating coverage has been found to be grain-size dependent. Coarser-grained sandstones have higher coating coverage due to smaller surface area and smaller amount of illite required to form complete coatings. The study has found that thick illite coating (up to 27 μm) and high illite-coating coverage (≥80%) effectively arrest quartz cementation. Total pore-filling cements threshold of >20% has a negative impact on porosity. The findings of this study can be applied to other reservoirs of similar depositional environments to improve their reservoir quality prediction.