Diagenetic Controls on Reservoir Porosity of Aeolian and Fluvial Deposits: A Case Study from Permo-Carboniferous Sandstones of Saudi Arabia

Continental sandstones of aeolian and fluvial systems are major reservoir targets for petroleum and CO2 storage in several places worldwide. The Permo-Carboniferous sandstones in Saudi Arabia are significant aeolian and fluvial, owing to their huge hydrocarbon reserves. Previous studies have documented the sedimentology, stratigraphy, and hydrocarbon potentials of the reservoirs. Nevertheless, the roles of grain-rimming illite, other clays (kaolinite and chlorite), and cements on porosity evolution of the sandstones are poorly constrained in the literature. We utilized a multi-technique approach consisting of petrography, X-ray powder diffraction, and scanning electron microscope to study the composition, textural features, diagenesis, and role of grain-rimming illite in arresting quartz cementation within aeolian and fluvial sandstones. Our findings indicate that illite coatings inhibited quartz cementation, and 40% illite-coating coverage efficiently restricted the growth of authigenic quartz cement. Intergranular illite and quartz cement were the most abundant cements that significantly impacted reservoir quality. However, porosity was chiefly reduced owing to compaction rather than cementation. Fluvial sands have poor reservoir quality due to low mechanical infiltration to emplace grain-coating clays and high amounts of co-deposited clays that occluded intergranular porosity and blocked pore throats in the sandstones. The grain size was found not to influence illite-coating coverage. Overall, carbonate and Fe-oxides (hematite) cements have less influence on reservoir quality. A total intergranular cement (clays, carbonates, and Fe-oxides) volume between 20 and 30% has a detrimental effect on intergranular porosity. The results of the study can be utilized to understand better and predict reservoir quality in sandstones of similar depositional settings around the world.