Factors Controlling Reservoir Quality of a Paleozoic Tight Sandstone, Rub' al Khali Basin, Saudi Arabia

The heterogeneity of depositional and diagenetic controls on the reservoir quality of tight sandstones is a major challenge. Understanding which factors are the most influential is significant for improving reservoir evaluation and prediction. This study utilized sedimentological core descriptions, thin-section petrography, quantitative evaluation of mineralogy by scanning electron microscope, X-ray diffractometry, scanning electron microscopy and porosity-permeability measurements to define and characterize the factors that control reservoir quality in the Upper Ordovician Sarah Formation of the Rub' al Khali Basin. Four facies associations (FA), including fluvial (FA1), glaciolacustrine delta (FA2), subglacial (FA3) and glaciofluvial (FA4), have been recognized. The results show that the highest reservoir quality was enhanced by feldspar dissolution in FA1. The development of its reservoir quality was, however, partially limited by compaction, and authigenic pore filling and grain coating with the clay mineral, illite. The lowest reservoir quality, related to FA3, appears to have been influenced by the detrital matrix content, compressibility and siderite cement precipitation. The presence of a detrital matrix, anhydrite and barite cement, and grain compaction reduced the reservoir quality in FA2. On the other hand, intergranular porosity, and fractures that facilitated the development of dual porosity and permeability systems in this FA, might have slightly enhanced its reservoir quality. Factors such as grain-size distribution and sorting influenced the reservoir quality of all the FAs, but these were the main controls on the reservoir quality of FA4, in addition to compaction. The reservoir quality of this FA might have also been improved by fractures and the dissolution of unstable grains.