Rock Property of the Upper Ordovician in the Rub’ al-Khali Basin, Kingdom of Saudi Arabia

Abstract

Abstract In the Rub' al-Khali Basin, the Late Ordovician glacial sediments were deposited in a vast water body where laterally shifting depositional environments resulted in lateral facies changes, from well-sorted glacio-fluvial reservoir sandstone to diamictite deposits. The predominantly fluvial channel deposits, consisting of medium to very fine grained sheet sands and interfluve muddy siltstones, were widely distributed, the thicknesses of which ranged from tens to several hundreds of feet. In core, glacial outwash channels, braided plain sands, and lacustrine shale layers with drop-stones from melting ice were observed. The net sand-to-gross ratio is generally high due to the sedimentation dynamics, whereby moving icepressured meltwater jets became sorting agents in the process of glacial sedimentation. In the central Rub' al-Khali, core studies from drilled wells and associated log responses show variable reservoir quality. The overall reservoir quality is controlled by a number of primary and secondary factors. Primary controls are related to sedimentation and the depositional environment, which are manifested in these type of facies, grain size and sorting. Well-sorted, coarse sandstone is associated with high energy currents, and is mainly present in the fluvial system. Non-reservoir facies such as lacustrine shale and diamictite deposits are associated with low energy lacustrine or deep water deposits. The characteristic high net-to-gross ratio observed in the Rub' al-Khali Basin is primarily due to grain sorting. Thin sections from core indicate that mechanical compaction and silica cementation had, in areas, effected reservoir quality. Post-deposition diagenesis, particularly clay coatings, preserved intergranular porosity by preventing continued silica growth on sand grains. Mineral dissolution is not prevalent, and likely did not have significant effect on porosity. Evidence of pressure solution, such as micro-stylolites and sutured contacts between grains are present, indicating sandstone grains were subjected to significant mechanical compaction. Compaction is related to the depth of burial, and its impact on reservoir quality varies throughout the Rub' al-Khali Basin. Silica cementation appears to be sourced from stress inducing stylolites, where the silica cement is formed around the grains.