The Lower Silurian Qalibah Formation consists of two members, the (upper) Sharawra Member and the (lower) Qusaiba Member. The Qusaiba is the main hydrocarbon source rock for the known Paleozoic hydrocarbon accumulations in central Saudi Arabia. The Qusaiba Member is composed mostly of claystone and shale with interbeds of siltstone and sandstone. Depositionally, the Qusaiba Member is interpreted to represent the delta-toe clays, whereas the Sharawra Member was deposited as pro-delta sandstones of an immense fluviodeltaic system that dominated the Silurian–Carboniferous of Arabia. The Qalibah Formation accumulated syndepositionally in at least two rapidly subsiding depocenters. One depocenter was located in central and southern Arabia, and the other depocenter was located in northwestern Saudi Arabia. The former trough began subsiding during the Late Ordovician following the Taconic uplift. The latter trough began subsiding during the Early Silurian. At least 10,000 ft (3100 m) of Upper Ordovician–Lower Devonian strata accumulated in the trough located in central and southern Saudi Arabia. About 3300 ft (1000 m) of Lower Silurian sediments accumulated in the depocenter found in northwestern Saudi Arabia. The development of these troughs documents a period of crustal extension probably related to the rifting of the Lower Paleozoic continental margin of Saudi Arabia.
The Qusaiba Member organic-rich facies occurs near the base of the unit and has up to 8% total organic carbon content. The development of favorable source rock facies in the Qusaiba is observed along the flanks of the depositional thicks associated with the “lower” Qusaiba Member. The basal Qusaiba Member organic-rich facies was produced during periods of elevated primary productivity of organic matter within the water column in an open-marine, nonsilled basin setting. Deposition of the basal organic-rich facies of the Qusaiba Member occurred as a condensed sequence along a sediment-starved continental margin. The basal Qusaiba Member organic-rich facies accumulated in normally to partly oxygenated bottom-water settings; however, the sediments below the sediment/water interface were probably preserved in an anoxic setting. Geochemical data also demonstrate that the organic-rich Qusaiba Member sediments found in the depositional thicks have characteristics consistent with a more oxidized sediment column. Geo-chemical data also show that the basal Qusaiba Member organic-rich facies deposited more distally to these depocenters are more dysoxic to anoxic in their characteristics. The organic-rich facies of the basal Qusaiba Member found in depositional thicks was produced under conditions that either reworked the organic-rich sediments, thereby oxidizing them, or diluted the organic matter, resulting in less preservation of potential source rock.