Depositional and tectonic setting of the Lower Silurian hydrocarbon sourcerock facies, central Saudi Arabia

The Lower Silurian Qalibah Formation consists of two members, the (upper) S harawra Member and the (lower) Qusaiba Member. The Qusaiba is the main hydr ocarbon source rock for the known Paleozoic hydrocarbon accumulations in ce ntral Saudi Arabia. The Qusaiba Member is composed mostly of claystone and shale with interbeds of siltstone and sandstone. Depositionally, the Qusaib a Member is interpreted to represent the delta-toe cla)is, whereas the Shar awra Member was deposited as pro-delta sandstones of an immense fluviodelta ic system that dominated the Silurian-Carboniferous of Arabia. The Qalibah Formation accumulated syndepositionally in at least two rapidly subsiding d epocenters. One depocenter tvas 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 upl ift. 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 (10 00 m) of Lower Silurian sediments accumulated in the depocenter found in no rthwestern Saudi Arabia. The development of these troughs documents a perio d of crustal extension probably related to the rifting of the Lower Paleozo ic 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 so urce rock facies in the Qusaiba is observed along the flanks of the deposit ional 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, n on-silled basin setting, Deposition of the basal organic-rich facies of the Qusaiba Member occurred as a condensed sequence along a sediment-starved c ontinental margin, The basal Qusaiba Member organic-rich facies accumulated in normally to partly oxygenated bottom-water settings; however, the sedim ents below the sediment/water interface were probably preserved in an anoxi c setting. Geochemical data also demonstrate that the organic-rich Qusaiba Member sediments found in the depositional thicks have characteristics cons istent with a more oxidized sediment column. Geochemical data also show tha t the basal Qusaiba Member organic-rich facies deposited more distally to t hese depocenters are more dysoxic to anoxic in their characteristics. The o rganic-rich facies of the basal Qusaiba Member found in depositional thicks was produced under conditions that either reworked the organic-rich sedime nts, thereby oxidizing them, or diluted the organic matter, resulting in le ss preservation of potential source rock.