The Lower Silurian Qalibah Formation of Saudi Arabia: An Important Hydrocarbon Source Rock (1)

Abstract

The Lower Silurian Qalibah Formation represents a distinct, lithostratigraphic, time-transgressive sequence. It is disconformably bounded below by the Upper Ordovician to possibly Lower Silurian Sarah Formation, and above by the continental to marginal-marine Lower Devonian and possibly Upper Silurian Tawil Sandstone Formation. This paper redefines the Qalibah Formation within these boundaries. In its reference section near the town of Al Qalibah in northwest Saudi Arabia, the Qalibah is 499 m thick, whereas in a subsurface composite section it reaches 955 m. The Qalibah Formation is a coarsening-upward, progradational sequence consisting of a lower Qusaiba Shale Member and an upper Sharawra Sandstone Member. The basal hot of the Qusaiba is a black, euxinic, bottomset shale (up to 70 m thick) which was deposited during the Early Silurian sea level rise following the deglaciation of Gondwana. This Paleozoic shale is organic rich and the most likely source rock of the low-sulfur, high-gravity oil, condensate, and gas discovered in recent years in the Paleozoic rocks of central and eastern Saudi Arabia. Lithologically similar Silurian sequences were deposited over most of the broad stable shelf of Gondwana from the Middle East to the African Sahara.