The palaeo-valley infilling glaciogenic Sarah Formation, an example from Rahal Dhab palaeo-valley, Saudi Arabia

The Sarah Formation is a glaciogenic sedimentary unit deposited along the Gondwana margin during the latest Ordovician ice age and represents a major hydrocarbon reservoir in northern Saudi Arabia. Large-scale glacial palaeo-valleys cut into the Qasim Formation and were infilled by the Sarah Formation. Post-glacial transgression in the earliest Silurian resulted in the deposition of the Qusaiba Shale Member and associated organic-rich basal source rocks, which cap the Sarah Formation infilled palaeo-valleys. This unique setting makes the Sarah Formation an important emerging exploration target in Saudi Arabia. This study focuses on the facies and depositional architecture in seismic-scale outcrops of the Sarah Formation in north-western Saudi Arabia. The Rahal Dhab palaeo-valley provides a 100km long dip-oriented cross-section which has been covered by 24 vertical sections, sedimentary architectural analyses at metre to kilometre scale and by three cored shallow boreholes. In the Rahal Dhab palaeo-valley, the Sarah Formation was deposited in a proglacial setting that ranged from marginal marine to offshore prodelta and is made up of three units: (i) the Sarah Sandstone; (ii) the Sarah Shale; and (iii) the Uqlah Member. This study shows the relationships between these three units and architectural controls on reservoir quality in this system. This paper contributes to the regional understanding of the Sarah Formation, and the new depositional model of the Rahal Dhab palaeo-valley provides an outcrop-reservoir analogue for hydrocarbon exploration in adjacent areas.