Sudan geological study was focused on the surface geology mainly for surface mapping and limited shallow mining activities. With the recent discovery of commercial hydrocarbon, extensive subsurface data has been acquired both offshore and onshore. These data revealed existence of several sedimentary basins offshore in the Red Sea and onshore in the interior Sudan. Main sedimentary Basins are shown in Figure (2).
Fig (2) Geological map of Sudan
These basins are all rift basins, owing their existence to the rifting activities of Western, Central and East African Rift Systems.
Exploration is still at early stages and the data collected is scarce. Based on the available data and from analogy to other basins it can be concluded that the major conditions for Petroleum accumulations have been met.Hundreds of metres of rich source rocks have been penetrated in Muglad, Melut, Blue Nile, Red Sea, Khartoum, and White Nile Basins.
Excellent and extensive reservoir quality sands have also been drilled. The oldest sedimentary basins encountered so far in Sudan are of Cambro - Ordovician age? These occur within narrow grabens formed by rifting, which preceded consolidation of
Pan African structures in north and north- western Sudan, Fig (2) shows the
distribution of sedimentary basins in Sudan. Much attention has been given to an already explored Mesozoic rift related basin
systems, in the south and central Sudan, while the Palaeozoic basins in the NW Sudan have not been explored. Data from these basins such as surface geology, regional gravity and magnetic which indicate that the basins are deep and filled by thick
pile of marine and continental sediments during Palaeozoic and Mesozoic times e.g. Murdi basin (4.5 km depth) , and South Wadi Howar basin (5 – 6 km). The gravity, seismic and drilling data acquired in interior Mesozoic basins in the central and southern Sudan indicated that
more than 30000 feet of clastic sediments occur within the deepest central trough of the three major rift basins.
Fig (3), shows generalized stratigraphic columns for the Kaikang trough, which runs along the western flank of the Muglad basin and that of Unity in the Eastern flank, whereas, Fig (4) shows the stratigraphy of the Melut basin. The sediments are interbedded sandstones, claystones, siltstones, mudstones and shales. Intrusive rocks (Sills) were encountered in some wells such as in Garad-1, Sobat–1,Tabaldi-1…etc.
Changes in lithofacies primarily reflect variations in the subsidence rates of various sub-basins. The lack of significant magmatism during active rifting is attributed to the shallow depth of fault detachments at a brittle – ductile transition zone. Abu Gabra Formation is the main source rock, consists of dark lacustrine shales containing a typically waxy kerogen and proved to be a reservoir in block # 6. Bentiu formation, Darfur group sandstone members, Amal and Tendi formations are the principal reservoirs. Shales and claystones within Darfur Group as well as shales and claystones within Abu Gabra Formation act as seal to underlying oil bearing horizons. None of the wells drilled in the Melut basin, has encountered the source rocks, however, based on the crude oil biomarker distributions and characteristics, it is believed that the source rock in Melut basin is equivalent to the Abu Gabra Formation found in the Muglad basin. Yabous Formation is the main reservoir in Melut
basin. Time of oil migration is uncertain, but it seems to be during mid to late Tertiary in the Muglad basin and during mid – Cretaceous to Late Cretaceous time in the Melut basin. The present temperature gradients over these basins are normal and no over pressure zones have been recorded so far.
Fig (3) Stratigraphic summary of Muglad basin
Fig (4) Stratigraphic summary of Melut basin