Outlines of the geology of Sudan

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