PETROLEUM GEOLOGY OF THE FULA SUB-BASIN, MUGLAD BASIN, SUDAN

The Fula sub-basin is a fault-bounded depression located in the NE of the Muglad Basin, Sudan, and covers an area of about 3560 km(2). Eleven oilfields and oil-bearing structures have been discovered in the sub-basin. The Lower Cretaceous Abu Gabra shales (Barremian - Aptian), deposited in a deep-water lacustrine environment, are major source rocks. Reservoir targets include interbedded sandstones within the Abu Gabra Formation and sandstones in the overlying Bentiu and Aradeiba Formations (Albian - Cenomanian and Turonian, respectively). Oil-source correlation indicates that crude oils in the Aradeiba and Bentiu Formations are characterized by low APIs (<22 degrees), low sulphur contents (<0.2%), high viscosity and high Total Acid Number (TAN: >6 mg KOH/g oil on average). By contrast, API, viscosity and TAN for oils in the Abu Gabra Formation vary widely. These differences indicate that oil migration and accumulation in the Fula sub-basin is more complicated than in other parts of the Muglad Basin, probably as a result of regional transtension and inversion during the Late Cretaceous and Tertiary. The Aradeiba-Bentiu and Abu Gabra Formations form separate exploration targets in the Fula sub-basin. Four play fairways are identified: the central oblique anticline zone, boundary fault zone, fault terrance zone and sag zone. The most prospective locations are probably located in the central oblique anticline zone.