ABSTRACT

In early 1975, Chevron Overseas Petroleum Inc. commenced what became a major petroleum exploration effort in previously unexplored interior Sudan. With the cooperation of the Sudanese government, Chevron has acquired a vast amount of geological and geophysical data during the past 12 years. These data include extensive aeromagnetic and gravity surveys, 36,040 mi (58,000 km) of seismic data, and 86 wells. This information has defined several large rift basins now recognized as a major part of the Central African rift system.

The sedimentary basins of interior Sudan are characterized by thick nonmarine clastic sequences of Jurassic(?)-Cretaceous and Tertiary age. Over 45,000 ft (13,716 m) of sediment was deposited in the deepest trough and extensive basinal areas are underlain by more than 20,000 ft (6,096 m) of sedimentary rocks. The depositional sequences include thick lacustrine shales and claystones, floodplain claystones, and lacustrine, fluvial, and alluvial sandstones and conglomerates. Those lacustrine claystones deposited in a suboxic environment provide good oil-prone source rocks. Reservoir sandstones have been found in a wide variety of nonmarine sandstone facies.

The extensional tectonism that formed these basins began in the Jurassic(?)-Early Cretaceous. Movement along major fault trends continued intermittently into the Miocene. This deformation resulted in a complex structural history that led to the formation of several deep fault-bounded troughs, major interbasinal highs, and complex basin flanks. This tectonism has created a wide variety of structures, many of which have become effective hydrocarbon traps.

During the past eight years, several important oil discoveries have been made. Significant accumulations have been delineated in the Heglig and Unity areas, where estimated recoverable reserves are 250–300 million bbl of oil.

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