In early 1975, Chevron Overseas Petroleum Inc. commenced what became a major petroleum effort in previously unexplored interior Sudan. With the cooperation of the Sudanese government, Chevron has acquired a vast amount of geologic and geophysical data during the past 11 years. These data include extensive aeromagnetic and gravity surveys, 36,000 mi (58,000 km) of seismic data, and results from 86 wells. This information has defined several large rift basins that are now recognized as a major part of the Central African rift system.

The sedimentary basins of interior Sudan are characterized by thick Late Jurassic-Cretaceous and Tertiary nonmarine clastic sequences. More than 35,000 ft (10,671 m) of sediment has been deposited in the deepest trough, and extensive basinal areas are underlain by more than 20,000 ft (6,098 m) of sediment. The depositional sequence includes thick lacustrine shales and claystones; flood-plain claystones; and lacustrine, fluvial, and alluvial sandstones and conglomerates. The lacustrine claystones that were deposited in an anoxic environment provide good oil-prone source rocks. Reservoir sandstones have been found in various nonmarine sandstone facies.

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

During the past 7 years, Chevron has made several important oil discoveries. Significant accumulations have been delineated in the Heglig and Unity areas, where estimated recoverable reserves of 250-300 million bbl of oil have been established.

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