The nonmarine, pre-salt sequence of offshore Cabinda, Angola, is a prolific hydrocarbon producer. Thick, organic-rich, lacustrine shale was the source of the oil trapped in four distinct, nonmarine, reservoir rocks.

Subsidence followed by the formation of deep grabens introduced the Early Cretaceous rifting of the South Atlantic. The resultant basins of west Africa were filled with up to 2,500 m of nonmarine deposits. Aptian salt overlies the nonmarine deposits. Pre-rift and synrift strata are distinguishable in the pre-salt sequence and are separated by major unconformities. Hydrocarbons occur in both the pre-rift and the syn-rift strata.

The mostly clastic, pre-rift strata rest unconformably on metamorphic basement and were deposited in fluvial-lacustrine environments prior to active rift faulting. Today, the pre-rift strata are in tilted fault blocks dipping 20° or more. The Lucula Formation, a fine-grained sandstone in the pre-rift strata within these blocks, is one of the major productive reservoirs of West Africa.

The syn-rift strata, which unconformably overlie the pre-rift rocks, gradually filled the deep graben lakes produced by initial rift faulting. Organic-rich dolomitic shale, the source rock for both pre- and syn-rift reservoirs, grades upward into shallow-lacustrine green shale and carbonate rocks. The shallow-water, nonmarine carbonate rocks are important reservoirs. Final rifting caused gradual westward tilting and erosion of the syn-rift topography. Nonmarine carbonate rock and sand unconformably filled the resultant surface. Along the eastern margin of this basin fill, carbonate reservoirs produce oil.

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