Abstract

Significant amounts of oil are produced from Lower Cretaceous pre-evaporite, nonmarine sequences of these west African marginal basins. Gabon, Cabinda, Congo Brazzaville, and Angola. Organic-rich lacustrine source beds attain thicknesses up to 900 m. Their geometry and sediment similarities among several basins indicate a small number of large Early Cretaceous lakes extending along the South Atlantic rift with dimensions and conditions similar to Lake Tanganyika. The organic-rich facies is underlain by a sandy facies deposited during initial rifting. The lacustrine phases deposited green clays and fluviolacustrine-deltaic sands, which were abruptly terminated by marine incursions in the Aptian.

Our study of conventional whole cores from the Melania Formation of Gabon provides further evidence that these source beds were deposited in brackish to freshwater environments in a deep lake. Logs show characteristic low bulk densities and high resistivity which allow correlations over 80 km. Large-scale cyclic preservation of organic matter in a stable, low energy environment with anoxic bottom conditions is interpreted. There is little clay in the organic-rich “shales” which typically comprise finely laminated, carbonaceous and dolomite-rich rhythmic couplets. Bulk organic carbon concentrations up to 20% are not uncommon. Palynomorphs, ostracods, and algae also indicate temperate conditions around a low salinity environment. The slope sequences are richest in uniform laminates with some intercalated thin turbidites and enterolithic slump folds, whereas coarse turbidites were funneled into the deep basin plain. These hydrocarbon-saturated turbidite sandstones are derived from fluvial systems draining crystalline hinterland.

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