RIFTED MARGIN SOURCE ROCK DEPOSITION - A CARBON ISOTOPE AND BIOMARKER STUDY OF A WEST AFRICAN LOWER CRETACEOUS LACUSTRINE SECTION

Lower Cretaceous passive margin sections, penetrated in wells peripheral to the deep basinal development of pre-salt sediments of the Liawenda-Ponta Vermelha Troughs in the Lower Congo Coastal Basin, revealed sustained source potential. Kerogen type, kinetic (E(a)/A) and carbon isotopic data plus rock extract biomarker signatures confirmed large, systematic organofacies variations within these rift phase sediments. An inverse correlation between 25,28,30-trisnorhopare and gammacerane abundances suggested three major depositional regimes reflecting salinity and water depth control. C-13-depleted basal sediments, showing strong gammacerane/4-methyl sterane signatures, were segregated by a major isotopic excursion. This event represented the transition between saline playa lake to deeper water stratified conditions supporting a high level of anoxia. For these intermediate sediments, sterane and tricyclic diterpane abundances, plus sterane/hopane ratios, had marine connotations and could be interpreted as the result of intermittent marine incursions. Fore-shadowing irreversible oceanic ingression, a resurgence of gammacerane abundance in the uppermost sediments typified littoral-shallow marine depositional conditions. Recognition of a possible non-ubiquity in lacustrine depositional conditions during the opening of a rifted margin provides further insight into the complexity and attendant controls on hydrocarbon generation in such habitats.