AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Geochemical Characterization of Rocks and Fluids from Liberia and Sierra Leone Offshore.


The study area is a portion of the South Atlantic passive margin, implying a complex thermal history (rifting), located at the oceanic-continental transition. A rapid sedimentation with important lithological contrasts and important water depth variations through time are characteristics of this margin. Volcanic intrusions and sub-aerial activity are known to have occurred during the basin evolution. Sierra Leone and Liberia basin are limited by the Sublima and Liberia pateaus. The Sublima plateau is the result of the intersection of the Sierra Leone TF and the African continental margin. The Liberia plateau is a basement high forming at the intersection of the North Atlantic and South Atlantic transform systems with the continental margin. The objective of this study was to carry out the geochemical characterization of rocks, and fluids (oil and gas samples) from Montserrado-1 (Liberia) and Jupiter-1, Mercury-1, Mercury-2, Venus-1B wells (Sierra Leone). The geochemical evaluation of rock samples included Total Organic Carbon (TOC), Pyrolysis Rock-Eval, Visual Kerogen Analysis, Vitrinite Reflectance (Ro%), Thermal Alteration Index (TAI) and Pyrolysis-GC. Gas samples were analyzed by Gas Chromatography and Isotopic Analysis, and oils/ organic extracts were analyzed by Gas Chromatography (GC) and Gas Chromatography–Mass Spectrometry (GC-MS). Campanian and Coniacian to Albian units show the best petroleum potential based on organic matter richness (TOC). Turonian to Albian source rock quality (Type II Kerogen, marine) increases toward the northwestern part of the studied area (in Venus-1 and Jupiter-1 wells, Figure 1). Maturity of Cenomanian to Albian rocks increases towards the northwestern area. These sequences present a maturity that ranges between the peak oil generation to late mature. Thermogenic gas was identified in the deeper part of the Venus-1B Well (4050-5630m), at 4590-4850m in the Mercury-1 Well, below ~4400m in the Mercury-2 Well and below ~5000m in the Jupiter-1 Well. Important thermogenic contributions in the total hydrocarbon gas from the middle-deeper part of the Montserrado-1 Well were observed (from ~3800m). The thermogenic component is mainly oil-condensate associated gas. Different biomarker ratios support oil-oil correlations for Mercury-1 oils indicating one genetically petroleum system. The source rock that generated these oils is characterized by high marine organic matter contributions. Analyzed organic extracts at 4564m and 4568m in the Mercury-2 Well show mixed organic matter with significant mature terrestrial organic matter input. Mercury oils are associated to a mature shaly source rock (marine shale) probably of Late Cretaceous age.