THE PALEOENVIRONMENTAL SIGNIFICANCE OF PYRITIC NODULES FROM LOKPANTA OIL SHALE INTERVAL IN THE PETROLEUM SYSTEM OF LOWER BENUE TROUGH, NIGERIA
Petrographic, Scan Electron Microscopy (SEM), X-Ray Diffraction (XRD), rare earth element (REE) and sulphur isotope (d 34 S) studies were carried out on the pyritic nodules collected from the Lokpanta oil shale interval of the Lower Benue Trough, Nigeria. This is to ascertain the origin of the nodules and their implications in the hydrocarbon generation and accumulation in the region. The mineralogical composition (from SEM and XRD) of the nodules indicates calcite, pyrite, barite and anhydrite
... arite and anhydrite as common minerals. The petrographic studies show that the pyrite is subhedral to euhedral in texture. Barite and anhydrites occur as pores, veins and cavity fillings and together with pyrite constituted the only sulphate minerals. Calcite is the only carbonate mineral and occurred as the matrix. The sulphur d 34 S values range from +0.9 to +1.6‰ with an average of +1.6‰ which correspond to authigenic pyrite. The REEs result show high enrichment of LREEs relative to HREEs suggesting seawater source while the positive europium (>1) anomaly values indicate anoxic event, and reducing diagenetic conditions in a significantly high temperature. Although, the Lokpanta oil shale interval is deposited under an Oceanic Anoxic Event (OAE) and is capable of generating hydrocarbon in the study area, the authigenic nature and euhedral texture of the pyrite grains suggest metamorphic or thermal alteration for the formation of the nodules. Moreover, the volcanic intrusions and hydrothermal activities within the study area could also enhance the formation of the pyritic nodules. The generation and accumulation of hydrocarbon in the Lower Benue Trough could further be investigated using heat flow model.