Geochemistry and organic petrography of Cretaceous sediments of the Calabar Flank, southeastern, Nigeria

• Published in: Marine and petroleum geology 2012-08, Vol.35 (1), p.252-268
• Main Authors: Ekpo, B.O., Ibok, U.J., Essien, N., Wehner, H.
• Format: Article
• Language: English
• Subjects: Calabar Flank; Carbon; Degradation; Deposition; Earth sciences; Earth, ocean, space; Exact sciences and technology; Flanks; Geochemistry; Hydrocarbons; Marine; Marine geology; Niger Delta; Nigeria; Organic petrography; Paleoenvironment; Sedimentary rocks; Sediments; Shale; Stratigraphy; Sulphate reduction
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2012.03.010

Detailed bulk geochemistry and organo-petrography of outcrop Cretaceous sediments (with no significant effects of weathering) from the Calabar Flank, southeast Nigeria were performed to understand the organic carbon source, accumulation and degradation, and paleo-climatic, paleoceanographic and paleoenvironmental conditions in West Africa during Early Cretaceous (Aptian) to Maastrichtian times. This study was based on microscopic, elemental analyses (organic carbon, nitrogen, iron and sulphur), Rock-eval pyrolysis and carbon-isotope analyses. In general, the Calabar Flank shales are characterised by highly variable total organic carbon (TOC) contents, which range between 0.1% in Aptian–Albian Mfamosing Limestone and 9.9% in the Awi Formation sediments. The organic matter (OM) is a mixture of immature to early-mature marine and terrigenous OM of types III and IV. This is indicated by low hydrogen indices (HI value (10–190 mg HC/g TOC), Tmax (417–460 °C), vitrinite reflectance %Ro (0.39–0.62 %Ro), low to high C/N ratios (3.4–1158.0) and high amounts of terrigenous macerals (vitrinite + inertinite). Based on carbon isotope, C/N ratios and sulphate reduction index (SRI), OM degradation (up to 70%, SRI > 2.5) is most pronounced for shales deposited in a marine environment. The geochemical and petrographic data indicate that local factors such as low bioproductivity, down slope transport and redeposition of sediments from a fluvial–deltaic basin to nearshore facies, shallower, oxic and mildly oxygen-deficient environments, humid–arid paleogeographic conditions, specifically controlled the amount and quality of the OM during Aptian–Mastrichtian stages where marine sediments have been assumed to be deposited during the global anoxic events. Therefore, the order of the main factors controlling OM content in sediments are: input of terrigenous material transported from the land > low OM productivity by marine photoautotrophs > low preservation. ► Organo-petrography and geochemistry of Cretaceous Calabar Flank, Nigeria. ► The OM is immature to early-mature of marine and terrigenous types III and IV. ► Sulphate reduction (SRI > 2.5) of initial OM indicate degradation up to 70%. ► Paleoenvironment of deposition is interpreted based on the integration of all data. ► Amount and quality of OM are specifically controlled by local environmental factors.