Provenance and paleoenvironment of organic matter within the Fika sediments in Chad (Bornu) Basin, northeastern Nigeria: An integrated organic geochemical and palynofacies approach

The Upper Cretaceous Fika sediments in the Chad (Bornu) Basin were characterised to define paleoenvironment and source of the organic matter, using an integration of organic geochemical analyses and palynofacies observations. The present-day TOC contents of the Fika sediments are in the range of 0.41–2.37 wt%, while the Hydrogen Index (HI) values are largely below 200 mg HC/g TOC, suggesting that the sediments generally have fair to good generative potential and contain mainly Type III and Type IV kerogens. This is supported by palynofacies observations whereby the kerogen is characterised by large amounts AOM and phytoclasts. The Fika sediments have vitrinite reflectance and Tmax values in the range of 0.70–1.34% and 429–465 °C, respectively, indicating early-mature to late-mature stage of oil-window. The level of organic maturation in Fika sediments was further supported by the several biomarker maturity indicators. Biomarker distributions and palynofacies data provide evidence for a mixed aquatic algae and terrigenous organic matter input deposited in a marginally marine, moderate salinity, proximal setting with a strong influx of terrestrial plant under environmental conditions that were mostly suboxic. Compound specific isotopic analysis of n-alkanes fraction were also used for the paleoenvironmental inferences. The flat portion pattern of the n-alkane profile between n-C21 and n-C26 in Fika samples is an indication of marine environment and it shows that Fika sediments consist of both terrestrial and marine organic matter deposited in fluvial/deltaic to shallow marine settings.