Organic Source Input, Thermal Maturity and Paleodepositional Conditions of Imo Formation in the Anambra Basin, Nigeria

This investigation examines the samples of Imo Formation from Nzam-1 well, Anambra basin, to identify the source of organic matter, the depositional environment, thermal maturity and tectonic setting. The sediments consist of grey to dark grey, less gritty fissile shales, with no distinct grainy appearance or layered structure, and white to grey sandy shales with intercalations of fine grained sandstones. Biomarker results indicate short to middle chain n-alkanes, low to medium CPI values and narrow range of Pristane/Phytane ratio. Major oxides in the samples follow the trend: SiO₂ > TiO₂ > Fe₂O_3. Trace elements such as Sr, Ba, V, Ni, Co and Cr were identified. The biomarker results suggest thermally immature to early mature sediments deposited in anoxic to sub-oxic environment. The biomarkers of terpanes and steranes indicate derivation from a mixed marine algal source with minor contributions from terrigenous inputs. Trace element ratios also indicated oxic to dysoxic bottom water conditions, reflecting mixed terrigenous and marine conditions. Major oxide ratios indicated that samples were sourced from a passive continental margin under humid paleoclimatic conditions. Geochemical parameters (Cr/Ni, and Th/Cr ratios etc.) suggested a felsic dominated basement source.