The Occurrence of Lacustrine Depositional Setting in the Mamu Formation, Anambra Basin, Nigeria

Coal and carbonaceous shale samples were collected from Okaba and Onyeama in Mamu formation, Anambra basin, Nigeria. This study evaluated the organic materials and depositional environment of coal deposits of Anambra basin based on their elemental (C, H, O, N, and S) constituents and carbon isotopic compositions of biomarkers (n-alkanes). Elemental analysis in the samples was studied using a Carlo-Erba 1108 CHNS-O analyzer. Total organic carbon and total sulphur were measured by LECO analyses. The carbon isotope analysis of individual n-alkanes in the aliphatic fraction was performed using gas chromatography-combustion-isotope ratio mass spectrometer. The majority of the samples were characterized by low sulphur contents (>0.5 wt%). The low sulphur contents indicate coal derived from non-marine organic matter. The total organic carbon/total sulphur ratios range from 12.9 to 234.7 in the samples indicating non-marine coal. The total organic carbon/total nitrogen ratios also range from 20.49 to 31.51 in the samples. These ranges of values reflect land plant organic matter input. The elemental compositions of the samples indicate that the organic materials are derived majorly from terrestrial sources. The isotopic distribution of individual n-alkanes showed that the samples were derived from mixed organic materials (terrestrial and marine) deposited in fluvio-deltaic (Onyeama) and lacustrine-fluvial/deltaic (Okaba) settings.