Light Hydrocarbon Geochemistry of Niger Delta Crude Oil: Implication for Source, Thermal Maturity, and In-Reservoir Alteration Processes

Gas chromatography-mass spectrometry was used to analyze 26 oil samples from several sub-basins for their light hydrocarbons (LHs) fraction to ascertain any compositional differences in terms of source inputs, thermal maturation, and in-reservoir alteration. Mango's invariant K1 parameter clearly delineates the Niger Delta oils into two homologous sets, consistent with the plot of ph/nC18 vs pr/nC17. The studied oils appear to be modestly distributed within Thompson's (Kerogen Type) curves model, with heptane values ranging from 0.64 to 22.04%, on average 16.19%. The oils are mostly plotted within the aromatic and aliphatic curves of Thompson's model. The LHs fraction of the studied oils shows an overall 6-ring-member preference, suggesting a major contribution from higher plants. Conclusively, the studied Niger Delta oils reflect expulsions from different source rocks formed from terrigenous and mixed terrigenous/marine organofacies that were originally deposited in oxic to sub-oxic settings and of early to peak oil thermal maturity. Varying degrees of evaporative fractionation were established in most samples from Greater Ughelli as evidence of gas washing in the depobelt.