Light Hydrocarbon Geochemistry of Niger Delta Crude Oil: Implication for Source, Thermal Maturity, and In-Reservoir Alteration Processes.
Description
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.
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26 oil samples were collected from several depobelts in the Niger Delta region and stored in glass containers that were sealed and kept at a cool temperature before being transported to the laboratory for analysis. An Agilent 7890A Gas Chromatography split/splitless injection system (280 °C) was used in conjunction with an Agilent 5975C MSD for the GC-MS analysis of the light hydrocarbon components present in the oils. Full scan mode (10–235 amu/sec) analysis was performed on all of the samples. The oil specimen was fed directly inside the GC in split mode, and the temperature of the GC was automated to rise at a rate of 5 °C per minute, ranging from 30 °C (where it was kept for 5 minutes) to 310 °C, which was maintained for 15 minutes. Meanwhile, helium was employed as the hauler gas. Standard Mango and Thompson parameters were then determined.