Source rock geochemistry of central and northwestern Niger Delta: Inference from aromatic hydrocarbons content

Abstract

The influence of origin, depositional environment and thermal maturity of organic matter on the occurrence of aromatic hydrocarbons in source rocks from the central and northwestern Niger Delta was investigated. Eighty two source rock samples from four oil wells in the central and northwestern Niger Delta were analyzed for the aromatic hydrocarbon content using gas chromatography-mass spectrometry (GC-MS). The results of analysis of aromatic hydrocarbon fractions of the source rock extracts show the presence of many classes of aromatic hydrocarbons, which include naphthalenes, phenanthrenes, biphenyls, dibenzothiophenes and fluorenes. Trimethylnaphthalene (TMN) is the most abundant among the naphthalenes while dimethylphenanthrene (DMP) is the most abundant among the phenanthrenes. Among the biphenyls, 3-methylbiphenyl (3 MB) is the most abundant while 4-methyldibenzothiophene (4MDBT) is the most abundant among the dibenzothiphenes. Only two fluorenes were detected, and fluorene is more in abundance than 1-methylfluorene (1 MF). Depositional environment indicators of aromatic hydrocarbon organic matter reveal that the organic materials in the source rocks in these wells were deposited in marine-to-swamp depositional environments under reducing to suboxic conditions. Thermal maturity indicators calculated from the abundance of the aromatic hydrocarbons indicate that the source rock samples are thermally mature. Most of the source rocks are at the peak of oil window while a few at the early oil window. Source rocks from wells GB, OP and OT in the central Niger Delta are thermally more mature than those from well AW in the northwestern Niger Delta.