Petroleum & Coal ISSN 1337-7027 Available online at www.vurup.sk/petroleum-coal Petroleum & Coal 57(5) 526-531, 2015 GAS CHROMATOGRAPHY-MASS SPECTROMETRY (GC-MS) COMPOSITIONAL ANALYSES OF AROMATIC HYDROCARBONS IN NIGER DELTA CRUDE OILS Mark O. Onyema * , Leo C. Osuji Petroleum and Environmental Geochemistry Research Group, Department of Pure and Industrial Chemistry, University of Port Harcourt, Nigeria E-mail: onyemark@yahoo.com (Corresponding author). Received August 14, 2015, Accepted November 30, 2015 Abstract The composition of aromatic hydrocarbons used as diagnostic tools in oil correlation studies was investigated in Niger Delta crude oils. Crude oil samples collected from producing wells were fractio- nated and gas chromatography-mass spectrometry (GC-MS) used to analyze the aromatic fraction. The analysis revealed high abundance of aromatic hydrocarbons which consist of polycyclic aromatic hydrocarbons (PAHs) 92.8% - 93.7% and aromatic steranes 6.3% - 7.2%. PAHs identified include C 1 - C 3 naphthalenes, C 0 - C 2 phenanthrenes, C 0 - C 1 biphenyls, C 0 - C 1 fluorenes and C 0 - C 1 dibenzo- thiophenes. The considerably high abundance of naphthalenes and low dibenzothiophene/phenan- threne ratio from 0.09 - 0.11 indicated contribution of terrestrial organic matter to source rocks which were deposited in oxic environment. The identified aromatic steranes, monoaromatic steranes (MAS) and triaromatic steranes (TAS), were characterized by the short-chain C 20 - C 22 and long- chain C 26 - C 29 series. The abundance of TAS over MAS from 2.08 - 2.99 suggest high maturity crude oils for the Niger Delta region. From these results, the characterization of aromatic hydro- carbon compounds in Niger Delta crude oils have provided substantial evidence of their suitability as geochemical markers for crude oil correlation studies. Key Words: GC-MS; composition; polycyclic aromatic hydrocarbons (PAHs); aromatic sterane; oil correlation. 1. Introduction The source of crude oil, burial environment and thermal maturity can be inferred from the composition of hydrocarbons in the oil [14,7] . Earlier, focus was placed on the saturate hydrocarbons, such as normal alkanes (n-alkanes), isoprenoid alkanes and the biomarkers tri-, tetra-, and pentacyclic triterpane hydrocarbons [2-3,11] . With improvement in analytical methods, crude oil studies are now supplemented with geochemical data from aromatic hydrocarbons [23-24] . Aromatic hydrocarbons in crude oil include naphthalene, fluorene, phenanthrene, dibenzo- thiophene and their alkylated isomers, as well as anthracene, fluoranthene, benzopyrenes and aromatic steranes [16] . They originate from chemical and geological transformation of natural product molecules, deposited during sedimentary processes, by diagenetic rearran- gement or dehydrogenation or by aromatization or fragmentation during catagenesis [21] . Consequently, the occurrence and distribution of certain aromatic geochemical hydrocarbons are diagnostic of the crude oil in which they occur and have proven effective in oil correlation studies [20,17-18] . Ratios of dibenzothiophene/ phenanthrene used in a cross-plot with pristane/ phytane classified crude oils from different source rocks and ages into their paleodepositional environments [8] , while the distribution of phenanthrene and its methylated isomers have proven effective in oil-oil correlation by identifying differences in timing of oil generation, maturity and post-generative alteration [1] . In Nigeria’s Niger Delta region, studies on crude oil correlation use the distribution of saturate hydrocarbons, such as n-alkanes, isoprenoid alkanes and triterpanes [5,10] . Also, the composition of light hydrocarbons, invariance ratio of isoheptanes and ring preference