Organic and elemental elucidation of asphaltene fraction of Nigerian crude oils

Abstract

This paper provides data on the chemical composition of the asphaltene component of Nigerian crude oils in order to ascertain its properties which can assist in the refining and processing paths of the natural resource. Crude oil samples were collected from three major oil fields in Nigeria and were deasphalted using n-pentane. The organic characteristics of the precipitated asphaltenes were determined using Fourier Transform Infrared (FT-IR) Spectrometry, while the elemental contents were measured by Inductively Coupled Plasma Atomic Emission spectroscopy (ICP-OES) and Carbon and Nitrogen Analyzer. Comparison of the average content of the Nigerian crude oils asphaltene with that of other nations shows that the Nigerian crude oil asphaltene content (3.65%, w/w) is less than that of other nations, except for Russian crude oil, but slightly higher than that of Brazilian crude oil, indicating that asphaltene deposition may pose less problems during Nigerian crude oil development than most of the crude oils from other nations. The IR spectra of the asphaltenes indicate the presence of alkanes, aromatic rings, phenyl rings, alcohols, ethers, carboxylic acids, esters, aldehydes, ketones, anhydrides, organic sulphoxides, sulphate salts and thiobenzophenones, confirming the fact that asphaltenes are composed of high molecular weight polycyclic constituents comprising of nitrogen, sulphur and oxygen heteroatoms. Ten elements (Cr, Cu, Fe, Mn, Ni, S, V, Zn, C and N) are examined and their concentrations determined. The results show that the values of all the elements are comparatively higher in the asphaltenes than the crude oils due to the presence of comparatively high concentrations of porphyrins in the asphaltenes than the crude oils. This is confirmed by their T-test values. Cross plot analysis result between the asphaltenes and crude oils using their elemental mean concentrations as variables indicates that moderate and positive correlation (R2=0.53) exists between them, indicating inter-element and geochemical relationships between them. Elemental clustering analysis results present two groups which are also fairly correlated. As expected, the elements show very close clustering because all the elements are known to associate with petroleum hydrocarbon formation. Also, apart from N, S and C; other elements are transition metals with similar chemical affinity.