Abstract
Correlations of bulk geochemical properties such as density, API gravity, saturates, aromatics, resins and asphaltenes (SARA) compositions, aliphatic hydrocarbons (from nC8 to nC38) and their diagnostic ratios (Pr/Ph, Pr/nC17 and Ph/nC18) were evaluated for two Niger Delta crude oils (samples A and F) and their mix at different proportions of 4:1, 3:2, 2:3 and 1: 4 (samples B, C, D and E respectively). Pearson correlation showed strong relationships (> 0.99) between sample pairs in their bulk geochemical properties and diagnostic ratios, but variable relationships (0.20 - 0.85) in the aliphatic hydrocarbons which increased with increase in the proportion of sample F. Assessment of aliphatic hydrocarbon compositions by hierarchical cluster analysis (HCA) showed two major groups of clearly differentiated clusters with negative similarity and their plots revealed the aliphatic hydrocarbon compositions increased/decreased from samples A to F and exhibited variable strength for correlation of the oil samples. Plots of ratios of the aliphatic hydrocarbons were used to depict the geochemical compositional variations of the oil samples. Coefficient of determination (R²) of the plots indicate the ratios C 14/C14 + C25, C15/C15 + C24 and C17/C17 + C23 fits well and could account for 99.4%, 99.3% and 99.2% of the geochemical compositional variations of the two Niger Delta crude oils and their proportional mix. The results suggest these aliphatic hydrocarbon ratios could be useful for estimating the composition of two Niger Delta crude oils in their mix.
Highlights
Crude oil is a naturally occurring liquid mixture of hydrocarbon compounds
Agilent’s gas chromatography (GC) system 7890B equipped with an HP-5 silica capillary column 30 m x 320 μm ID and 0.25 μm film thickness and fitted with a flame ionization detector (FID) was used for aliphatic hydrocarbon analysis
Results of Pearson correlation of the crude oil samples for bulk geochemical properties and diagnostic ratios is presented in table 1 and for the distribution of aliphatic hydrocarbons (C8 - C38) is presented in table 2
Summary
Crude oil is a naturally occurring liquid mixture of hydrocarbon compounds It is derived from the chemical and geological transformation of organic matter in the depositional environment under the influence of heat and pressure changes over geological time scale [1, 2]. Ratios of aliphatic hydrocarbons such as pristane/n-heptadecane (Pr/nC17), phytane/n-octadecane (Ph/nC18) pristane/phytane (Pr/Ph) and carbon preference index (CPI), are commonly used as diagnostic tool for oil correlations and for interpreting geochemical data [10] They provide valuable information on organic matter source (marine or terrestrial), depositional environment (oxic and anoxic), lithology of the source rock and maturity [11]. This could have important implications by identifying geochemical parameters for estimating the composition of two commingled Niger Delta crude oils
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