Terpane Characterization of Crude Oils from Niger Delta, Nigeria: A Geochemical Appraisal

Abstract

The geochemistry of crude oils from the Niger Delta, Nigeria, were evaluated using the characterization of C₁₉ to C₃₅ terpanes. Analyses of two representative crude oils from Western and Central Niger Delta showed abundances of C₁₉ to C₂₉ tricyclic terpanes constituted 12.09% and 29.74%, C₂₄ tetracyclic terpane 0.31% and 0.15% and C₂₇ to C₃₅ pentacyclic terpanes 87.61% and 70.12%, respectively. Diagnostic ratios of terpanes indicated relatively low abundances of C₂₃, C₂₈ and C₂₉ tricyclic terpanes, low abundances of homohopanes, a significantly high abundance of oleanane and that the Niger Delta crude oils were derived from terrestrial organic matter source rocks deposited in an oxic environment during the Tertiary period. Multivariate oil-oil correlation plot showed the Western and Central Niger Delta crude oils are not distinct, but moderately related (genetically). However, diagnostic ratios of C₂₄ tetracyclic terpane, which was high and moderate and gammacerane, which was low and high, revealed crude oils from Western Niger Delta were derived from predominantly terrestrial source and crude oils from Central Niger Delta were derived from terrestrial source with input from marine organic matter, respectively. Tricyclic terpanes/hopanes and isomerization ratios of C₃₂ homohopanes indicated the Niger Delta crude oils were generated at high maturity, at top of the oil generation window.