Tricyclic Terpane Fingerprint of Crude Oils from Niger Delta, Nigeria

Abstract

Tricyclic terpanes identified in two representative crude oils from the Central and Western Niger Delta (oil- RVAG and oil-DTIR, respectively) ranged from C19 to C29, including C24 tetracyclic terpane. Plot of the abundances of tricyclic terpanes show similarity in the distribution profile of the oil samples, characterized by the uncommon distribution of C20 and C21 tricyclic terpanes indicating both Niger Delta crude oils are derived from similar source organic matter. Ratios of C22/C21 and C24/C23 tricyclic terpanes indicate source rocks of the crude oil samples were deposited in a deltaic environment and derived from mixed marine/terrestrial organic matter. Additional, C26/C25 tricyclic terpanes, C24 tetracyclic/C23 tricyclic terpanes and C24 tetracyclic/C26 tricyclic terpanes indicate the crude oil samples are generated from shale source rocks derived from terrestrial organic matter, with oil-DTIR receiving a more terrestrial contribution, and marine facies, with oil-RVAG receiving a more marine contribution. Thermal maturity, determined from ratios of low to high molecular weight tricyclic terpanes, indicate the crude oil samples are mature with oil-RVAG more mature than oil-DTIR. The ratios of C19/C23 and C20/C23 tricyclic terpanes, which indicate high maturity of the crude oils, suggested terrestrial and marine source for oil-RVAG and oil-DTIR, respectively, contrasting with the relative abundances of tricyclic and tetracyclic terpanes. This implies the abundances of C19 and C20 tricyclic terpanes, particularly in crude oils at high maturity, as in the Niger Delta, has interfering effect on source interpretation and its ratios be used in combination with other parameters for evaluating Niger Delta crude oils.