Uncommon Distribution of C20 and C21 Tricyclic Terpanes in Niger Delta Crude Oils

Abstract

C20 and C21 tricyclic terpanes were investigated in crude oils from Niger Delta, Nigeria. From comparison of the mass chromatograms (m/z 191) of two crude oil samples (CEN-21 and WST-69) with related literatures on tricyclic terpanes which eluted from the gas chromatography (GC) within certain retention time, 12 peaks were selected. Peaks 1 - 5 were selected as C20 tricyclic terpane isomers (TR20a-e) and peaks 6 - 12 selected as C21 tricyclic terpanes (TR21a-g). This distribution is usually as crude oils show a single peak each and indicate the distribution of C20 and C21 tricyclic terpanes in the Niger Delta crude oils is uncommon. Total abundances showed the C20 and the C21 tricyclic terpanes were more in CEN-21 than WST-69 with ratios of 3.07 and 1.84, respectively, with TR20a, TR20b, TR20d and TR21d being the most abundant isomers in both oil samples. These abundances indicate CEN-21 was thermally more mature than WST-69 and that the C20 tricyclic terpanes were generated more than the C21 tricyclic terpanes with increasing thermal maturity of the crude oils. Significant similarities observed in the normalized composition profiles and 66 derived ratios suggest the Niger Delta crude oil samples were genetically related and were predominantly derived from terrigenous Tertiary deltaic petroleum systems. Some differences observed in the composition profile and derived ratios indicate minor input from a different source, depositional environment and/or maturity level and are suggestive as indicators for evaluation, correlation and/or discrimination of the Niger Delta crude oils.