The Distributions and Geochemical Significance of Triaromatic Steroids and Aromatic Dinosteroids in Crude Oils from Niger Delta Basin, Nigeria

Abstract

The crude oils from the Northern and offshore Niger Delta basin, Nigeria have been characterized by gas chromatography-mass spectrometry (GC-MS) in terms of their origins, depositional environments and thermal maturity based on the distribution of triaromatic steroids and aromatic dinosteroids. The oil samples from ADL and MJI oilfields were characterized by higher abundance of C26 20R + C27 20S TAS, C28 20S and C28 20R triaromatic steroids while OKN, MJO and WZB oil samples were characterized by the predominance of C26 20R + C27 20S TAS peak. Among the compounds identified in m/z 245 mass chromatograms, C21methyltriaromatic steroids, C22methyltriaromatic steroids, C27, 4-methyltriaromatic steroid, C27, 3-Methyltriaromatic steroid + C28, 3,24- Dimethyltriaromatic steroid and C27, 4-methyltriaromatic steroid + C29, 4-methyl-, 24- ethyltriaromatic steroid were the dominant compounds in the oil samples from ADL and MJI oilfields while the oil samples from OKN, MJO and WZB oilfields were characterized high abundance of all the compounds identified in m/z 245 mass chromatograms of the aromatic fractions of the oils. The crude oils were found to be formed from mixed origin (terrestrial and marine) but with significant contribution of dinoflagellates to the organic matter and deposited in freshwater-brackish/saline lacustrine environment. The oil samples were found to have early oil window maturity status based on the distributions and abundance of triaromatic steroids in the crude oils and this was further supported by well-established maturity parameters based on the saturate and aromatic biomarkers. This study showed that the abundance and distribution of triaromatic steroids and triaromaticdinosteroids can be used to assess the origin, depositional environments and thermal maturity of crude oils in the Niger DeltaBasin.