Biomarker application in the recognition of the geochemical characteristics of crude oils from the five depobelts of the Niger Delta basin, Nigeria

Abstract

Biomarker fingerprints of crude oils are useful indicators of origin of organic matter input in source rocks and depositional conditions which are useful indices for petroleum systems development within a hydrocarbon producing horizon. Twenty five (25) crude oil samples from the five depobelts of the Niger Delta basin, Nigeria were studied to describe their biomarker fingerprints, provide information on the origin of organic matter input in the source rock(s), determine depositional environmental conditions and thermal maturity of the crude oils. The study was based on biomarkers (steranes and hopanes), normal alkanes and acyclic isoprenoids (pristine and phytane) obtained from the gas chromatography–mass spectrometry (GC-MS) analyses performed on the saturated fractions of the crude oils. The results of pristine/phytane (Pr/Ph) ratios, Pr/n-C17 ratios, Ph/n-C18 ratios, C29/C27 sterane ratios, sterane/hopane ratios, %C27, %C28 and %C29 regular steranes, oleanane index, waxiness index, Ts/Tm ratios and Carbon Preference Index (CPI) values, indicated that the analyzed crude oils belong to the same family of oil and originated from terrigenous clastic source rock (s) containing land plant organic matter with minor marine organic matter input, deposited under oxic to sub-oxic paleoenvironmental conditions. The Pr/n-C17 versus Ph/n-C18 cross plot indicated marine algal type II and a mixed type II/III kerogen. The C32H:22S/(22S+22R) values together with Ts/(Ts + Tm), C29:20S/20S + 20R, oleanane index and CPI values indicated that most of the crude oils have reached thermal equilibrium with high thermal maturity levels.