Ten crude oils samples from onshore and offshore Niger delta were characterized based on their biomarkers and isotopic composition by Gas chromatography–Mass spectrometry and Isotope mass spectrometry. Abundance of pentacyclic triterpanes of hopane and oleanane skeletons and C₂₇–C₂₉ steranes in the oils indicates that they were formed from a mixed source (marine and terrestrial kerogen). The oleanane concentrations permit source grouping of the oils into three families. Various maturity parameters computed for the oils, especially those of aromatic biomarkers and aromatic sulfur compounds (thiophenes) to some extent reveal an increasing thermal maturity with increasing reservoir depths. The carbon isotope values lacked sufficient variation to distinguish maturity differences in the oils. This observation is quite revealing in that oils whose associate gases had earlier revealed a consistent increase in maturity with increasing depths, also have similar maturity values. The implication of this is that the oils and associated gases might have been formed at different levels of thermal maturity before being pooled together during migration into their present habitat.