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The Niger Delta Basin is one of the most prolific hydrocarbon provinces in the world, yet the origin of the vast amounts of oil and gas found in the numerous sub-basins across the delta remains contested. A total of 180 oil samples from more than 40 oil fields in the Niger Delta were analyzed by gas chromatography (GC), GC-mass spectrometry (GC-MS) and selected samples by GC-tandem mass spectrometry (GC-MSMS). The interpreted thermal maturity and source depositional environments of these hydrocarbons show significant variation depending on the components analyzed, and allow no clear correlation to a single source rock but rather imply extensive mixed contributions. Diamondoid hydrocarbon parameters were used for the first time on these Paleogene–Neogene reservoired oils to investigate source, thermal maturity and mixing effects and to perform cross-correlations of these oils. The diamondoid abundances and distributions support the hypothesis of co-sourcing of oil from a thermally cracked, sub-delta, Type II marine source, which was then mixed with oils of relatively lower maturities in the Paleogene–Neogene reservoirs. Future geochemical interpretations should treat the Niger Delta oils as potential mixtures of oils of variable maturities from different sources, often with the most important source biomarkers depleted because of the extent of thermal cracking.