Organic geochemical (Biomarker) method using gas chromatography-mass spectrometry was used to access the thermal maturity and biodegradation levels of the Agbada, Afowo, Yolde, and Bima Formation oils. Results obtained reveals that the Sterane and homohopane maturity parameters for the studied oils and their corresponding trisnorhopane thermal indicators (Ts:Tm ratio) are consistent with the thermal maturity values of petroleum source rocks within the oil window. This suggests that all the studied oils were generated as conventional light oil, some of which later degraded into heavy oils. Total Ion Current (TIC) fragmentogram of the saturate fractions of the studied oils shows progressive depletion of chromatographically resolved hydrocarbons relative to the unresolved hydrocarbon mixture, forming an unresolved complex mixture (UCM) hump consistent with oils that have undergone biodegradation except for the reservoired Agbada1 oil which was deeply buried beyond 2000 m and hence not biodegraded. Where these reservoir sands occur at depths greater than 2000m, it is possible to find non-biodegraded oils in them as long as they are properly sealed and connected to active petroleum source rocks.
Key words: Northern Benue trough; Dahomey Basin; Niger Delta Basin; Oil Geochemistry; Thermal maturity; Biodegradation.