A total of 17 crude oil samples from producing wells in the eastern Niger Delta were analyzed for their thermal maturity status. The study is aimed at establishing maturity trends across the area and identifying maturity parameters that can be reliably used for maturity assessment of the area. On the basis of the geography, the study area has been subdivided into eastern, western, and central areas. The oils were analyzed using gas chromatography (GC) and gas chromatographymass spectrometry (GCMS). The results showed that the pristane/phytane (Pr/Ph) ratio ranges from 2.3 to 5.1, which indicates that the oils were derived from mixed terrigenous/marine organic matter. The sterane and hopane results based on thermal maturity parameters were unreliable for oil maturity assessment. Aromatic maturity parameters worked better for thermal maturity assessment of the oil samples analyzed. The conclusion was based on the plots of 4-methyl dibenzothiophene/1-methyl dibenzothiophene (MDR), C-27 17 alpha (H)/C-27 18 alpha (H) (Tm/Ts), methyl phenanthrene ratio (MPR), monoaromatic steroid C-21/triaromatic steroid C-21 + monoaromatic steroid C-28 (MAC(21)/TAC(21) + MAC(28)), and 20S/(20S + 20R)C-27 maturity parameters, which are in good agreement with regard to the maturity status of the oils analyzed and are, therefore, suitable for use in the thermal maturity assessment of oils in the study area. Maturity of the oils is found to increase from the east to west area.