Crude oils in the Thrace Basin (western Turkey) and western Turkmenistan are believed to have been generated by a common Oligocene siliciclastic source rock. This widespread source rock extends over the area between western Turkey and the eastern South Caspian Basin. Oils from three Eocene reservoirs in the Thrace Basin and from four Pliocene reservoirs onshore western Turkmenistan were analyzed to investigate and compare their source-rock characteristics. bn order to understand control on the timing of hydrocarbon generation and source-rock maturation in both basins, the results of quantitative basin modelling were compared with those of geochemical analyses. The results indicate that all the oil samples exhibit similar geochemical characteristics, such as IP13-IP20 acyclic isoprenoid, terpane, regular sterane, methylsterane, and dinosterane profiles. The low tricyclic / pentacyclic terpane ratios, low C-20 norhopane /C-30 hopane ratios and low diasterane / regular sterane ratios, and the presence of 18 alpha (H) oleanane and gammacerane further support a common source or source facies. Based on these observations, it is concluded that shallow-marine clastics of Oligocene age constitute source rocks in both basins. The oils are of low maturity (Req (%) < 0.60), as indicated by their low ethylcholestane 20S/20S+20R, 17 alpha (H), 21 beta (H)-bishomohopane 22S/22S+22R, and high 17 beta (H), 21 alpha (H) moretane/l 7 beta (H) 21 alpha (H) hopane ratios. However, oils from west Turkmenistan appear to be move mature than those from the Thrace Basin. This is consistent with their earlier generation, which resulted from the higher sedimentation rate and higher heating rate. Present-day reservoir depths and temperatures appear to play only a minor role in determining the oils＇ maturities.