Several genetic mechanisms of less mature oils have been proposed by previous researchers, and solvable organic matter generating hydrocarbons in early evolution stage has been suggested as one of the most important mechanisms for considerable immature reserves in rifted-basins such as the Jinhu depression, North Jiangsu-South Yellow Sea basin and Bamianhe oilfield, Bohai Bay basin of East China. However, our recent investigations show that some of the genetic models of less mature oils cannot be used to guide oil exploration. Oil-rock corrections have indicated that the so-called immature oils of the two basins are mainly derived from intervals located in normal oil-windows. There is hardly correlation between the alga-rich Es4 shale developed in brackish environments of the south slope of the Nuzhuang sag, Dongying depression and Bohai Bay basin and the crude oils in Bamianhe Oilfield in terms of the distribution patterns of steroids especially dinostanes. The results suggest that solvable organic matter in shale with buried depths less than 2700 m of the area, which was considered to be the main source rock of Bamianhe Oilfield in previous studies, is unlikely to be the essential source of the oils. Immature oils mostly coexist with adjacent favorable oil-kitchens in East China and the fact that the so-called immature oils of the typical immature oilfield i. e. the North Jiangsu-South Yellow Sea basin have features of mature oils infer that oil-mixing is a common phenomenon in some Tertiary rifted-basins of East China. Some methods of resource assessment may not be suitable for less mature oils if other genetic mechanisms rather than the traditional kerogen degradation account for immature oils. A new assessment method showes that the amount of immature resources in the area studied is not as large as previously calculated. Reinvestigation is necessary in basins bearing immature oils, adopting new analysis techniques such as biomarker quantification. It will be useful in determining the real maturity and the proportion of mixed oils with different sources.