Abstract: The widespread Low?resistivity oil reservoirs in Qionghai swell in western Pearl River Mouth basin of South China Sea are often associated with the underlying high?resistivity reservoirs. This paper makes comparisons of the crude oils from such two types of reservoirs,and suggests that the low?resistivity reservoirs are normally lower in oil density and sulfur content, with slightly higher maturity, more terrigenous organic matters in kerogen and enriched clay mineral deposited in an oxidizing environment, and such two types of crude oils have different reservoir?forming processes: the early formed mixed oil dominated by the Wenchang oil is preferential to charge into underlying high?permeability reservoir with low charging pressure, thus forming high?resistivity oil reservoirs; with more and more oil saturation of high?resistivity reservoir, the great amount of late formed mixed oil dominated by Enping oil of Wenchang?A sag is then charged into the upper low?permeability reservoirs with higher charging pressure, thus forming low?resistivity oil reservoirs. It is concluded that the wide distribution of the low?resistivity oil reservoirs in Qionghai swell indicates its surrounding or marginal hydrocarbon?generating sags should have higher hydrocarbon?generating intensity and be regarded as targets for further petroleum exploration