Abstract: As a principal part of the Western Pearl River Mouth Basin,Wenchang Sag mainly includes Sag A and B.It is about 4000km².We discovered that its accumulative total petroleum reserves exceed 1.0×10⁸m³,The discovery of natural gas reserves are about 300×10⁸m³ and proved rich hydrocarbon depressions.It mostly developed two kinds source rock:Wenchang group and Enping group.It embodied three sets of source:Wenchang group of deep lacustrine source rock,Wenchang group of shore shallow lacustrine source rock and Enping river swamp facies source rock group.The recent WC 19-1M-Xwell drilled firstly revealed the being of high quality Wenchang group lacustrine source rock following the hydrocarbon generating threshold.So it deepened to the study of the geochemical characteristics and the redefinition of hydrocarbon generation potential,through the contrast research between the coming into hydrocarbon generating threshold and not,combining basin simulation.This paper analyzes that the character of the ancient lake gyre made the WenChang group turn into the high quality hydrocarbon source rock.And