Abstract: The geochemical characteristics of extracts from the crude oils and reservoir bed taken from a well drilled in Pearl River Mouth Basin(PRMB) were investigated in detail. The trap on which the well was drilled was cut by a sub-EW direction normal fault. And study of the seismic section passed the well revealed that the fault has ever had three main developments. Fractionation results indicated that both the crudes and the extracts were richer in saturates(63.34-73.03%) and poorer in polar fraction(non-hydrocarbon+asphaltene, 11.31-20.43%).And saturate to aromatic ratios being larger than 3(the maximum up to 6.21) suggested their higher rank of maturation level. GC patterns of the saturates showed that there are tow major types of crudes in the trap. One is minor biodegrated, which occurred in the upper section(2005m and its above), and the other was normal, which distributed in the lower section(below 2005m).Furthermore, because of the injection of the oil with much higher maturity and lower molecular weights the top two ranked the highest maturity of the biodegraded oils, thus their n- alkane carbon number distribution appeared saw- toothed shape. Therefore, the biodegraded oil can in turn be divided into two sub-types: mixed vs. non-mixed. GC/MS analyses for the saturates confirmed that the crudes have ever experiienced higher geotemperature due to the discovery of diadrimanes and 8. 14-secohopanes. And the presence of oleanane implied the terrestrial origin of the oils while the dominant C₃₀ 4-methyl steranes in the sterane kingdom revealed evriched algal organic input in the source rocks. The fault development survey revealed that the essential cause responsible for the varying geochemical features of the upper biodegraded oils was resulted from the reaccumulation of the oils in the trap. The development of the fault resulted in the escaping of the underlying highly mature crudes from their initial reservirs and immidiately migrating upward along the fault. They favorably entered the shalowest reservoirs owing to differential accumulation, resulting in the ever-biodegraded oils upgrated their maturity and enriched in lihter carbon isotope composition and lower molecular weight n- alkanes because of mixed with the reaccumulated hydrocarbons.