Abstract: In this srudy, some representative samples of normal crude oils in Biyang basin and heavy oils of Northwestern slope of the basin, as well as the major possibly source rocks have been investigated by means of organic geochemical technigues. The main results are as follows: l.By conparison with normal crude oils occurred in Biyang basin, the heavy oils located in NW slope of the basin possess higher viscosity( 280 mPa. s), higher concentration of resins plus asphaltenes( 34%), lower wax( 15%)and lower condensation point( 24℃), besides they have bigger specificgracity. 2.Physical properties of heavy oils located in different part of the NW slope exist some differentiations. The colser to the NW margin of the basin, the bigger spesific gravity. With increasing spesific gravity of the sample, its viscosity and concentration of resins plus asphaltenes increased, while its wax decreased. Besides the common biomarkers, some special biodegredating products, such as(aaabbb)-25-norhopane, 17 β-28-norlupane, 17β-23, 28-bionorlupane and so on have been also deteceed in saturated hydrocarbon fractionations of heavy oils. 3.According to the input imformation, such as the ratios of steranes/terpanes, gammacerane/C₃₀x β-hopane and maturity parameters, such as the ratios of motane/hopane, C₂₉-sterane-ααα 20S/20(S+R) and C₂₉-Sterane-ββ /(αα+ββ), the normal crude oils can be divided into three groups. The B86-B4062 group collected at the centre-southern part of the basin is much similar toheavy oils, as far as original molecuilar parameters is concerned. 4.The 3rd member of-fietaoyuan Formation(Eh_a) situated in NW slope of the basin is still immature source rock. So far it hasn't been entered into the oil generation threshold. Besides, the various biomarker parameters of it are different from that of heavy oils. Based on the comprehensive study of the characteristics mentioned above,the authors suggeste that the heavy oils of NW slope of Biyang depression originated from secondaryalteration(include oxidative degradation and biological degradation) pf the normal crudeoils, i.e both heavy oils of NW slope and normal crude oil of center-southern part of the basin have the same source. While the normal crude oil was migrated, accumulated and preserved in the shallow reservoirs of NW slope of the basin, it suffered secondary alteration and formed the heavy oil progressively.