A detailed organic geochemical analysis of six oil samples from the Baiyinchagan depression in the Erlian basin, Northern China, wascarried out in order to evaluate their origin. The oils are reservoired at a very shallow depth (223-560 m subsurface) and their chemical andphysical properties vary greatly, ranging from normal to extremely heavy oil. The preservation of non-biodegraded oil in such a shallowreservoir is possibly related with palaeo-pasteurization of the reservoir before uplift. Maturity difference is not the primary control on thechemical and physical properties of the oils and there is considerable geochemical evidence to suggest the additional influence of inreservoir/post-accumulation processes such as biodegradation, water-washing and (possibly) evaporation. Whereas some oils appear to beless affected, others are moderately biodegraded up to level 4 on the Peters and Moldowan (1993) scale, with sterane distributions largelyunaffected and 25-norhopanes undetected. Contrary to classical biodegradation, the unusual heavy oil shows little evidence of biodegradationfrom aliphatic components. Water-washing is suggested to be the primary process leading to its formation since the severe alteration ofsoluble aromatic hydrocarbons is observed. In addition, since the oils have been uplifted significantly after accumulation, evaporation and/orleakage to modify oil compositions cannot be ruled out.