Abstract: The heavy oil genetic mechanism is studied by analysis of the petroleum geologic and geochemical characteristics in Huoyanshan central uplift belt in Tuha basin. The result shows that the existence of 25-norhopane and 25, 30-bisnorhopane in the Triassic crude oil reveals the hydrocarbons in this area generally undergo biodegradation with different degrees in different areas. In Tuyuke area in low position of the structure, it is relatively weak; in Yingye'er area it is higher for it is in high position of it. Also, the fluid inclusion study indicates that in Shengjinkou area, normal crude oil occurs when oil charging followed by tectonic rising, biodegradation and forming heavy oil. The fluid inclusion from Yudong-1 Well appears in twice crude charges including normal oil and heavy oil. The fluid inclusions from Lu-8, Lu-9, Lu-11, Lu-2, Lu-10 wells reveal that from west to east the oil charges gradually change by migration from normal oil into complete heavy oil with the burial depth becoming shallower. Lu-10 Well could be the key point for crude oil to be thickened or oil composition to be dispersed at that time.

Key words: Tuha basin, crude oil, biodegradation, fluid inclusion, hydrocarbon migration, heavy oil origin, genesis