Abstract: Reservoir bitumen have been found in Lower Ordovician carbonates from some wells in Lunnan low up-lift of Tarim basin. It occurred in the pores among mineral grams, with a content of 1%~3%. The measured reflectance of the bitumen ranges from 1.05% to 1.10%. With the combination of the burial history and palaeogeothermal evolution in this area,it is guessed that the bitumenization of the related crude oil took place at Early Hercynian,the oil reservoir was formed during Late Devonian to Early Carboniferous according to its petroleum geologic background. It is concluded that this set of reservoir bitumen was formed by the thermal maturation of bitumenized crude caused by denudation, weathering andoxidatjon owing to uplifting of this area during this period. The reservoir bitumen from the Lunnan low uplift has a similar origin with the Lower Silurian bitumen sandstones in Tabei uplift and Tazhong area. The discovery of the reservoir bitumen of Early Ordovician has extended the distribution of palaeo-oil and gas reservoirs of Early Paleozoic, and provided a new evidence for recognizing the complicated origin of oil and gas reservoirs in Lunnan area.

Key words: Tarim basin, Lunnan area, Ordovician, Carbonate rock, reservoir rock, bitume