Abstract: The Jurassic petroleum system has been being the exploration target because most oils found in north margin of Qaidamu Basin are thought to sourced from the Jurassic source rocks for a long time.The Carboniferous distributed scatteredly in west part of north margin of Qaidam Basin has not been minded for the thought of poor capability of hydrocarbon generation.There are some disputes on the attribution of the stratum deeper than 4 666 m after the drilling of Well Lengke1 in Lenghu 5th structure.The stratum is the Carboniferous based on study on lithology,electricity,paleontology,and it is realized that what has been drilled to in Well Shen86,Lenghu area is also Carboniferous.Thick Carboniferous in Well Gaqiu1 drilled before Luliang Mountain in 2004 suggested the exploration perspective of the Carboniferous.It is realized that there is not only one set of source rock,the Jurassic for the discovery of the Carboniferous in the west part of north margin of Qaidam Basin.The Carboniferous and the Jurassic source rocks are very different based on their geochemical characterization.The most remarkable difference is high maturity with low abundance of disasteranes and diahopanes for the Carboniferous source rocks.The Carboniferous and the Jurassic can be distinguished well by geochemical characterization,combing with lithology,electricity and paleontology.Correlation results of oil and source indicate that some petroleum is sourced from the Carboniferous in west part of north margin of Qaidam Basin rather than all petroleum is from the Jurassic,indicating new petroleum exploration area in north margin of Qaidam Basin.