Qaidam basin has experienced four major tectonic movements since Early Jurassic

including late Himalayan stage(end of Middle Pleistocene)

middle Himalayan stage(end of upper Youshashanian)

late Yan-shanian stage(end of Cretaceous) and early Yanshanian stage(end of middle Jurassic).The maximum principal stress values during the 4 major stages measured with acoustic emission(AE) method are 35.6～41.4

54.8～62.8

73.8 and 58.3 MPa

respectively.The current effective value of principal compressive stress is in the range of 15.6～112.3 MPa.The effective value of principal compressive stress during the main reservoiring stages of Tertiary oil and gas is in the range of 54.8～62.8 MPa.The sealing history of the NWW fault in western Qaidam basin is studied through simulating the stress fields during the major reservoiring stages and analyzing the typical oil and gas reservoirs in this area.It is found that the NWW reverse fault had dual effects on hydrocarbon migration and accumulation in late Miocene.It acted as hydrocarbon migration pathway during the active stage

and acted as seal during the ceasing stage

making hydrocarbons to accumulate both in the hanging wall and footwall of the fault.Hydrocarbons have accumulated mainly in the areas with moderate shearing stress.