Keywords: 
• Hailaer basin /
• Beier depression /
• geothermal gradient /
• apatite fission track /
• geothermal history

Abstract: Beier depression is the highly explored depression in Hailaer basin with complete and thick sedimentary sequence. The producing oil condition is very excellent, and it is the main oilgas depression in Hailaer basin.The main hydrocarbon source rocks is from the Nantun group, next is Damoguaihe group and Tongbomiao group.The average paleogeothermal gradient recovered by the vitrinite reflectance and homogenization temperatures of fluid inclusion and fission track method is about 3.7～5.8℃ /100m,it is obvious higher than present geothermal gradient 3.35℃/100m, the paleotemperature is higher than present geothermal, the paleotemperature research and thermal history simulation show the max paleogeothermal value were reached in late Cretaceous,paleogeothermal field controlled maturity of hydrocarbon source rocks of Lower Cretaceous.Results of the geothermal history on the basis of illite dating results and hydrocarbon generated and hydrocarbon expulsed process show that the forming time of oilgas reservoirs have two stages in Beier depression: late Cretaceous and early Tertiary. Late Creatceous is the main stage for Beier depression. The second stage is early Tertiary，it happened Lifting disintegration function in the stage,the goal stratum temperature started to reduce, the intensity of hydrocarbon production began to weaken, on the one hand,the oilgas reservoirs formed in the first stage was adjusted;one the other hand, it was the second formation process of oilgas reservoirs.The second forming stage of oilgas reservoirs is important for the formation of oil and gas reservoirs in Beier depression and also the important factor for oilgas accumulation in Beier depression.