The Hulunhu Depression with thick deposits and well-developed strata is an important replacement exploration area in the Hailaer Basin.The Nantun Formation is its primary hydrocarbon source rock and the Tongbomiao and Damoguaihe Fms are the secondary source rocks.These source rocks have supreme oil-generating conditions

but no systematic study has been performed on its geothermal evolutionary history so far.The present geothermal gradient is 30.7 ℃/km in the Hulunhu Depression

belonging to the moderate temperature field.The average paleogeothermal gradient recovered by the vitrinite reflectance

homogenization temperature of fluid inclusions and apatite fission track method is about 42-56 ℃/km

obviously higher than the present geothermal gradient.That is to say

the paleogeotemperature is higher than the present geotemperature.The paleogeotemperature reconstruction and geothermal history simulation showed that the Hulunhu Depression reached the maximum paleogeotemperature in the Late Yimin period(90Ma)

and entered a cooling process since the Paleogene and Neogene.The maturity of the Lower Cretaceous hydrocarbon source rocks was mainly controlled by the paleogeothermal field.The results from the studies of the paleogeothermal evolutionary history and the authigenic illite dating of reservoirs showed that the Late Yimin period was a very important stage for hydrocarbon accumulation in the Hulunhu Depression.In this period

the depression uplifted and got denudated

the temperature of strata reduced

the burial depth of source rocks decreased

and the intensity of hydrocarbon generation weakened.