Abstract: The Eogene mudstone caprock in Kuche Depression is very thick and distributed widely in the depression. It is of strong capillary seal ability, overpressure seal ability and weak restraining hydrocarbon concentration seal ability. The comprehensive seal ability of the Eogeneb mudstone caprock in Kelasu, Dongqiulitake and Yinan Structures is good, its comprehensive seal ability gradually become bad from north to south The distribution of the mudstone caprock controls the distributions of oil or gas in the Cretaceous system. The gas reservoirs distribute under good caprock, oil reservoirs distribute under middle caprock. The seal period of mudstone caprock of Eogene system is later than the period that a great deal of oil expelled from sourcerock of the Triassic and Jurassic system and is not favor able for a great deal of oil to accumulate. The seal period of mudstone caprock is earlier than the period that a great deal of gas expelled from sourcerock of the Triassic and Jurassic system and is favorable for a great deal of gas to accumulate.