Abstract: There are five sets of thick mudstones with wide distribution and low sediment change in the Lower Tertiary of the Nanpu Depression,which were deposited in medium-deep lake environment.The mudstones show efficient capping on thermal flux and hydrocarbon fluid,which results in development of high paleogeothermal gradient,high salinity and deep corrosion in the interior of mudstones and underlying formation.It is the main cause for formation of overpressure.Some lithologic hydrocarbon pools can be formed in the thick mudstones.As a result of its capping,thermal evolution of organic matter in abnormal belts was enhanced and hydrocarbon was generated.Deep corrosion accelerated forming of secondary pores in sandstones in the abnormal belt,so physical properties of reserviors was improved.A reasonable prediction of overpressure based on abnormal hydrothermal fluid activity in Beipu-Laoyemia area provided important guide for hydrocarbon exploration in the area.