Abstract: According to previous studies,we applied a serious of techniques,such as X-ray diffraction,thin section analysis,pyrolysis analysis and biological marker analysis,and then compared the related characteristics between the study area and adjacent normal pressure areas.Based on the previous work,we made a further analysis on the distribution of overpressure in LD block and its effects on clay mineral transformation and thermal evolution of organic matter.The results show that:①In study area,the depth of the top interface becomes deeper from basin center to margin.②Over the interface of overpressure,with the increase of depth and geotemperature,the content of montmorillonite,illite mixed montmorillonite and kaolinite gradually decreased,while the content of illite and chlorite increased.However,below the interface of overpressure,content of every component in clay mineral almost has no change which inhibited by formation overpressure.③With the increase of depth,the ratio Pr/nC₁₇ and Ph/nC₁₈ decreased.While below the interface of strong overpressure,the trend reversed and even appeared slight increase.R_o values,the maturity of organic matter,which are measured below the overpressure interface,also appeared significantly abnormal lower.Overpressure indirectly weakens the reservoir's cementation by inhibiting the transformation of clay minerals,which promotes the formation of deep and high-quality reservoir.Overpressure in the study area inhibits the thermal evolution of organic matter.The inhibiting effect of overpressure on hydrocarbon generation enlarges position and volume of the later valid source rock.It delays the hydrocarbon-generating and expulsion times of main source rock,which contributes to the late hydrocarbon accumulation.