The reservoir in the huge thick pre-salt lacustrine carbonate of M Oilfield in Brazil Santos Basin is silicified，and there are no relevant researches to clarify its geneses. Based on the data of core，thin section，fluid inclusion and X-ray fluorescence，the various silicification geneses in M Oilfield are discussed and their effects on reservoir are analyzed. The results show that the silicification in M Oilfield is common；more than half of samples in reservoirs are affected by the silicification，but only 17% of them are moderately and strongly silicified；the silicification can be divided into three categories：the sedimentation，early diagenesis and late hydrothermal fluid. Among them，the precipitation and early diagenesis silicification have limited effects on reservoir physical properties. The comprehensive evidences show that the strongly hydrothermal silicified intervals are closely related to the spatial distribution of intrusive rocks. Meanwhile，the main oxide contents of strongly silicified intervals are between those of the intrusive rocks and the surrounding carbonate rocks，showing a mixing characteristics of the two. Furthermore，the homogenization temperatures of the primary quartz fluid inclusions are high in the range of 87-193 ℃. These comprehensive evidences indicate the hydrothermal fluids resulting in silicification were originated from magmatic activity. Post-magmatic hydrothermal solution tended to move along the weak structure zones，including faults，fractures and the high-porosity matrix pores connecting with fractures，which leaded to strong silicification in reservoirs. When the silica content is more than 10%，reservoir physical properties are dramatically affected.