Tazhong 45 reservoir, lying in the Middle-Upper Ordovician strata of Tazhong area, is a fluoritized carbonate reservoir. There exist many fluorite veins filled with crude oils in its reservoir bed. The well core and microscope observation indicates that fluorite is formed by replacement of calcite. There are many hydrothermal minerals accompanying fluorite, such as pyrite, quartz and chlorite. Two generations of fluid inclusions are observed in fluorite, namely the primary and the secondary inclusions. Most of the primary fluid inclusions have homogenization temperatures between 260 and 310℃, which represent the temperature of hydrothermal fluid responsible for fluorite precipitation. The secondary fluid inclusions have homogenization temperatures between 100 and 130℃, which represent the hydrocarbon charging temperatures because there are many petroleum inclusions among the secondary inclusions. Characteristics of the mineral assemblage and primary homogenization temperatures indicate that the precipitation of fluorite is related to hydrothermal activities in Tazhong area. Theoretically, the volume will be reduced by 33.5% after the replacement of calcite by fluorite, which is likely to raise the reservoir quality to a great extent. There are also numerous intergranular pores filled with hydrocarbons in fluorite. Therefore, the hydrothermal activities and the precipitation of fluorite in Tazhong area greatly improvethe reservoir property and make the Tazhong 45 reservoir more suitable for hydrocarbon accumulation.