Abstract: Hydrocarbon compositions of the separated gas and oil from the Well Gaotan-1 are analyzed by using gas chromatography. The features of the surface oil from the Well Gaotan-1 is studied and in-depth research is carried out for formation fluid physical properties with PVT experimental instrument. The results show that for the formation fluids in the Well Gaotan-1, the content of C1—C5 components reach 71.41%, the content of light hydrocarbons is relatively high, the content of heavy components is low, the wax content and freezing point are medium, the density and viscosity are low, indicating characteristics of volatile oil. The crude oil is single-phase liquid under the formation conditions, the formation pressure is abnormally high, the difference between formation pressure and saturated pressure is large, the formation has sufficient energy, the shrinkage rate of the formation oil is relatively high and the elastic energy is relatively strong. Combining with the PVT data of the formation fluid and using a triangle chart of three elements in reservoir fluid and reservoir identification parameters (φ1), the paper considers that the reservoir in the Well Gaotan-1 is a low-saturation volatile oil reservoir