Gas hydrate deposits formed on the pipe wall can lead to blockages of oil and gas production flowlines. Similar to wax and frost deposits, hydrate deposits formed along the pipe wall also effectively work as an insulating layer. In the present study, to quantitatively analyze the thermal properties of hydrate deposits, hydrate formation and deposition experiments were systematically performed in a gas-filled deadleg system. During the experiments, water vapor first condensed on the cold pipe wall and then formed hydrate deposits. After the experiments, thickness profiles of hydrate deposits along the pipe wall were measured by borescope detection. The average porosity and average dryness of the hydrate deposits were also calculated. On the basis of the measured hydrate deposit thickness, the effective thermal conductivity of the deposits was calculated according to the heat balance between the natural convection on the surface of the deposit growth front and the heat transfer through the interior of the hydrate deposits. Then, based on the calculated hydrate deposit porosity, dryness, and effective thermal conductivity, the thermal conductivity of the pure hydrates inside the hydrate deposits was determined. The effects of gas temperature and deposit thickness on the calculation results of the thermal conductivity were also investigated in this work. The conclusions in this work add knowledge to the key properties for hydrate management in deadlegs and flowlines.