Microgravity experiments and molecular dynamics simulations have been performed to study thermodiffusion of ternary mixtures composed of methane, n-butane and n-dodecane in petroleum reservoirs conditions. The simulations showed in both cases that methane and n-butane tend to migrate towards the hot side while n-dodecane tends to migrate towards the cold side. Some of the experimental results showed the same trend but with much larger amplitudes. Finally, a simulation of an idealized fluid column composed of one of the ternary mixture emphasizes that thermodiffusion can have a large impact on the vertical distribution of the components in a petroleum reservoir.