Abstract: A method for reducing the viscosity of a liquid heavy hydrocarbon feedstock comprising the steps of bringing said feedstock to a temperature capable of cracking of at least a portion of the hydrocarbons present in said feedstock and introducing said feedstock into a bottom portion of a maturation device (i.e. a "soaker"), where the feedstock is displaced from the bottom upwards, to get evacuated from a top portion of the soaker towards a fractioning unit. The invention also relates to an apparatus comprising a soaker in which are located, transversely to the direction of displacement of the feedstock to be treated, a plurality of annular discs spaced apart from one another, each disc comprising a circular central passage substantially coaxial to the soaker, the treated feedstock circulating from the bottom portion upwards in the soaker through the central passages of the various annular discs.