Accretion during subduction, in which one tectonic plate moves under another, forms a wedge where sediments can be transformed into hydrocarbons. We utilised realistic computer models to investigate this and, in particular, how accretion affects mobility in the wedge and found that the evolution of the wedge and the thrusts it develops fundamentally control the thermal maturity of sediments. This can help us better understand the history of subduction and the formation of hydrocarbons in wedges.