The hydrocarbon systems of the West Netherlands Basin are analysed using the two-dimensional forward modelling. Three source rock intervals are considered in the modelling. The history of heatflow is calculated from the forward modelling of tectonic subsidence. Modelling results show that the oil accumulations were filled just after the Sub-Hercynian inversion event (∼80Ma), and that in the Westphalian gas system a potential exists for undiscovered reserves. Modelling of the Namurian gas system shows that mixing with Westphalian gas is likely. The existence of a Namurian hydrocarbon system may also imply that hydrocarbons are trapped at deeper, unexplored levels in the Westphalian and Namurian succession.