Abstract: A comprehensive review of the data on gas source rock, source-reservoir-cap assemblage and trap characteristics, the formation condition of large gas field in Southern North Sea Basin was made in this paper in order to understand the basic mechanisms of oil accumulation. In this area, the Westphalia coal measures of the Late Carboniferous is the major gas source rock, with the Rotliegende sandstone as the main reservoir. The Lower Triassic sandstone is also a reservoir in some areas. The Upper Permian evaporite deposits provide a unique regional cap. Most large gas fields are preserved fractured structural traps. The formation of gas field depends upon the combination of the availability of Carboniferous gas source rock, distribution of Rotliegende sandstone and the effective sealing by regional evaporite cap rock.