Four sets of stacked amplitude anomalies are described from a three-dimensional seismic survey acquired in Block A of the Dutch North Sea. The amplitude anomalies (AAs) have a sub-circular planform and within each set they are stacked vertically: they have a high degree of spatial overlap in the vertical succession. These sets of AAs are interpreted as vertical anomaly clusters (VACs) composed of five to seven major AAs each. Seismic interpretation of the VACs and quantitative analysis of the reservoir intervals reveal that the VACs are gas-bearing silt-rich reservoirs hosted in the upper section of the Upper North Sea Group, a mixed clastic succession of Pliocene to Pleistocene age. Novel analysis based on the geometry of the individual anomalies reveals that these are likely to be the result of a gas migration process characterized by sequential upward gas charge into reservoir units and that the flow across the seals separating these reservoirs is controlled by central regions of focused fluid flow. These regions function as seal-bypass systems and are most likely formed by hydraulic fracturing.