This report is a contribution to the Strategic Environmental Assessment (SEA3) conducted by the Department of Trade and Industry (now Department of Energy and Climate Change) and is an addendum to "SEA2 Technical report 008 - North Sea Geology, covering subsurface geology, sea-bed sediments, sediment mobility and seismicity." by same authors. The geology of the North Sea from the Palaeozoic era to the present day is reviewed, with reference to petroleum geology. Geological factors affect the environmental consequences of oil and gas exploitation in many different ways. For example, in the case of the Ekofisk oilfield in the Norwegian sector of the North Sea, oil production led to seafloor subsidence of a few metres due to its chalk reservoir, but production-related subsidence on this scale is atypical. The evolution of the geomorphology and composition of the shallow and seabed sediments is discussed. The distribution of mud, sand, gravel and hard substrates influence the nature of the benthos and can affect the way in which contaminants are accumulated and transported. Shallow seabed sediments support the foundations of structures ranging from platforms to pipelines. Hydrogeological conditions and the risk of pollution to aquifers are also reviewed.