DTI Strategic Environmental Assessment 2001 (SEA2) : North Sea Geology

This review presents a summary of published data and their interpretation from areas in the mature oil and gas areas of the UK North Sea occurring to the east and north of the British Isles. The basis for this review is the premise that the modern environment is a synthesis of past environmental conditions. The purpose is to review (1) the evolution of the deeply-buried sediments with reference to the petroleum geology and production-related seabed subsidence (2) the evolution of the shallow seabed sediments with reference to present sediment distributions and seabed features (3) the evidence for possible hydrogeological exchange across selected onshore/offshore areas (4) the history of earthquakes and the hazard that they may pose. It is intended that the review will provide a basis for a better understanding of the impacts of possible future changes in the natural environment. The overall modern topography of the North Sea sea bed has originated from the influences of deep geological structure on the patterns of basin subsidence, uplift and climate on sediment input. The smaller-scale seabed geometry of the continental shelf is a relict of several glacial periods when large volumes of material were eroded from the adjacent mainlands and from the continental shelf itself. This material was then re-deposited on the shelf or in the deeper waters on the adjacent continental slope. The modern sedimentary environment of the North Sea continental shelf is now dominated by very low sediment input and the reworking of the seabed by near-bottom currents. A precursor to the submarine evolution of the North Sea occurred more than 375 million years ago with the deposition of marine limestones. Subsequently, subsidence and burial under thick accumulations of basin sediments has generated gas from coal source rocks, possibly commencing prior to approximately 140 million years ago. Oil and gas has been generated from deeply-buried mudstone source rocks from approximately 65 million years ago to the present day. Commercial petroleum reservoirs occur in almost every sedimentary succession ranging in age from approximately 410-36 million years. Exceptionally, the extraction of oil and gas has lead to production-related seabed subsidence, the effects of which are locally felt. This process appears to be restricted to a few types of reservoir and to date does not appear to have had major environmental impact. Extreme changes from arctic to temperate climates have been the dominant control on sediment type and the overall very high rate of sediment input into the North Sea from approximately 800,000 years ago to the present day. The overall effect of the repeated glaciations during the cold periods has been to keep the North Sea basin filled with sediments during a time when there was very rapid basin subsidence. The bulk of the modern seabed sediments comprises substrates that are more than 10,000 years old and have been reworked from strata by currents that have been generated by tides and sea waves. The reworked sediments typically form large areas of seabed sand and gravel. Such sediments also form the large-scale sandbanks and ridges and smaller sand waves. These characterise much of the seabed topography in the southern North Sea and are of strategic environmental interest. The largest ridges and banks have formed subparallel to the dominant tidal currents and occur as open-shelf ridges, estuary-mouth ridges or headland-associated banks. Many of these nearshore sand banks are mobile, others show little evidence for long-term mobility except at seabed where sandwaves appear to indicate that there is modern clock-wise circulation of sand around the bank. Hard substrates which are resistant to reworking are of interest to environmentalists and developers alike as they form areas of stable seabed for biota and may present problems for seabed site developments. The main types of hard substrate occurring at or near seabed comprise the unconsolidated gravel spreads, boulders, hard cohesive sediments which were formed during the glaciations and rock outcrops. All these commonly occur together in the nearshore western margins of the North Sea. The distribution patterns of rock, gravel spreads and the hard cohesive gravelly Quaternary sediments that have been deposited under or adjacent to the last ice sheet are quite well known and have been mapped by regional surveys. Other, usually older, seabed or superficial hard cohesive substrates are patchily developed mid-shelf and are therefore relatively unpredictable. Soft muds typically cover wide flat areas in the deeper waters of the continental shelf. In the central and northern North Sea the spreads of soft muds are locally characterised by small depressions or ‘pockmarks’, most of which appear to have been formed at times of fluid escape at seabed. The areas of seabed pockmarks have therefore originated by unusual processes resulting in seabed excavation and soft-sediment mobility. Few data are available with which to assess the possible effects of development operations on onshore and offshore aquifers. What data there are indicate that saline water ingresses inland locally from restricted zones offshore from East Anglia whilst the predominant movement elsewhere is that of freshwater movement offshore. There is a local risk of groundwater contamination if developments are superimposed on areas at seabed outcrop. There is a negligible risk of contamination of onshore supplies of freshwater from the mature areas of the oil and gas development provinces in the central and northern North Sea. Overall, the risk of onshore aquifer contamination decreases with increasing distance from the offshore to developments. The regional distribution patterns of earthquakes occurring under the North Sea are related to the deep geological structure. Expectations of earthquakes with magnitude of 4 or higher may require special structural design and are therefore also of environmental concern. In the North Sea as a whole, the expectations for a magnitude 4 natural seismic event is approximately every 2 years and a magnitude 5 natural seismic event every 14 years.