Genetic sequence stratigraphy for the North Sea Late Jurassic and Early Cretaceous: distribution and prediction of Kimmeridgian–Late Ryazanian reservoirs in the North Sea and adjacent areas

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Abstract
Sediments of Kimmeridgian to Late Ryazanian age form a group of key hydrocarbon play fairways in the syn-rift Jurassic of the North Sea. The perceived yet-to-find reserves of these often subtle plays, lying at or below seismic resolution, have attracted considerable industry attention over the past few years. Reserves are currently estimated by BP Exploration at 1 to 5 billion barrels of oil equivalent, reservoired in three play systems: (1) apron fans (e.g. Brae type); (2) basin floor fans (e.g. Miller, Galley, Ettrick and Magnus types); (3) shallow marine shelf (e.g. Ula, Gyda, Fulmar, Piper, Clyde types). In order to assess the future exploration potential of this play fairway, a high resolution, predictive, sequence stratigraphy was erected for the North Sea Late Jurassic. The stratigraphic framework combines data from over 500 exploration wells with seismic and field data (Magnus, Brae, Miller, Ula, Gyda and Clyde). In the Late Oxfordian to Late Ryazanian, a total of 11 genetic stratigraphic sequences have been defined. They are bounded by maximum flooding surfaces which, within the limits of the biostratigraphy, represent basin-wide isochronous events across NW Europe and can be recognized in exploration wells and at outcrop from Greenland to the Wessex Basin. The maximum flooding surfaces have been biostratigraphically calibrated to provide a consistent and easily identifiable stratigraphic framework. Candidate sequence boundaries have been interpreted within this stratigraphic framework, from basin-ward shifts of facies belts, using sedimentological and wireline log data. The combination of these stratigraphic methods has produced a very powerful tool to predict the presence and distribution of potential reservoirs and play types across the entire North Sea Basin from outcrop in East Greenland to the offshore Netherlands. The model suggests that three major cycles of sand input into the basin can be recognized with an overall marked decrease in net sand content with time. Each cycle is bounded by tectonically enhanced maximum flooding surfaces representing major periods of basin floor reorganization. The intervening maximum flooding surfaces temporarily switch off sediment supply to the basin but do not offset depocentres. These events can form important, field-wide permeability barriers. It is proposed that the tectonically enhanced maximum flooding surfaces are a response to tectonic subsidence during maximum relative sea-level rise, whereas maximum clastic progradation occurs from basin margin uplift during relative sea-level fall. The model is considered to have application at regional and field-specific scales; for example, prediction of both basin floor fan distribution and potential intra-reservoir permeability barriers.

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