Controls on hydrocarbon column-heights in the north-eastern North Sea
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Knowledge about the controls on hydrocarbon column-heights is important in hydrocarbon exploration, as such column heights are the main controlling factor for in-place volumes of prospects. The north-eastern Viking Graben comprises an overpressured and a close to- normally pressured area, and commercial and sub-commercial discoveries as well as dry structures have been drilled in both areas. Both oil and gas is present in reservoirs in the study area and the aim of the present study was to investigate the controls on the hydrocarbon column-heights and the distribution of oil and gas. A regional seismic interpretation of the main reservoir unit in the area, the Brent Group, has been carried out based on 3D seismic and published exploration well data. Mapping of spill points and fluid contacts within the different reservoirs in the area resulted in identification of both filled and underfilled traps. The seismic amplitude variations of cap rocks above underfilled and dry traps were investigated in search for evidences of underlying and column-restricting leaky faults or fault intersections. An overpressured area without lateral pressure communication between hydrocarbon-bearing and dry structures has been identified in the deep basinal parts, while a close to normally pressured area with possible communication between the reservoirs is present on the eastern flank of the Viking Graben. Previously open fill-spill routes are suggested to be closed at present day in the overpressured area due to extensive quartz cementation during burial. This has resulted in a limited supply of gas from the deep grabens to the shallow terraces. It is inferred that the oil that is preserved in the shallow structures is present because of closure of the migration routes for gas from the graben areas, combined with oil charge from shallower areas. A total of six structures in the study area have been found to leak, of which four are underfilled. One of the reportedly dry structures within the study area is suggested to contain hydrocarbons up-dip from the well location. Leakage through faults and especially fault intersections are suggested to control the hydrocarbon column heights in the leaky structures, and such leakage can be the cause of emptied reservoirs. The suggested leaky faults have different orientations from the faults that delineate the filled structures. However, most of the leaky faults are not critically oriented for failure in the present day stress regime. Seismic bright amplitude anomalies in the Kyrre Formation are present above four of the four column-restricting leaky faults or fault intersections in the deep overpressured area. It is suggested that analysis of amplitude variations in the Kyrre Formation should be applied to future volume and prospect risking of structural traps in the study area.