Mixed deep- and shallow-water depositional model for the Forties Sandstone Member in the South Central Graben, North Sea
pp. 191-233

The Upper Paleocene Forties Sandstone Member is a prolific hydrocarbon reservoir in the Southern North Sea. It has for decades been regarded as a 'classic' submarine fan depositional system, containing mainly high-density turbidites formed during a period of coeval tectonic uplift of the East Shetland Platform and lowstand deposition in basinal areas to the south and east. This view is partly challenged by the sedimentological, palynostratigraphical, and 3D seismic data presented in this study. A revised model is proposed in which the Forties Sandstone Member in the South Central Graben is interpreted as a deep-marine depositional system, which due to the presence of intra-basinal topographic highs locally was transformed into a more shallow-marine setting. Within the study area, several fault trends and a number of salt diapirs can be shown to have been active during deposition, and marginal- to shallow-marine conditions may have developed in association with these tectonic elements. The sub-basins between the highs became sites of rapid sedimentation of slope to basin plain turbiditic successions. Non-marine to nearshore palynofacies and the presence of root traces and wave-formed structures are the prime indicators of nearshore conditions, with the presence of presumed shallow-water trace fossils and seismic clinoforms possibly indicative of shelf-to-slope transition being secondary arguments.

The shallow-marine lithosomes mainly formed in shoreline and shoreface environments that formed as rims around diapirs and fault block crests. They are sand-dominated and relatively thin, and have a more sheetlike reservoir geometry than their deeper-water counterparts. Hence, thickness anomalies are no longer a pre-requisite for postulating sand in plays involving the Forties Sandstone Member. Moreover, the presence of local coastlines several tens of kilometers east of where it was located in previous models increases the probability of Upper Paleocene reservoir development in the Norwegian sector of the Southern North Sea.