The Iapetus Ocean during the Caledonian Orogeny (Ziegler, 1975).

The geologic structures related to CaledonianOrogeny formed during the closure of the Iapetus Ocean when Baltica, Laurentia,and Avalonia plates collided from Early Ordovician to Early Devonian (Figure2-A).

During the Late Silurian, the North Searegion was dominated by a compressional regime due to the closure of IapetusOcean during the Caledonian Orogeny (Ziegler, 1975). Lower Paleozoicsediments metamorphosed during the Caledonian Orogeny to form the basement of the Southern North Sea(Cameron et al., 1993). Middle Devonian to Early Carboniferous siliciclasticsand carbonates unconformably overlay the Caledonian basement (Wong et al.,2007).

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              Theseparation of peri-Gondwanan plates resulted in the development of the RheicOcean during the Ordovician along the southern flank of the Iapetus Ocean(Nance et al., 2012). The Variscan orogeny structures formed during the closureof the Rheic Ocean when the Laurussian and Gondwanan plates collided to formthe supercontinent Pangaea, which took place from Late Devonian to Late Permian (Figure 2-B).

In the LateCarboniferous Period, the southern North Sea became a foreland area of thesouth-west extended Variscan mountains. With the collapse of the Variscan Mountains, its northernforeland was subjected to post-collisionalextension. The extensional tectonics resulted in subsidence of east-west oriented northern and southern Permian Basinsin the Variscan foreland basin. The creation of these basins are coevalpost-Variscan events (Glennie, 1986). The northern and southern Permian Basins are separated by the Mid-North Sea High and Ringk?bing-Fyn High (Ziegler, 1977). The Netherlands are included in the southernPermian Basin (Geluk, 2017). Deltaic sedimentation prevailed during the latestages of the Variscan orogeny leading to the deposition of Upper Carboniferouscoal measures, which formed the source rock for gas occurrences mainly in the southernNorth Sea (Ziegler, 1977).

By the end of the Permian, the North Sea area was subjected to arid conditions, whichresulted in the deposition of 1,000 m of Zechstein salt in the northern andsouthern Permian Basins (Ziegler, 1977).            Duringthe Triassic, the region was covered by awidespread and shallow sea. The Mesozoic North Sea rift formed an integral partof the Arctic-North Atlantic rift system (Ziegler, 1977). The rifting startedin the Late Triassic in the northern North Sea and spread southwards. ThePermian framework was overprinted by a north-south trending rift system. In the MiddleJurassic, continued rifting lead to the development of the Viking Graben to thenorth and the Central Graben to the south.             During Oligocene-Miocene times, the Alpine orogeny and its compressional stresses causedreactivation of several basement faults and inversion of pre-Tertiary basins inthe southern North Sea Basin (Figure 2-D). During the inversion,basement faults’ reactivation in the southern North Sea triggered further halokinesis (Glennie et al.

, 1981). Thehalokinesis generated intrusive salt structures such as stocks, diapirs, domes,and sills (Glennie, 2009). Rift system in the North Sea became inactive withthe onset of sea-floor spreading in the Norwegian-Greenland Sea. The North SeaBasin started to subside, and high sedimentation in theTertiary caused accumulation of a 3,500-m thick succession in the Central NorthSea (Ziegler, 1977). Tertiary and Quaternary deposits formed the Lower, Middleand Upper North Sea Groups (Figure 4), which together form the North SeaSupergroup (Wong et al., 2007).             A combination of induced mantleconvection and intraplate stresses resulted in domal uplift of theFennoscandian Shield in the Oligocene time (Rohrman et al., 1995, 1996).

Totaluplift reached around 4,000–4,500 m in southern sectors of Norway (Riis, 1992,1996; Sales, 1992; Lidmar-Bergström et al., 2000). During the Neogene,simultaneous uplift of the Fennoscandian Shield and subsidence in the SouthernNorth Sea Basin resulted in the development of the Eridanos fluvial-deltaic system (Overeem et al.

, 2001; Figure3). Sediments from Fennoscandian Shield in the north and the Variscan Massif tothe south filled the Southern North Sea Basin and built an immense delta (Bijlsma,1981). Differential loading throughout the region caused further movement ofthe Permian Zechstein salt.

This remobilization strongly influenced allelements of the petroleum play in the North Sea region (Harding and Huuse,2014).