Thestructural framework of the North Sea is the result of a rift in the UpperJurassic – Lower Cretaceous. In Carboniferous to Permian periods, riftingoccurred due to volcanism followed by a deposition of reddish eolian andfluvial sandstones, also known as the Rotliegendes. This caused two basins todevelop with sedimentation of thick Zechstein evaporate sequences. Afterdeposition of younger sediments, the buoyancy force displaced the salt to thetop- essential in closing structures and trapping hydrocarbon as in the southof North Sea. This influenced the topography and further sedimentation.
In theTriassic period, the sea floor spreading took place, trending from the N-S toNE-SW. Subsequently, deposition of thick coarse fluvial sediments at the riftmargins which grade into finer-grained sediments, and lake sediments in thecentre of the basins took place (Balson, Butcher, Holmes, Johnson, Lewis, andMusson, 2001). Aftera transgression that marked the transition from Triassic to Jurassic, avolcanic dome grew beneath the triple point between the Viking Graben, CentralGraben, and Moray Firth Basin which caused uplift and erosion. This wasfollowed by another phase of rifting. This produced several fields. In thenorthern North Sea and Horda Platform (Brent Group), large deltaic systemsconsisting of coal, sand, and shale were deposited (Balson et al., 2001). Themost notable rifting phase occurred in the Late Jurassic to Early Cretaceous.
During this rift-phase, major fault blocks were uplifted and tilted whichcreated sediment supply by erosion. Anoxic basins in the North Sea, withaccumulated shales created a crucial source rock, and also the DraupneFormation- an important seal for hydrocarbon traps. Oncethe rifting has ceased during the Upper Cretaceous, thermal subsidence occurredand gave way to two distinct lithologies- chalk deposition in the south, andsiliciclastic, clay-dominated sediments in the north. Amajor phase of basin inversion took place at the end of the Upper Cretaceous whichaffected many basins in the NW of Europe and may have reactivated some basementfaults during the Oligocene to Miocene times (Glennie and Boegner, 1981). Inthe Cenozoic period, subsidence in the North Sea formed a syncline towards adepositional axis- extending from the Viking Graben, through the CentralGraben, to the Netherlands (Balson et al., 2001).TheDire Straits acreage is located in the northern North Sea. Research of thepetroleum system of the northern North Sea at present have been detailed.
Itconsists of an Upper Jurassic source rock, migration pathways, and reservoirsranging from Devonian to Eocene strata, traps, and mudstone seals (Balson etal., 2001).Thesource rock is known as the Kimmeridge Clay Formation of the Humber Group. Itis from the Upper Jurassic syn-rift, consisting of organic-rich marine mudstones.The post-rift thermal subsidence and burial of the Cretaceous and Cenozoictimes have created an optimal environment for the source rocks to mature forhydrocarbon generation, beginning from the Palaeogene times (Johnson andFisher, 1998).Thehydrocarbon migration has travelled vertically, and is restricted laterally tothe Upper Jurassic and Palaeogene successions. According to Balson et al.
(2001), hydrocarbon extractions can be made in successions of clastics orcarbonates ranging from the Devonian and Eocene strata. Brookset al. (in press) suggest that the reservoirs with producing fields are of UpperJurassic to Lower Cretaceous syn-rift reservoirs with various trappingmechanisms such as tilted fault block, domes and stratigraphic closures. Additionally,the post rift Lower Cretaceous mudstones also provide a regional seal for manytraps. The reservoir of both periods comprises of both shallow and deep marinesandstones.