The the Rotliegendes. This caused two basins to

The
structural framework of the North Sea is the result of a rift in the Upper
Jurassic – Lower Cretaceous. In Carboniferous to Permian periods, rifting
occurred due to volcanism followed by a deposition of reddish eolian and
fluvial sandstones, also known as the Rotliegendes. This caused two basins to
develop with sedimentation of thick Zechstein evaporate sequences. After
deposition of younger sediments, the buoyancy force displaced the salt to the
top- essential in closing structures and trapping hydrocarbon as in the south
of North Sea. This influenced the topography and further sedimentation. In the
Triassic period, the sea floor spreading took place, trending from the N-S to
NE-SW. Subsequently, deposition of thick coarse fluvial sediments at the rift
margins which grade into finer-grained sediments, and lake sediments in the
centre of the basins took place (Balson, Butcher, Holmes, Johnson, Lewis, and
Musson, 2001).

After
a transgression that marked the transition from Triassic to Jurassic, a
volcanic dome grew beneath the triple point between the Viking Graben, Central
Graben, and Moray Firth Basin which caused uplift and erosion. This was
followed by another phase of rifting. This produced several fields. In the
northern North Sea and Horda Platform (Brent Group), large deltaic systems
consisting of coal, sand, and shale were deposited (Balson et al., 2001). The
most notable rifting phase occurred in the Late Jurassic to Early Cretaceous.
During this rift-phase, major fault blocks were uplifted and tilted which
created sediment supply by erosion. Anoxic basins in the North Sea, with
accumulated shales created a crucial source rock, and also the Draupne
Formation- an important seal for hydrocarbon traps.

Once
the rifting has ceased during the Upper Cretaceous, thermal subsidence occurred
and gave way to two distinct lithologies- chalk deposition in the south, and
siliciclastic, clay-dominated sediments in the north.

A
major phase of basin inversion took place at the end of the Upper Cretaceous which
affected many basins in the NW of Europe and may have reactivated some basement
faults during the Oligocene to Miocene times (Glennie and Boegner, 1981). In
the Cenozoic period, subsidence in the North Sea formed a syncline towards a
depositional axis- extending from the Viking Graben, through the Central
Graben, to the Netherlands (Balson et al., 2001).The
Dire Straits acreage is located in the northern North Sea. Research of the
petroleum system of the northern North Sea at present have been detailed. It
consists of an Upper Jurassic source rock, migration pathways, and reservoirs
ranging from Devonian to Eocene strata, traps, and mudstone seals (Balson et
al., 2001).The
source rock is known as the Kimmeridge Clay Formation of the Humber Group. It
is from the Upper Jurassic syn-rift, consisting of organic-rich marine mudstones.
The post-rift thermal subsidence and burial of the Cretaceous and Cenozoic
times have created an optimal environment for the source rocks to mature for
hydrocarbon generation, beginning from the Palaeogene times (Johnson and
Fisher, 1998).The
hydrocarbon migration has travelled vertically, and is restricted laterally to
the Upper Jurassic and Palaeogene successions. According to Balson et al.
(2001), hydrocarbon extractions can be made in successions of clastics or
carbonates ranging from the Devonian and Eocene strata.

Brooks
et al. (in press) suggest that the reservoirs with producing fields are of Upper
Jurassic to Lower Cretaceous syn-rift reservoirs with various trapping
mechanisms such as tilted fault block, domes and stratigraphic closures. Additionally,
the post rift Lower Cretaceous mudstones also provide a regional seal for many
traps. The reservoir of both periods comprises of both shallow and deep marine
sandstones.