1.1.1 – 210 million years ago) and Late

1.1.1  Geological History

The North
Sea sedimentary basin is overlaid by lower palaeozoic crystalline and
metamorphic basement rocks assembled during the Caledonian Orogeny (about 420 –
390 million years ago) to form the Caledonian basement through the closure of
the Iapetus Ocean and the Tornquist Sea, at the Iapetus Suture and
‘Trans-European Fault Zone’, respectively (Andrews et al. 1990; Johnson et
al. 1993; Gatliff et al. 1994; Glennie and Underhill 1998). Many of
the major faults within the Caledonian basement formed lines of weakness that
experienced significant reactivation during subsequent phases of earth
movements.

During
the Devonian (about 410 -360 million years ago), there was widespread red-bed molasse
and lacustrine sedimentation as the newly formed Caledonian mountain ranges were
eroded. Mid-Devonian (about 375 million years ago) marine limestone in the south
of the Central North Sea were formed during an early rift phase. This was a
precursor to the main phases of Permo-Triassic (about 290 – 210 million years
ago) and Late Jurassic rifting (about 160 – 140 million years ago) and
associated strike-slip movements.

During
the early Carboniferous (about 360 – 325 million years ago), fluviodeltaic and shallow-marine
sediments accumulated in parts of the Central North Sea at times of regional
crustal extension. These Carboniferous rocks were gently folded, faulted,
uplifted and eroded during the Late Carboniferous Variscan Orogeny
approximately 300-290 million years ago.

During
the Late Permian (about 270 – 250 million years ago) red beds (Rotliegend
Group) accumulated within the widespread Northern Permian Basin. Following
marine transgression, cyclical evaporitic successions (Zechstein Group) were
deposited and locally reach over 1000m in thickness. The evaporites have been
deformed by halokinesis intermittently since mid-Triassic times (about 230
million years ago),

In the
Mesozoic, firstly Triassic, there was a return to arid, continental climate
conditions and both sandstone and mudstone-dominated redbed successions were
laid down. During the Early Jurassic there was a spread of marine deposits over
much of the North Sea during a phase of thermal subsidence following
Permo-Triassic rifting.

During
the Middle Jurassic, regressive, paralic sediments accumulated when a major subaerial
thermal dome formed within the Central North Sea (Underhill and Partington 1993).

The Late
Jurassic was a time of major extensional faulting. The rifting was initially
most intense at the extremities of the present graben system and as time elapsed,
it propagated back towards the centre of the dome (Rattey and Hayward 1993;
Fraser 1993). The onset of major rifting probably occurring in the middle
Oxfordian to early Kimmeridgian (approximately 157 – 155 million years ago)
(Underhill 1991; Glennie and Underhill 1998).

Regional studies of this area of the Northern North Sea
have shown that there are two primary plays that can work in this province. The
notional prospectivity in Middle and Upper Jurassic reservoirs along the margin
of the East Shetland bounding fault and in zones where a possible strike-slip
element has been recognised offsetting the platform. Some of the channel
sandstones associated with these fault junction entry points have been found to
be oil bearing and productive in the Hutton field which is located in the
Northern north sea sourced majorly by the prolific Kimmeridge Clay Formation.