Location of the Southern North Sea provided by GeoMapApp
Aerial image of the North Sea provided by the United States Geological Survey
The North Sea basin is located in northern Europe and lies between the United Kingdom, and Norway just north of The Netherlands
and can be divided into many sub-basins. The Southern North Sea basin
is the largest gas producing basin in the UK continental shelf, with
production coming from the lower Permian sandstones which are sealed by the upper Zechstein salt.
The evolution of the North Sea basin occurred through multiple stages
throughout the geologic timeline. First the creation of the Sub-Cambrian peneplain, followed by the Caledonian Orogeny in the late Silurian and early Devonian. Rift phases occurred in the late Paleozoic and early Mesozoic which allowed the opening of the northeastern Atlantic. Differential uplift occurred in the late Paleogene and Neogene.
The geology of the Southern North Sea basin has a complex history of
basinal subsidence that had occurred in the Paleozoic, Mesozoic, and Cenozoic.
Uplift events occurred which were then followed by crustal extension
which allowed rocks to become folded and faulted late in the Paleozoic. Tectonic movements allowed for halokinesis to occur with more uplift in the Mesozoic followed by a major phase of inversion occurred in the Cenozoic affecting many basins in northwestern Europe.
The overall saucer-shaped geometry of the southern North Sea Basin
indicates that the major faults have not been actively controlling
sediment distribution.
Geological history
Paleozoic era
Two major orogenic events occurred in this era, the Caledonian Orogeny and the Variscan Orogeny, allowing a complex geologic history to begin. During the late Silurian and early Devonian the Caledonian Orogeny occurred with episodes of uplift and erosion leaving unconformities. The Caledonian event occurred due to the collision of three land masses – Laurentia, Baltica, and Avalonia – which would eventually lead to the creation of Pangea.
This collision allowed for a mountain belt to form NW–SE in the
northern portion of the current basin, and in the south extending SW–NE. Following the Caledonian Orogeny approximately 380 Ma the Variscan Orogeny started and ended near the Permian. During this time period the orogeny caused Carboniferous rocks to become folded and faulted. The last collision occurred in the late Carboniferous where two super continents collided leading to the Varsican mountain range, Laurasia and Gondwanaland. Late Permian deposition of evaporites created the Zechstein supergroup which act as a salt cap for the fine grained sediment.
Mesozoic era
During
this era the end of extensional tectonics had been well constrained in
the southern North Sea basin; the extension occurred from the late Carboniferous to the Triassic.
There had been some reactivation of Varsican basement faults due to the
subsidence of the Sole Pit Basin and allowing basin tilts creating a
peripheral graben system around the basin. Due to the reactivation of the basement faults it led to the beginning of halokinesis in the basin. The halokinesis allowed major uplift during the Mesozoic
because of the presence of salt and the reactivation of basement
faults; the thrusting permitted the sediment to thrust over the diapers
and float on top of the Zechstein salt.
Due to the Kimmerian phase uplift in the northern portion of the North
Sea, it allowed subsidence and deposition to fill the basin, creating sandstone.
Due to differential loading along the faults, salt diapers developed
and played a huge role in the southern North Sea basin and all salt
tectonic structures.
Reverse faulting associated with late Carboniferous basin inversion is
recorded by a wide range of Carboniferous stratigraphy subcropping the
Permian sediments.The subcrop pattern indicates a strong influence of
NW–SE tectonic trends during this inversion. This inversion event was
followed by deposition of upper Carboniferous red beds, which pass up
into sands of the Permian Rotliegend Group; these are overlain by
evaporites of the Zechstein Supergroup.
A major phase of basin inversion during or at the end of the Late
Cretaceous affected many basins in northwestern Europe, including the
Sole Pit Basin and the Cleveland Basin, and has been attributed to
strike-slip reactivation of basement faults.
Cenozoic era
During the end of the Mesozoic and into the Cenozoic era the Alpine orogeny occurred which led to reactivation of faults and structures. In the beginning of the Tertiary,
inversion involving basin tilt and reactivation of basement faults
transpired. The center part of the southern North Sea basin comprises
the Silver Pit and Sole Pit trough and the Cleaver Bank High, which are
all distinguished by a series of salt swells and walls which occurred in
the Tertiary.
A reversal of basin tilt during the Tertiary uplifted the thick
sedimentary wedge in the Sole Pit Trough to form the Sole Pit High. Since the orogeny reactivated the Mesozoic rifts it permitted the Zechstein salts to act as a buffer or detachment layer separating two structural regimes, which can lead to traps for natural resources.
Tectonic phases
Caledonian phase
During the Paleozoic there were three major landmasses that collided, Laurentia, Baltica, and Avalonia closing the Iapetus ocean. The event created a mountain chain trending North to South in the northern portion and an East to West trend in the South. The reason being that there is a North to South trend in the North is because Laurentia coming from the West and Baltica coming from the east meeting at the center to create a compressional regime. Through time eventually Avalonia
coming from the south closing the Iapetus ocean, collided with the two
landmasses to create a T-junction giving an East to West trend in the
southern portion. This event is the first major event that would lead to the creation of Pangea.
The tectonic event comprised the entire Ordovician and into the early
Devonian, the Caledonian rocks are the basement of the current North
Sea.
Variscan phase
From the late Devonian to the end of the Permian ending in the Paleozoic era the Variscan Orogeny occurred. The super continents of Gondwanaland and Laurussia collided creating an extensive mountain range just east of the pre-existing Caledonian mountains and creating Pangea the super continent at the end of the Variscan phase. The collision of these plates plays an important role in the potential of hydrocarbons in the Southern North Sea basin.
The start of this phase is the collapsing of the Caledonian orogeny and
a general extensional regime which would cause a depression to fill
with sediment.
There are four major phases in this orogenic event. First phase known
as the Bretonian reflected in changes in the sediment input and the
reactivation of a south plunging subduction zone.
The second phase, the Sudetian, was of volcanic event and extrusive
metamorphic and igneous rocks with uplift and mild folding of grabens in
the vicinity which lead to inversion.
The Asturian tectonic phase created fragmentation of the Variscans and
its foreland due to the complex fault system of conjugate shear faults
and secondary extensional faults. The last major phase, the Staphanian, caused the majority of faulting and deformation expressed in wrench faults.
The accumulation of hydrocarbons in the south was permitted due to the
basin that was formed, the foreland basin was barely disturbed by
tectonic events in the northern region and eventually sealed up by the
salt caps of the Zechstein formation. Since the Caledonian and Variscan orogeny are closely related in time both events helped create Pangea and the Caledonians slowly phase into the Variscan orogeny.
Kimmerian phase
Pangea animation
The break up of Pangea occurs during the Kimmerian tectonic phase for most of the Mesozoic, until the early-mid Cretaceous, this marks the start of creating the present position of our continents today.
During the Jurassic, rifting activity reaches its maximum and North
America starts to move apart from Eurasia following that event in the Cretaceous the southern part of North America starts to open up the Atlantic Ocean with the separation of South America and Africa. At the end of the Mesozoic the North Sea reached its final position where it lies in present day. Throughout the Cretaceous
rifting eventually slowed down and came to a halt which later created
the North Sea failed rift system because the regional stresses had
shifted on to North America. The Jurassic is probably the most important geological time for hydrocarbon exploration in the North Sea.
Many accumulations are in Jurassic reservoir, the Kimmeridge clay is
considered the most important source rock and structures formed during
rifting form excellent traps.
In the first place rifting was responsible for the deposition of
organic rich source rock due to anoxic conditions in the deep isolated
rift basins. Possibly the most important phase to create structures and traps for the natural resources we try to collect today.
Alpine phase
This phase is currently active today and started in the Cretaceous.In the late Cretaceous and in the Tertiary inversion phases in the Southern North Sea region occurred due to the Alpine orogeny and its compressional stresses. Since there had been inversion the Zechstein salt played a huge role by acting as a buffer between two structural regimes. Although the phase reactivated pre-existing faults it allowed the salt tectonics to remain active during the Tertiary as the sediments were deposited, and later became penetrated by the salt diapers.
The Alpine phase did add more structural confusion to the geologic
history, but it also help create more traps with the Zechstein salt.
Sedimentary formations
Main Formations
Rotliegend group
The sandstones of the Southern North Sea region form gas reservoirs. Deposition started in the early Permian, and near the end of the early Permian finer sediment was deposited in an environment of lacustrine and saline/sabkha.
Zechstein group
The
Zechstein group consists of evaporites which sealed the Rotliegend
group for reservoir formation. Sedimentation was dominated by the
development of mixed carbonate-evaporite depositional system throughout
the southern Permian basin.
Climatic conditions allowed the deposition of five major sedimentary
cycles of progressive progradation and desiccation of the basin after an
initial recharge through basin flooding.
Cromer Knoll group
The Cromer Knoll is deposited on top of an unconformity at the base of the Cretaceous period.
Regional uplift and erosion allowed the unconformity to appear in the
late Triassic and depositing the Cromer Knoll and chalk groups.
Salt tectonics
Zechstein salt cap
Salt tectonics
is the movement of a significant amount of evaporites encompassing salt
rock within a stratigraphic sequence of rocks. Within the southern
North Sea basin this plays a huge role in the oil and gas industry
because the tectonic events throughout the geologic timescale allowed
these halokinesis structures to trap areas of natural resources. The
major salt basins were clearly deposited by gravity driven measurements with three basinal areas: the German, English and Norwegian basins.
The southern North Sea basin concerns the English and German Zechstein
salt basins. The German basin can be categorized as a salt wall which is
a linear diapiric structure possibly related either to basement
faulting or to the controlling effect of regional dip, and the English
basin is categorized as a salt pillow type of structure, developed in
association with thinning of overlying beds but without diapiric
effects. The major types of salt structures in this basin are salt pillows or swells which lie in the cores of buckle fold structures.
Petroleum geology
Location of oilfields (green dots) and gasfields (red dots) in the Southern North Sea
In general the reservoir potential is restricted to aeolian sandstone, although poorer quality potential reservoirs are found in fluvial sediment. About 85% of the gas production in the southern North Sea basin comes from the pre-Zechstein Permian sandstones and 13% from the Triassic fluvial sandstones.
The sandstone deposited prior to the Zechstein evaporites are
essentially the area in which the oil industry is pulling the natural
resources from due to high quality seal from the salt diapers and
pillows which acted as a buffer between structural segments. Triassic
sequence fluvial sandstones are of lesser quality of a reservoir because
it was not sealed in a trap such as the Rotliegend.
