The Wessex Basin is a rift system located in southern England that covers approximately 40,000 square km. It overlies basement rock from the Variscan orogeny in the late Paleozoic. The basin formed in the late Paleozoic and contains sedimentary sequences from the Paleozoic to Cenozoic. Fault-controlled subsidence created four main sub-basins. Source rocks are found in the Lower Lias clays, Oxford Clay, and Kimmeridge Clay from the Jurassic period. The main reservoirs are the Triassic Sherwood Sandstone and Jurassic Bridport Sands. Impermeable mudstone and clay layers like the Mercia Mudstone and Kim

1. Petroleum System of the Wessex Basin, Southern England
Raynaldi Inaray
Louisiana State University
Geologic Background
Located in Southern England, within the counties of Dorset and Hampshire and
extends to the English channel, the Wessex basin covers approximately 40.000 square
km of Southern England. Roughly a north-south rift system, extensional reactivation of
the Variscan compressional structure is believed to be one of the major components of
the early evolution of the basin. The relative motion of the African plate relative to the
European continental plate however, was recognized to be a major driving development
of the basin during the late Cretaceous. The red cross section line (above) is roughly the
location of the main sub-basins located from north to south.
The Wessex is a part of a wider intracontinental basin that overlies a sector of the
Variscan foldbelt system which was metamorphosed and consolidated in the late
carboniferous period which makes an effective basement for the basin. The basin itself
was initiated in the late Paleozoic and contains various sedimentary sequences of
Paleozoic-Cenozoic sedimentary sequence.
It was first perceived that the Wessex basin was thought to be a single depositional
basin with the Weald basin, a neighboring basin located close to the Wessex, because
the boundary between these two were unclear during the first petroleum exploration.
However, differences can be seen from the sedimentary stratigraphy of the permian until
the early Triassic between both basins.
Geologic Cross Section
Fault controlled subsidence that leads to mainly normal faults developed the Wessex-Weald basin as an
asymmetric half grabens and horst structures. Initiated around the Permian-triassic period, the basin underlies a
regional upwarps that were formed by bulk shortening of the graben-fill. Reversal movement of the normal faults
during the tertiary compression creates folds that are in the form of monoclinal and periclinal flexures.
This eventually leads to the formation of four main half graben sub-basins of the Wessex-Weald. The
southernmost Portland-wight sub-basin, the Dorset sub-basin that is concealed by the Cranbourne fault, the
Mere (wardour) sub-basin, and the northern most Pewsey sub-basins.
The basement of the Wessex Overlies a sector of the Variscan foldbelt, formed post the Variscan thrusts,
however some suggest extensional reactivation of Variscan thrust influence the Wessex basin overall structure.
Structural Evolution Reservoir-Seals-Traps
1.) During the Permian, semi arid/desert
sedimentation currently known as the
Exmouth and Dawlish group consisted of
mostly breccias and sandstones marked
the beginning of a volcanic series that the
basin was entering. Most of the Permo-
Triassic strata that is present in the in basin
are desert facies.
2.) The upper Triassic marine
transgression marked the first onset of
marine sedimentation within the Wessex.
Shallow marine sedimentation then
appeared at the Jurassic period which
starts at the lower Liassic strata up to the
Kimmeride clay formation.
3.) Concurrent sedimentation along with an
ongoing tectonic activity, these strata were
effected by syn-sedimentary extensional
faulting trending east-west. This leads to a
tectonic inversion that began on the early
tertiary where folding due north of the
structure is very distinctive.
4.) The upper cretaceous chalk group
deposition is followed by further faulting
and tilting trending north
5.) Early tertiary marks the tectonic
inversion that subsequently ended the
development of the basin. In the late
Oligocene, the Alpine collision lead to
uplifting of the region and folding due
north. The picture on the left shows an
area along the coast of southern England
(Durdle door, west of Lulworth cove) that
shows the distinctive folding caused by the
Alpine collision
Source rocks References
Source rocks of high potential hydrocarbons in the
Wessex basin are found in various sub-basins because
of migration pathways formed. These source rocks are
found mainly in the Jurassic period and are commonly
grouped into 3:
1.) The lower Liassic clays which contains 7.36% of
TOC. Deposited in an oxygen deficient waters, it lacks
benthic faunal activity and are found in various level of
maturity on which sub-basin it is found.
2.) The upper Jurassic Oxford clay that contains up to
12.21% of TOC. Containing bituminous shales,
mudstones, and siltstones. The changing lithology as it
continues upwards were caused by the re-oxygenation
of the water during deposition; kerogen type II, III are
present although the oxford mostly falls in the oil
generation window (immature)
3.) The Kimmeridge clay with a TOC up to 20% are rich
in organic matter. It is mostly derived from marine
planktons that contains an oil prone sapropelic kerogen.
It is mostly compried of calcareous mudstones,
limestones, sandstones, and siltstones. The kimmeridge
is largely immature.
Reservoir:
There are currently 2 existing reservoirs in the Wessex basin: The
Triassic Sherwood sandstone group and the Jurassic Bridport
sands group shown in the blue arrow on the left picture.
1.) The Triassic Sherwood sandstone group has a thickness of
approximately 100-300 m of a thick red bed succession that was
deposited in semi arid conditions. It indicates a variety of past
geologic environments but suggests mainly an alluvial or a
lacustrine environment deposition. This group produces an
economically recoverable petroleum production only at Wytch
farm oilfield with porosity up to 29% and up to several darcies
2.) The Jurassic Bridport sands are mostly comprised of a finer
grained sediment. Porosity ranges up to 32% and permeability
ranges up to 300 mD. Unlike the Sherwood sandstone, the
Bridport sands were deposited under shallow marine conditions.
With a thickness approximately 35-100+ m, the Bridport sands act
as the main reservoir in smaller discoveries in the county of
Dorset.
Seals:
An effective mudstone layer and clay formation acts as a seal to
keep the hydrocarbons in both of the reservoirs intact.
1.) The Mercia mudstone group acts as a seal of the the
Sherwood sandstone reservoir. This group comprises of various
lithology mixing. Mudstones, sandstones, siltstones and halites
are some of the few identified.
2.) The Kimmeridge clay acts as a seal of the Bridport sands
reservoir. Deposited as a fossiliferous marine clay, the
Kimmeridge is a very economically important rock unit as it is also
a major source rock for various oilfields around the North sea
hydrocarbon province.
Traps:
The traps in the Wessex basin are differentiated in 2 different
types:
1.) Upfaulted tilt blocks and horst initiated during the Triassic
times and were formed during the active extensional phases of
basin subsidence. The strata are tectonically undisturbed hence
the seals are unbreachable
2.) Monoclines and periclinal structures formed during the tertiary
inversion structures. These beds are cut through by a reverse
fault which can be unaffected, but mostly it alters hydrocarbon
prospects. Hydrocarbons may migrate through the fracture
system and/or water may invade from the surface.
Stoneley, R. Review of the Habitat of Petroleum in the Wessex Basin:
Implications for Exploration: Proceedings of the Ussher Society 8(1)
1992 P.1-6. London: Ussher Society, 1992. 1-6. Print.
Harvey, Toni, and Joy Gray. Hydrocarbon Prospectivity of Britain's
Onshore Basins. Department of Energy & Climate Change, 2014. 1-
77. Print.
Lake, S. D. The Structure and Evolution of the Wessex Basin.
Durham: U of Durham, 1985. Print.
West, Ian. "Oil South England - Introduction." Oil South England -
Introduction. 28 Oct. 2014. Web. 8 Nov. 2014.
<http://www.southampton.ac.uk/~imw/Oil-South-of-England.htm>.
"Geology of Dorset." Wikipedia. Wikimedia Foundation, 2 Nov. 2014.
Web. 8 Nov. 2014. <http://en.wikipedia.org/wiki/Geology_of_Dorset>.