Poster final Raynaldi Inaray Wessex
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 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 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>.
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Transcript of Poster final Raynaldi Inaray Wessex
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-Traps1.) 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 ReferencesSource 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>.
