West, Ian M. 2016. Petroleum geology of the south of England - Introduction to Portland - Isle of Wight, Offshore, Basin. http://www.southampton.ac.uk/~imw/Petroleum-South-Portland-Wight-Basin.htm. Version: 15th November 2016
Petroleum Geology of the South of England - Portland-Wight-Basin

Ian West,
Romsey, Hampshire

and visiting Scientist at:
Faculty of Natural and Environmental Science,
Southampton University,
Webpage hosted by courtesy of iSolutions, Southampton University
Aerial photographs by courtesy of The Channel Coastal Observatory , National Oceanography Centre, Southampton.
Website archived at the British Library

..| Home and List of Webpages

|Kimmeridge Bay | Kimmeridge - Oil Shale |Kimmeridge Bibliography. |Lyme Regis |West Bay, Bridport| Osmington - Oil Sand | Durlston Head Geology. |Stair Hole, Lulworth |Poxwell Quarry.

Click here for the full LIST OF WEBPAGES

(You can download this educational site to SurfOffline or similar software to keep an offline copy, but note that updating of the live version takes place periodically.) See also associated webpage:

Petroleum Geology - South of England - Introduction.

Bibliography of Petroleum Geology of the South of England.

|Petroleum Geology of the Western Weald and South Downs (Shale Oil etc.)




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The Chalk cliffs at Harry Rocks and the Needles, Isle of Wight, mark the northern boundary of the English Channel Inversion Structure, photograph from Studland beach, 5th  June, 2014

The offshore, mature, Lias shale basin is beneath the sea shown in this view across to Atherfield, Isle of Wight, from Durlston Castle, Swanage, November 2013

Oil and gas prospects, and fracking possibilities, of Poole and Christchurch Bay and surrounds, Bournemouth, Dorset area, south of England - map


The offshore Beacon Oilfield, near  Hengistbury Head, Poole Bay, Bournemouth, Dorset and problems of producing the oil


A simple diagram showing the main stratal units in the Wessex Basin

The diagram above is intended to be introductory for the benefit of those not familiar with the local stratigraphy. It is very simplified and not accurately to scale. However, it shows the general positions of the important stratigraphical units. More detail is provided in other associated webpages with regard to details of parts of the stratigraphical column. Further information will also be given below, in due course.

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A generalised geological map of the central English Channel from the Isle of Wight to the Cherbourg Peninsula

A simplified overview of Wessex Basin structures, oil and gas fields and boreholes

A map showing the oilfields, gas fields and oil kitchens of the Wessex Basin and the western part to the Weald Basin, southern England, Dorset

Major tectonic features of the Wessex Basin and adjacent area, in relation to petroleum geology

An isopachyte map showing stratal thicknesses for the Sherwood Sandstone, the lower and main reservoir for the Wytch Farm Oil Field

Lulworth Cove, Dorset in relationship to the Portland-Wight Basin or English Channel Inversion

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The Wessex Basin, Inversion Structure.

"Inversion Structure" is a popular oil industry term. The local Wessex Inversion Structure is a classic. There follows a brief summary; details will be given below, or in other associated webpages.

1. A deep basin is developed in the Jurassic and Cretaceous. Where the basin subsides deep enough, actually in this case to the SW of Atherfield Point, Isle of Wight, the organic rich source rocks (Kimmeridge and Lias) go beyond 2 to 3 km down (i.e. rather to boiling point of water). This produces oil and gas, but it does not happen out of the basin. No major oil or gas has been produced from oil shales at Lyme Regis or Kimmeridge because both are too shallow (although Kimmeridge is getting nearer). Result - oil and gas in the basin centre but not at first migrated.

2. Oil and gas migrate out of the basin to the high margins in the Cretaceous. i.e. northward to the Wytch Farm oil fields area. This was happening in the Lower Cretaceous - Wealden to Aptian (Lower Greensand). There is the complication of much faulting - blocking some oil movement.

3. There is great erosion in the late Cretaceous (Upper Greensand, Albian) of the "high" rising area to the north so that the Gault and Upper Greensand are lying on an eroded surface of Late Jurassic and Early Cretaceous rocks (much faulted).

4. Much later due to pressure from the south (probably African Plate moving north, Atlas Phase ) the basin then "Inverts", i.e. it rises and becomes a structural high. This is the real Inversion process and it happened in the local area in Tertiary times, and as we now know, much more specifically, in Bartonian times (but not only Barton - that was just the maximum). The new highs are Chalk anticlines or monoclines shedding debris off to the new basin in the north (explains the mid-Barton pebble bed at Alum Bay).

Barton palaeogeography of the Isle of Wight, New Forest and adjacent area showing the first effects of Tertiary compression, Atlas Phase, from the south


5. So in the final Inversion Structure, the oil is not in the axial area where it originated but is in the relatively high area to the north (Wytch Farm) .

6. Theoretically it could stop now. However, continuing compression from the south (African Plate moving north) means that the basin does not just level out but part actually rises substantially above sea level. Tertiary Monoclines ("Purbeck, Isle of Wight Anticline) develop. Theoretically, oil could move back in to the central zone in Bartonian times, but it does not seem to have done so, probably, largely because of fault-blocking. Theoretically the central monoclinal areas could become major, late reservoirs, but in practice this does not seem to have happened.

7. So, the exploration areas are like Wytch Farm, north of the main source area. However, how far northe could they extend? Why has no substantial oil been found in the northern Isle of Wight? (There is an apparently "dead" area from northern Isle of Wight to Lymington to Lyndhurst region).

8. The Weald Basin is another Inversion Structure, with a similar history.



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PW 3. LICENSING BLOCKS - 26th Seaward Licensing Round (2011)

The second tranche of awards in the 26th Seawward Licensing round was announced by DECC (the UK Department of the Environment and Climate Change) on the 30th December 2011.

Licence Block map, Offshore Oil and Gas Licensing, Central English Channel, 26th Seaward Round, May 2011

Offshore geology of Portland, Weymouth, Lulworth and Kimmeridge, simplified geological map showing the major structures relevant to offshore petroleum geology of the area

A deep cross-section, east of Weymouth, showing the major Late Cimmerian listric fault, phasing out in Triassic Salt, redrawn after part of a BGS section

Offshore Oil and Gas Licensing - May 2011. For some information regarding the 26th Seaward Round of petroleum exploration licence blocks in the central English Channel see the following publication which is available as a pdf online. It is not a geological publication but contains environmental information:

Department of Energy and Climate Change. 2011. Offshore Oil and Gas Licensing, 26th Seaward Round, Central English Channel, Blocks 97/14, 97/15, 98/06b, 98/07b, 98/08, 98/11, 98/12, 98/13 and 98/14. Appropriate Assessment (AA) [environmental assessment], May 2011. 91 pp.

Extract, summarised from Background and Purpose:
On the 27th January 2010, the Secretary for State for the Department of Energy and Climate Change (DECC) invited applications for licences in the 26th Seaward Licensing Round. The Secretary of State undertook a screening exercise on whether the Blocks would be likely to have a significant effect on a relevant European conservation site. This is the "Appropriate Assessment" (AA), that is an environmental assessment regarding conservation, for the Central English Channel petroleum licensing blocks. It includes a version of the map above.
Notes on Production Licences, granted for Blocks:
The three types of Seaward Production Licences are explained on page 5. Traditional Production Licences are the standard type of Seaward Production Licences and run for three successive periods or Terms. If the production is successful and there is compliance with the regulations then a company can hold the licence for many years. However, the inital term lasts only four years and it will only continue into the Second Term if the agreed Work Programme has been completed and if 50% of the acreage has been relinquished. Two other type of licences, Frontier Production Licences and Promote Licences have different rules which are particularly designed to encourage exploration. See the document, page 5, for details.
Drilling Commitments (summarised notes from p. 6)
A Firm Drilling Commitment is a commmitment to the Secretary of State to drill a well.
A Contingent Drilling Commitment is a commitment to drill a well.. A Drill or Drop (D/D) commitment is a conditional commitment with the proviso that the licence is relinquised if a well is not drilled.
"Up to 3 Drill and Drop wells have been proposed as a combined total number for the central English Channel Blocks applied for. ..Exploration wells would be drilled from land, although subsequent activities could involve offshore drilling."

If the exploration wells are drilled from land, as mentioned in the extract above they would probably involve Extended Reach Drilling (ERD) like those at Goathorn Peninsula, Wytch Farm Oilfield (to about 10 and a half kilometres eastward into Poole Bay). Possibly, ERD boreholes could give access to large parts of the blocks south of Swanage to Weymouth (97/14, 97/15 and 97/11). Some blocks might be reached by similar drilling from the Isle of Wight or Hampshire mainland (98/13, 98/14, 98/08, 98/07b).

Reservoir rock targets are unknown, but a reasonable similarity to nearby Wytch Farm would be expected. There the upper reservoir is the Bridport Sands and the major and more important potential reservoir is the Sherwood Sandstone.

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Release of Offers in January 2012.

Offers given for Nearshore Licence Blocks, Dorset and Isle of Wight area

A map of some Wessex licence blocks, as in early 2013

Licence blocks of Northern Petroleum, offshore to the southwest of the Isle of Wight


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PW 4A. LICENSING BLOCKS - 26th Seaward Round (2011) - Companies

Wessex Exploration. PlC. [with Mirrabooka]

[Go to their website for more information

[shortened extract]

"Wessex Exploration PLC is pleased to announce the provisional award by the DECC of varied interests in five "PROMOTE" offshore Southern England, located within the English Channel. .. Awards anounced 30 Dec. 2011. Wessex and NWE Mirrabooka (UK) Pty Ltd will hold 35% and 65%. NWE Mirrabooka will be the operator."
Wessex and Mirrabooka will explore the blocks with seismic for two years (i.e. to Dec. 2013). Then the interests can be converted from "PROMOTE" status to "TRADITIONAL" status. This would mean "DRILL OR DROP" commitment (they would have to drill or loose the licence blocks).

The blocks are:
98/6b - (small nearshore block off the Southbourne coast of Bournemouth)
98/7b - (off Hengistbury Head and most of Christchurch Bay, all nearshore)
98/8 - (Western half of the West Solent, west of Lepe Beach)
98/12 (Part) - (Northern half of a large block south of Christchurch Bay)
98/13 (Split) - This block is Southwest of the Isle of Wight.)

These blocks are mostly east or southeast of the Wytch Farm Oilfield, offshore part. They are mostly south of Christchurch Bay and west of the Isle of Wight. No blocks south of the east Dorset coast, Purbeck etc., have been taken.

Wessex Exploration PLC have stated "The acreage is believed to contain at six different structural leads with a geological chance of success of these leads ranging from 15% to 32%."

{The implication of this to the general industry public may be that there is much less than 25% chance of an Extended Reach Drilling rig being constructed somewhere in the Christchurch Bay coastal area. Wessex Exploration (and Mirabooka) also have blocks on and adjacent to the Isle of Wight. Thus the Isle of Wight cannot be eliminated as a possible base area for exploration offshore into the geologically interesting, sea area south of the Christchurch Bay coast. Incidently, Northern Petroleum have in the past considered a particular site for a drilling rig in the region between Barton and Milford, re the Hurst Castle Prospect; the location, originally selected was at Downton, not far from Taddiford Gap region. Taddiford Gap is, in fact, now the place where the Navitus Bay windfarm electricity pipes are planned to come onshore. This is in a residual undeveloped space between Barton-on-Sea and Milford-on-Sea.]

[more on other companies and blocks will be added]

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Locations of thick source rock sequences

Simplified map of Jurassic and Cretaceous sedimentary depocentres in the south of England

The simplified map above shows the combined Jurassic and Cretaceous thicknesses, either proven or estimated. The oil shales are mainly in the Lias at the base of the Jurassic and in the Kimmeridge Clay in the upper part. The relative thicknesses of the oil shales and their thermal maturity is likely to be closely related to the total Jurassic thickness. The map above shows combined Jurassic and Cretaceous thickness. This is relevant to maturity, but thermal maturity of the Lias in the Portland - Wight Basin was probably developed by Lower Cretaceous. The Kimmeridge oil shale is not mature onshore in Dorset, but it might be in the middle of the depocentres. It is mature in the Weald depocentre.

The overburden is very limited in southern Isle of Purbeck but in the southwestern Isle of Wight there is a significant Wealden cover. With regard to the south Isle of Purbeck, See details of 26th Round.

A map showing depth to Pre-Permian Basement in the Wessex and English Channel Basins, modified after DECC, and in relation to shale gas

This map, based on, but modified after, a part of DECC map, shows the general offshore areas where oil shale occur. It is also present near surface at Portesham, Dorset.

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The English Channel Inversion - An Example Cross-Section

The English Channel Inversion Structure or the Portland-Wight Basin in a cross section, nearer to Durlston Head than to Atherfield, Isle of Wight


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Kimmeridge Oil Shale crops out on the sea floor from the Kimmeridge area (east of Kimmeridge) to the Isle of Portland (northern part near the Castle). It passes under the Chesil Beach and out into Lyme Bay. It is present beneath younger rocks under the sea floor over a more extensive area (towards the Isle of Wight).

A generalised map showing the approximate location of oil shale resources offshore from the Dorset coast

The following comment is of interest:
"in Quadrant 97 [i.e. Blocks 97/14, east of Portland and 97/15, Kimmeridge area] and Block 98/11 [Swanage] there may also be the potential for shale gas."

It is well-known that what is probably the best oil shale in Britain is at Kimmeridge and the nearby offshore area (see the classic work of Gallois with regard to Kimmeridge oil shale). The oil shale has been worked on land for gas production in the 19th century.

Simplified geological map of part of the Central English Channel

See also:
DECC; Department of Energy and Climate Change. 2011b. Promote UK 2011. The Unconventional Hydrocarbon Resources of Britain's Onshore Basins. 33pp. Available online as a pdf.

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[brief extract from DECC 2011]
"The Kimmeridge Clay Formation of onshore eastern and southern England (Fig. 21) is a potentially prospective shale (for oil and probably biogenic gas) because it contains ubiquitous oil-shale beds (Gallois 1979). Its background shale has TOC up to about 10%, its bituminous shale has TOC values up to 30% and its oil-shales have TOCs reaching 70% (e.g. the Blackstone Bed, Stocks and Lawrence 1990) [70% is actually the total organic matter, not the total organic carbon]. Five basins show thickening of the formation in response to syn-sedimentary faulting (Weald, Wessex, English Channel, Cleveland and Lincolnshire-Norfolk).

The Kimmeridge Clay Formation is immature for thermogenic gas generation onshore (Scotchman 1991) and it is only marginally mature for oil generation in the Weald Basin depocentre. Its carbonate content varies from 12% (oil-shales) to 94% (limestones, Farrimond et al. 1984). Scotchman (1991) showed that high TOCs and sedimentation rate result in high phytane/TOC values. The low maturity is confirmed by TMAX 403-437 C on pyrolysis. Samples confirmed kerogen of Type II or mixed Type II-III on the Van Krevelen diagram (Tissot et al 1974).

Exploration for oil-shale deposits was conducted in Dorset at Corton after World War 1 (Strahan 1920). Exploitation of these Kimmeridge Clay oil-shale beds was uneconomic because of the thinness of the beds and high sulphur content. As mentioned above, however, the Kimmeridge oil shale is not generally thermally mature, whereas in places (offshore) the Liassic bituminous shales probably are.

The English Channel Basin, particularly south of Purbeck and on the southern Isle of Wight, contains more mature source rocks at all levels than in the area near Wytch Farm oil field (Colter and Havard 1981), because of Alpine inversion. There are already precedents for deviating wells from the onshore to the offshore (Amocos Down Barn and the Wytch Farm development wells in Dorset) to access the main part of the basin for shale gas." [end of extract from DECC 2011. Shale Gas.]

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Natural fracturing of Liassic Shales-with Beef at Monmouth Beach, Lyme Regis, Dorset, superficially resembling the artificial fracking of shales, July 2013

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The English Channel Inversion

Lulworth Cove, Dorset in relationship to the Portland-Wight Basin or English Channel Inversion

The classic cross-section of the Wytch Farm oilfield and Purbeck Disturbance by Colter and Havard, 1981

Cross-section of the northern margin of the English Channel Inversion Stucture at Lulworth Cove and its relationship to the Wytch Farm Oilfield

The diagram above is just a simple cartoon for educational purposes, and does not show the numerous faults or other details. See accurate and technical cross-sections for details. The key work on this subject is the impressive seismic study of Underhill and Paterson (1998) - Genesis of tectonic inversion structures: seismic evidence for the development of key structures along the Purbeck - Isle of Wight Disturbance, Journal of the Geological Society, London, vol. 155. This paper provides various accurate sections through the Purbeck Disturbance, and is essential reading. This new evidence is consistent with the better of the older views, but eliminates some unsatisfactory hypotheses. The many diagrams cannot be reproduced here, so the paper must be obtained by anyone studying the area seriously. See also the related work of Underhill and Stoneley (1998). Underhill and Paterson Paper - Lyell Collection, Geological Society of London.

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Recent Exploratory Boreholes

Location map for boreholes for oil in the western part of the Isle of Wight

Most of the recent exploratory wells of the 1980s and 90s etc seem to have been targeted at fault structures north of the main Purbeck-Isle of Wight Disturbance (i.e. north of the line of the re-activated Variscan faults). There are major unconfomities beneath the Albian, as shown in a sub-Albian map above. The structural similarity with the Wytch Farm area is obvious, and the hope of finding another highly-successful Wytch Farm Oilfield has clearly been a driving force. Most boreholes seems to have been concentrated in the western part of the island, and thus closer to Wytch Farm. Theories of migration from an area with mature source rocks southwest of the Isle of Wight is a factor. Surprisingly the wells have proved dry so far. The problem might be one of migration. Even in the area south or southeast of Wytch Farm there are problems about a migration fairway. Appropriate juxtaposition of permeable strata at the right time was essential.

Another possible complication might be the regional easterly tilt during Aptian-Albian times. This is seen in the coast from Dorset to Devon as the westerly overstep of the Albian. If migration coincided with this movement, then there may have been not just a tilt up towards the high north of the basin, but also a tilt up westwards. In simple terms there could have been migration in the Cretaceous towards the northwest, rather than just northward. Undoubtedly though there is no simple pattern and the details have to be established.

The state of the reservoirs or potential reservoirs is not considered in any detail here. The Isle of Wight is fairly close to the eastern margins of the Triassic Basin and thus Triassic reservoir facies may be different from Wytch Farm (i.e. more marginal in facies). The Bridport Sand facies is finer-grained and a less satisfactory potential reservoir at the Marchwood Borehole, compared to the Dorset area. The Middle Jurassic is "muddy" according to the records of the Arreton Borehole. There are only very general comments here though, without technical detail and without conclusions. Obviously those who have studied the Isle of Wight boreholes will have a much clearer knowledge not only of the reservoir aspect but also of the possible migration aspect.

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Isle of Wight Licence Blocks

Licence blocks of Northern Petroleum, offshore to the southwest of the Isle of Wight


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Sandhills - 2 Borehole

Location map for the Sandhills Borehole into the Middle Jurassic and the Sherwood Sandstone, Isle of Wight

A exploratory borehole Sandhills No. 2 has been sited at Sandhills near Porchfield, in the northern part of the Isle of Wight, not far from Newport (A Sandhills No.1 Borehole had been drilled before 2001). The licences here had exploration targets for Sandhills No. 2 in both the Middle Jurassic Great Oolite (a major reservoir rock in the western Weald area such as Horndean and Singleton) and in the Triassic Sherwood Sandstone formations (the major and lower reservoir of the Wytch Farm oilfield). Before drilling the estimated potential recoverable reserves at Sandhills-2 were believed to be 10-15 million barrels in the Great Oolite and 20-80 millions barrels in the Sherwood Sandstone. Before the drilling the structure was well-established from seismic and the formations which could be oil-bearing were expected to be present.

In 2005, Northern Petroleum began exploratory drilling for oil, with its Sandhills-2 borehole at Porchfield. However, there was no success in finding useful oil reserves, even though the prospect initially seemed very promising. Northern Petroleum ceased operations at Sandhills in October 2005.

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(Hengistbury Head, not favoured as a well-site?)


Hydrocarbons, oil, gas and shale occur south of Hengistbury Head, Bournemouth, Dorset, photograph 6th July 2013

Much of Hengistbury Head is a former industrial area, a type of "Brownfield Site". This should not be forgotten, even its use now is largely recreational and for conservation. However, the place has not lost its economic potential. It is relatively far south, between Poole Bay and Christchurch Bay. It conveniently close to the offshore Beacon Field with oil and gas potential in the Sherwood Reservoir. Not surprisingly this has resulted in consideration being given to Hengistbury Head being an onshore well-site for extended reach drilling to an offshore oilfield or fields in the English Channel. Furthermore it is only a little more than 10km south of the area of mature oil shale, the source rocks of the Wytch Farm oilfield. These lie in the Portland - Isle of Wight Basin, an area in which there is the possibility, and only the possibility, of shale gas hydraulic fracturing, i.e. "fracking". The potential "fracking area could be reached from Durlston Head, Swanage, or the southwest coast of the Isle of Wight (rather more probable for technical reasons). Thus Hengistbury Head may have no involvement with "fracking", but I wonder if it is for ever ruled out as a place for extended reach drilling into nearby oil and gas fields of Poole and Christchurch Bay. Perhaps this will never be allowed.

If there really is oil potential in or south of Christchurch Bay (or eastern Poole Bay) where else might be the best site for a drilling rig; Highcliffe Castle area, Becton Bunny, Taddiford, Hordle Cliff, Milford? A forest, an old quarry, some little-used place, near to the coast might be useful. The Navitus Bay Wind Farm electricity supply is planned to come ashore at Taddiford Gap, so it may in a sense be already a chosen site for a link to an offshore industry. However, all this regarding oil is speculation and nothing at all may happen.

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More on:

More on the English Channel Inversion Structure of the Portland - Isle of Wight Basin

Durdle Door, near Lulworth Cove, Dorset, is in a zone of compression at the northern boundary of the Portland - Isle of Wight Basin, or English Channel Inversion Structure


Subcrop map of the Carstone (Lower Albian), Isle of Wight, based on work of Radley

An interesting feature related to the Inversion Structure, partly explained in the diagram above, is the occurrence of reworked Jurassic (Kimmeridge Clay) ammonites in the Cretaceous strata of Redcliff, Sandown. Beneath the southern Isle of Wight, concealed and known only from boreholes and seismic is a thick sequence of Lower Cretaceous and Jurassic strata; in the northern part of the island, however, about a kilometre of this is missing. This large gap is the result previous extensional movements on the same faults producing depression to the south and uplift to the north. The northern uplifted area, similar to the structure at Wytch Farm Oilfield, was eroded in Cretaceous times. The southern area is part of the English Channel Basin and this was uplifted, thus becoming an inversion structure during the Tertiary tectonism.


Northern Hinge of the Inversion - Lulworth Area
(subject to be expanded)


Inversion tectonics shown photographically and diagrammatically at the eastern end of St. Oswald's Bay and at Dungy Head, 27th March 2014

The hinge area at St. Oswald's Bay and Dungy Head is shown in a labelled photograph above. This is an important area for study the northern margin of the Inversion in good coastal exposures. There are oil seeps from here, Dungy Head, to Stair Hole, Lulwort Cove and on the Mupe Bay. They are all on a small scale though. The matter will be discussed futher

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PW 12. SHALE GAS (hydraulic fracturing)

Burial diagenesis and thermal maturity of Jurassic shales in the south of England, simplified

Diagenesis burial zones and fracking potential of British Jurassic bituminous shales and associated strata, shown in a simplified diagram

There are no plans for hydraulic fracturing ("fracking") in Dorset, that is the onland area, at present. In any case this does not seem likely because good mature source rocks are not deeply buried on the Dorset mainland. It might not necessarily be out of the question with regard to biogenic gas, though, although there is no reason at the moment to discuss this. The deep basin offshore, the Portland - Isle of Wight Basin or Inversion Structure is more promising. Jurassic strata in the central part might be as mature as Jurassic strata in the central Weald, as at Balcombe. The offshore area would be most difficult and costly to reach. However, they probably could be reached by ERD, Extended Reach Drilling (as at Wytch Farm), if that was appropriate. All that is needed is suitable place such as an old quarry or a small forest on the nearby land.

In spite of this, there seems to be some public concern about something which may not happen. The deep basin is not onshore in Dorset; it is the Portland-White Basin offshore. The first offshore drilling around Britain took place within it, south of Lulworth Cove, in 1963 before the North Sea was developed. Whether or not any hydraulic fracturing ever takes place in the offshore basin is unknown. The Kimmeridge Clay is probably too near the surface south of Dorset (insufficient overburden as a seal) and the Lower Lias has not been shown to have the oil shale quality of the Kimmeridge Clay. Of course, one cannot predict what might happen in the distant future, but a possible future target area may not be directly south of the Dorset coast and almost certainly not on the Dorset mainland.

A good oil shale like the Kimmeridge Oil Shale would be needed. It must be deeply buried. It should have natural joint fractures, but not, as has the Kimmeridge Clay on the Dorset, numerous, rather open extensional faults. The obvious targets in Dorset, then, are the Kimmeridge Clay and Lower Lias. The Lower Lias is older and more deeply buried, but is less organic-rich than the Kimmeridge Clay. The units vary laterally in thickness and are better quality and thicker in the basins. The main Kimmeridge oil shale, the Blackstone, is about 60cm thick at Kimmeridge and probably much the same towards Durlston Head. This is part of the English Channel Inversion area.

There is, as the maps show, a basin is in the English Channel south of the Isle of Purbeck to southeast of Swanage. The Isle of Portland is on a western extremity of this, but overburden is thin here. There is a sub-basin, not as deep as the offshore one north of Weymouth in the Upwey - Portesham area. The Kimmeridge Oil shale has been worked at Portesham, but is quite near the surface. Note that the present burial depth of an oil shale is not the key factor with regard to its thermal maturity. Within inversions the former burial depth may have been much greater. Thus the maturity of an oil shale may far exceed that expected from its present burial depth.

The following paper by Davies, may suggest that perhaps there will be no shale gas hydraulic fracturing from the Kimmeridge Oil Shale or Blackstone in south Dorset, at the places where it is thickly developed.

See: Davies, et al., 2012. See also Durham (2012)
The Kimmeridge Blackstone or oil shale is too close to the surface in the southern Isle of Purbeck, and at Portesham and on the Isle of Portland. It is not at the necessary two to three kilometre depth in these area. Liassic oil shales are different, but generally have a much lower organic content.

So there are no plans for hydraulic fracturing for gas in Dorset, and not good prospects onshore. Details of the offshore basins are not fully understood.

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Obviously, gas can escape more readily than oil. Although there are large quantities in Dorset, that of the largest proven onshore oilfield in northwest Europe, there is very little natural seepage of oil. There are only minute amounts lost at Osmington Mills and the Lulworth Cove area. Gas bubbles on a small scale on the sea floor above the Purbeck Gas Field (Infrastrata) at Anvil Point near Durlston Head, Swanage It also bubbles on the Lulworth Banks, south of Lulworth Cove, according to a diver. No large-scale gas seeps are known in the area

Considering gas escape, the Dorset Jurassic is much faulted, though, and a series of roughly north-south extensional faults are present from Gad Cliff to Durlston Head. Many such faults can be conveniently seen in the Kimmeridge coastal section. They are present in the Portland Stone cliffs of the south of the Isle of Purbeck, but are less accessible except by boat or by rock climbing. It is not known whether these are sealed satisfactorily at depth. They may not always be so.

Many extensional faults cut the Kimmeridge Blackstone or oil shale

The Kimmeridge Oil Shales are the most famous of the oil shales in Britain, and are important as the source rock for North Sea Oil. At present there is a general interest, about possible hydraulic fracturing of oil shales deep underground and extracting large quantities of gas. The Kimmeridge oil shale in the south of England might, at first, seem an obvious target. However, these oil shales are not deeply buried and not thermally mature at the coast. It is not proven that they are mature in the deepest part of the basin. However, they may be submature, or possibly almost becoming mature in places. In general they may not be buried deeply enough to be of direct use for gas production by hydraulic fracturing.

The bituminous shales of the Lower Lias, however, are much deeper in east Dorset and the adjacent English Channel. They have become thermally mature and have resulted in the formation of oil. At the coastal exposures at Lyme Regis these are not so obviously rich in organic matters as is the Kimmeridge Clay with its well-defined oil shale. An anecdotal point is the Shales-with-Beef of the Lower Lias, though are sufficiently rich in TOC to have been involved in a natural cliff fire, the well-known "Lyme Volcano" of about 1908.

The Liassic shales are the source oil for oil of the Wytch Farm oilfield, and this oil has migrated northward, probably in the Cretaceous, into the Sherwood Sandstone reservoir south of Bournemouth. It is not known whether hydraulic fracturing of Liassic strata would be worthwhile. If undertaken it would presumably be in or near the deep parts of the English Channel Basin, as shown on maps here.

A possible problem is that most of the Jurassic of the Dorset coast, and thus probably offshore, is fractured with many extensional faults. The most obvious of these are the approximately north-south trending faults seen in the cliff sections at Kimmeridge and elsewhere. Some faults in the region are growth faults, the most well-known of which is the Eype Mouth Fault, which has been active since early Jurassic times. Obtaining commercial quantities of gas offshore by hydraulic fracturing may never happen. It is less likely on the land of Dorset.

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See also:

Durlston Head Webpage.

Oilfields and prospects in the southeast Dorset and adjacent sea area, revised version, 9th October 2016

Above is shown a new, simplified map of the oilfields and prospects in the southeast Dorset area and adjacent offshore. This has been revised to take account of data published by Infrastrata in 2016. (earlier maps may be slightly different)

The Durlston Head Gas Field in relation to source rock areas and oil fields in the East Dorset coastal region

Durlston Head, and Durlston Castle, Dorset, photographed from the Paddle Steamer Waverley by Dr Clive Boulter on 19 September 1997

Multibeam bathymetry of the sea floor south of the Isle of Purbeck from Durlston Head to St. Aldhelm's Head, Dorset, courtesy of the Channel Coastal Observatory

There is a methane gas field beneath the Durlston Head area. The limits are not know exactly and the location map above is probably not correct. Another map shows it extending into the sea area. The offshore geology as shown by multibeam bathymetry is indicated in a map above. This gas field has been investigated many years ago by BP who put down a deviated borehole into this, from a quarry south of Swanage. Long before this exploration, a gas seep into the sea at Durlston Head had been found by divers, and is obviously a small leak from the gas reservoir.

A fairly old record of the Durlston Head gas seep was given by (Hinchcliffe, 1978).

"On the seabed directly beneath Durlston Castle, above whose cliffs is poised the giant stone globe of the world, a further curious phenomenon is found. Here the seabed is bubbling. in 10 metres of water, long columns of bubbles ascend. Some sources are continuous, some spasmodic. I recently collected a sample of this gas and it proved to be an inflammable natural gas. An even more curious phenomenon here is the large numbers of huge bass and pollock which tend to swim about near these bubbles. Do they mistake the hydrocarbon gas for oxygen?"

Later, the diver, Mike Markey, who drew my attention to this article, reported on the 7th Dec. 1990 that the gas seep was still bubbling at that time. He had also found a similar gas seep on the Lulworth Banks, near the axis of the offshore anticline. According to Judd (2004), gas bubbles lose methane to the water as they rise, so deep water seeps are unlikely to contribute to the atmosphere. However, bubbles break the surface above some shallow water seeps. I do not know whether this happens at Durlston Head or whether the bubbles disappear before they reach the surface.

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[Durlston Head Gas Field -]

Southard Quarry Borehole, Swanage
[SZ 02347775] (drilled by British Petroleum in 1989)


Triassic salt thicknesses in East Dorset, with particular reference to Durlston Head, modified after part of a salt map by Harvey and Stewart (1998)

Note the extensive Triassic salt development beneath the Durlston Head area. It is greater than 300 metres thickness of salt. The proposed InfraStrata borehole may go into this salt, as an InfraStrata borehole did on the eastern side of the Isle of Portland. That was part of a plan for storage of gas in cavities dissolved out artificially in the salt.


The borehole at Southard Quarry by BP in 1989 was a predecessor to the planned California Quarry Borehole. There is only limited information easily available to the public. It has been said that the borehole lost drilling fluid, and some may have assumed that it was not a deep borehole. Gallois 2000 (Proceedings of the Geologists' Association, vol. 111, 265-280 named a stone band, close below the Grey Ledge Stone Band (in the Wheatleyensis Zone), of the Kimmeridge Clay as the Southard Quarry Stone Band. It is shown on a vertical section.
Harvey and Stuart (1998, pp. 242-244 and fig.3), have given some further limited information regarding the Southard Borehole. It would seem that this was a deep well and it penetrated the Triassic salt deposits. It seems to have shown that the salt thickness was about 320 m. in thickness, south of the major, mid-Durlston Bay fault (downthrowing south). This compares with estimates for zero under Swanage town and with more than 400m. in Lyme Bay.

"Generally the thickness of individual units of pure halite range from 21 to 40m., but can exceed 60m. in thickness, as proved in the Martintown, Chickerell and Southard Quarry Boreholes." "Depth to the uppermost halite bed - 2141m. at Southard Quarry." [from D.J. Evans and R.A. Chadwick, 2009. Underground Gas Storage: Worldwide Experiences and Future Development in the UK and Europe. 1st June, 2009. 283pp. Geological Society Special Publication, No. 313. Price 90 pounds, Fellow's price 45 pounds]

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Durlston Head Gas Field -

California Quarry Borehole, Swanage

(On land - but for drilling seaward into the basin)

Approaching the proposed location of the California Quarry, gas exploration wellsite, west of Durlton Head, Dorset, November 2013

The California Quarry, Swanage, site of the exploratory borehole by Infrastrata into the offshore Durlston Head Gas Field, seen from the north

The entrance to California Quarry, California Farm, Swanage, Dorset, a locality that will probably become the wellsite for exploration of the Durlston Head gas field by Infrastrata

California Quarry, Swanage, with work in progress, prior to the drilling for gas at this site by InfraStrata

A borehole will extend laterally from this quarry out to Durlston Head and beyond offshore. It will extend down into the Sherwood Sandstone Reservoir of the Wytch Farm oilfield. It is apparently not intended, at least at present, for Shale Gas hydraulic fracturing, and in any case the Liassic shales may not be mature so close to Durlston Head (and not necessarily in the main Inversion block). However, in the future the site may perhaps be of use when serious exploration of the offshore mature Liassic shale is taking place. There might perhaps be a long Extended Reach Borehole into the main deep Inversion Structure which lies to the southeast. However, if the mature shale suitable for hydraulic fracturing can not easily be reached from California Quarry or adjacent quarry, then an offshore drilling rig might be needed. Thus it is by no means certain that in the future this area will be a base from exploiting the main mature Liassic shale. It is even possible that the offshore shale reserves could be worked by a long Extended Reach Borehole from the Atherfield area of the Isle of Wight. This is quite a promising possibility because the Liassic shales are just about mature (RO = 0.5) at Arreton on the Isle of Wight and the potential Late Cimmerian overburden (i.e. Lower Greensand) is thickly developed in the Atherfield area.

The the California Farm borehole is intended to investigate gas reserves at Durlston Head. The new exploration could in addition be very useful in helping to understand the margins of Portland-White Basin or English Channel Inversion Structure and its future economic, shale-gas potential, even if this was not its main objective.


California Quarry 1 Well Approved

"On 29 November 2013 Dorset County Council resolved to grant planning permission for the California Quarry-1 well, the site for an exploration well to be drilled from onshore to offshore into petroleum licence P1918. Commenting on the news, Andrew Hindle, CEO of InfraStrata said: 'InfraStrata is pleased to be able to add additional exploration areas adjacent to its existing licence areas in Dorset and Northern Ireland. The approval for the well in Dorset is an important step in building a multi-well programme across our portfolio for the coming year.'

[After - Energy-pedia Licensing - online, 3rd December 2013. See this article.]



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Wealden Conglomeratic Oil Sand - the Mupe Bay Palaeoseep

Oil sand in the lower part of the Wealden Group at Mupe Bay, Dorset

Conglomeratic oil sand - oil sand blocks in the bottom of a Cretaceous, Wealden river Channel, Mupe Bay, Dorset

A schematic section through the channel sandstone with oil-saturated clasts, and associated beds, Wealden, Mupe Bay, Dorset

As mentioned above, a conglomeratic oil sand, outcropping near the steps is one of the most interesting features of the Wealden section. It has long been regarded as the consequence of a natural example of "palaeo-pollution", i.e. a river into which oil seeped in Cretaceous times. West (1975, p.211) , believed the seepage to have occurred up an unexposed intra-Cretaceous fault.

See Wimbledon, Allen and Fleet (1996) for some more recent studies. Selley and Stoneley (1987) and Stoneley (1992) followed this theory further but Miles et al. (1992) argued against the view. See also Cornford et al. (1988) for organic geochemistry of the Mupe Bay oil sand.

Mupe Bay palaeoseep?

This diagram, based on Selley and Stoneley (1987), was captioned as "Geophantasmogram to illustrate the evidence for late Jurassic and Early Cretaceous movement along the Purbeck disturbance or fault, and the genesis of the Mupe Bay palaeoseep." Note that this theory is slightly different from that of West (1975) which invoved an additional, local fault transverse (i.e. roughly north-south) to the main east west structures shown here. According to Miles et al. (1992) penecontemporaneous oil seepage is a myth. However, the latest new information on the topic below shows that there really was transportation of oil-cemented clasts into the bed, and that there is also a modern active oil seep, here, at Mupe Bay!

In spite of some arguments against the Mupe Bay conglomeratic oil sand being evidence of a palaeoseep, it has generally been considered that oil migrated from the Portland - Isle of Wight Basin (offshore) in Cretaceous times. This is the conventional theory for the origin of the oil in the Wytch Farm Oilfield (with some additional Tertiary migration). It is now important to note that the original theories of early Cretaceous oil seepage and migration at Mupe Bay just here have been proven again.

See the key paper by Emerton et al. (2013) - A Magnetic Solution to the Mupe Bay Mystery. The abstract is given below, as a convenient summary, but the full paper is available online and this should be read. The reference and abstract (with highlights) is given below:

Emerton, S., Muxworthy, A.R. and Sephton, M. A. 2013. A magnetic solution to the Mupe Bay mystery. Marine and Petroleum Geology, vol. 46, pp. 165-172. September 2013. By Stacey Emmerton, Adrian R. Muxworthy and Mark A. Sephton. Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.

The clasts hold a magnetic signal unlike the matrix indicating two phase staining.
The clasts show similar directions and thus were soft when deposited into the bed.
The clasts indicate the onset of oil migration in the Early Cretaceous.
The matrix is consistent with today's magnetic field and is an active oil seep.

An outcrop of Wealden beds at Mupe Bay (UK) is associated with a key piece of evidence for the timing of hydrocarbon migration in the Wessex Basin. A conglomeratic bed contains oil-stained clasts and matrix that appear different upon superficial observation. Conventional interpretations assign differences to the erosion and transport of oil-cemented clasts by Wealden rivers before their incorporation into a later stained conglomeratic bed. This scenario constrains the onset of oil migration in the basin to the Early Cretaceous; however, arguments have been put forward for single phase staining.
Magnetic information may provide new ways to examine the Mupe Bay record of oil migration. Migrating fluids such as hydrocarbons have been shown to cause chemical conditions suitable for the alteration or formation of authigenic magnetite resulting in associated chemical remanent magnetization (CRM). Magnetic characterization reveals both the matrix and clasts contain multi-domain magnetite but abundant hematite only exists within the clasts. Hysteresis parameters show the matrix has more multi-domain and likely larger magnetic grains than the clasts.
Magnetic directions are different in the clasts and matrix supporting a two-phase oil-staining event. Moreover, paleomagnetic directions for the clasts after tilt correction (82.6N and 155.2E) are consistent with biodegradation processes in the Early Cretaceous. Consistent directions in separate clasts imply the biodegradation and magnetite formation took place following transportation and incorporation of the oil-cemented clasts into the conglomerate bed. Magnetic directions reveal that the Mupe Bay matrix has a remanent magnetization corresponding to today's magnetic field, confirming the matrix represents an active oil seep.
This study represents an unprecedented use of magnetic data to date the onset of oil migration in a basin. The classic two-stage oil-staining scenario, which constrains the onset in the Wessex Basin to the Early Cretaceous, is supported by our data.

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Burial diagenesis and thermal maturity of Jurassic shales in the south of England, simplified

Offshore potential source rocks in well 98-11-4Z, east of Ballard Cliff, Swanage, Dorset, modified after Buchanan, 1998

Some maturity information regarding southern England oil shales, based largely on old data of Ebukanson and Thornton, 1986, with some additional data

Offshore - Kimmeridge Clay Maturity

In Dorset the kerogens in the outcrop samples of the Kimmeridge Clay to the south of the Purbeck - Isle of Wight Disturbance, that is the Kimmeridge cliffs, are relatively more mature than those to the north of that line, for example the Ringstead Bay area ( Ebukanson and Kinghorn, 1986). The Kimmeridge Clay thickness in the cliff sections and boreholes at Kimmeridge was thought to be about 508m according to (Cox and Gallois (1981). Later work, taking into account seismic and borehole data suggested that the full thickness of the formation was between 535m and 585m (Gallois, 2000). A single exact figure is unlikely to be obtained because the coast section is not far south of the Purbeck - Isle of Wight disturbance, and thus, in this not completely stable area, there are differences in detail in thicknesses east (further out towards the basin) and west (i.e. Brandy Bay, closer to the fault). The Kimmeridge Clay thickness is about 7% thinner at Swanworth Quarry (Gallois, 2000).

Kerogen samples for the lower parts of the Kimmeridge Clay Formation and those from the whole of the Oxford Clay Formation in the Arreton No. 2 Borehole of the Isle of Wight are marginally mature (estimated R0: 0.50 to 0.60%) (Ebukanson and Kinghorn, 1986).

Thus, the most mature contemporaneous organic matter in the Kimmeridge Clay and the Oxford Clay occur in two parts of southern England: the Isle of Wight area to the south of the line of disturbance and the central part of the Weald of Sussex and Kent (Ebukanson and Kinghorn, 1986). The Lower Lias is mature over a wider area. Kerogen in the lower part of the Lower Lias (Shales-with-Beef etc?) is in (at least just in)the catagenic stage in Arreton-2, Winterborne Kingston, Marchwood, Winchester-1, Henfield, Warlingham and Penshurst Wells. However, Lower Lias samples are in the main phase of oil generation only at Arreton-2, Henfield, Winchester-1 and Penshurst (Ebukanson and Kinghorn, 1986). The most mature Lower Lias sections occur in Arreton-2 (estimated R0: up to 0.90%) and Penshurst (estimated R0: up to 0.85). At Penshurst the total Jurassic thickness is 1.259m. It is interesting to note that Ebukanson and Kinghorn, (1986) drew attention to the areas of the central Weald in the eastern part of West Sussex and southern Surrey where the total Jurassic attains thicknesses over 1.424m, according to Gallois (1965). Here they expected higher maturity values than reported for the Penshurst Borehole. This is now the area of the planned Balcombe, hydraulic fracturing borehole.

A hypothetical burial history diagram for Arreton No. 2 Borehole, Isle of Wight, based on Ebukanson and Kinghorn (1986), with minor modifications

[More needs to be known about apparent anomalies at Arreton; conflicting figures have been given and there are two boreholes. The Winchester-1 figure, if correct, is surprisingly high for an area not within the centre of a subsiding basin. Efforts will be made to obtain more accurate data. A table will be provided below in due course.]


[to be constructed]

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This topic is briefly mentioned here in a webpage on hydrocarbons, because it is strange that the proposed wind farm site seems to be located in that it, or at least partially overlaps the oil source area ("Oil Kitchen")of the English Channel. This is the region from which the Wytch Farm oil and gas has come, and is one of only two regions (the other being the central Weald Basin) in the south of England where Jurassic shales may be sufficiently mature for hydraulic fracturing ("fracking"). It is suprising that the central part of this oil generation basin has been chosen for part of the proposed wind farm site.


The Navitus Offshore Wind Park, English Channel, Community Consultation Document for Phase 3

[Start of the document on p.3:]

[p.3] "About the Project.
The proposed Nativus Bay Wind Park is an exciting 50-50 joint venture between Eneco Wind UK Ltd (Eneco)[a wind farm company, with a wind farm at Tullo, Aberdeenshire and planning another one in Scotland] and EDF Energy [a gas and electricity supplier]. At the moment, we are developing plans to build and operate an offshore wind park off the Dorset and Hampshire coasts to the west of the Isle of Wight.
Navitus Bay could produce enough clean, safe energy to power over 790,000 homes ever year, that is about nine times the number of homes in Bournemouth or 13 times the number of homes on the Isle of Wight. The the wind park could also lower harmful carbon dioxide (CO2) emissions by 1,150,000 tonnes a year.
All proposed works described in this document are subject to development consent being granted by the Secretary of State for Energy and Climate Change. The purpose of this document is to provide details of the proposed project we intend to seek development consent for and to seek feedback from the local community prior to the submission of an application for development consent. ...."[continues with "Community Consultation - Phase 3, running from 1st Feb. 2013 to 5th April 2013, and thus this part of the programme is completed.][The report discusses bats, dormice and archaeology etc, but not geology. It says nothing about the shale gas potential under at least part of the area, in spite of the fact that one of the two companies is a gas supplier.]


The Navitus Bay Wind Park in relation to the Wessex Basin Oil Kitchen and thermal maturity of Jurassic shales, with fracking potential


Navitus Wind Park, planned location offshore, in relationship to the Jurassic-Cretaceous thicknesses and possible thermal maturity of Jurassic shales

It is notable that the proposed Navitus Bay Wind Park partially overlaps, or seems to overlap, the so-called "Wessex Basin Oil Kitchen", the thermally mature area of Jurassic source rock shales. The exact boundaries of the "oil kitchen" may not necessarily be precisely known at present, but there is very much seismic information in existence.


So, why is the wind farm location, partially over the mature source rock area? Is this by chance? Will its position favour or reduce the potential for offshore fracking (hydraulic fracturing of bituminous shale), if this will ever be planned?

The eastern part of the Portland-Isle of Wight Basin is the region from the which the oil and gas of the great Wytch Farm oilfield has come. If hydraulic fracturing or fracking of bituminous shales of the Lias and possibly the Kimmeridge Clay (and Oxfordian) takes place offshore from Dorset, western Hampshire and the Isle of Wight, this is a promising region. It has similarities to the central Weald Basin where fracking is to take place, initially at Balcombe, and probably later in other adjacent area. Fracking may or may not actually takes place within the wind farm boundaries, but if not, it may well take place nearby. In any case, precautions of gas leaks etc will be needed at the Navitus Bay site, because there are offshore gas seeps at two places in the region. One is at the Durlston Head gas field, not far away, and another is at the Lulworth Banks. Oil seeps occur in Corallian strata at Osmington Mills, Dorset. The Wealden (lower part) has oil seeps at Dungy Head, Stair Hole, Lulworth Cove and Mupe Bay. There is minor seepage at Peveril Point, Swanage. Although all these seeps are small it might be necessary to check the Navitus Wind Park for oil or gas seepage. This is particulary because it is mainly on Wealden strata. However, it should be noted that Wealden oil seeps do not seem to occur in the Wealden, Wessex Formation of the Isle of Wight. The lowest part is not exposed there, though. The question is probably one of detailed geology of the Wind Park and information on the depth of foundations of the wind turbines and the extent to which the foundations are cemented and sealed in.

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Acknowledgements (draft to be expanded).

Discussion with many geologists on field trips has proved very helpful. I thank Ramues Gallois and Steve Etches with regard to Kimmeridge Clay matters in particular. Various field trips with oil company staff have very useful. I much appreciate the advice and help of my daughter, Tonya Loades of Bartley West, Chartered Surveyors.

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Dr Ian West, author of these webpages

Webpage - written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.


at his private address, Romsey, kindly supported by the School of Ocean and Earth Science, Southampton University,and web-hosted by courtesy of the iSolutions of Southampton University. Any views expressed here are those of the author and do not necessarily represent the views of Southampton University. The website is written privately from Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University.