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SHERWOOD SANDSTONE RESERVOIR (Lower and Major Reservoir)
Sherwood Introduction
The principal reservoir of the Wytch Farm Oilfield is the Sherwood Sandstone of Triassic age. It contains about 416 million barrels of oil reserves. The western part of the reservoir lies beneath Poole Harbour and adjacent area, while the large eastern part is under Poole Bay. More than half of the Sherwood reserves lie in the offshore area (Poole Bay). This caused the drilling of ERD, Extended Reach Drilling wells, beginning in 1993. The large drilling derrick is situated at Wellsite M on the Goathorn Peninsula.
The Sherwood Sandstone Reservoir is at about 1585 metres depth (true vertical depth subsea - TVDSS) (Hogg et al., 1999). There is a maximum 110 metre column of oil-bearing sand above the oil-water contact in the Sherwood Sandstone Reservoir.
The Sherwood Sandstone Reservoir at Wytch Farm is about 150 metres thick. It is divided into oil-producing and oil-barrier zones. These are numbered down, not upward as in normal stratigraphical procedure. There are effectively three reservoirs within the Sherwood Sandstone. The sequence from top downward is as follows:
(Zone 20 - muddier barrier)
ZONE 30
(with lower permeability and less net to gross oil)
(Zone 40 - muddier barrier)
ZONE 50
(Zone 60 - muddier barrier)
ZONE 70
.
The Sherwood Sandstone was deposited in E-W Early Triassic rift system, which developed as a result of extension along a pre-existing Variscan or Hercynian Thrust front. This Triassic rift system was broad scale and should not be confused with the local extensional faulting of the Cretaceous (Late Cimmerian or Sub-Albian), important for development of fault-traps. This later faulting resulted in three extensional faults running east-west under Poole Harbour and Poole Bay (south of Bournemouth). They are from north to south, the Northern Fault, the Arne Fault and the Wytch Farm Fault. These are shown in a map on p. 1158 of Hogg et al. (1999). The Sherwood Sandstone rests on playa mudrocks of the Permian Aylesbeare Group (seen at Littleham Cove in Devon) and is conformably overlain by playa mudrocks of the Triassic Mercia Mudstone Group (seen near Sidmouth, Devon). This acts a caprock or seal.
The Sherwood reservoir is formed from a complex and heterogeneous arkosic alluvial succession deposited in a semi-arid, red-bed palaeoclimatic setting (Bowman et al. 1999). It was deposited near the confluence of two major braided alluvial tracts draining northward from France and westward from the Brabant Massif (Belgium). It is interesting to note that these Triassic sediments are immature (i.e. arkosic with much feldspar), whereas the later sandstones such as the Bridport Sands are more mature (higher percentage of quartz). Because of the climatic conditions there has been development of calcrete and calcrete conglomerate. These palaeosol carbonate and, in addition, some beds of mudstone form baffles which affect the vertical flow of fluids (Bowman et al. 1999). Reservoir Simulator Models take into account the complications of vertical and horizontal fluid flow and enable reservoir management decisions about water injection strategy and artificial lift (Bowman et al. 1999).
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SHERWOOD SANDSTONE RESERVOIR continued:
Production History
Detailed information regarding the Sherwood Sandstone Reservoir, particularly in terms of oil production, was given by Hogg et al. (1999) and Bowman et al. 1999). Some more recent updated, summary information has been provided by Wahidiyat (2010). This later publication shows how the reservoir has reached a later and more mature stage in terms of oil production.
Production from the Sherwood Reservoir accounts for 85% of the total Wytch Farm production. As the diagram above shows, it has passed the phase of peak oil production. It is still producing a large quantity of oil and gas but the Water Cut (percentage of water produced during pumping) has risen to a relatively high level. Measures have been taken and more can be taken to extend the life of this productive reservoir.
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SHERWOOD SANDSTONE RESERVOIR continued:
Architecture, Facies and Origin
The Sherwood Sandstone Group reservoir at Wytch Farm has been described by McKie, Aggett and Hogg (1998) - Reservoir Architecture of the upper Sherwood Sandstone, Wytch Farm field, southern England. This paper is in the Geological Society Special Publication, No. 133, Development Evolution and Petroleum geology of the Wessex Basin.
It consists of about 150 metres of arkosic [i.e. feldpar rich] sandstone that originated in a continental environment, part of the supercontinent - Pangaea. It is not marine and thus quite unlike the Bridport Sands reservoir. The sandstones have been deposited in a variety of fluvial, lacustrine and aeolian depositional environments. The general setting was in the arid zone north of the equator (which has moved southward over time), and rather like the Sahara desert. The date was round about 245 million years ago. It should be noted that although the casual visitor to the North African deserts will be impressed by the general aridity, the sandy ground and the blown dust, a large proportion of the sediments are reworked by water during flash floods. Thus the impression from the geology is not necessarily representative of just what the surface would have looked like most of the time. Of course, in British Triassic strata aeolian environments are recorded, and so too are lacustrine deposits. Lakes can be permanent but are often empheral in desert environments, and in such a situation are likely to precipitate evaporites.
McKie et al. found three orders or trends of facies variability in the Sherwood Sandstone here. The first order evolution trend over the entire Sherwood Sandstone Group is as follows:
1. Perennial braidplain [i.e. from perennial braided rivers - not very arid] to:
2. Ephemeral sheetflood systems [a common modern environment in the dry Sahara with dry wadis and broad sandflats which may flood with sheets of water, suddenly after rare, torrential rain storms] to:
3. Ephemeral lacustrine conditions [temporary desert lakes - sometimes salt lakes].
This trend through the red Sherwood Sandstone culminated in the deposition of the red, finer-grained, Mercia Mudstone [the "Keuper Marl" of the old literature]. McKie et al. considered that this represented a long term waning of sand supply and increased "flashiness" of the fluvial system [i.e. more flash floods].
The trend, discussed above, is further divided into second order cycles defined by five areally widespread floodplain and lacustrine deposits containing minimal development of fluvial sandstones.
The petrography of the Sherwood Sandstone in the Wessex Basin has been the subject of a study by Strong and Miladowski (1987). Early diagenesis was characterised by carbonate (calcrete cementation) with non-ferroan dolomite and some evaporites towards the basin centre. Later diagenesis involved some grain dissolution of feldspar and anhydrite cementation (later subject to some dissolution). There are some quartz cements in places. Later weathering processes have also been discussed by these authors.
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SHERWOOD SANDSTONE RESERVOIR continued:
Oil Field Brines
The brines of the Wytch Farm Shewood Sandstone Reservoir have been studied by Worden et al. (2006). This Triassic reservoir contains very salty water.
They found that the brines are of sodium chloride type, as you would expect in the Permo-Trias underground. They are not, however, as might have been expected, saturated for halite, but reach up to 230,000 mg/litre (saturation for salt is near 350,000). There is some decrease in salinity from the flanks to the crest, but the water there is still very saline water.
These authors recognised two phases of influx of meteoric water (i.e. freshwater ultimately of rainfall origin), one probably of the Lower Cretaceous age and before the oil influx into the present site of the reservoir. The later was of Eocene age and was after or during Eocene oil influx (i.e. this is assuming that although oil could have been migrating earlier, probably during the Cretaceous, it moved into the present site in the Sherwood Reservoir in the Eocene). There is no evidence for fluid movement during the last 40 million years.
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SHERWOOD SANDSTONE RESERVOIR continued:
NORM, Natural Radioactivity - Sherwood Reservoir
It is not surprising that NORM, Naturally Occurring Radioactive Material is present within the produced fluids, the oil and water of the Wytch Farm Oilfield.
This happens in many oilfields, as in the North Sea.
In any sample of oil, water or rock the quantity of radioactivity may be very small. However, because large quantities of fluid pass through the pumps and pipes and they can concentrate certain substances which are present in very low proportions in the fluids. This topic has been much discussed in relation to North Sea drilling rigs. The report of Sniffer (2004) provides much detail regarding the topic and is available online. It discusses the disposal of these oilfield radioactive wastes.
For more information see also the website: NORM Scale: Source of Emissions for further understanding of NORM and the development of radioactive scales on equipment. Reference is made to the Radioactive Substances Act 1993. NORM is mainly a technical problem for engineers within the oil industry.
The NORM at the Wytch Farm Oilfield is in the Triassic Sherwood Sandstone Reservoir and not in the Bridport Reservoir. It is unusual (Worden et al. 2000). It consists of thin metallic coating of 210Pb (lead 210, part of the uranium 238 radioactive decay series) and of 210Pb (polonium 210 which decays rapidly to lead 206). There is very little radium in the metallic scale. The lack of radium is explained by the fact that the brines have very high sulphate, and thus radium will precipitate out with barite as radium sulphate before it reaches the oil wells. However, undersaturation of the reservoir brines for lead sulphide, means that lead can remain in solution. Thus the 210Pb (lead 210) can be carried in solution into the downhole pumps and processing pipes.
(Worden et al. 2000) reported the occurrence of some local high radioactivity of thousands of becquerels. However, according to Sniffer (2004) because lead 210 is weak gamma emitter and polonium 210 is an alpha emitter and these sources of radiation would not be detected outside a steel vessel wall with standard NORM monitoring equipment. Thus it would seem that the radioactivity is confined within steel boreholes, pumps and pipes is not exposed. It is, thus, not generally any hazard to the public because it is shielded in pipes and in most cases it is also deep underground in distant, extended-reach boreholes. Nevertheless it is an expensive matter for the oil industry which has to deal with it by removing components and decontaminating them elsewhere. Decontamination work would have to take place during the final stage of the oil the tear-down (dismantling).
Oil source rocks, oil shales, can be enriched in 238U [uranium] and 232Th [thorium] and provide an ultimate source of radioactive material. In the Dorset area there does not seem to be high concentrations of uranium and thorium in Liassic bituminous clays (or in the Kimmeridge Clay), so this is unlikely to be the explanation. In addition, the NORM is not present in the Bridport Reservoir which is Liassic and occurs above the Liassic Clays
Wytch Farm radiation probably has a special explanation associated with the Permo-Triassic red-bed facies. It is of significance that the Sherwood Sandstone lies above the Aylesbeare Mudstone which has the well-known uranium-thorium nodules at Littleham Cove near Budleigh Salterton. It is not known whether any such nodules occur in the area of the Wytch Farm Sherwood Sandstone reservoir. However, the Sherwood Sandstone is in part playa origin, like the Aylesbeare Mudstone. The conditions were broadly similar at times and the sediments are mostly reddish and oxidised.
Worden et al. (2000) pointed out that uranium may have been sourced from weathered material from the Hercynian granites (e.g. Dartmoor Granite), mobilised by oxidation and groundwater flow and then precipitated where oxidised U-bearing fluids met deeper reduced water (Durrance, 1984). There are variations on this theory involving local reduction, but the presence of granites and oxidised water are key factors. An arid environment, proximity to playa lakes and a regional aquifer able to import uranium are also important. The similarity in source to the radioactive nodules of Devon seems almost certain, but the details are not necessarily understood.
For further details on the radioactivity of the Sherwood Reservoir at Wytch Farm read the paper by (Worden et al. 2000). It is available online.
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STRUCTURE:
Northern Margin of the English Channel Inversion
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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OIL LEAK (of 2010 to 2011).
A small oil leak was reported from a pipeline at Wytch Farm on the 27th January. See Rowena Masson - BP shuts UK Field after Oil Leak, The Telegraph Newspaper. See also other newspaper reports of this date, including that of Durkin (2011) in the Bournemouth Daily Echo. The oil field was closed down for two months. The Dorset leak occurred just before warnings from the Health and Safety Executive about corrosion risk in the oil industry, specifically in relation to North Sea Oil platforms. A BP spokesman reported that a small amount of oil from a pipeline, encased in concrete and sand, had risen to the surface. The section of pipeline has been replaced and all key pipelines were being inspected, with the aid of robots. The leak was reported to have occurred at one of the wellsites, but it is not known which one. Dr. Susie Baverstock, Wytch Farm's communications and land management leader, assured the (Bournemouth) Daily Echo newspaper that the leak was small and there was no contamination to the surrounding water or countryside. It was contained and dealt with very promptly. No oil left the site whatsoever.
It would seem that a small oil leak, that caused no contamination or environmental problem, resulted in the safeguard of substantial checking and maintenance work that shut down the oilfield for two months. It does not seem to have been a major threat to the environment and the oilfield is probably all the safer now because of the maintenance work.
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OIL LEAK of 2010 to 2011 - Further Notes.
It is not know where oil leak occurred. Was it at an oil well such as the Goathorn Extended Reach Drilling Site or was it in the vicinity of the Gathering Station? The reports state that the pipeline was encased in concrete and sand. A small amount of oil had risen to the surface of the sand. Thus it seems to have been a near-surface event.
WYTCH FARM continued:
Borehole: Old Harry - 1,
(Borehole 2000 in Old Harry Rocks Prospect in Block 98/11, relinquished to DECC in 2009)
The report reference and some details are given in the Petroleum South of England, Bibliography:
DECC (2009).
(For details of the Chalk Cliffs and coast at and around Harry Rocks see:
Harry Rocks and Ballard Cliff webpage.)
.
There was an Extended Reach Borehole from Wellsite M, in 2000, to a prospect to the east of Old Harry and Harry Rocks, between Studland and Swanage (near Ballard Down and Ballard Point). It seems not to have been successful in economic terms but is of interest in showing some structural and stratigraphic features beneath the sea floor east of Harry Rocks.
The well was drilled by the Operator of Wytch Farm at the time, BP (since sold by BP to Perenco UK). The objective was a prospect of limited size in the the northern, downthrow side of the Purbeck faulted Monocline or Purbeck - Isle of Wight Disturbance. In relation to the cliff section, it was effectively north of the Ballard Down Fault.
A brief technical report is available as a pdf file on the internet: DECC, Department of Energy and Climate Change, Government of UK. Relinquishment Document of DECC, Government of UK, Licence Block P1022 (Block 98/11).
The general comments here are based on this report, but written in non-technical language. The illustration above is based on, with modification, on part only of the map: Fig. 4, Map displaying Block 98/11 Discoveries, Leads and Prospects.
The well, entering obliquely from the direction of Goathorn Peninsula, drilled through 60 metres, vertical thickness, of Sherwood Sandstone. This is he lower and major reservoir of the Wytch Farm Oilfield. It entered the Triassic sandstone from a subsea depth of 1738 metres, that is quite deep, deeper than at Wytch Farm. The highest 9 metres of the Sherwood Sandstone was shaley, not suprising since it is just beneath the argillaceous Mercia Mudstone.
A good porous sand was encountered not far down in the sandstone at 1748-1754 metres depth. There may be gas or oil, but there no oil shows were found. Because of a technical point with the seismic interpretation, the borehole seems not to have entered the Sherwood Sandstone at the level intended. This problem arose because the seismic velocity in the Chalk in the Harry Rocks area was higher than expected. Perhaps this was because of the hard, well-cemented Chalk, known to be present beneath the Ballard Down Fault, but I do not know the details. The result of this problem is the top of the Sherwood Sandstone proved to be 40 metres deeper than expected. Presumably the borehole was not, therefore, in the best and intended position.
With regard to reservoir potential, the Sherwood Sandstone did not have the good fluvial (river) channel sands, as are present at Wytch Farm. These have particularly good porosity and permeability features. The authors of the report thought that the area is off the main Sherwood sand channel axis. Thus the well was plugged and abandoned.
With regard to details, there was a good quality, 6 metre, hydrocarbon-bearing sand at the top of Zone 30, the top production zone of the Sherwood Sandstone at Wytch Farm. Below this there are a series of thin, individual hydrocarbon-bearing sands. This sequence, with natural gas, extends to the total depth of the borehole. This depth was at 44 metres below the mapped spill point, the theoretical base of the oil. The thin sand bodies are probably individually stratigraphically trapped or effectively closed off. However, if they are linked together and part of a connected reservoir body there is a question of how the structure would be sealed (from leakage of oil). A linkage or connection of faults would apparently be needed to seal the structure down to at least 1800 metres subsea. This is below the oil-water level in Licence Block 98/11-1 to the east.
For more detailed information there is also a report [not seen]:
Watts, G. 2000. Old Harry Review: Review of results from Exploration Well L98/06-M18 and L98/06-M18Z in 98/11. By Giles Watts, June 2000. Internal Report of BP?
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Wareham Field
In 1964 Wareham was drilled, following the Kimmeridge No.1 discovery in 1959. This Wareham well produced 23 barrels per day of oil (with much water) from thin, 4.9 metres, of Inferior Oolite and the top 2.9 metres of Bridport Sands. Wareham No.2 produced oil at a rate of approximately 14.5 brl per day from the Cornbrash after fracturing and acidizing. It was later abandoned (Colter and Havard, 1981). Thus initial production from the Wareham Field was very poor.
Later, however, in 1991, BP brought the Wareham Oil Field into production and it became very successful.
It currently produces about 300-400 barrels of oil a day. The reservoir now in use is the Bridport Sand and there are reserves of about six million barrels. It is not connected to the Wytch Farm oil field. There are two wellsites (WC and WD) are located to the west of Wareham at Worget. WC has 5 producing wells and 2 water-injection wells; WD has 2 producers and 2 water-injectors (information from a BP webpage).
WYTCH FARM continued
Pipelines
The oil export pipeline from the Gathering Station in the Isle of Purbeck to the BP marine terminal at Hamble on the east bank of Southampton Water was laid down in 1988. It is 400 mm diameter and 90 kilometres long. From the Gathering Station it swings round to the west of Wareham and then goes north of Wimborne Minster. From there it proceeds southeast to Sopley (near Christchurch). Up to this point there is a parallel methane gas pipeline but this terminates at an installation at Sopley (where the gas joints the national gas supply grid). The oil pipeline then continues eastward in the southern part of the New Forest or just south of the New Forest. It does not cross the open heathland. It crosses the Beaulieu River estuary south of Exbury and then heads for Fawley. However, it does not terminate in the Fawley Oil Refinery but proceeds beneath Southampton Water to the BP Hamble Terminal. Maps of the pipeline are available in BP publications. The pipeline is buried and would difficult to find at the surface.
There are four valve stations along the pipeline and automatic processes that will shut down the flow in case of a sudden drop of pressure (safety measures against leaks). The oil takes 25 hours to travel from Wytch Farm to the Hamble Terminal. LPG is separated at the Gathering Station and is transported by rail.
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Sale of Wytch Farm Oilfield to Perenco UK (2011)
Wytch Farm Oilfield and associated nearby fields are being sold from BP to Perenco (according to an article . See: Energy Pedia News - UK: BP agrees sale of Wytch Farm assets to Perenco. (report - 17th May 2011)
Extract:
"BP has agreed to sell its interests in the Wytch Farm, Wareham, Beacon and Kimmeridge fields to Perenco UK for up to $610m in cash. The price includes $55m contingent on Perenco's future development of the Beacon field and on oil prices in 2011-13.
The sale of these interests is part of BP's plan, announced in July 2010, to divest up to $30 billion of assets by the end of 2011. Before today's agreement, BP had already announced sales agreements totalling around $25 billion."
See also Durkin (2011b) who explains the matter more fully in the (Bournemouth) Daily Echo (available online):
"Energy giant BP has announced the sale of its Wytch Farm oilfield in a deal worth a staggering $610 million. The Purbeck plant, the largest onshore oilfield in Western Europe, has been sold to Perenco UK Ltd, the British arm of an Anglo-French independent oil and gas company with interests in 16 countries.
It is expected that all employees based at the Wytch Farm plant will continue to work for the new company.
An immediate $500m payment has already been made, with a further $55m to be paid on completion, expected at the end of 2011. The remaining $55m is contingent on submission of Perenco’s future development of the Beacon field and on oil prices in 2011-13."
"Wytch Farm, which is sited in one of the most environmentally-sensitive areas of the UK, produces around 15,000 barrels of oil a day. However, at its peak production was closer to 110,000 barrels."
END OF WYTCH FARM OILFIELD SECTION
DORSET OIL AND GAS FIELDS - OTHER THAN WYTCH FARM AND KIMMERIDGE
DORSET OIL AND GAS FIELDS -
Durlston Head Gas Field
See also:
Durlston Head Webpage.
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.
HAMPSHIRE OIL AND GAS FIELDS
LOCATIONS:
Portsdown No. 1
One of the first boreholes for oil in the region, Portsdown No. 1 in 1936 was located at what was thought to be a very promising locality. However the well was dry. It was on the Portsdown Anticline between the Wessex Basin and the Weald Basin. It was thought at the time that oil was likely to be trapped in Tertiary anticlines (as in Iran) but in fact most of the recent oil discoveries have been in fault traps beneath the sub-Albian or sub-Aptian unconformity.
LOCATIONS:
Hasley Hill, New Forest - Fordingbridge No.1 Petroleum Exploration Borehole
An exploratory borehole for oil, Fordingbridge No.1, was put down by BP (The British Petroleum Company Limiited) at the western end of Hasley Hill (map reference near SU192118), near Ogdens, in 1959. The location is at 229 feet or 69.7 metres above OD. It was a major borehole, especially for that time, extending down to 1368 metres, and penetrating the Mercia Mudstone. There must have been a large drilling rig on Hasley Hill.
Fordingbridge No.1 is the only borehole for petroleum exploration within the New Forest and was drilled at a time when planning and environmental regulations were less strict. However, there was an application for a borehole at Denny Lodge by Shell in 1982. That application was rejected. An important deep borehole for geothermal exploration at Marchwood was drilled at the former Marchwood Power Station. This also provided useful information to the oil industry.
There follows an extract from Falcon and Kent (1960), pp. 48-49, giving summary information on the well at Hasley Hill. Norman Leslie Falcon was the Chief Geologist of BP. and Dr. Percy Edward Kent (well-known as Peter Kent, F.R.S.), Geological Advisor of BP Exploration (Canada).
"MORE RECENT BORINGS IN SOUTHERN ENGLAND
...
(c) Fordingbridge, Hampshire
Lat. 500 54' 17.7"; long. 010 43' 59.6"; elevation 229 feet O.D. [69.8m.] [Mr A.P. Terris was Resident Geologist at the borehole]
The section of Fordingbridge No 1, drilled on a gravity and seismic high at Ogdens (Hasley Hill), near Fordingbridge, is still being worked out, but the following brief account may be given.
Drilling began in the Tertiary beds of the Hampshire basin; the Chalk was reached at 678 feet and. the Upper Greensand and Gault penetrated from 1997 to 2251 feet. Beneath the Gault the well passed directly into Kimmeridge Clay, shown by a core 20 feet below the top to be Lower Kimmeridge (with Aulacostephanus, etc.). Only 210 feet of Kimmeridge Clay remained beneath the Gault. The unconformity eliminates the Lower Greensand, Wealden, Purbeck, Portland and Upper Kimmeridge, measuring approximately 4000 feet in eastern Dorset (the nearest outcrops) and some 2700 feet at Portsdown on the other side of the same Tertiary basin. This represents a very considerable pre-Albian uplift.
Beneath the Kimmeridge the boring proved a normal sequence of Corallian (about 170 feet), Oxford Clay (about 430 feet), Cornbrash (35 feet) and Great Oolite Series (404 feet). The last-named was mainly limestone, with only a minor development of argillaceous beds which could be ascribed to the Fuller's Earth, in marked contrast to recent borings nearer the Dorset coast (see the account of the Radipole borings). Below, 118 feet of sandy and ferruginous limestones and ironstones are ascribed to the Inferior Oolite, and beneath this the well proved an Upper Lias development of Bridport Sands type, 250 feet thick. The remainder of the Lias was normal, except that the Middle and Lower subdivisions totalled only about 450 feet and rested upon 60 feet of beds ascribed on fossil evidence to the Rhaetic. The well was completed at 4487 feet, 106 feet into the Keuper [Mercia Mudstone]."
Borehole Succession in Listed Format:
Rotary Table (?) - 229ft. - 69.8m. ht. above O.D.
Tertiary - middle of Bracklesham Group downwards
(678ft - 206.7m. thickness)
Base of Tertiary and top of Chalk - 678ft. - 206.7m. depth.
Chalk (1319ft. - 402m. thickness)
Base of Chalk and top of UGS - 1997ft. - 608.6m. depth.
Upper Greensand & Gault
Unconformity - 2251ft. - 686m. depth.
Lower Kimmeridge Clay (210ft. - 64m. thickness)
Base of KC and top of Corallian - 2461ft. - 750m depth. depth.
Corallian (about 170ft. - 51.8m. thickness)
Base of Corallian and top of Oxford Clay - 2631ft. - 801.9m. depth.
Oxford Clay - (about 430ft. - 131m. thickness)
Base of Oxford Clay and top of Cornbrash - 3061ft. - -932.9m. depth.
Cornbrash - (35ft. - 10.7m. thickness)
Base of Cornbrash and top of Great Oolite - 3096ft. - 943.7m. depth.
Great Oolite - (mostly lst. but with some Fullers Earth) (404ft. - 123.1m. thickness)
Base of Great Oolite and top of Inferior Oolite -
3500ft. - 1066.8m. depth.
Inferior Oolite - sandy and ferruginous limestones and ironstones - (118ft. - 36m. thickness)
Base of Inferior Oolite and top of Bridport Sands 3618ft - 1102m. depth.
Upper Lias - Bridport Sands - (250ft. - 76.2m. thickness)
Base of Bridport Sands and top of Liassic Clays - 3868ft. - 1179m. depth.
Middle & Lower Liassic Clays - (450ft. - 137m. thickness)
Base of Middle & Lower Liassic Clays and top of Penarth Group (Rhaetic) - 4318ft. - 1316m. depth.
Penarth Group (Rhaetic) - (60ft. - 18.3m. thickness)
Base of Penarth Group and top of Mercia Mudstone (Keuper) - 4378ft. - 1334.4m. depth.
Mercia Mudstone (Keuper) penetrated - (60 feet - 18.3m.)
TD = actual - 4487ft - 1367.6m. (4438ft. - 1352.7m. - calculated on above figures).
LOCATIONS:
The Marchwood No.1 Geothermal Borehole, 1979-1980
The Marchwood No. 1 Geothermal Borehole was commenced on the 8th November 1979 and completed on the 27th February 1980. It preceded the other geothermal well of the area, which was Southampton No.1 Borehole. That was started on the 26th September 1981, and the well head is located in central Southampton north of the West Quay centre and is in now in the car park of the children toy store - Toys-R-Us.
Thus the Marchwood well was the first geothermal experiment in the region and a drilling rig was put in place so as to be capable of drilling to the Total Depth (T.D.) of 8580 feet (2615m.). The Marchwood and Southampton area is part of a region of high heat flow through the underlying strata. At Southampton the rate is 59 milliwatts per square metre compared to 40mW/m
2
for more northerly parts of Hampshire (Edwards and Freshney (1987). This is almost a 50% increase, and seemed worth exploiting. I was at the Geology Department of Southampton University at the time and followed, to some extent, the progress of both wells. The initial hope was that the Sherwood Sandstone reservoir, a porous red, Triassic sandstone exposed in the Sidmouth to Budleigh Salterton area, would contain hot water at about 100 degrees Centigrade. The usual geothermal gradient is about 30 degrees per kilometre, so that without considering the details, a temperature like this would be expected at round about 3 kilometres. Thus, if the Sherwood Sandstone had been near the total depth of the well, about 2600m, water with a temperature near that of boiling water might have been obtained, and this could have been useful. It was intended to use the heat produced to supplement the energy supply for the Marchwood, coal-fired Power Station, to which the well was adjacent.
However, the Marchwood Borehole, did not produce hot water at this temperature. We shall consider, below, why the temperature was lower than expected. A further complication was that a year or so after the drilling, and for other reasons, the Marchwood Power Station was demolished. Thus, there was no real use nearby for the warm water. However, in spite of the lack of commercial success with regard to geothermal energy, the borehole was certainly a successful research venture. It provided much information on potential oil reservoirs in the area, and particularly helped the understanding of the Bridport Sands, the Upper Reservoir of the Wytch Farm Oil Field.
The Sherwood Sandstone of the Trias was the selected target for hot water. This is because it was the deepest formation in the area that was expected to be sufficiently porous and permeable. This was proved to be the case. An older sandstone, the Devonian Old Red Sandstone lies lower in the borehole sequence, but it is tight, that is, non-porous and non-permeable from a practical point of view. The Sherwood Sandstone can transmit water from a considerable distance and is recharged up-dip in Devon. It should be appreciated, though, that the water is saturated brine. This is because the Trias contains substantial salt deposits in places and some evaporites were proven in the Marchwood Borehole. Even if hot water is pumped from a well it cannot be used directly but has to be directed through stainless steel heat exchangers with the aid of downhole, stainless steel pumps. The brine may also contain heavy metals in solution, and has to be discharged somewhere. The ideal is to have two adjacent boreholes, a production borehole for producing hot water and an injection borehole for disposing of used brine. There was no second borehole at Marchwood. At Southampton Geothermal Well there is only one borehole but the used brine and any associated trace element content is discharged into Southampton Water (already a polluted estuary). Thus the matter is not necessarily simple.
Note that the Sherwood Sandstone Reservoir in the Wytch Farm oilfield contains very saline water with NORM, naturally occurring radioactive material. See the section of this webpage on Sherwood Sandstone NORM at Wytch Farm. There is not necessarily a likelyhood that NORM is present so far to the east of the Wytch Farm oilfield, and, in any case, the pumping at Southampton is on a much smaller scale than at Wytch Farm.
For technical information on the monitoring and modelling of the Southampton reservoir see the four BGS reports by Barker et al. (1988; 1989a; 1989b and 1991, final report). These may not be easily obtained, but copies are present in Southampton University, Hartley Library, Cope Collection.
The British Government department, BERR, have commented that the geothermal aquifer resource within the Wessex Basin, under the Bournemouth area, appears to be the most attractive for possible future exploitation, but the commercial risks of speculative drilling remain high. At 3.5p/kWh or more, the cost of heat from the aquifer resource is still significantly higher than that from conventional industrial boilers (approximately 1.44p/kWh). See also Rollin, for more information on low temperature geothermal energy in the UK, including the Wessex Basin.
Returning to specific matters regarding the Marchwood No. 1 borehole, there was, as mentioned above, a problem regarding depth of the geothermal aquifer. The part of interest is the succession from the Lower Greensand downward. It is well-known that there is usually, in this region, a Pre-Albian unconformity at the base of the Gault, and the Basal Gault Pebble Bed is an indication of this. At the base of the Gault in the Marchwood Borehole (depth - 2240ft. - 682.8m.) , the pebble bed does not seem to have been identified (there was coring in the Gault but without recovery). Under the Gault is a very thin sequence of Lower Greensand. It is from 2240ft. to 2267ft.,only 27ft. or 8.2m (compare to the southwestern Isle of Wight where it is, at Atherfield, about 800ft. or 243m.). There is an unconformity, as is common, at the base of the Lower Greensand. It is lying directly on top of Kimmeridge Clay. The Wealden, which can reach more than 600m in the subsurface, is missing. So too is the Purbeck Formation which reaches about 120m. in Durlston Bay. The Portland Stone and Portland Sand is missing. These units can reach about 70m in total thickness. Thus if about the greatest thicknesses could have present, then the Sherwood Sandstone would have been about 790m. lower, although this would have been the extreme. If you add this to the base of the Sherwood Sandstone in Marchwood No. 1 then you obtain 2515m, just a little less than the total depth of the well. Of course the unconformity present means that some of the Kimmeridge Clay is probably missing, and in any case, the Lower Greensand is abnormally thin. A large drilling rig, shown above, was chosen and the plan for drilling the Marchwood Geothermal Borehole was appropriate to deal, if necessary, with a thick Jurassic-Cretaceous sequence including Wealden, Purbeck and Portland. This was a sensible plan, and a reasonably thick sequence was expected so that the Sherwood Sandstone would prove to be deep and hot. It was therefore a disappointment when the unconformity at the base of the LGS was discovered. It was thus possible to use the spare capacity of the drilling rig to penetrate the Devonian and to gain an understanding of the strata below the economic basement (i.e. below the base of the Permo-Trias). So that is why so much Devonian has been drilled.
LOCATION:
Southampton No.1 (Western Esplanade) Geothermal Well, 1981.
[Notes, December 2011. I have been told that the pipe and pumping equipment at the top of the Southampton No.1 well in the car park adjacent to Toys R Us seems to have been removed and the well capped. I have not yet been there to confirm this.]
Summary Well Data
(after Thomas and Holliday, (1982)):
Well Name: Southampton No.1 (Western Esplanade)
Location: Western Esplanade, Southampton
National Grid Reference: SU 41559 12018
Latitude: 50° 54' 24''
Longitude: 1° 24' 33''
Hydrocarbon Exploration Licence: Amoco/Ultramar XL 138
Client and Operator: Department of Energy and Institute of Geological Sciences (now British Geological Survey, BGS)
Drilling Contractor: Kenting Drilling Services Ltd.
Rig: Kenting No. 12.
Spud Date: 26 September 1981
TD Reached: 20 November 1981
KB Elevation: 24.92 ft. AOD
Ground Level: 10.96 ft. AOD
Terminal Depth (TD): 5994 ft. below Kelly Bushing
Hole Diameter (depth below KB)
26 inch to 165 ft.
17.5 inch to 3005 ft.
12.5 inch to 5994 ft.
Casing and completion depths below KB:
20 inch casing to 158 ft.
13 and 3/8 inch casing to 2995 ft.
9 and 5/8 inch liner was hung from 2826 ft. to 5635 ft.
Cores - six cores were taken with total core length of 226 ft. See Thomas and Holliday (1982) for further details of coring.
Wireline logging: Schlumberger Inland Services Inc., Tessel Well Services Ltd.
Mud Logging: Exploration Logging (UK) Ltd.
Drilling Superintendent: Mr. D. McIntyre, Drilcon Ltd.
In addition to the Marchwood Borehole, there is another deep borehole nearby which provides useful information on the geological succession and lithologies. It has provided information relevant to petroleum exploration in addition to its primary purpose in providing geothermal energy and data on geothermal prospects. This well is the Southampton No.1 Geothermal Borehole. It was drilled the year after the Marchwood No.1 Geothermal Well so that this was the second borehole in the area for geothermal energy. The location is Western Esplanade and the well-head is easily seen by parking in the car park of Toys-R-Us and Babies-R-Us, northwest of the West Quay Centre, Southampton.
As a geothermal well it is unusual in that there is only one well, a producing or extraction well, and no well to re-inject the brine from the Sherwood Sandstone, with its limited content of undesirable trace elements. The output fluid is discharged into Southampton Water. At the time when the well was drilled it was not thought that any possible polution involved was important in an estuary already much affected by sewage and industrial pollution. A more modern procedure is to re-inject the output fluid into another well, an injection well and return it to the deep natural reservoir. This is the normal procedure in the case of the geothermal wells of the Paris region. Sherwood Sandstone fluids are well-known from the Wytch Farm Oilfield, and contain various trace elements. These are not normally allowed to enter Poole Harbour (see also Wytch Farm section regarding NORM).
In many respects Southampton No.1 proved to be similar to Marchwood No.1 and most of the formations are similar and at similar depths. A slight difference is the occurrence of Portland Group beneath the sub-Lower Greensand unconformity. At the time it was hoped that, a sub-Albian fault occurring between the two sites might have led to a significantly deeper level for the Sherwood Sandstone at Southampton, and, consequently, higher temperatures. However, in fact, the situation proved to be similar to that at Marchwood and the downhole brine temperature was at about 76 degrees C. A better figure near 100 degrees might have been obtained had the Cretaceous sequence been without major unconformity and was as thick as on the southern Isle of Wight and at Swanage Bay, Dorset (i.e. if, therefore, the Sherwood Sandstone was about another half kilometre or one kilometre lower). The hot rather than boiling temperature was accepted as satisfactory, and this time, unlike Marchwood, the wellbore was not
continued deep into the Devonian (Old Red Sandstone) and merely went into the top part of it before TD'ing.
In spite of the moderate, rather than high temperature, the hot brine could be used at Southampton to provide geothermal energery for central heating. The heat has been used to some extent for local buildings, such as Southampton Civic Centre, West Quay Centre and the Royal South Hants Hospital. The brine is brought to the surface by stainless steel, downhole electric pumps. It is not only saturated for sodium chloride and therefore corrosive, but it also contains very small quantities of undesirable trace elements, heavy metals. The brine is thus not usable directly for central heating but is processed through stainless-steel heat-exchangers before being discharged into Southampton Water (ideally there should be another borehole into which the used brine is discharged downwards, but there is not another borehole at Southampton). Thus, the hot brine can usefully provide geothermal energy, but it is a complex matter and is not cheap or easily. In practice the geothermal energy is used in combination with heat from fossil fuel sources at the power station near the West Quay Centre (note the conspicuously red building).
Pumping is from a downhole turbo pump at about 600m., and at about the level of the Gault and the Lower Greensand. The geothermal aquifer is the Sherwood Sandstone which is much deeper ranging from 1729m. to 1767m. Thus the brine is rising under hydrostatic pressure for about two/thirds of the height, and is pumped up for the last part.
The pumping scheme is explained in basic form in a large colour poster, kindly provided by Utilicom and Southampton Corporation. At the well head, shown above, there is an electrical pump which forces a certain proportion of the brine back down a central pipe so as to provide motive power for the downhole turbo pump. This long, narrow, fluid-driven turbine is fairly deep in the borehole and it pumps the hot brine up an outer concentric pipe to the surface (together with the recycled brine used for the power to drive it). All the equipment has to be stainless steel to avoid corrosion from the brine.
LOCATION:
Lee-on-the-Solent, Hampshire
A borehole at Lee-on-the-Solent was drilled in 1984 by the North Sea Sun Oil Company, to a TD of 6091 feet (1857 metres). It ended in Devonian Sandstone under rather thin Sherwood Sandstone. The Tertiary sequence, the Chalk and the Upper Greensand and Gault are of usual characteristics and not remarkable. An interesting aspect is that the main unconformity is not just beneath the Gault but beneath the Carstone of the Lower Greensand. The unconformity is as 2191 feet (668 metres).
The Carstone is directly on an almost complete Purbeck sequence but, unexpectly, with no Wealden. The Purbeck is 166 ft. thick (51 metres), in other words rather thin. It seems of "basinal" (Durlston Bay) type with regard to the Lower Purbeck and with anhydrite well-developed. The Caps are not conspicuous but there is some slightly dolomitic limestone in the basal anhydrite strata. The Middle Purbecks contain small ("juvenile") gastropods.
There is no well-developed Portland Stone. The Portland Sand is glauconitic to some extent, as is usual, but has some limestone in the upper part. The Kimmeridge Clay is from 2819 ft down to 3608 (789 feet or 240 metres, a moderate thickness for this formation). The Corallian is of normal type with some ooids. The Oxford Clay is of normal facies. There is some thin Cornbrash Limestone. The Forest Marble includes mudstone.
The Great Oolite is a fairly thick (4809-4990 ft.) well-developed fossiliferous limestone with some ooids. It is in the facies characteristic of the Western Weald oilfields (and unlike Dorset). The Fullers Earth (s.l.) is calcareous in the upper part, but is the usual mudstone in the lower part. The Inferior Oolite is finely crystalline, micro-sucrosic with some vuggy porosity in parts. Unusually it has some chert. There is no mention of berthierine but some yellow staining is referred to with regard to the lower part.
The Bridport Sands are fine-grained with some subordinate claystone in part. The Lias, in general, has much limestone alternating with mudstones. It seems to be more calcareous than in Dorset. The Penarth Group is in normal Dorset facies with limestones above mudstones.
The Mercia Mudstone is thick with much sandstone in addition to mudstone. It is anhydritic towards the base. The Sherwood Sandstone is from 6532 feet (1990 metres) to 6609 feet (2014 metres), only 24 metres thick. It was recorded as:
"Sand/Sandstone, mottled, off white/red-brown, friable, poorly sorted, generally medium to coarse-grained (though locally fine-grained), translucent/ smokey, angular and sub-angular quartz, with a ferruginous/argillaceous matrix and/or calcite/silica cements; and interbedded with minor argillaceous and micaceous siltstone."
This does not seem to be a thick and good quality reservoir as at Wytch Farm. It was noted that in the testing summary:
"Recovered fluid indicative of formation water with no evidence of hydrocarbons. Zone abandoned." There seem to be no mentions of oil shows or gas shows in the well.
LOCATIONS:
Lyndhurst - A, Formerly-Proposed, Petroleum Exploration Borehole, Denny Inclosure
Oil exploration has taken place in the New Forest and, as noted above, BP has drilled a well at Hasley Hill in 1959 (Fordingbridge No. 1). It proved to be dry, however, and the interest in oil exploration moved to Dorset, particularly Wytch Farm in the 1970s. Shell UK Ltd. acquired the licence blocks for much of the New Forest. Block PL.181 from Lyndhurst eastward was surveyed seismically using the White Lias, the top of the Penarth Group (Rhaetic) as a good reflector. Look at the map above and you will see that the best high area that seems to have closure (no lateral escape) is where the 4400 and 4300ft contours loop round adjacent to south of a fault. This type of half-dome is very promising for trapping upward-moving oil and gas. It is situated exactly where it would not be welcomed - under the Denny Wood Nature Reserve!
Of course, there was controversy regarding an application to drill in Denny Inclosure, even though the Hasley Hill well seems to have gone through without much argument. There was a Public Inquiry and no permission for drilling was granted. A result of the Public Inquiry is that some interesting information of the subsurface became available at the meeting (see above).
Shown above are maps and a cross-section through the strata beneath Denny Inclosure. The proposed trajectory of the Lyndhurst - A Borehole is shown, on the basis of a diagram on a handout sheet provided by Shell UK Ltd at the time of the Public Inquiry. The diagram has been redrawn in colour and labelled with the probable stratal units that are present. The drawing shows the possibility of oil (and gas?) in all the three main reservoirs of the area. However, it is not know whether anything worthwhile is actually present in any or all of the reservoirs, and the matter remains an interesting open question.
Of the reservoirs, the Sherwood Sandstone Reservoir (Trias) is the important reservoir at Wytch Farm and it the reason that the field is one of the largest onshore oil fields in NW Europe (with nearly 500 million barrels reserves). The Sherwood Reservoir, however, requires that special migrations conditions have enabled hydrocarbons to reach the sandstone, and this may or may not have happened in the New Forest area. The Bridport Sands reservoir is the original reservoir of Wytch Farm and is largely operated by water injection to increase the reservoir pressure. The Great Oolite and possibly adjacent Cornbrash reservoirs are those that have proved successful in the eastern Weald area, as at Humbley Grove, Horndean, Singleton etc. It is not known, of course, whether there is any oil or gas in these potential reservoirs at Denny Inclosure. Conditions might have been unfavourable for migration into the area in Cretaceous time (the general migration date for the region was Cretaceous). The extensional fault which provides closure on the north side might not necessarily be sealing (i.e. oil could have escaped upwards in the Cretaceous). The oil industry probably knows much more now about the likelyhood of oil reserves in this New Forest area, although different companies and different geologists may have varying views. Porosity and permeablility of the reservoirs are interesting aspects that students might wish to consider. Fortunately there is much detailed information on Marchwood No.1, the important borehole at Marchwood Power Station. The log of this should be examined in detail, and any information of Fordingbridge No. 1 and Lockerly No. 1 would be useful. Particular aspects to consider would be the marginal facies changes in the Trias towards the southeast, where it thins out, and the improvement in the reservoir characteristics of the Great Oolite in the same direction.
At the time when Lyndhurst - A was planned deviated wells could be drilled but not laterally from any great distance. Now the new Extended Reach Technology, used so successfully at Wytch Farm to obtain oil at the Goathorn Well from the vicinity of Bournemouth Pier means that most of the New Forest can now be drilled from drilling rigs situated beyond its official limits, or from urban areas within the forest region. If it was thought worth-while and cost-effective the Denny Inclosure prospect could now be drilled by extended reach (i.e. obliquely) from a large rig at Lyndhurst, Ashurst or Marchwood.
(This might be a good topic for student discussion and projects on the future of the New Forest National Park in terms of petroleum geology and petroleum industry. The New Forest has been greatly affected by the petroleum industry in various ways, and even the oil from Wytch Farm passes through it by buried pipeline.)
LOCATION:
Hurst Castle Prospect
(Lymington and New Milton, Hampshire - Petroleum Exploration License - PDL089 - Wessex Exploration Ltd.)
Part of the New Forest region near Lymington has been under investigation for petroleum resources by Wessex Exploration Ltd. Details will be found by going to their website at:
Wessex Exploration Limited (not necessarily available now).
Here is a small extract from the start of the webpage to draw attention to their work. Particularly see the good maps. Note the information on the Hurst Castle Prospect.
"Wessex Exploration Limited, bidding on its own in the 9th Landward Bid Round was awarded Petroleum Exploration and Development License (PEDL) 089 on 4 September, 2000. PEDL 089 is located in southern Hampshire near the towns of Lymington and New Milton, on the mainland opposite the western end of the Isle of Wight. The work obligation for the initial term of the PEDL was met when Northern Petroleum drilled the Bouldnor Copse 1 well, and fifty percent of the PEDL was relinquished in September, 2006. The PEDL is now in its second exploration period.
Wessex on 11 September, 2002 made an "Out of Round" application for a Petroleum Production License over the area immediately offshore from and adjacent to PEDL 089. Wessex was awarded License P1153 over this offshore area, effective 3 October, 2003. The primary term of this license expired in October, 2007, but was renewed by DBERR into a second exploration period.
A preliminary structural map of the Hurst Castle Prospect at the Sherwood Sandstone level is shown. Estimated P10 oil-in-place is of the order of 190 million barrels for the Sherwood reservoir alone, with possible recoverable reserve of 36 million barrels. A separate structural map shows four-way dip closure offshore, with possible P50 recoverable reserves in the 16 million barrel range." ... [continues]
See further details of the
Hurst Castle Prospect.
They report that the Hurst Castle Prospect has potential recoverable reserves of the order of 36 million barrels from the Sherwood reservoir, in a Wytch Farm type fault-block feature. The primary reservoir is the Triassic Sherwood Sandstone. The Bridport Sands is a secondary objective as is the Frome Limestone (Great Oolite). In the same region an exploratory well was drilled at Boulder Copse across the West Solent on the Isle of Wight. The results were negative but Wessex Petroleum did not consider that this impacts on the prospectivity of the Hurst Castle objective.
Note that it is of interest regarding the New Forest National Park that the reservoir objectives are the same as those of Shell at Denny Inclosure (Lyndhurst - A).
Note that a Sherwood Sandstone Reservoir complication regarding NORM, naturally occurring radioactive material, exists and is successful dealt with at Wytch Farm. This does not mean that it necessarily exists in the Hurst Prospect, but its relative proximity to Wytch Farm would probably require it to be investigated. See the section, above, on NORM in the Sherwood Sandstone of the Wytch Farm Oilfield. Because the NORM precipitates are shielded by steel pipes they are not generally any risk to the public.
Another comparison can be made with Wytch Farm regarding water pumping. Water injection, if needed, could require a large source of water (fortunately the sea is not far away). With regard to pumping, in the later stages of oil production from the Sherwood Sandstone at Wytch Farm there has been a 95% water-cut (in other words almost 20 times as much water as oil is pumped). Thus disposal methods for surplus water might also have to be considered. However, these are later issues, only of any relevance if substantial oil is discovered.
LOCATION:
Hoe Lane, North Baddesley, Hampshire.
(Hoe No.1, Hoe Lane, North Baddesley, near Southampton and Romsey)
(If interested, compare to
- drilling in the 1920s.)
ISLE OF WIGHT OIL AND GAS FIELDS
PETROLEUM GEOLOGY - ISLE OF WIGHT
The Arreton Borehole (old studies)
Several exploratory boreholes for oil have been drilled in the Isle of Wight. Before 1960 the exploration commenced at Arreton (Falcon and Kent, 1960). The structure was a closed, east-west elongate dome south of the Chalk Ridge. The strata at the surface is Lower Greensand and the borehole extended down to the Inferior Oolite at 5161 feet (1573 metres). This is not as deep as most modern petroleum boreholes in the region, and therefore did not test Bridport Sands or Sherwood Sandstone (the reservoirs at Wytch Farm).
The first exploratory wells in the Wessex region in 1936 in obvious anticlines at Portsdown and Poxwell were disappointing and proved dry. Unlike Iran, where the geologists had been exploring, there was no significant oil in these anticlines. None was found in other similar folds in the area. Already, in 1960,
Falcon and Kent (1960), both very notable petroleum geologists, realised that most of the Tertiary structures investigated (Poxwell Pericline, Portsdown Anticline, Chaldon Anticline etc) had developed too late to act as traps for oil migrating towards the edge of the Wessex Basin. Without enormous quantity of seismic data that exists today they did not have a good knowledge of the Cretaceous structure at depth, and the English Channel Inversion theory so familiar now, was not well-understood at the time (although intra-Cretaceous tectonism had been recognised since well before the 1930s).
Of course, it well-known now that oil did migrate towards the margin of the English Channel (Wessex in part) Basin. The Wytch Farm Oilfield is proof of this. Similarly oil migrated towards the margins in the Weald Basin (or Inversion). The Middle Jurassic oilfields of West Sussex and East Hampshire are proof of this. Now it is known that the migration was Cretaceous in age and was into Cretaceous fault traps (the intra-Cretaceous or Late Cimmerian structure are largely extensional faults, unlike the compressional folds of the Tertiary). Thus, although the Arreton Borehole was not targeted at Cretaceous traps, it was planned in the 1950s to find oil that had migrated to the edge of the basin. The general theory was correct but the exact location was not. Oil had not accumulated in the Corallian (as seen in small quantities at Osmington Mills) or the Cornbrash (as at Kimmeridge), and nothing seems to have been found in Middle Jurassic limestones, the reservoirs of the West Sussex and East Hampshire fields. These Middle Jurassic limestones were found to be more muddy and less suitable as reservoirs than in Portsdown and Henfield (Weald Basin).
Of general geological interest, it was noted that the Wealden had a thickness of 2010 feet (612 metres) and thus was like the thick succession at Swanage, Dorset. Only the upper part of the Wealden is exposed in the Isle of Wight cliff sections. The Purbeck was found to be 343 feet (104 metres), only a little thinner than in the type section at Durlston Bay. Data for depth of formation tops will found in
Falcon and Kent (1960), Table 1. There is not a detailed log in this report though.
PETROLEUM GEOLOGY - Isle of Wight
Recent Exploratory Boreholes
(See Also - Sandhill 2 Borehole, below)
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.
PETROLEUM GEOLOGY - ISLE OF WIGHT (continued):
Sandhills - 2 Borehole
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.
ISLE OF WIGHT
The English Channel Inversion Structure
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.
ENGLISH CHANNEL OFFSHORE OIL AND GAS EXPLORATION
.
.
WESTERN WEALD (WEST SUSSEX - EAST HAMPSHIRE) OIL FIELDS
Thickness of Great Oolite and Oil Well Locations
The White Carbonate Shoal Sands (Middle Jurassic, Western Weald)
Many factors are involved in the presence of oil fields in the western Weald area of West Sussex and East Hampshire. One of these factors is the existence of a good reservoir rock. This is the Great Oolite of the Middle Jurassic, the major oolitic limestone of the Cotswold and Bath area. The distribution of the white sands of the ooid shoal in the warm blue water of about 165 million years ago is shown above in a modified version of a palaeogeographic map by Sellwood et al. (1989).
(If it existed today it would be a holiday resort, but rather dangerous with ichthyosaurs and plesiosaurs in addition to sharks and crocodiles.) There are obvious similarities to the modern Florida - Bahamas area (also to the southern Arabian Gulf but that is too arid for good comparison). At this time, of course, the southern British region was still attached to America, and was part of Pangaea (stretched, pulled apart a little and soon to open!).
WESTERN WEALD OILFIELDS
Humbley Grove Oilfield, Hampshire
Humbly Grove Oilfied is near Lasham, Hampshire, 4 km. north of Alton, on Production License PL116B. The discovery well was drilled by Carless in 1980. Oil and gas was found in the Bathonian Great Oolite limestones. Gas was discovered in a Penarth Group (Triassic) reservoir (depth to crest - 1292 metres) and this was also found to contain some oil (Gluyas and Hichen, 2003).
Development commenced in 1985. Thirteen deviated wells were drilled from three sites between March 1985 and May 1986 and a gathering centre constructed. An estimate of stock tank oil initially in place (STOIIP) is 43 million stock tank barrels (MMSTB). Cumulative oil production to 2003 was 5.7 MMSTB oil and 6 BCF gas.
The depth to the crest of the Great Oolite reservoir is 981 metres. Oolitic and pelletoidal grainstones, particularly in the Humbly Grove and Herriard Member of the Great Oolite are the main producing horizons. The Hoddington Member is a vertical permeability barrier.
There are some hydrocarbons in the Cornbrash and Forest Marble. In the Great Oolite oilfields of the Western Weald it is Storrington and Humbly Grove that have high permeabilities (Gluyas and Hichen, 2003).
In 1995 the oil field was developed into an underground gas storage facility, with a gas pipeline linking it to the national gas grid at Barton Stacey. The replenished gas cap on the oil field increased the pressure on the remaining oil, boosting production and increasing the lifetime of the extraction.
For more information and photographs see the:
StarEnergy Website - Humbly Grove. "We have already built a successful 10-billion cubic feet gas storage facility at our Humbly Grove oilfield near Alton in Hampshire. The store operates by taking natural gas from the national transmission system (NTS), operated by National Grid Gas plc, at Barton Stacey through a 17-mile pipeline to the Humbly Grove oilfield. The gas is then pumped into the reservoir for storage. During periods of high demand, the gas is returned to the NTS after processing.
The Humbly Grove gas store, which started operations in November 2005, is also re-pressurising the oil reservoir and so extending the remaining life of the field from less than 10 years to now around 20 years."
(Listed in Oil-South-of-England Bibliography as:
StarEnergy (2011).)
WESTERN WEALD (SOUTH DOWNS) OILFIELDS
Singleton Oilfield, Sussex
Summary Data
(mostly after Trueman (2003))
Location: Near village of Singleton, West Sussex, north of Chichester. Installation is in a forest north of Singleton.
Discovered: 1989 by Regneneses Group.
Structure: E-W elongated, faulted horst. Effectively a continuation of the Horndean Trend.
Depth to crest: 1234 metres (4050ft.) for northern, 1271 metres (4170 ft.) for southern.
Reservoir: Great Oolite, Forest Marble and Cornbrash. Great Oolite (Bathonian) is a series of prograding oolite shoals deposited on a broad carbonate ramp.
Production Licence PL 240.
Porosity and permeability is low. Porosity average: 13%; permeability average: 0.5mD (milliDarcies). Porosity is high below the Hoddington Member (i.e. Humbly Grove and Hesters Copse Members)
STOIIP - Stock Tank Oil Initially in Place. (Great Oolite Reservoir) estimate: 70 MMSTB (70 million stock tank barrels in place - not all recoverable). (Compare with STOIIP for Stockbridge, the large field, - 171 MMSTB).
Production: Was 450 barrels of oil per day, now about 600.
Cumulative production to 2003: 2.85 MMSTB, million barrels of stock tank oil (i.e. 2.85 million barrels of oil). Gas content uncertain.
Oil density: 39.15 degrees API.
Formation water - brine about half full halite saturation level - 150 parts per thousand (seawater is about 35 ppt).
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General Notes on the Oil Field and the Environment
:
The Singleton Oilfield (map ref. SU 884154) is inconspicuously situated in Singleton Forest about 10km. north of Chichester, Sussex. It is within the picturesque Chalk hills and forests of the South Downs. It is near the A286, the Midhurst road, north of Chichester, and about 2km. north of the village of Singleton. You cannot drive to the well site but it is not far from a public footpath in the woods. The site is fairly small, well-fenced and guarded and effectively hidden in the woods. Little can be seen from the outside.
The beam pumps are painted a dull, vegetation colour. There is control over noise to a limit of 33 decibels; the trees help reduce noise, particularly when they are in leaf. It is, thus, relatively inoccuous from an environmental point of view and has hardly any visual impact.
The oilfield is one of several in the West Weald, that is the West Sussex and East Hampshire region. It is within onshore licence PL 240 and is largely owned by Providence and it is operated by Star Energy Group PLC (STAR). The field has been producing since 1986. Estimates of STOIIP [oil in place] in the Great Oolite reservoir are approximately 70 MMSTB [million barrels]. It is trucked to the Holybourne Rail terminal for sale (Providence Oil Company, October, 2007 - Singleton Oil Field Acquisition Effective).
The site, has at present six wells, including five beam pumps or pumpjacks (nodding donkeys) and a jet pump well. A few years ago the site was producing about 450 barrels of oil per day Anonymous (2008), but the figure is now higher and at about 600 BOPD (Providence Oil Company). Planned expansion will increase production to between 850 to 1,000 barrels per day. Providence Energy will be increasing the one-hectare site by 0.3 hectares and installing four new jet pumps. Singleton Oilfield has been in operation since 1990 and has expanded several times over the past 18 years. It has planning permission to drill for oil until 2016. For more information on ownership see Providence (2007).
It should be noted that Star Energy Group PLC (STAR) are also involved with Providence in Natural Gas production at the Singleton Oilfield.
The main reservoir rock at Singleton, as in adjacent areas of the Western Weald (Sussex-Hampshire border) is the Great Oolite, although the limestones of the Forest Marble and Cornbrash above are also in use.
The field consists of two east west trending elongate horst blocks divided by a narrow graben (Providence Oil Company, October, 2007 - Singleton Oil Field Acquisition Effective). The wells
produce from the Great Oolite formation, consisting predominantly of oolitic lime grainstones deposited during transgressive/regressive episodes and has been sub-divided into three members. Note that there can on occasions be some problems with terminology, in that the Great Oolite Limestone Formation was considered by Sellwood et al. (1985) to be part of a larger Great Oolite Group which also includes the Forest Marble. It seems here that from the reference to three members, the scheme of Sellwood et al is being used, and that the members are the Humbly Grove Member (at the base), the Hoddington Member and Herriard Member. The Forest Marble lies above the Herriard Member, and consists, in the region, of skeletal oolitic packstones and grainstones with interbedded claystones (mudstones). It tends to be more skeletal than the Great Oolite Formation beneath, and has a typical Forest Marble fauna, including crinoidal debris.
An isopach (thickness) map of the Great Oolite Limestone Formation (i.e. Great Oolite sensu stricto) has been provided by Sellwood et al. (1985). He gave the Great Oolite Limestone Formation as between 60 and 75m. thick in the area of Singleton and Horndean. The Singleton X-9 well was drilled in this main Great Oolite reservoir in the southwest flank of the field. However the Singleton X-8x well is situated on the northern flank of the field and a 2,000 foot (609m.) horizontal section was drilled and completed in the Cornbrash and Forest Marble sequences. Wells in the area have similar oil water contacts within the Great Oolite
reservoir, indicating that the prospects may be connected Providence (October, 2007) have provided a map of the oilfield showing that the wells are greatly deviated to the south, the southwest and to the southeast for two or three kilometres. Some of these pass close to the village of Singleton, which is south of the well site. The map suggests that the location for the facilities could have been at the village, or on open fields south of its present site, but an environmentally concealed place in woodland was chosen.
In the long term future the oilfield could be used for gas storage (Providence, October, 2007). This means that it is not a temporary operation but it likely to be an almost permanent feature, at least in some form.
WESTERN WEALD (SOUTH DOWNS)
Markwells Wood No.1, Sussex
Location:
Location: West Sussex, near the Hampshire Boundary. Map reference SU758133. Six kilometres east of Horndean (and about 4km ENE of the Horndean Oilfield). North of Emsworth. Nearest village West Mardens. In the southeastern part of Markwells Wood, near Forestside and Northwood Farm.
Summary Data:
Well completed: December 2010 or early January 2011.
Reservoir: Great Oolite - vertical thickness 44.5 m (thinner than expected - 73m.)
Reservoir porosity: 12 to 13 percent, normal for Great Oolite in this region.
Great Oolite Reservoir
This seems to be a small to medium oilfield in the Great Oolite Reservoir and in some respects rather like Singleton, but at an early stage of development. There has been several articles in the press regarding this in January 2011, at a time when the first borehole has been completed and the drilling rig removed. The notes below come mainly from press reports. The depth to the oil-water contact is similar and the reservoir, the Great Oolite, is the same. The first well has been drilled by Northern Petroleum. Live oil has been found in a cored portion of the reservoir. Apparently the mean potential is between 35 million barrels of oil in place and 61 million barrels of oil in place (compare to Singleton with 70 million barrels of oil in place). However full information is not yet available and a programme of testing for production is to take place.
The well is interesting from more than one aspect. It is a very deviated well, at 56 degrees according to Stockopedia. This means the location at which the oil reservoir was drilled into is not under the well site at Markwells Wood and may be about about a kilometre or a mile away. Because the well was very oblique to the reservoir the thickness recorded in the borehole is much greater than the true vertical thickness. The gross hydrocarbon-bearing interval intersected in this sloping borehole was 83.8 metres (275 ft). However the calculated true thickness is only 44.5 m (146 ft), compared to an expected estimate of 73 m (240 ft). The original estimate would have been compatible with Horndean Oilfield, not far away. See also the isopach map of
Sellwood et al. (1985). It is not clear as to why the Great Oolite is apparently thinner in the Markwells Wood borehole than in adjacent areas. This might be the result of local faulting, possibly the intersection of a fault by the borehole, or perhaps, less likely, the presence of a local penecontemporaneous high in Middle Jurassic times. No doubt more information will be available on this later.
Although the reservoir thickness is different, the porosity is of about the same as for other oilfields in the region in the same reservoir Stockopedia.
Environmental Aspects
Because this well is in the South Downs National Park, there are environmental issues. At present, as shown by photographs included here it is very inconspicuous, and at the present (January 2011) there is just a square area of concrete or tarmac with a single well-head in the centre, and one or two huts. There is no direct public access from the road. However, if you go to the nearby village of West Mardon and walk westward through the woods on the footpaths you can probably find it within about an hour. Unless there is a drilling rig on site, you will not see it until you are within about 100 or 200 yards.
WESTERN WEALD OIL AND GAS FIELDS:
Storrington (north of Bognor)
(Details to be added)
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CENTRAL ENGLISH CHANNEL (Near the English Coast)
26th Seaward Round - Offshore Oil and Gas Licensing - May 2011
(Future Plans for Seaward Drilling in Dorset, Hampshire and the Isle of Wight.)
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 (another four years) if the agreed Work Programme has been completed and if 50% of the acreage has been relinquished. In other words there has to be some success with oil discovery and production within four years or the company looses the licence for that block.
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, but includes specific provision for DEC to waive the commitment in the light of further technical information.
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.
The following notes on drilling (p. 6) are important:
"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."
Notes - Speculation on Future Drilling - Comment
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 (and discussed above). A very large drilling rig is needed and is used with both top drive and a steerable and driven bit. This has allowed drilling to about 10 and a half kilometres eastward into Poole Bay. Perhaps, and I am unaware of the detailed plans, similar boreholes could give access to large parts of the blocks south of Swanage to Weymouth (97/14, 97/15 and 97/11). Some blocks could be reached by similar drilling from the Isle of Wight or Hampshire mainland (98/13, 98/14, 98/08, 98/07b). For the Hurst Castle Prospect in 98/08 there has already been discussion of ERD drilling from Downton. ERD boreholes from the land of the Dorset coast would, perhaps, be most likely, although not necessarily, to take place from sites where there have already been drilling rigs, albeit smaller ones. These include NE Portland (drilling to explore gas storage prospects), Kimmeridge (several boreholes for oil), Swanworth Quarry near Worth Matravers (borehole to investigate the Kimmeridge Clay), and a quarry at Swanage (a borehole was drilled south into the gas field at Durlston Head by BP). There may be other possible sites around the Weymouth area, but Lulworth Cove, Durdle Door to White Nothe may be less likely and presumably drilling would be unlikely within the Army Firing Ranges near Lulworth.
Relatively small oilfields on land such as those in the Sussex, Hampshire and Wiltshire South Downs and Western Weald area, tend to use smaller drilling rigs which do not stay on site for long, and are subsequently replaced by low, inconspicuous pumps. This is also true from much of the Wytch Farm oilfield that is producing from the Upper Reservoir, the Bridport Sands. The only drilling rig which has been semi-permanent for years is the very large, ERD rig at the Goathorn Peninsula. Photographs of this interesting feature are shown above. Extended reach drilling southward from the Isle of Purbeck, southward from the Kimmeridge or Weymouth area, or eastward from Portland is presumably likely to involve similar large and semi-permanent rigs. I have no specific knowledge on this and might be wrong; however, I am merely speculating that there might be similarity to the notable example at Wytch Farm.
Reservoir rock targets are unknown, but a reasonable similarity to nearby Wytch Farm would be expected. Thus the upper reservoir, the Bridport Sands would no doubt be examined. However, the major and more important potential reservoir is the Sherwood Sandstone which is deeper and less well-known except at the Wytch Farm oilfield. There are some other minor potential reservoirs. The Middle Jurassic Great Oolite (the reservoir of the western Weald oilfields is probably not of major importance here, because of a lateral facies change.)
Problems of migration and traps are fairly complicated and not discussed here. The Wytch Farm oil has migrated largely in the Cretaceous and remigrated to some extent during the Tertiary. The mapped seafloor faults south of Swanage to Weymouth tend to be N-S, unlike the more E-W structures of Wytch Farm and the Sussex oilfields.
NEARSHORE OIL EXPLORATION BLOCKS, 26th ROUND;
Release of Offers in January 2012.
OIL SHALE - FUTURE ECONOMIC POTENTIAL?
