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Romsey, Hampshire
and Visiting Scientist at:
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..| Home and List of Webpages |The Petroleum Geology of the Portland - Isle of Wight Offshore Basin |Petroleum Geology - Balcombe and the Central Weald |Bibliography of Petroleum Geology of Southern England |Kimmeridge Bay | Kimmeridge - Oil Shale |Kimmeridge Bibliography. |Lyme Regis |West Bay, Bridport| Osmington - Oil Sand | Durlston Head Geology. |Stair Hole, Lulworth |Poxwell Quarry.
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.
The Wealden sands and the Purbeck Formation are most notable for oil indications in the Lulworth Cove area. Good oil sands can found in a faulted block of Wealden near the base of the formation. It is easy to find and it is quite near small faults near the Purbeck-Wealden junction. A photograph above shows an oil geologist from PGS Reservoir Ltd. holding a sample. Lees and Cox, mentioned below, found good oil indications at the cliff top just to the west of Stair Hole. However oil sand is very easily seen (and smelt) in Stair Hole. Traces of oil are also present in Purbeck Marble, a gastropod limestone at the top of the Purbeck Formation. They are generally not present in the Lower Purbeck, but are mostly in proximity to the Purbeck-Wealden Boundary, both here and at Mupe Bay. It was back in 1937 that geologists of the D'Arcy Exploration Company, later to become part of BP, investigated the Dorset coast in a search for oil (Lees and Cox, 1937). They said " The Purbeck Beds at their outcrops on the Dorset coast show a number of impressive oil indications; these include bituminous limestone, as at Peveril Point, and fractured limestones at Mupe Bay, Lulworth Cove, and Durdle Door which show oil residues on the faces of cracks and which ooze a little soft bitumen in warm weather. " Their plate 11 has an arrow pointing to this part of Stair Hole. Here in a crack or joint of a block of Purbeck limestone is one of these oil residues of the type which started the oil exploration in this region.
Oil was discovered in the Bencliff Grit at Osmington Mills in 1937 by Mr. Taitt, a geologist of the D'Arcy Exploration Company, Ltd., a subsidiary of the Anglo-Iranian Oil Company, Ltd. The geologist was said to have discovered this when back on leave from his work in Iran, the major region for British oil supplies at that time (which were produced by the Anglo-Iranian or British Persian Oil Company, i.e. predecessor of BP). The exploration was much influenced by the knowledge of the Iranian (Persian) oil traps. The following is from Lees and Cox (1937): "The presence of a rich impregnation of oil in the Corallian Bencliff Grits outcropping near Osmington Mills in Dorset was recently discovered by Mr. A. H. Taitt. A specimen of the sand from the exposed cliff face has given 12.1 per cent of oil, and where these beds pass below high-tide level at Bran Point there is a small but active seepage of free oil. This seepage is, so far as we are aware, a new discovery, and its existence does not appear to be known even to the local inhabitants."
The Bencliff Grit east of Osmington Mills contains oil-sand and sometimes some surface oil seepage. It is best seen near the western end of the main cliff section. The field leader may wish to pass round one or two dark brown specimens from this Bencliff Grit exposure, to avoid excessive hammering by the party. Notice that the oil sand smells strongly of oil. The oil sand is not at the top of a sandstone reservoir as is usual underground, but instead it is here above the water-table (i.e. in the vadose zone). It seeps downward and is usually concentrated above clay laminae. In some weather conditions there is some small seepage in the form of small trickles from the cliff, as shown on the right, but this surface appearance varies from time to time. Recognise the oil sand by a dark brown colour and any surface oil seepage. Try breaking of a piece and smelling it; it has a strong bituminous smell.
Further east at the eastern end of the outcrop in the cliffs of the Bencliff Grit, just after it has disappeared under a ledge of the next limestone in the sea there is a small active oil seep, although it is often difficult to see. Bubbles of oil emerge in shallow water at intervals, probably rising from joints in the limestone, and the oil disperses as a film on the surface. To observe this it is best to look when there are quiet water conditions and a tide rising from low water. This small but interesting feature is a classic aspect of Dorset geology because the finding of this in the 1930s was one of the reasons for commencement of oil-exploration in Dorset. That led eventually to the discovery of the great Wytch Farm oil-field, containing almost 500 million barrels of recoverable oil.
KIMMERIDGE CLAY - OIL SOURCE ROCK
Production of Shale Oil and Gas in the 1890s
The Kimmeridge clay is famous as the major oil source rock in the North Sea. It is deeply buried there and thermally mature. In Dorset it is not mature and is not gnerally the source rock for the local oil (it was not buried deeply enough during Cretaceous times). The deeper Liassic bituminous shales, in the English Channel Basin to the south, are usually regarded as the source rock for this region. Nevertheless the Kimmeridge Clay has been worked by an oil company back in the 1890s, for production of shale oil (particularly paraffin) by pyrolysis, and for gas and other products. It is still a potential source of shale oil but has a very high sulphur content, actually containing pyrite nodules and when burnt produces particularly unpleasant sulphurous and oily fumes. It has been investigated by Gallois in terms of reserves. It occurs as a relatively thin bed, but over a very large area. It seems very unlikely that the Victorian mining and pyrolysis will recommence in the near future; however, it might be necessary to use it at some time in the distant future when oil resources are very limited. Production of shale oil from oil shale is sometimes regarded as a promising new process. In fact, it is a very old procedure, and the Kimmeridge Oil and Carbon Company was doing this in Dorset in late Victorian times. The factory was at Wareham, Dorset, near the railway station.
About 1.6km east of Kimmeridge Bay is the small headland once known as Blackstone Point but usually referred to as Clavell's Hard (map ref. SY920777). The Kimmeridge oil-shale or Blackstone was worked here and further the east, near Rope Lake Head, it descends to the beach.
The Kimmeridge Oil and Carbon Company reported that in 1890 there were five thousand feet of underground tunnels or levels at Kimmeridge (see Legg, 1884, p. 23 for a map of industrial Kimmeridge showing the various levels and shafts). Two of the levels, No. 3 and No. 4 were opened onto this ledge. The old photograph shows the state of the ledge in the 1890s (based on Strahan, 1898). Next to the hut is No. 3 Level, covered by debris now. The oil shale, which just above the floor of the ledge or platform, is almost a metre thick (0.86m according to Strahan, 1898). Slabs of this have been piled up ready for removal. You will notice the railway track and some wooden posts, probably part of a pier or jetty, no longer in existence. The five levels or adits in the cliff are from the 1890 workings of the Kimmeridge Oil and Carbon Co. Ltd. in the final phase of operation (Legg, 1984). Galleries branched off the main levels, and between No. 1 and No. 2 the Blackstone was worked out for the whole of the 150 feet closest to the cliff (Legg, 1984). Worked out shale can be easily seen close to Clavell's Hard.
For more information on Kimmeridge Oil Shale and the history of its workings, please go to: Oil Shale at Clavell's Hard, Kimmeridge and Kimmeridge - Cliff Fires
DORSET - KIMMERIDGE OIL, GAS AND OIL SHALE
.
KIMMERIDGE OIL SOURCE ROCK Composition and Geochemistry of the Blackstone, the Oil Shale. As a potential oil-source rock it is appropriate to consider the geochemistry of the main oil-shale, the Blackstone. Various analyses have recorded a total organic content of between 20% and 74% (e.g. Cosgrove (1973), Dunn (1973), and Legg (1984)). Strahan (1898) referred to an analysis which gave 61% volatile matter and 13.15 % carbon (coke). Thus the total organic content was 74.15 % with ash of 28.85 % Thus, in general, the purest high quality Blackstone can be considered as consisting of about 70% organic matter in the form of kerogen. A very organic-rich, sample of the "Best Blackstone" (at the base) which has been analysed for trace elements contains 61.6% organic matter and only 5.6% CO 2 . There are the following trace elements in ppm: V - 25; Cr - 20; P - 1580; Zr - 7; Ni - 35, Cu - 25, Mo - 20, Ga - 3, Mn -20, Li - 15, Rb - 50, Cs - n.d., Sc - 2. This data is from Dunn (1973), who has provided tables of analyses for this and adjacent strata. The trace element composition is not unusual for a Kimmeridge Clay rock with low clastics. V and Cr are low compared to adjacent strata while P is high for the Kimmeridge Clay. Zr is as low as would be expected for a rock with low clastics, Ni and Cu are similar to values for the shales, and Mo and Ga are not unusual. Mn is decidedly low, probably because of deficiency of carbonate. Li is very low and Rb, as would be expected, is much lower than in the shales. Cs and Sc are not present in significant quantities. Generally then, elements contents are low compared with the adjacent bituminous shales probably because of dilution by organic matter trace. The high phosphorus is the notable feature. Additional trace element data has been provided by Cosgrove (1973). He analysed various Kimmeridge Clay samples including a museum specimen of Kimmeridge Coal or Blackstone. This particular specimen gave 71.4% 'organic C' (but this is presumably this total organic matter, not just carbon). CO 2 was 3.7%, and these figures are not very different from the values given by Dunn for the richest oil-shale. The trace element composition is broadly similar to that given by Dunn. There are some additional elements, including Zn - 65, Br - 16, Sr -154 and Y - 13 ppm. The most notable feature is the iodine content of 72 ppm which makes this one of the most iodine-rich combustible rocks known. Other Kimmeridge rocks have lower iodine contents but they are still quite high for shales. The iodine was attributed (Cosgrove, 1973) to a marine plant origin. It was not detected in Carboniferous oil-shale from Broxburn, Scotland, which is of freshwater origin. More trace element data is available in Gallois, (1978). In addition to comparable results to those discussed above, Ba is unremarkable being near 250 or 300 and U is very low and not concentrated in the oil-shale. In terms of organic maturity the Kimmeridge Clay of the Kimmeridge area is still immature but has R o (estimated vitrinite reflectance) of about 0.45, higher than the value for the Kimmeridge Clay of Ringstead of 0.34 (Ebukanson and Kinghorn, 1986). See this work, too, for information on aliphatic hydrocarbons in the Kimmeridge Clay. Gas production from Kimmeridge Blackstone has been found to be 11,300 cubic feet per ton, after purification of the gas. This is said to be only 700 cubic feet less than the output from a ton of coal. This information is from Legg, (1984) quoting Dr Ted Ward, who, in turn, was referring to the work of the German chemist August Wilhem von Hoffman in the 19th Century. Strahan (1898) stated that one ton yielded 9,000 cubic feet of gas. At 60° C methane is the gas produced. At 100° C methane, ethylene, ethane ande propylene are produced. C 4 and C 6 hydrocarbons are also produced at high temperatures (Gallois, 1978). A potential hazard is that hydrogen sulphide is produced at 200° C. Older studies (Strahan, 1898) showed that the Blackstone can yield 120 gallons of oil to the ton in laboratory tests, or on a large scale 66 gallons to the ton. Associated 'common shale' gives 33 gallons to the ton. A thorough, more recent study of oil yield has been made by (Gallois, 1978). Up to 20.9 US gallons per short ton are the potential oil yields from samples in various boreholes through the Kimmeridge Clay in southern England. The Blackstone at Kimmeridge has given oil yield of 15 wt/wt %. Hydrocarbons produced by the pyrolysis of Kimmeridge Clay oil shales are mostly viscous oils and tars with a high sulphur content (Gallois, 1978). Oil shale density is much less than associated sedimentary rocks can be as low as 2.2 (and probably lower in the case of the Blackstone). Clay minerals in Kimmeridge oil-shales are dominantly illite, but also with some kaolinite, some mixed-layer mineral and some chlorite (Gallois, 1978). Incidently, it is interesting to note that ammonia was a by-product of distillation, and salammoniac has been recorded as produced by an oil-shale cliff-fire (Cole, 1975) . I do not at present have information available on the nitrogen content of the Blackstone. Gallois (1978a) has considered the practicalities of large-scale oil shale distillation. The oil shale has to be retorted to about 500° C and the volume of spent shale after retorting has a similar volume to the raw shale. There is therefore much to dispose of and it has few uses and may include small quantities of carcinogens. Its disposal therefore presents formidable problems. There has been some important recent work on the organic geochemistry of various Kimmeridge lithologies, including the Blackstone, by Van Kaam-Peters et al. (1998) . They found that organic matter had been preserved by sulphurisation. The organic matter of all sediments was deposited under euxinic conditions as revealed by the occurrence of isorenieratene derivatives indicating (periodic) photic zone euxinia. They suggested that at times of reduced run-off from the hinterland, represented by so-called condensed sections, the flux of reactive iron was relatively small compared to the flux of reactive organic matter, which resulted in the formation of relatively small amounts of pyrite and an excess of hydrogen sulfide capable of reacting with fresh organic matter. Within the condensed sections, variations in the degree of sulfurisation of organic matter are probably due to both differences in primary production and differences in the supply of reactive iron. Their findings demonstrate that climatic changes, probably driven by Milankovitch cycles, can have a large impact on the molecular and carbon isotopic compositions of the sedimentary organic matter in an otherwise relatively stable stratified basin. They also show that large amounts of labile carbohydrate carbon may be preserved through sulfurisation.
There are significant resources of oil shale offshore to the south of Dorset. In fact there is probably much more Kimmeridge oil shale offshore than onshore. The Kimmeridge area is a small landward relic of a far larger outcrop under the sea. It is likely that even most of the old quarry sites of the Romans are under the sea now (up to half a kilometre offshore). The outcrops in the cliffs and ledges at Kimmeridge, Burning Cliff near Weymouth, and the Isle of Portland are, of course, well-known. The sea floor has been mapped acoustically many years ago by Donovan and Stride (1961). There has been more recent sea-floor mapping of the oil shale near to Portland by Gallois. Thus, the general pattern of the outcrop is well-known and there should, of course, be much more Kimmeridge oil shale beneath younger strata (Portland Group etc) in the English Channel to the south. Lyme Bay is not shown on this map but there should be more Kimmeridge oil shale in the offshore Kimmeridge outcrop there (southwest of Chesil Beach - see BGS West Fleet and Weymouth Sheet 341 and part of 342, 2000 edition). It seems very unlikely that any attempt will be made to use this oil shale in the near future. The environmental problems of blasting and dredging the sea floor would probably be enormous, and it is not known whether it would be at all economically worth-while. Only in desperate times in the future might this be even be considered. Subsurface retorting or burning and distilling underground in this shallow offshore area might at some time be possible from shallow boreholes, but would probably be uneconomical. Even if the technology was developed to do this, there would be a major problem of sulphate ions forming from the oxidised sulphide (pyrite nodules and dispersed pyrite occurs abundantly in the oil shale). Very acid and potentially polluting fluids would be developed in large quantities and would need neutralising (this might be done using chalk or even limestone or dolomite within the Kimmeridge Clay Formation, to produce gypsum).
The particular multibeam bathymetry images of the Channel Coastal Observatory, shown above, reveal only a little of the oil shale outcrop. However they indicates the level of detail that can be obtained with regard to the sea-floor. The Kimmeridge Clay has numerous, recognisable stone bands of dolomite and limestone and also harder oil shales. These project as ledges or ridges on sea floor and can be mapped by modern techniques. It will soon be a very simple matter to determine the detailed outcrop of the Kimmeridge oil shale or Blackstone on the sea floor to the south. It is quite likely that that potiential economic resource has already have been mapped in detail offshore. Of course, no plans are known for such sub-sea dredging of the Kimmeridge oil shale and they do not seem likely to happen in the near future. However, they might take place at some stage when there is a severe shortage of mineral sources for energy production.
KIMMERIDGE NO.1 OIL WELL
Kimmeridge Oilfield - Introduction Northwest of Kimmeridge Bay is the oil well of British Petroleum Co. Ltd. This well site is small with a single beam pump or "nodding donkey". It has a high wire-mesh fence around it but it can be clearly viewed and there is an explanatory notice provided by BP on the front. This is probably the oldest continuously producing well site in the UK and was drilled in 1959. This was the original successful Dorset oil well before the much large Wytch Farm oil discovery was made. It has produced 350 barrels per day from fissured shelly limestone of the Middle Jurassic Cornbrash at about 320m depth. Production now is only 80 barrels per day (12,720 litres), according to the BP information notice. The oil is collected twice a week by tanker and transported to the main BP Gathering Station at Wytch Farm near Corfe Castle. It is then stabilized and exported by pipeline to a terminal at Hamble on Southampton Water. From there it travels by tanker to oil refineries for production of high quality fuels. The pump is a conventional beam pump or sucker-rod pump, although this one is very quiet because it is electrically driven rather than having the more common diesel engine for power. At depth in the anticline, at about the level of the Cornbrash, both oil and gas enter the casing which is perforated (this is usually done by firing shots out of the well).The oil is pumped up by a long string of narrow rods within the tubing (the inner piping) of the well on a system operated by valves. There is a significant weight of rods and oil to be lifted at each movement of the beam so there are two heavy, half-moon-shaped, counter-weights which rotate. You can see these in the photograph. The gas rises to the surface between the tubing and casing and is piped away separately at the surface. There is, according to the notice, intention to use the gas for fuel in the future to provide energy to drive the pump mechanism. Some bushes or small trees are being grown in front of the oil well and this single pump causes very little disturbance to the environment and, indeed, is something of a tourist attraction. Had a large quantity of oil been found here then more screening of the pumps by trees, as at Wytch Farm, might have been necessary. Of course it is obvious that the oil is not coming from the Kimmeridge Clay which is at surface in the cliffs at Kimmeridge. However, such an origin is sometimes a misconception amongst the non-geological public, probably because the Kimmeridge Clay is the major source rock for the oil in the North Sea. There is no liquid oil in the exposed Kimmeridge cliffs, only oil-shale and bituminous shale containing kerogen, a brown waxy substance. The Kimmeridge Clay at Kimmeridge is not thermally mature (it has not been suffficiently heated by sufficiently deep burial at this particular locality). The real source of this oil is probably the deeper-buried, Lower Jurassic (Lias) bituminous shales in the offshore English Channel Basin. The Oil Field - General The search for oil in Dorset started in 1935. The first prospecting licences under the 1934 Petroleum Production Act were granted to the D'Arcy Exploration Company, who worked with BP. Exploration wells on whaleback anticlines at Poxwell and Portsdown in 1936 were unsucessful. Generally oil was not discovered in quantity in the Tertiary anticlines, which were compared to those in Iran, from whence Britain obtained its oil. It is now known that the oil of the large oilfields like Wytch Farm are in concealed fault traps of Cretaceous age (Late Kimmerian), not generally in the Tertiary anticlines. These can only be detected by seismic methods. The Kimmeridge field was discovered in 1959, long after the initial search for oil in Dorset. It began producing in 1961 ( (Gluyas et al. 2003). For a short time the production was up to 350 barrels a day, but later declined to about 100 BOPD and is now about 60 BOPD. The Kimmeridge Field was developed under a Mining Licence (ML5) granted in 1964. The field still produces under that licence and it is due to expire in 2014 (Gluyas et al.). Between 1959 and 2000 six wells were drilled in Kimmeridge Bay (these are shown on a map above and listed below). The reservoir is a fissure system in the Middle Jurassic Cornbrash Limestone. This belongs to the top Bathonian to basal Callovian, and includes the zones of Cydoniceras discus and Macrocephalites macrocephalus. It is about 27 metres thick here. The nearest surface exposures are at the shores of the Fleet Lagoon (Shipmoor Point near Abbotsbury, access difficult; Butterstreet Cove, near East Fleet, access easy). History of the Oil Well The search for oil in Dorset started in 1935. The first prospecting licences under the 1934 Petroleum Production Act were granted to the D'Arcy Exploration Company, who worked with BP. Exploration wells on whaleback anticlines at Poxwell and Portsdown in 1936 were unsucessful. Generally oil was not discovered in quantity in the Tertiary anticlines, which were compared to those in Iran, from whence Britain obtained its oil. It is now known that the oil of the large oilfields like Wytch Farm are in concealed fault traps of Cretaceous age (Late Kimmerian), not generally in the Tertiary anticlines. These can only be detected by seismic methods. Volume of the Oil Reservoir It is interesting to note that by 1986 the well had produced 2.7 x 10 6 bbl House (1993) . This was thought to be greater than the original volumetric capacity of the reservoir according to ( Stoneley and Selley (1986). They considered that the reservoir was being replenished by migration of oil from lower horizons. However, Evans, Jenkins and Gluyas (1998) in a paper entitled - The Kimmeridge Bay oilfield: an Enigma Demystified - have provided new information. New calculations have made on the reservoir. Further details are given below, in the Structural Closure section. The oil column was known to be 14.3 metres (47 feet) to 17.4 metres (57 feet) thick in about 20 metres of Cornbrash under a seal of Oxford Clay. Porosity The porosity of the Cornbrash limestone here is very low (around 1 percent) and permeability is negligible. However fracture porosity is present, and this not well understood. Different estimates were made for the average porosity including that provided by the fractures. It is very difficult to average out the variable volumes of fracture porosity in relation to the volume of almost non-porous limestone. The details of the fractures are not known, and the extent of fracture porosity is one of the uncertain variables. Structural Closure The main structure is a tight, structurally complex, monoclinal faulted fold, known as the Purbeck Disturbance Gluyas et al. (1998). The Kimmeridge trap is south of this. The northern limit of the trap, as shown above, is an east-west fault. The other limits are dips, east, west and south off the dome-like structure. A problem lies with the closure, in effect, the size of the trap or structure from which oil cannot escape. This has been difficult to calculate because of uncertainties regarding fracture porosity. Different closures have been calculated, usually larger, and sometimes much larger, than the originally mapped closure of 120 to 160 acres. Problems of estimating closure is why the hypothesis of resupply or of much extended fractures systems originated. Evans, Jenkins and Gluyas (1998) having studied the decline behaviour believe that the reservoir contains a finite ultimate recoverable reserve of 3.5 million barrels. They have stated that: "Despite being the most popular myth surrounding the field, the reservoir is neither fed from a deeper accumulation nor is migration into the field still occurring. Neither is oil being produced from a single isolated fractured zone which was only penetrated by the first well." They point out that if migration was continuing then the production history would not show the conventional decline which it does. The well results are consistent with a fractured reservoir contained within an anticlinal trap. Nevertheless the extent of the reservoir system and effective porosity are still not fully understood. All other similar traps in the Wessex Basin seem to have either been breached or never been charged. Their final remarks are: "Perhaps, like the Wytch Farm field to the north, it is the sole survivor of a much more significant play which was wiped out during inversion" [inversion was the uplift of the basin to the south]. The Low Pressure Problem A special aspect of the well, and one which has generated some discussion, are the abnormally low reservoir pressures, actually below hydrostatic. The initial pressure in Kimmeridge - 1 well was 400 PSI at 520m SS. This pressure would be expected in a reservoir several hundred metres shallower. Brunstrom (1963) suggested that oil may have been sealed in the fracture prior to Miocene folding and that the fissures were physically enlarged during folding leading to reduced pressures. Evans, Jenkins and Gluyas (1998) comment that some such explanation is necessary because there is no history of recent burial which might otherwise explain the pressures. The Succession in the Boreholes Other boreholes have been drilled in the Kimmeridge Bay area and although not useful in terms of production have provided more information. See Evans, Jenkins and Gluyas (1998) for details. The succession in the original borehole was given by Brunstom, 1963 as follows:
Lower Kimmeridge Clay
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0-243m
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Corallian
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243-340
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Oxford Clay
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340-519
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Kellaways Beds
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519-537
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Cornbrash
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537-564
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Forest Marble and Fuller's Earth
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564-889
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Inferior Oolite
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889-909
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Bridport Sands
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909-1042
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Broadbench 1. 1936-1937. Intended to reach Bencliff Grit, Corallian, because of the oil in that at Osmington Mills. Found a joint in Sandsfoot Grit wet with light oil, but did not get to the Bencliff. -----
Broadbench 2. (later renamed Kimmeridge No.1) Oil oozed from partially-leached calcite veins in the Cornbrash at 512m. subsea. (Why were the calcite veins partially leached?). Production tests yielded 30bbl/day. Two acid treatments caused the well to flow briefly at the very high rate of 4300 barrels a day. Completed as a producing well. This is now the producing Kimmeridge No.1, with the nodding donkey and shown in photographs above. -----
Kimmeridge No.2. Drilled 1960, east of Kimmeridge No.1. Top Cornbrash at 583m. Some oil in sands within the Oxford Clay. Very small quantity of oil from Cornbrash. Pressures showed Oxford Clay and Cornbrash in communication. -----
Kimmeridge No.3. Drilled 1959-60, prior to Kim 2. Location 762m. to SW of Kim 1. Small test production from Cornbrash. -----
Kimmeridge No.4. 1960. Mechanical breakdown. -----
Kimmeridge No.5. In 1980, post Wytch Farm Sherwood discovery. Weak gas shows throughout Jurassic. Important for going into the Triassic Sherwood Sandstone at 2272. Sherwood has only weak oil shows and reservoir quality was poorer than at Wytch Farm. (Useful information on Sherwood). -----
The RGGE (Rapid Global Geological Events) Boreholes. Three continuously cored boreholes for research were drilled through the Kimmeridge Clay by 2000. Two boreholes were in Swanworth Quarry, near Worth Matravers, and the third, Metherhills No.1 was at Metherhills, adjacent to the road down to Kimmeridge Bay. Metherhills No.1 went into the Sandsfoot Clay of the Corallian. These boreholes were not intended for petroleum exploration. See Gallois (2000) for details.
WYTCH FARM OILFIELD, DORSET
This is the largest known onshore oilfield in northwest Europe. It was once owned by Gas Council; for many years it was owned by BP; recently (2010) it was sold to Perenco UK Ltd. This is not a technical description. Only a general and elementary account for the student or non-petroleum geologist is given here. The oilfield has been in operation for about forty years and any data given here is likely to be out-of-date, and is suitable for educational purposes only. All photographs are at least a few years old and no precise data on the present situation is given. The broad scheme and location of well sites has not changed much though. . SUMMARY INFORMATION [Wytch Farm]- BRIDPORT SANDSTONE FORMATION RESERVOIR
Bridport Sands Reservoir.
This was the original reservoir, but it is now the minor one.
Marine Lower Jurassic, Toarcian to Aalenian, carbonate-cemented sandstone, compartmentalized.
Depth at about 900m depth.
Thickness is 45m to 73m (compared to coast exposure at Bridport of 43m). Porosity. At the coast section the porosity is 15% to 37% in the friable sands. The mean friable sand porosity is 26%. Porosity. The original porosity was 43% as shown by the cemented beds, but the porosity of these hard beds is negligible now. The hard carbonate-cemented beds may be baffles (like islands in a stream), that is they allow some flow around them, but in some cases they may be barries (like dams in a stream) which are more restrictive to flow.
(Wellsite M on Goathorn Peninsula)
. 
Probably the most obvious and conspicuous feature of the Wytch Farm Oilfield is the very large drilling rig that is used for Extended Reach Drilling. This is situated at Wellsite M on Goathorn Peninsula. It has drilled up to 11.3 kilometres to the east and into the great Sherwood Sandstone reservoir about 3 kilometres south of Bournemouth Pier. This rig has Top Drive and Downhole Powered Steerable Drive. Top Drive is a moveable mechanism for rotating the great length of drilling rods and the bit. On older, traditional systems this rotation was conveyed to the drill string via a Rotary Table (a circular rotating central part of drilling floor) and a Kelly, a square-section sliding rod turned in the centre of the rotary table. The Top Drive descends (lowered by the travelling block) from the top of the rig each time a drilling rod descends (which is then uncoupled). A (yellow) Torque Track prevents the Top Drive mechanism box from rotating back against the torque. The toolpusher (drilling engineer) can choose to rotate the drill string by the Top Drive, especially when drilling in a straight direction. However when changing the direction of the borehole and producing a curve the top drive can stop rotating and the Powered Steerable Bit is brouht into use. The drill string now does not rotate but the bit rotates separately, powered from above. In addition the bit is steered from the wellsite by hydraulic control and turns the direction of the well. In this manner a well can swerve round under Sandbanks, for example, and descend beneath the Sherwood Reservoir. It can then come up into at about 11 kilometres away, south of Bournemouth into the reservoir at a depth of about one and half kilometres below the surface. The well will be cemented, perforated at the bottom, and fitted, from the Goathorn wellsite, with a Downhole Submersible Electrical Pump. This is fixed in place at the bottom of the well south of Bournemouth, with cables for the heavy electrical power. There are now many of these pumps in Poole Bay.
[Note regarding the photographs:
Traditional beam pumps or sucker rod pumps or "nodding donkeys" are shown above. These are well-known characteristic features of oilfields. However, they are only used at Wytch Farm for the upper reservoir, the Bridport Sands, which was discovered long ago. The main and newer reservoir is the Sherwood Sandstone which is lower. This is pumped by downhole Electrical Submerged Pumps, ESPs, tubular machines in the borehole and not of course visible at the surface. These operate in 31 wells and produce 85% of Wytch Farm oil. The electric motors are tandem and sometimes triple tandem. These machines, at the bottom of the wells, are very powerful and range up to 1400 horse power. They consume much electricity but continuously pump large quantities of oil and are an efficient method of production. For details and history of ESPs at Wytch Farm see: Wahidiyat (2010).] Wytch Farm is Europe's largest onshore oilfield. It has reserves of 467 million bbl - 74 million cubic metres. There are three reservoirs: the Frome Formation (Middle Jurassic), a relatively minor reservoir, at about 800m; the Bridport Sands (Upper Lias, Lower Jurassic) at about 900m; and the Sherwood Sandstone (Triassic) the most important reservoir at about 1600m. It is on the south side of Poole Harbour, Dorset, extending eastward into Poole Bay, off Bournemouth.
Offline and not available to the public for copyright reasons. Privately held at archive: STUDLAND-NEW-2011-c-psp/11STD-Middlebere-Topo-Map-lab.psp JPG-STUDLAND-NEW-2011-Compaq/11STD-Middlebere-Topo-Map-lab.jpg JPG-STUDLAND-NEW-2011-Compaq/11STD-Middlebere-Topo-Map-lab.jpg .
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. The Gathering Station of the Wytch Farm Oilfield is the nerve centre of the oilfield with a computerised control room governing its entire operation (old BP brochure). It is near the Hartland Moor National Nature Reserve, as can be seen on a map above. It is thus in an environmentally sensitive area. One of the conditions attached to the Planning Permission was nothing on the site may be above the tree line. One consequence of this is that the fractionation towers where the gases separated had to be redesigned in pairs, rather than have conventional taller units. In fact the Gathering Station is so well hidden by coniferous forests that the there no obvious environmental impact until you are very close by to the installation. The facilities are painted a brown colour and are not high. They are not visible above the tree line. Its location is well-known and has been a subject of discussion in the local Dorset region for many years. However, if you did not know about it in advance you would not even notice its presence. In this respect it is, and always has been, environmentally successful. Oil Processing (historic information) The mixture of hydrocarbons and much water coming from the wells is passed through a series of vessels where heating and lowering of the pressure separates the oil, gas and water. Production separators of an oilfield are usually horizontal chambers, which usually work in stages, perhaps three stages, with an inlet temperature of about 100 degrees C and an initial pressure of something like 30 to 50 times atmospheric pressure (Oil and Gas Production Handbook). In separators water settles at the bottom, oil floats and taken out in the middle, th gas bubbles up and is taken out at the top. Crude oil is produced at the end of the process and finally passes through desalters to reduce the salt content. It is then cooled and the crude oil passes through a 40.5 cm. export pipeline to Hamble on Southampton Water. It passes through part of the New Forest, but the route is almost invisible. Gas Processing (historic information) The gases are first cooled and dehydrated before being passed through a series of fractionation towers (mentioned above). These have a de-ethaniser which separates off the methane [CH4 - natural gas] from ethane [C2H6]. There is also a depropaniser to remove propane [C3H8] and a debutaniser to remove butane [C4H10]. The methane and the ethane are conditioned to the correct specification for sale, although some is used as fuel gas on site. The sales gas, methane, is odorised [given a smell for safety reasons], metered and pumped along a 20.3cm pipe to Sopley where it connects to the National Gas Grid (information from an old BP brochure for the public). The general visitor would not know which pipes are which, and those containing water would look similar. As mentioned below, there are very large quantities of water in use her, often more water than oil or gas. Water Processing (historic information) The water in the Gathering Station pipes is of two types. One of these is the seawater from Cleavel Point in Poole Harbour and this is pumped down into the reservoirs. The other water is produced water coming up the wells. Water is discussed further in a section below.
Renewed interest in the search for British oil in the 1930s led to the Petroleum (Production) Act 1934. This declared all oil and gas reserves to be the property of the Crown, with exploration and production licences issued by the government to suitable operators. It was about this time that D'Arcy Exploration, later to be become BP Exploration, started looking for oil in the UK.
It took until 1939 for the first commercial discovery onshore to be made - at Eakring in Nottinghamshire. Smallr finds followed nearby. Eakring alone supplied two million barrels of oil during the Second World War.
D-Arcy Exploration Work D'Arcy Exploration started its search for Dorset oil with a shallow well drilled at Broadbench, in Kimmeridge Bay in 1936. Other wells were drilled at Poxwell, Ringstead, and Lulworth, and between Worbarrow and Mupe, all without success. Surveys in the 1950s shifted attention to a structure known as the Weymouth Anticline, and so BP drilled at Radipole in 1958. Analysis suggested that a deeper well should be drilled at Kimmeridge, and so oil was discovered there in March 1959. Production started in September 1960 and the Kimmeridge well continued to produce about 100 barrels of oil per day in the early 1990s (but perhaps less now). (The barrel it equivalent to a standard unit of 42 US gallone or 35 imperial gallons or 159 litres).
An application was made for the drilling of an exploration well for oil at Goathorn Peninsula, well-site F, by BG(E) [British Gas, Exploration] in 1979. There is much details are given in the document regarding the scheme, and although something of the distant past now, it still provides interesting reading. Dorset County Council. 1979. Wytch Farm Oilfield. Goathorn Peninsula. Report of Working Party on the environmental implications of an exploration well site for oil or gas. May 1979, 75pp with various maps and plans. Go to the Oil South of England - Bibliography. Use the link above or alternatively scan down to "Dorset County Council" for more details. Here are some small extracts. Introduction:
1.1. Following the discovery in 1973 of a commercially viable oil field at Wtych Farm, Gas Council (Exploration) Limited - GC(E) - are now developing facilities at the various well sites, gathering station and railhead at Furzebrook. (Map 1). 1.2. During exploration of the Wytch Farm field [i.e. Bridport Reservoir], a second deposit [i.e. Sherwood Reservoir] of oil was found in 1977 which lies beneath and probably displaced to the east of the original field. 1.3. To further investigate both deposits of oil GC(E) submitted a planning application to drill two evaluation wells on Goathorn peninsula. Following advertisement of the applications, objections wer received by Dorset County Council, many of which relate to environmental factors. .. 1.10. [The Studland Well was proposed but this development never happened and was probably refused permission; it is just mentioned here out of interest.] The working party were also asked to to pay regard to the possibility of a similar proposal being made on the Studland peninsula at some time in the future and to consider any relevant interactions between the two proposals [a map is given showing the possible location of the Studland (South Haven Peninsula) well site. The Studland well was to be at approximately SY 027855, 0.5km west of Redhorn Quay and on the heath just on the northwest side of Ferry Road. On page 43, there is further discussion of the, then-considered, Studland well. It was to be positioned on the west side of the Studland Toll Road (Ferry Road), but it is mentioned that a site east of the Toll Road may be possible.] 26.5 [p. 35. An item of interest on blow-outs. No blow-out has actually occurred and there has been only one known oil leak (in 2010). The safety procedures are mentioned below.] 1. The maximum formation pressure likely to be encountered will be accurately estimated. 2. The well will be circulated with a suitable quantity of properly conditioned and weighted mud to compensate for this pressure. 3. The rig will be fitted with "blow-out" preventer equipment, which consists of three valves fitted one above the other. They are hydraulically operated and can be activated locally or remotely. (These valves are tested daily and pressure-tested weekly). [Note that in the case of flowing wells, as opposed to pumped wells, there is a also a downhole safety valve which can automatically close.] 4. Estimates by GC(E) engineering staff indicates by comparison with Wytch Farm that, in the unlikely event of a "blow-out", then a flow in the region of 3,000 barrels of oil per day could result. The holding capacity of the well site within the bund wall is 17,500 barrels. This would allow a period of about 5 and a half days to to bring in any additional measures needed to control the pollution. [reference is made on p. 40 to the safety record of BP for having drilled 10,000 wells on land for some ten years without the occurrence of a "blow out". The failure of a "blow-out" preventer is estimated at 1 in a million. The "blow-out" risk at Wytch Farm is estimated at 1 in a million. In fact from 1973 to 2013, forty years, there has never been a "blow-out" at Wytch Farm. Oil Pollution in Poole Harbour in 1961, not from an oil well (see map 12), page 41: "In the first incident, 6000 gallons (170 barrels) of oil escaped from a land installation at Holton Heath [adjacent to Poole Harbour] covering the Spartina in that area, but fortunately did not spread very far. The second incident, which occurred on 25 January [1961] was much more serious. On this date an oil tanker and another ship collided in the area between the mouth of the harbour and Brownsea Island releasing 61,561 gallons (1,750 barrels) of oil at about the time of low water. Easterly winds drifted the oil on the flood tide towards Brownsea Island, Goathorn Point and the island shores to the west, to approximately the area labelled "heavily polluted" on the map (map 12). As a result of heavy rain and south-west gale force winds which developed two days after the collision, spring tides flooded above their predicted levels, and the oil, caught temporarily by the stems of Spartina, was lifted again and spread far afield. Fortunately, it soon became trapped and this time held by Spartina, so that by 2 February [1961] when the tide was 1 ft. above predicted levels the oil was submerged and very little was seen floating. Relatively few birds frequent Spartina marsh, and the oil was safer there than on the water, on the beaches or on the mudflat feeding grounds. At least 300 birds of 32 different species are known to have been affected by the oil pollution and at least 148 birds of 28 different species are known to have died as a result of it. .... [continues on to p. 42.] . [CONTINUES]
. The diagram above shows the layout of a conventional well-site of 1979. This is based on, but redrawn from a plan in the Goathorn document, mentioned above. The well-site plan shown probably indicates the general pattern for traditional-type exploration wells in southern England. The site for the much larger, Extended Reach Boreholes, is site M on Goathorn Peninsula and not the one shown. The ERD site M is, as would be expected, on a larger scale. The flaring facility is necessary when drilling a new borehole because there would have been initially no gas pipeline, and therefore any gas encountered during drilling is normally flared off to dispose of it. Later when oil production takes place then any gas is piped off and processed before being made available for domestic use. Wytch Farm now supplies gas to the domestic system by pipeline to Sopley, near Christchurch. .
- The Sherwood Reservoir and the Main Production Phase As noted above the oilfield was initially producing only from the Bridport Reservoir, the higher stratum. In 1978 additional deeper wells had been drilled from Wytch Farm's existing wellsites. Afterthe change of ownership BP drilled into the Sherwood Sandstone. It has already been suggested in the days of British Gas that this was a probable reservoir, but it has been said that BP were initially doubtful about this. In any case it proved to be very sucessful. Much larger reserves were discovered in the Triassic Sherwood Sands Formation at 1585 metres (5200 feet). From the beginning of 1988 to the end of 1990, Stage II in the development of the Wytch Farm Oilfield took place. The expanded facilities produced an annual average of 65,000 barrels per day in about 1992. Production peaked at 101,000 barrels per day in 1996. It has been in decline since then, but new reserves are in sight. It now produces round about 17,000 barrels per day, mostly from the Sherwood reservoir. In 1984 British Petroleum announced plans to drill on Furzey Island which it purchased from Algy Cluff (one time owner of the Spectator and who had earlier founded his own private oil company) (Legg, 1987). From Furzey Island there was to be directional drilling into both the Bridport Sands and the Sherwood Sandstone. Drilling at up to 60 degrees meant that it was possible to go further laterally into the deeper Sherwood Reservoir. There is now a substantial area of well heads and oil pumps on Furzey Island, visible on aerial photographs, but almost entirely shielded from Brownsea Island and other parts of Poole Harbour by coniferous trees. Furzey Island was used and is in use, and according to planning applications may be in use until at least 2037. Another aspect of the applications was to site a major well on the Studland Peninsula at Shell Bay. However, there was sustained opposition because of fears of oil spillage, interferance with the natural drainage pattern and the unacceptable nature of the roads and traffic that it would require. A further problem was that Shell Bay had become National Trust land in 1982 ( (Legg, 1987). If the objections had not been made then the very large Extended Reach Borehole might have been sited at Shell Bay. This would have increased the chances of reaching distant locations such as the Beacon Field. Environmentally the problems would have been great, though. In the 1990s, steering of borehole bits became possible and then long, Extended Reach Boreholes, could be made. There had always been a problem of how to extract oil from the eastern part of the Sherwood Reservoir. This extends into Poole Bay, almost reaching a position south of Bournemouth Pier. At one stage the Wytch Farm oilfield held the record for ERD with a borehole extending eastward for 10.1 km. A later borehole reached 10.5 km.
There is a range of techniques available for the management of production levels and in this case both reservoirs [NB. Both reservoirs, the Bridport and the Sherwood Reservoirs] will be served by pumped production wells with pressure support by peripheral water injection through dedicated wells drilled for this purpose. Due to more favourable geological conditions the predicted recovery from the Sherwood Reservoir presently stands at 38%.
Since the date of that document many injection wells have been drilled. The "injectors" have always been shown on maps of the oilfield produced by BP, and there is no secrecy about this. In about 2002 there were 8 Bridport Injectors and 13 Sherwood Injectors. These carry high pressure water from Poole Harbour to the periphery of each oilfield. For supply of Poole Harbour water (seawater) there is a pipeline from Furzey Island to wellsite M on Goathorn Peninsula. The original plan in 1984 for the Sherwood Reservoir was to have 30 pumped production wells and 12 water injection wells. It was suggested that a condensate injection well might be required [condensate is condensed gas, and if there are unneeded low levels of gas from some wells this can be reinjected into the oilfield, partly for disposal and partly to increase the pressure in the reservoir.]. It is not known whether a condensate injection well was put into the oilfield. The produced water (in large proportion in the Sherwood Oilfield) contains some oil and some fine sand. These contaminants and dissolved gases are removed. The water is primarily used for reinjection into the reservoirs to maintain high hydrostatic pressures (to drive out oil and/or open up the rocks). On occasions the injection plant may be unavailable, and then subject to discharge consent the spare water may be disposed into Poole Harbour. These details are from p. 16 of the 1984 Consultative Document. Seawater extraction rates from Poole Harbour of 80,000 barrels per day (i.e. about 3 million gallons per day) would be required to supplement the production water reinjection quantities. The seawater from Poole Harbour requires treatment to remove sand, marine organisms and dissolved gases. Note that if this level of injection to locations around the periphery of the oil field is taking place and has been taking place for decades, then the questions arises at to whether the area is being "pumped-up", like a car tyre on a large scale. However, because of the large area of the oil fields this may not necessarily be noticeable, and it has be considered with the understanding that oil and gas is being pumped out and piped away. [No claims is made here to understand the details or the final balance of this, but something might be happening but it would have to be on a very small scale. There is no obvious evidence yet of land-level changes. In any case, water injection can be beneficial. There are suggestions of raising Venice 25 or 30 centimetres by water injection into an underground reservoir - see the internet on this topic. Subsidence in an oil field is generally considered to be more of a problem than than land raising. Twenty-eight oil fields world wide have been affected by subsidence after extraction of hyrocarbons. After 9 metres of surface subsidence took place above the Wilmington Oil Field in California, water injection was used to ameliorate the problem. See: Mayuga (undated about 1970?). An interesting question is what if anything will happen when the water injection stops. This might not be until at least 2037 because Perenco have planning permission to operate the oil field until then, and, of course, in some way it might continue. The end of the Sandbanks Peninsula does not show obvious signs of sinking or rising. Shold there be significant changes of land level, it is likely that the margins of Poole Harbour might reveal this. The subsidence of the Groningen Reservoir of the Netherlands might have some relevance to the Sherwood Reservoir of Dorset, because both are of sandstones in the Permo-Trias. See: Geertsma, J. 1973. "Subsidence Due to Hydrocarbon Production in the Netherlands" - available online. (Depth of the Groningen Reservoir is at 2750 to 2900m compared to 1600 for the Sherwood Reservoir of Dorset. Its thickness is 100 to 200m and in this respect it is similar to the Wytch Farm Sherwood Reservoir of about 150m thick.) The matter of possible land raising or sinking will be left here for the present. It is not necessarily implied that major movement up or down affecting property etc has occurred or will occur in relation to Wytch Farm Reservoirs. Nothing drastic has happened since the oilfield operations commenced in 1973!
A BP spokesman said: ' ........ It was noticed when somebody observed a small amount of oil, from a pipeline encased in concrete and sand, had risen to the surface of the sand. We've replaced that section of pipe and as a precaution we are inspecting all the key pipeline down there.'
The oilfield was shut down for about three months. Much work was done so that it was apparently in safe condition when shortly afterwards sold to Perenco.
By Barremian (Cretacous, top Wealden) times the Blue Lias and Shales-with-Beef, the source rocks for the Wytch Farm oilfield had been buried, WSW of the Isle of Wight, to more than 2250 metres, as shown by the map above. This seems marginal for oil generation, although an additional minor factor was that this was at about the time of the Cretaceous thermal maximum. Thus the average surface temperature was higher than normal. The Lias source rocks are believed to thermally mature in the dark blue area of the map, but not so to any signficant distance beyond this. The Kimmeridge Clay was probably not, in general, thermally mature.
Wytch Farm is Europe's largest onshore oilfield. It has reserves of 467 million bbl - 74 million cubic metres. There are three reservoirs: the Frome Formation (Middle Jurassic), a relatively minor reservoir, at about 800m; the Bridport Sands (Upper Lias, Lower Jurassic) at about 900m; and the Sherwood Sandstone (Triassic) the most important reservoir at about 1600m. The oil field is situated on the south side of Poole Harbour, in the northern part of the Isle of Purbeck, with extension seaward, to the east, into Poole Bay. The original and main site is on a branch road from Norden (near Corfe Castle). The oil field is well-concealed in the Purbeck Forest and most of it is in private land inaccessible to the public. Very little of it can be seen without special arrangements. The huge Goathorn drilling rig projects well above the trees and can be seen from the South Haven Peninsula, and from Brownsea Island and Sandbanks. The public cannot get close to this. Some pumps, successfully concealed from environmental impact, are on Furzey Island in Poole Harbour. There is also a gathering station, a rail terminal and a pipeline to Southampton Water. All this is very well-known, but it is hardly seen at all by the public because of a prize-winning environmental policy by the operators - BP. The Bridport Sands reservoir was in production since 1979. The Sherwood Sandstone reservoir is extensive and continues eastward under Poole Bay to near Bournemouth Pier. An early plan to obtain oil from the relatively shallow water of Poole Bay was to construct an offshore island, Hook Island. This scheme was rejected and difficult to implement in a such an environmentally sensitive and valuable residential and holiday region. Later there was much success with Extended Reach drilling from the Goathorn Peninsula, northwest of Studland village. This type of drilling accelerated production by three years and continues very successfully. The southern half of Brownsea Island, in Poole Harbour, lies above part of the Sherwood Sandstone reservoir (the lower reservoir) of the Wytch Farm Oilfield. The Bridport Sands reservoir approaches the south shore of the island. Across the channel to the south of Brownsea Island is the Goathorn Peninsula. Here the Extended Reach Drilling Borehole of BP has broken records for lateral drilling. Boreholes penetrate the Sherwood Sandstone reservoir from beneath and extend more than 10 km. eastward into Poole Bay. The terminate south of Bournemouth, not far from Bournemouth Pier. The Goathorn drilling rig can be seen from Brownsea Island with the naked eye but it is at a moderate distance and is not conspicuous or obtrusive. The photograph above was taken with a low-power telephoto lens. It is looking southward to the BP drilling rig from the B-P (Baden Powell) Scout Camp area of Brownsea Island. Beyond are the Purbeck Hills of Chalk. From Brownsea Island the rig is just a subject of interest projecting above the forest. Little else of the Wytch Farm oilfield is visible from here, even though Furzey Island, just to the southwest of Brownsea Island has many boreholes and is a major terminal for oil wells. Almost everything is concealed by coniferous trees.
The Bridport Sands are well known as the Upper Reservoir of the Wytch Farm Oilfield, Dorset. As a marine Jurassic sandstone, with good porosity and permeability, they form a good, conveniently accessible example of a Jurassic reservoir, and are not greatly different from North Sea sandstones, such as the Brent Sandstone. To see details of the Bridport Sands in good cliff exposures at the coast, as shown above, go to: Bridport - Bridport Sands, East Cliff and Burton Bradstock - Bridport Sands.
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).
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. 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.
BP have explained how the oil is pumped from the Wytch Farm field in their excellent brochure of 1992b. There are submerged electrical pumps. The diagram shows a pump in a vertical well, but similar pumps are used in the several Extended Reach Drilling, horizontal wells beneath Poole Bay.
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.
(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?
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.
"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痴 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."
DORSET OIL AND GAS FIELDS - OTHER THAN WYTCH FARM AND KIMMERIDGE
For information on the Durlston Head Gas Field and the proposed Infrastrata borehole at California Quarry please go to the webpage;
Petroleum-South-Portland-Wight-Basin
In the late 1930s the importance to the country of making major oil discoveries on the south coast of England was obvious. Second World War ships ran on oil in accordance with Winston Churchill's policy. the D'Arcy Exploration Company was very successful in Iran (Persia) and was leading to the development of the British Persian (BP) Oil Company or Anglo-Iranian Oil Company. However, almost all oil was imported to Llandarcy in South Wales (partly named after the D'Arcy Exploration Co, but with a Welsh-sounding prefix). The famous anticlines in the Iranian fold belt are long and narrow and closed at both ends and in Tertiary and Cretaceous strata. They are usually faulted. In the 1930s D'Arcy Exploration Company geologists on leave in Dorset found an oil seep in the Corallian (Bencliff Grit) east of Osmington Mills. This is now very well-known to any student passing, but it was an exciting discovery then. They quickly realised that just inland from Osmington Mills is a small closed and narrow anticline, the Poxwell Pericline, and it is faulted. It looks rather like a small model of the Majid y Sulieman anticline, the most famous of the Iranian oil-bearing structures. However, the Poxwell Pericline was a failure. It produced no oil (the Portsdown Antcline, drilled in 1936 was also a failure). Thus Weymouth or Portland Harbour did not receive an oil refinery and major oil port. For more details and a summary of the borehole log, please go to: Poxwell Quarry webpage.
HAMPSHIRE OIL AND GAS FIELDS
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.
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).
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.
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.
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.
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.)
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.
(If interested, compare to - drilling in the 1920s.)
ISLE OF WIGHT OIL AND GAS FIELDS
PETROLEUM GEOLOGY - ISLE OF WIGHT
The Arreton Boreholes
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.
(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.
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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
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WESTERN WEALD (WEST SUSSEX - EAST HAMPSHIRE) OIL FIELDS
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!).
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).)
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). . 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.
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.
[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 ｰ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 (Amoco痴 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.]
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.
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.
[further information to be added]
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, Winteborne 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 "gas fracking" borehole.
(Including some brief information from:
Frack Free Somerset, available online.) The Somerset Coalfield stretches from the Mendips north to Pensford and Timsbury, and is centered around the town of Radstock, near Midsomer Norton. The southernmost part of the coalfield extends south to the Mendip Hills. It is an area well-known as a Coal Measures inlier just north of the Variscan Front of the Mendips. It is north of major thrust faults. In Somerset four PEDL licenses have already been granted. According to "Frack Free Somerset", the towns they cover include: Keynsham, Whitchurch, Saltford, Chew Magna, Pensford, Marksbury, Clutton, West Harptree, Chewton Mendip, Midsomer Norton, Radstock, Shepton Mallet, Wells, Wookey, Pilton and Evercreech. At a public debate in Bath on 14th March 2013 between UK Methane Ltd and Frack Free Somerset, company UK Methane confirmed that they have been working on plans in the Ston Easton area and also around Compton Martin for their next test drilling sites. They have said they intend to apply to apply for permission for full production in Keynsham towards the end of 2013. In Somerset four PEDL licenses have already been granted. Towns they cover include: Keynsham, Whitchurch, Saltford, Chew Magna, Pensford, Marksbury, Clutton, West Harptree, Chewton Mendip, Midsomer Norton, Radstock, Shepton Mallet, Wells, Wookey, Pilton and Evercreech. UK Methane own PEDL licenses 226, 227 and 228. They are partnered with Adamo Energy, the UK branch of Eden Energy, an Australian multinational. Fairfax Shelfco 320 Ltd own PEDL 225, covering Wells, Shepton Mallet and Bruton.
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.
