West, Ian M. 2014. Petroleum Geology of the Weald - Shale Oil. http://www.southampton.ac.uk/~imw/Petroleum-Geology-Weald-Shale.htm. Revised Version: 1st June 2014.

Petroleum Geology - Kimmeridge Clay and Lias of the Weald - Shale Oil

Ian West,
Romsey, Hampshire

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

..| Home and List of Webpages |The Petroleum Geology of the Portland - Isle of Wight Offshore Basin |Petroleum Geology of the South of England |The Petroleum Geology of the Portland - Isle of Wight Offshore Basin |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.

Click here for the full LIST OF WEBPAGES

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

Petroleum Geology - Portland - Isle of Wight Offshore Basin (Inversion)

Bibliography of Petroleum Geology of the South of England.

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Probable distribution of Jurassic shale oil in the Weald Basin and relationships to some adjacent oilfields, modified and redrawn, with additions, after a BGS diagram, May 2014

Map of Jurassic source rock maturity, modified after Butler and Pullan

CONTENTS: (revised list - 31 May 2014)

PGSB-3. INTRODUCTION - Topographic Maps
PGSB-4. INTRODUCTION - Stratal Succession
PGSB-5. INTRODUCTION - Geological Maps
PGSB-6. INTRODUCTION - Weald Basin - Hydrocarbons
PGSB-8a CONOCO BH - Balcombe No.1 Borehole, 1986
PGSB-9. BALCOMBE - Balcombe - Location (b)
PGSB-10. BALCOMBE - Borehole Techniques
PGSB-11. BALCOMBE - (in reserve)
PGSB-12. BALCOMBE - Kimmeridge Stratigraphy & Sedimentation
PGSB-14. BALCOMBE - Comparison with Dorset Coast
PGSB-13. BALCOMBE -Kimmeridge Clay - Horizontal Drilling
PGSB-14. Kimmeridge Clay - Dorset Comparison
PSGB-15 FERNHURST- Celtique Energie
PSGB-16 WISBOROUGH GREEN - Celtique Energie
PSGB-26 (not used)
PSGB-27 (not used)
PSGB-28 (not used)

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The Weald area is geologically an Inversion Structure, originally a basin that was subsequently uplifted into an anticline or elongate dome. It is of economic importance because it contains oil shales or bituminous shales at certain horizons within the Jurassic sequence. These are particularly developed within the Kimmeridge Clay Formation, but may also be present in the Lias and possibly the Oxford Clay. The Kimmeridge Clay is most important in terms of economic potential and the emphasis is on this well-known Formation. The Kimmeridge oil shale in Dorset has been used for armlets and other ornaments since the Iron Age and the Romans has a type of factory at Kimmeridge. It was much later, in the 17th century that it became used as a fuel for manufacturing alum, glass and salt. A furnace for example, burning Kimmeridge oil shale, was built in 1618. In Victorian times it was mined and Shale Oil and Shale Gas was manufactured from the Kimmeridge Blackstone. The present exploration for Shale Oil and Shale Gas in the Weald is therefore not a new procedure in southern England, although the technique of hydraulic fracturing or "fracking" was not available to the Victorian shale industry.

For information on the Dorset coastal exposures and the history of industrial use of Kimmeridge oil shale see the associated webpage on the Dorset coast cliff exposures and mines:
Kimmeridge - Blackstone, Oil Shale at Clavell's Hard

In the 20th century the Kimmeridge oil shale was investigated more than once for economic use. Plans to use it as fuel for the British Navy were not successful because the very high sulphur content caused a stink, that could not be tolerated by the crew of warships. Probably at night the British ships could have been detected by the enemy because of their smell of burning Kimmeridge oil shale.

In 2013 and 2014 there was a new flurry of interest in once again using again this valuable but difficult mineral resource, more than 100 years after production had ceased. This time it will be hydraulically fractured and not be burnt. No doubt modern techniques will be extract the sulphur content of the oil. The location is not Dorset, where the oil shale is not thermally mature (i.e. not naturally "cooked"), but some central parts of the Weald (Balcombe, Fernhurst, Wisborough Green etc) where it has been so deeply buried (prior to uplift) that it is thermally mature. This webpage is concerned only with the geology, so there is no serious discussion here about whether this is satisfactory from an environmental or local point of view. There is just geology for geologists and nothing more.

Incidently, the Kimmeridge Oil Shale of southern England is also of approximately the same age as the Kimmeridgian Haynesville Shale, a major hydraulic fracturing shale of the USA, where there are hundreds of wells. The Kimmeridge strata are the major source rocks of Saudi Arabia. Now an almost Kimmeridge oil shale to that of southern England has been found in Yemen and it has a very similar, hydrocarbon potential (see Hakami et al. 2013). So parts of the Weald may be comparable in some respects to some Middle Eastern in possessing similar Jurassic source rocks.

This educational webpage discusses some aspects of the petroleum geology of the central part of the western Weald area mainly of Sussex (rather than Kent), southeast England. It is very limited in scope and very brief for such important region. It is intended to expand progressively with regard to geological information on the Kimmeridge Clay and related subject areas, but not with regard to drilling techniques. This webpage and associated webpages are concerned only with the geology and are of a geological educational nature at intermediate level. The objective is to provide more information on the Kimmeridge Clay of the Weald Basin. This webpage is related to other Kimmeridge Clay webpages, which are listed below:

Geological Webpages - Re Kimmeridge Clay
Kimmeridge Bay and Introduction
Kimmeridge - Fossils
Kimmeridge - Kimmeridge Bay to Brandy Bay and Gad Cliff
Kimmeridge - Kimmeridge Bay to Gad Cliff
Kimmeridge - East - Hen Cliff, Yellow Ledge and Cuddle
Kimmeridge Bay - Westward
Kimmeridge - BLACKSTONE, OIL SHALE at Clavell's Hard
Kimmeridge - Burning Beach, Burning Cliffs
Kimmeridge - Rope Lake Head to Freshwater Steps
Kimmeridge - Egmont Bight, Houns-tout Cliff and Chapman's Pool
Kimmeridge - Kimmeridge Bay to Gad Cliff
Kimmeridge Clay Boreholes at Swanworth Quarry
Kimmeridge - Bibliography - Start
Kimmeridge - Bibliography Continued
Petroleum Geology of the South of England


For information on the Middle Jurassic oilfields of the western Weald, which are in Sussex and extend into Hampshire, please see the related webpage:
Petroleum Geology of the South of England
Go specifically to sections on: Storrington, Singleton, Markwell's Wood, Horndean, Stockbridge, Winchester etc.

Those oilfields contain oil and gas which has migrated marginally and westward out of the main Weald Basin during Cretaceous times. There are similarities to the Wytch Farm and other oilfields of Dorset which are the result of Cretaceous migration of oil from the English Channel Inversion or the Portland - Isle of Wight Basin. That is rather similar to the Weald Basin but is offshore.

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Most of this webpage is concerned with discusssing aspects of the general geology and petroleum geology of the Sussex, central Weald, region. The eastern part of the Weald into Kent is not discussed in any detail. This brief and partial account is descriptive. It is not an itinerary, and it does not suggest or recommend field trips or visits of any type to specific places. No advice is given to go to any particular place and no liability for accidents or incidents is assumed. There is no recommendation here to visit drilling sites, even if that was possible. Safety matters regarding drilling sites are not discussed here, other than that safety regulations in force at such places must be followed. Road safety in the vicinity of well-sites needs consideration, especially if heavy vehicles are entering and leaving. If geological exposures or cliffs are visited then the normal safety procedures should be observed. The Kimmeridge Clay reference section at Kimmeridge has risks of rock-fall and tidal cut-off and precautions must be taken, but they are discussed in webpages that relate to the cliffs.

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Topographic Maps

A location map for Balcombe, Fernhurst and Wisborough, potential drilling sites for shale oil or gas from the Kimmeridge oil shale, as at 2013

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Stratal Succession

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

The diagram above is only introductory and based on the nearby Wessex Basin rather than the Weald Basin. For the most part the sequence is similar. The vertical section is very simplified and not accurately to scale. However, it shows the general positions of the important stratigraphical units. More detail is provided in other associated webpages with regard to details of parts of the stratigraphical column. Further information will also be given below, in due course.

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Geological Maps

Mantell's map of the geology of southeast England, including the Weald of Sussex and Kent

Old, simplified, geological map of Sussex and Surrey with some additional notes regarding oil and gas

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Sussex Part of the Weald Basin - Hydrocarbons

Map of Jurassic source rock maturity, modified after Butler and Pullan

See the following key papers:

Gallois, R.W. 1979. Oil Shale Resources in Great Britain. Institute of Geological Sciences [now British Geological Survey]. Southern England and South Wales, Land Survey Division. 158 pp.

British Geological Survey. 2014. A new report of shale oil resources etc in southeast England is due to be published. It has not been seen at the date of writing (31st May 2014), but information will be added later. This will be important and should be read.

Butler , M., and Pullan, C.P. 1990. Tertiary structures and hydrocarbon entrapment in the Weald Basin of southern England. In: Hardman, R.F.P. and Brooks, J. (eds), Tectonic Events Responsible for Britain’s Oil and Gas Reserves, Geological Society, London, Special Publications, vol. 55, 371-391. The publication is available from the Geological Society.
The abstract of the Butler and Pullan work is given below as a brief summary of Weald structures and their relationships to hydrocarbon exploration:
The Weald Basin of southeast England was formed by rapid subsidence associated with thermal relaxation following early Mesozoic extensional block faulting. The basin appears initially to have taken the form of an easterly extension of the Wessex Basin but became the major depocentre during the Upper Jurassic and Lower Cretaceous, with associated active faulting. These movements appear to have ceased prior to Albian times and a full Upper Cretaceous cover is believed to have been deposited in a gentle downwarp which extended far beyond the confines of the Weald and Wessex Basins. Major inversion of the Weald Basin took place in the Tertiary, with both gentle regional uplift, which in the eastern part of the basin is estimated to have exceeded 5000 feet (1525 metres), and intense local uplift along pre-existing zones of weakness, which led to the formation of compressional features such as tight folds and reverse faults. Zones of Tertiary deformation appear to have been strongly influenced by underlying, particularly Hercynian, structural trends.
Lower Jurassic source rocks reached maturity in the early Cretaceous and initial migration occurred at this time, often over long distances, into traps closed by pre-Aptian faults. Tertiary tilting and uplift led to the breaching of many of these pre-existing traps and the formation of large folded closures. A second phase of hydrocarbon migration, particularly of gas, took place at this time, with significant vertical migration along fault zones. Major reservoirs located to date occur in Middle Jurassic carbonates and Upper Jurassic sandstones, but deep burial in the basin has caused considerable destruction of primary reservoir characteristics; changes in the temperature and pressure regimes and the mobilization of fluids within the basin resulting from the Tertiary uplift caused further diagenetic changes, particularly in the carbonate reservoirs.
Exploration of the Weald Basin remains at a very early stage, with a low drilling density to date. The more recent drilling has focussed on earlier structures, but traps formed or modified during the Tertiary movements represent important exploration objectives, although general deterioration in reservoir quality towards the centre and east of the basin makes large fold closures in these areas less attractive.

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Introduction (a)

Boreholes through the Kimmeridge Clay in the Weald to Dorset area

Kimmeridge Clay in the Central Weald Basin

1. The depocentre of the Weald Basin, the thick central area of deposition with maximum thicknesses of Jurassic strata is in East Sussex between Haywards Heath and Crawley. Balcombe, Bolney and Cowden have similar substatinal thicknesses. The Kimmeridge Clay is at maximum thickness at Balcombe, with 654 metres. This compares to the section at Kimmeridge, still in fact basinal, of about 505 metres. 3. In Dorset the thickness of the main oil shale, the Blackstone, varies roughly in relation to the total thickness of the formation. Thus thick Kimmeridge Clay in the central Weald Basin may mean more oil shale.

4. There is an equivalent location in the Portland - Isle of Wight Basin (particulaly southwest of the Isle of Wight and between the Isle of Wight and the Isle of Purbeck, i.e. Swanage). A similar thickness and thermal maturity of the Kimmeridge Clay is possible, and thermal maturity would be even better in the Lias further down. Thus good discoveries on land at Balcombe might be matched in the future by similar finds offshore. 5.

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Balcombe No. 1 Borehole, Conoco, 1986, Introduction

Coring of the Middle Jurassic by Conoco at Balcombe No.1 Borehole in 1986, photograph courtesy of Dr. Grindrod

This Balcombe No.1 borehole was drilled by Conoco to 1724m before finally being plugged and abandoned. Dr. Tony Grindrod tells me that two cores were taken in the Great Oolite from 4717 feet (1437m.) to 4834 feet (1473m.); see photograph above. The new Balcombe No.2 borehole is to go only 900m. In other words the new one is a rather shallow borehole of only about half the depth of the previous borehole and not deep enough to reach the main oil reservoirs of the Weald in the Great Oolite. The No.2 borehole, contrasts with No. 1 in that it is targeting a Kimmeridge argillaceous micrite (not very deep), probably of low porosity and permeability, and not something of interest in any of the previous oil exploration in the region. The Kimmeridge Clay has not been considered as a potential reservoir with regard to any previous borehole in Britain. Many boreholes have gone through it. Balcombe No.1 borehole has actually penetrated the "Kimmeridge micrite" or "Kimmeridge Limestone". Balcome No. 2 is going through the same strata as did Balcombe No. 1. Obviously it is difficult to understand the purpose in drilling a new borehole at the same site but to a shallower depth, unless some different technique to improve porosity and permeability is used (hydraulic fracturing or acidizing or both?).

However, Cuadrilla have apparently stated as follows, from papers quoted on the: Bishop Hill website .

"Balcombe-2 in PEDL-244 is planned as essentially a re-evaluation of the upper section of the Balcombe-1 well, drilled some 10m away on the same site in 1986. Balcombe-1 was drilled to a total depth of 5560ft into the Upper Lias, and was plugged and abandoned. The plan now is to drill a vertical pilot hole termed 'Balcombe-2' followed by a planned sidetrack to horizontal for non-hydraulically fractured completion and production testing."

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Sussex includes the western part of the Weald Basin. There is hydrocarbon source rock potential regarding the Lias, the Oxford Clay and the Kimmeridge Clay within the deep Weald Basin. There is some oil and gas in the Jurassic reservoirs of the region. The oil shale of the Kimmeridge Clay is particularly important. In addition the deepest and most mature of the Jurassic source rocks is the Lower Lias.


Go to the Mail Online article on Balcombe, August 2013, for aerial view of the Balcombe potential fracking site, Sussex, England


Balcombe is a well-known locality for exploration for shale oil. The environmental dispute here is well-known and is not discussed in this webpage. Records of activities and problems are in the press. Part of an example article is shown above. Search online for more information. There has been in early 2014 a local protest and objection from residents etc at Balcombe (details are not given here). This objection has been over-ruled by the authorities and drilling will go ahead here in the near future.

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Borehole Techniques

Possible Techniques for Obtaining Oil and/or Gas in the Central Weald Basn

Below is a simple outline, introductory classification of the main methods for obtaining oil and gas from rocks. This is show the Balcombe Kimmeridge Micrite source within broad perspective.

1. Conventional oil reservoirs in traps, mostly sandstones and limestones with good porosity and permeability. Examples - Structures in Sherwood Sandstone and Bridport Sands in Dorset and offshore, also Middle Jurassic limestones and dolomites in the western Weald (lower than the Kimmeridge Clay of Balcombe).

2. Unconventional - Thermally mature (i.e. naturally heated) oil shales or shaley limestones with a hight organic content. Presumably the aim is to produce oil and gas from these at Balcombe and Fernhurst and other places in between and around the area. Only in certain areas such as the central Weald Basin (Balcombe etc) and offshore from Dorset and Hampshire and the Isle of Wight are such rocks available. Thermally mature micrite (a type of limestone) in the Upper Jurassic Kimmeridge Clay is the subject of discussion in this webpage.

3. Unconventional - Thermally immature oil shales. These may have biogenic gas in some cases but details are not known. This is not discussed in any detail here. 4. Unconventional - Pyrolysis (effectively setting oil shale on fire underground to use the heat to release gas). This is very hazardous with many problems; it is not planned for the Weald area and it is not discussed here.




A fairly shallow borehole is to be drilled to only 900m vertically at Balcombe. This is only to the Kimmeridge Clay in the Upper Jurassic and is to only about half the depth of a normal Sherwood Sandstone Borehole in Dorset. It is not even to be planned to penetrate the Middle Jurassic which is has been drilled into at Storrington, Singleton, Markwell's Wood, Horndean, Humbley Grove etc of the western Weald. It may extend horizontally to about 750m. (2,500 ft) but this is small compared to the more than 10 kilometre horizontal boreholes from Wytch Farm Dorset. See comparison diagram below. The drilling rig at Balcombe is not large and not on the scale of the Dorset (site M) Goathorn Peninsula big drilling rig.

Balcombe well and a Wytch Farm Extended Reach borehole - comparison in vertical section

The large-scale operations which have proceeding without any appreciable problems at Wytch Farm Dorset for a very long time (starting in the 1970s). At one stage the very successful environmental concealment of the with the drilling and production in Dorset resulted in BP being awarded a prize. Thus although the scale of oil industry work in Dorset is that of the world's largest known onshore oilfield it has had public approval. The much smaller scale work at Balcombe in Sussex seems to have been less welcomed , probably because hydraulic fracturing of a limestone (not shale) is thought to be a process which might take place. Environmental may be mentioned in passing but are not discussed here in detail as the emphasis and objective in this webpage is new information on geology.

[For some limited information on the highly successful Wytch Farm oilfield go to:

Petroleum Geology, South of England.]

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(in reserve for future use)


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Balcombe - Kimmeridge Clay Stratigraphy and Sedimentology

Although the main objective of the Taylor et al. (2001) paper was sequence stratigraphy, it does provides useful general stratigraphic information, as used in the diagram above, relating to the Kimmeridge Clay in the south of England. Of particular interest is the Kimmeridge Clay summary log for Balcombe 1 borehole. The occurrence in the well of the two limestones, the "Middle Kimmeridge Micrites" 1 and 2 is shown. The Kimmeridge logs for the wells at Bolney 1 and Cowden 1 are very similar to the the sequence at Balcombe and represent a similar situation in some respects. Storrington and Chilgrove have thinner sequences and with regard to the Kimmeridge Clay are less promising. However, their objectives were different.

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Kimmeridge Clay - Horizontal Drilling into a Micrite

A diagram showing in simplified form the basic plan for the Balcombe-2 well, based on the Non-Technical Summary of the Environmental Permit Application for Cuadrilla Resources Ltd., and available online

The diagram above, based on a Cuadrilla publication, shows the general nature of the proposed borehole. Note, though, that the details of the geology in the simplified diagram (following the Cuadrilla diagram) are not necessarily precisely accurate. The thickness of the Purbeck anhydrite seems greater than usual for the area, but the stratigraphical details of the diagram are estimated, not proven. See the diagram below for stratigraphical information from previous boreholes in the area. For proven thicknesses and references to previous literature see the source publication - Taylor et al. (2001).

Borehole logs for the Kimmeridge Clay sequence with micritic limestones in the Balcombe No. 1 Borehole and also for nearby and very similar boreholes at Bolney and Cowden


Some Kimmeridge Clay borehole details in relation to unconventional hydrocarbon resources, at Balcombe, Sussex


Rope Lake Head seen from the shore on the west side, near Kimmeridge, Dorset, 2010

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PSGB-14 BALCOMBE, continued

Kimmeridge Clay - Comparison with Dorset Coast

Thus, the Kimmeridge Clay at Balcombe is at the moment a unit of particular interest. See also the details of the well-known Kimmeridge Clay succesion on on the Dorset coast. Webpages are provided in this website on the coastal sections. See particularly:

Kimmeridge - BLACKSTONE, OIL SHALE at Clavell's Hard
and associated field guides.


At Balcombe, drilling horizontally into a micrite, a fine grained limestone is planned and should started (or have started) soon [seems to have been postoned - November 2013]. The use of acidizing with 10% or 15% hydrochloric acid is a major part of the process. Micrite beds in the Kimmeridge Clay at Kimmeridge on the Dorset coast are shown below. There are at least three bands, but one of these, the White Stone Band is the thickest and most well-known.

The three White Stone Bands, or coccolith limestones in the Upper Kimmeridge Clay at Brandy Bay, west of Kimmeridge Bay, Dorset, October 2011


The three white coccolith limestones, the White Stone Band, the Middle White Band, and the Freshwater Steps White Band at Brandy Bay, Kimmeridge, Dorset, March 2012


The White Stone Band in the cliff can be seen to contain two, thin, black oil shale bands, east of Rope Lake Head,  Kimmeridge, Dorset, 2010


Standing above the White Stone Band, coccolith laminite ledge, east of Rope Lake Head,  Kimmeridge, Dorset, 17th September 2012


The White Stone Band, coccolith limestone, at beach level east of Rope Lake Head, Kimmeridge, Dorset, and showing some subdivisions

Above is shown a thin coccolith micrite, limestone band within the Kimmeridge Clay on the Dorset Coast (Jurassic Coast). Presumably the target horizon at Balcombe is rather similar to this. These thin limestone at Kimmeridge are formed of the calcareous remains of planktonic algae. These are coccoliths. It is interesting that in the Kimmeridge cliffs thin oil shales occur in association with these fine-grained limestones. The deposition of these is related to the deposition of bituminous shales or oil shales. The organic matter and the coccoliths were preserved in the deep stagnant sea conditions like those of the Black Sea at the present day.

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Location showing oil and gas discoveries in part of southeast England, within a Celtique Energie publication regarding Fernhurst, Sussex, 2013


Celtique Energie intend to drill into the Kimmeridge Clay Formation at Fernhurst in Sussex. They have provided much useful information in a brochure, and have given a public presentation. From their publications the scheme would seem to be broadly similar to that planned by Cuadrilla for Balcombe. There is the Kimmeridge oil shale, with bituminous, argillaceous limestone above at a rather similar depth. Fernhurst is in part of the same central Weald Basin as is Balcombe.

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Celtique Energie, Nine Acre Copse, Fernhurst, PEDL 231.


Simple location map for Fernhurst, Weald Basin, southeastern England, site for Celtique Energie borehole into the Upper Micrite of the Kimmeridge Clay, as at Balcombe

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Wisborough Green


Wisborough Green, a Sussex Weald village near which an exploration well site may be set up


An oil and gas exploratory well may be sited down Kirdford Road, near Wisborough Green, Sussex


At Wisborough Green an application made for only for an exploration borehole. This will determine the characteristics and prospects of the strata beneath, including the Kimmeridge Limestone or Kimmeridge Micrite.

"Celtique Energie" (Celtique Energie Weald Ltd., the oil and gas exploration company with its joint investment partner Magellan Petroleum (UK) Ltd. has applied to set up a temporary well site near the villages of Kirdford and Wisborough Green, Sussex. The proposal is to use a screened area of land.

The test is for the presence of commercial oil or gas in the: Kimmeridge Limestone (the "Kimmeridge Micrite") and the Great Oolite (a conventional oil reservoir at Storrington and elsewhere in the western Weald region).


[There is further discussion available online about Woodbarn Farm site, near Broadford Bridge, in West Sussex. This is almost between Wisborough Green and the Storrington oil field. It is apparently a conventional well to be targeted on the Triassic (the Triassic Sherwood Sandstone is the lower and main reservoir of the Wytch Farm oilfield, Dorset and as a conventional oilfield holds the greatest oil and gas reserves in southern England).]

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[To be added]

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[To be added]

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Arreton, Portsdown and Henfield Boreholes and problems of thermal maturity

The Rope Lake Head Stone Band and The Little Stone Band in the hudlestoni Zone, Upper Kimmeridge Clay, above the Kimmeridge Blackstone, east of Kimmeridge, Dorset

The limestones of the Upper Kimmeridge Clay (Hudlestoni Zone) are present in thick development only in the Weald Basin, and apart from some thin coccolith limestones (the White Band etc) and The Little Stone Band are not seen in the Dorset cliff sections or in boreholes of the Isle of Wight. However, there is a sequence near the base of the hudlestoni Zone which is generally fairly rich in carbonate. See the photograph above.

In the Weald they are only present in boreholes so they are not available to the field geologist in quarries or other exposures at the surface. In contrast the thin Dorset carbonates of the Kimmeridge Clay are well-known, but even there most of them are dolostones and were wrongly listed as limestones back in the past (and some of them still are!). Apart from the coccolith limestones most of the Dorset coastal Kimmeridge carbonates are ferroan microsparite dolomites, in some cases almost approaching ankerite. The well-known Basalt Stone of the Hudlestoni Zone is not a limestone but it an argillaceous dolostone. Petrographic and geochemical characteristics have been given by Bellamy (1977; 1980).

The details of the brown argillaceous limestones in the Huddlestoni Zone above the oil shale in the central Weald Basin are not know to the author at present [2013]. It is quite possible that they have affinities with the Short Joint Coal of the Kimmeridge type section (with coccoliths and elongate calcite) and particularly the dolomitic elongate-calcite limestones of the western Kimmeridge Clay section at Ringstead. The calcite of this is distinctive. It is of spindle shaped calcite crystals of microsparite size (not micrite) which can range from ferroan to non-ferroan. These distinctive crystals are associated with some dolomite crystals and with coccoliths. A similar lithology is also known from elsewhere in the Kimmeridge Clay, including Yorkshire, and it is also present in the Oxford Clay. It is not known to the author whether the Kimmeridge Clay limestone above the oil shale is similar to this or not, but it might well be so. This will be corrected if further information proves that it not of this type.

Bellamy (1980) has described bed R11 of the Upper Kimmeridge Clay of Ringstead as a typical example. He subjected this bed to detailed petrographic and geochemical analysis. It is a discontinuous, argillaceous carbonate bed outcropping above the Kimmeridge Blackstone or oil shale and below the coccolith limestone, the White Band (Huddlestoni) at Ringstead. It is stratigraphically from the appropriate part of the sequence.

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Micrite (i.e. microcrystalline limestone) is a limestone consists of minute calcite crystals generally less than 4 microns (4µ). This is less than 4 x a thousanth of a millimetre. A common example, although, in fact of a special type, is chalk. Micrites are lithified carbonate muds. Although initially a soft, fine-grained white mud, the calcite of micrites can become compacted, lithified and in some cases partly recrystallised into a coarser calcitic fabric. In other words the initial white mud can be converted with burial and time into chalk, or with deeper burial and/or tectonic activity into hard, practically non-porous limestone.

Carbonate muds accumulate in various modern environments. They are mostly formed in warm or fairly warm sea or lagoon areas, with tropical, subtropical or Mediterranean climates. The Kimmeridge Clay environment was a fairly deep, but not oceanic, sea at about 34 degrees N (the equator has been slowly moving away to the south, and the Weald area was fairly warm in Jurassic times). The particular type of micrite present in the Kimmeridge Clay of Balcombe and other parts of the Weald is a lithified coccolith ooze, that is probably quite argillaceous (clayey). It is slightly unusual because it is similar to a deep ocean ooze, but was formed in only a local (Weald-wide) sea depression, not in a deep ocean.

Its composition is believed to be broadly similar to that of the White Stone Bands of the Kimmeridge coast cliff sections. These, too are coccolith micrites. The most notable of them, the White Stone Band of the Pectinatus Zone is present not only in Dorset but also in the Weald area, and it can be recognised from the gamma ray and sonic log at Balcombe. The White Stone Band is a useful marker about the upper Kimmeridge Clay Micrite.

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Lithology of the Upper Micrite - a Coccolith Micrite Possibility.

The possibility that the limestones are elongate-calcite limestone has been briefly discussed above. Another possibility is that they are coccolith micrites (or some intermediate) which occur in the Huddlestoni Zone in the Kimmeridge type section. Thus the coccolith micrites of the Kimmeridge Clay are briefly discussed.

According to the Non-Technical Summary of the Cuadrilla Environmental Permit Application (June, 2013) "the wellbore is planned to drill through the Micrite which is [here] an argillaceous carbonate. The Celtique Energie project report on the proposed Fernhurst Borehole makes it clear that it is an argillaceous coccolith micrite, as might be expected. This micrite is the upper of the two micrites in the Huddlestoni Zone of the Upper Kimmeridge Clay at Balcombe (and the same at Fernhurst). This is "Middle Kimmeridge Micrite 2" or in Cuadrilla terms "Kimmeridge Micrite I" above the "Kimmeridge Micrite J" (Middle Kimmeridge Micrite 1).Now Kimmeridge micrites can be coccolith micrites and usually are. An SEM photomicrograph of the most well-known one of these, the White Stone Band of the Pectinatus Zone at Kimmeridge is shown below. This is not a surprising lithofacies for the Kimmeridge Clay, but the special feature of the Weald Upper Micrite is not only that it is extremely fine-grained but also it is argillaceous. Argillaceous coccolith micrites are not used as conventional reservoir rocks in general in southern England. Even if they have adequate porosity, the permeability in such a fine-grained rock would be remarkably low and unsatisfactory for normal reservoir purposes.

SEM details of a pure coccolith micrite, the White Stone Band, Pectinatus Zone, East of Rope Lake Head,  Kimmeridge, Dorset

The example shown above is a very pure micrite of coccoliths, i.e. almost pure calcium carbonate. It is not argillaceous, but is associated with thin oil shales. It is thus organic-rich. Notice that the porosity is relatively high, but because of the very small size of the pores the permeability may be very low. This situation applies to the thin coccolith micrites of the Pectinatus Zone of the Kimmeridge Cliffs.

A electron microscope replica of a Jurassic coccolith micrite with calcite cementation and low porosity, in contrast to the Weald, Kimmeridge coccolith micrites

Shown above for comparison and contrast is a Jurassic coccolith micrite with well-developed, micritic-scale, cementation. This shows coccolith shields rather than coccospheres (both are present in the Kimmeridge coccolith micrites). This example has very low porosity and permeability and is not a good reservoir rock, even with hydraulic fracturing. Notice that it also lack clay minerals and obvious kerogen.


(Further Note: On page 5 of the Non-Technical Summary of the Cuadrilla Environmental Permit Application, it states that "the well is planned to be drilled through the Micrite which is an argillaceous carbonate". So this clearly states that it is not a pure calcium carbonate micrite. It differs from the photograph above in containing clay minerals. This is what those familiar with the Kimmeridge cliff sections would expect. The gamma ray and sonic logs and for the very similar Bolney Borehole, shown in Gallois and Worssam (1993) seem to confirm this, and in the Bolney Borehole the "Micrites" are labelled as "Calcareous Mudstone".)

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Basic Principles

Porosity and permeability of potential reservoir rocks, simple introduction

The porosity of a rock is simply the percentage of pore space. Obviously this is very high in a poorly-cemented sandstone. It could hold water like a sponge. A solid, almost non-porous rock such as granite can hardly hold any water. The same principle applies to oil and to gas. The matter is not really entirely simple though, because in practice many oil reservoir rocks also have films of water around the grains, so the oil does not fill all the available space.

The practical value of an oil or gas reservoir depends on trapping of the hydrocarbons in a dome or fault structure. In conventional oil or gas production suitable closed structures have to be found. In these the oil or gas can move upward above water and be pumped out (or it may flow out) from the top.

In unconventional oil or gas exploration the oil or gas is in minute pores and will not flow out easily. In other words the porosity might be as high as in a conventional reservoir, but the permeability, the extent to which the fluid can flow, may be extremely low. Fine-grained rocks with minute pores, often partially blocks by flakes of clay were not used in the past as reservoir rocks. Now there are methods such as using hydraulic fracturing to open up fissures so that the fluids can flow out of the minute pores and travel to the oil well.

To understand this further the nature of the miniature pores needs investigating. In the case of Balcombe, it is known that the rock, part of the Kimmeridge Clay succession, is a micrite. It is very unlikely to be a pure micrite but the percentage of clay minerals present is not yet publicly known. The rock is probably an argillaceous (clayey) coccolith micrite (i.e. very fine-grained limestone). A further complication is the probable presence of kerogen, a brown waxy organic substance, and a potential source of oil. This substance can block pores.

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Permeability in oil-bearing rocks indicates the relative ease with which oil will flow through the rock. Most sandstones are very permeable; granites have extremely low permeabilities and may be impermeable for practical purposes. Limestones can vary greatly from low permeability in a hard, well-cemented limestone without much porosity (like some of the British Carboniferous Limestone) to high permeability in a porous oolite (such as the Portland Stone).

In terms of petroleum geology, permeability is is usually measured in millidarcies. A millidarcy is the permeability unit "darcy" divided by a thousand. The term "darcy" comes from water supply or hydrogeological studies based on the work of Henry Darcy. This French Engineer, lived from 1803 to 1858. He built a pressurised water distribution system for Dijon, France, that was very advanced and successful for its time. His principles can be applied to rocks with regard to water and they can also be applied to rocks containing oil.

Technically, a darcy is "the permeability that will allow a flow of 1 cubic centimeter per second of a fluid with 1 centipoise viscosity (resistance to flow) through a distance of 1 centimeter through an area of 1 square centimeter under a differential pressure of 1 atmosphere".

Darcy's Law as the basis of permeability measurements

Darcy's Law can be written in several different ways but a simple version, is given below. In petroleum geology the permeability in millidarcies is a measure of the relative ease of oil flow through a rock. A millidarcy (mD) is equal to 0.001 darcy and a microdarcy (µD) equals 0.000001 darcy. The nanodarcy is a minute unit that is only 1 x 10-9 of a darcy. In some places parts of the Kimmeridge Clay can have porosity values of about 20%, quiet substantial, but with permeabilities of only about 5 nannodarcys. This is normal for shales. It is not noted though that the Upper Micrite of the Kimmeridge Clay is a actually a limestone of very small particle size but probably with clay and kerogen. Thus the permeability, although low, may be higher than in the pure shales.


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Go to Kimmeridge Oil Shale webpage (Clavell's Hard, Kimmeridge coast, Dorset) for more information on this topic.


Sulphur [Sulfur] Content of Oil Shale

The Blackstone is an unusually sulphurous rock. This is the cause of its very objectionable smell when burnt. In 1912 the British Navy, with risk of war, raised the sulphur tolerance for use of oil in naval ships from 0.75% to 3%. This seems to have been an emergency measure. However the oil produced from the Kimmeridge oil shale was found to have about twice this, at about 6 to 7% of sulphur (Strahan, 1918). This stinking oil could not be tolerated by the Navy (perhaps the enemy could find the British ships by smell!). It was therefore unusable for naval purposes, although efforts were made to mix the sulphurous Kimmeridge shale oil with another lower-sulphur oil to bring the level down to an acceptable amount.

Later the high sulphur content of oil from Kimmeridge oil shale was confirmed by (Gallois, 1978, Table 7, p.14). He found the sulphur in oil from six Kimmeridge oil shale samples to range from 4.3 to 8.5 per cent. There is no question that oil from Kimmeridge Oil Shale is extremely sulphurous, much more so than that from other oil shales. Probably one reason is that the oil shale is a marinite deposited from seawater as opposed to fresh, lake water, and it was deposited in reducing conditions and in contact with an abundance of calcium sulphate. The abundance of pyrite confirms the surplus of sulphide ions in the oil shale sediments.

When the Kimmeridge oil shale is heated, hydrogen sulphide is produced. A preliminary experiment recorded by (Gallois, 1978, p. 16) reported that hydrogen sulphide is evolved from the Kimmeridge oil shale at 200 degrees C.


It was shown in (Gallois, 1978, p. 97) that although pyrite is present as discrete crystals and as large nodules in Kimmeridge oil shale, it not, as one might expect, the major source of sulphur in shale oil from this rock. A large proportion of the sulphur was found to be within the kerogen. Studies by Pearson et al. (1996) have shown that in Kimmeridgian strata in the Cleveland Basin, the kerogen sulphur is closely correlated with TOC [Total Organic Carbon - i.e. the proportion of organic matter] and it was found to be highest in laminated mudrocks [i.e. oil shales] consistent with most effective sulphurization of kerogen under anoxic conditions. If this is applicable to the bituminous Kimmeridge Clay of the Dorset coast then it accords with the very sulphurous nature of the Kimmeridge Blackstone which has the highest TOC in the formation. This confirms the earlier work, and again indictes that the sulphurous stench of heated Kimmeridge oil shale may be coming, at least in large part, from sulphurous kerogen, and not necessarily from pyrite.


Clay Mineralogy

Some clay mineralogy of the Kimmeridge oil shales and Kimmeridge Clay in general for the North Runcton (Norfolk) and Donington on Bain (Lincolnshire) boreholes has been given by Merriman in Gallois (1978). Illite and kaolinite were found to be the major components with illite dominant. Smaller amounts of mixed-layer clay was also found. These results are unremarkable. Aragonite (from ammonite shells etc.) was frequently present and so too was pyrite.

Dolomite was also recorded from some of the Kimmeridge strata of these boreholes. This is not surprising because in Dorset, ferroan dolomite is an important constituent of the stone bands ( Bellamy (1980)). This is particularly the case in the sequence down from, and including, the Basalt Stone (Pectinatites hudlestoni Zone) is very Mg-rich. The occurrence of palygorskite in one sample from the Donington on Bain Borehole 2 is compatible with this. Palygorskite is a magnesium aluminium phyllosilicate; it occurs with dolomites and evaporites in the Lulworth Formation, Purbeck Group. There is no reason to believe that its occurrence in the Kimmeridge as a minor constituent has any connection with evaporite, but is obviously associated with the excess of Mg in much of the Kimmeridgian strata.

More specific information regarding the clay mineralogy of the Kimmeridge cliff section (Kimmeridge Bay to Encombe) was given by Gallois (1979), p. 113. Illite was dominant, ranging from 44 to 70% (average 53%) of the less than 2 microns clay fraction. Kaolinite ranged from 15 to 27% (average 22%). Expandible (mixed layer)clay minerals ranged from 10 to 39% (average 25%). Overall this is a fairly unremarkable mix of clay minerals, and not in any way unusual for a British Jurassic clay.


The Kimmeridge Blackstone and its products have no reputation for causing cancers and there is no reason to be particularly concerned. Refined oil and petrol does not seem to be any major cause for concern, but there has been discussion about shale oil obtained by heating. The topic may worth consideration because workers involved with other oil shales have suffered from this. Thus the British Geological Survey, then the Institute of Geological Sciences, considered in the publication of Gallois (1979), p. 100 et seq the possible occurrence of carcinogens in early-stage products of the Kimmeridge oil shale. Pages 100 (in part), 102, 103 (table 10 on p. 101 is not specifically on this topic) are present in the pdf available from the Geologists's Association online and there is a good discussion. However, p. 104 is not present. It seems to be the only missing page in the whole volume. The last comments seen at the bottom of page 103 are as follows:

"The present work [i.e in 1979] suggests that carcinogenic PAHs [polycondensed aromatic hydrocarbons] may be present in Kimmeridge Clay shale oil. The concentration of these potentially harmful chemicals, if present, is likely to be small. Nevertheless it would necessary for their distribution to be "............[missing page 104].

[A discussion on the possibility or not of carcinogens occurring in the ash of burnt Kimmeridge oil shale might be on the missing page, but I do not know.]

Cases of skin cancer were reported from the Scottish (Carboniferous) oil shale industry. Users of shale oil in the textile industry seem to have been at risk. Apparently "615 fatal cases of scrotal cancer, a proportion of which were believed to have resulted from mineral oils were recorded in the industry between 1911 and 1938 (Henry, 1946). The Mule Spinning Regulations of 1952 therefore introduced a restriction on the use of lubricating oils to those which had been drastically refined with sulphuric acid to remove the known carcinogens."
The carcinogens are apparently not present in the natural oil shale, but they were formed during pyrolysis. They may be destroyed during refining. See the full carcinogen section in Gallois (1979) for more information on this topic. (See also Wikipedia on "Mule Spinners' Cancer).

See also:
International Agency for Research on Cancer (IARC) - Summaries & Evaluations - SHALE-OILS. [Extract: "Inhalation of either raw oil shale or spent oil shale produced lung tumours in rats. Application of an extract of spent oil shale produced skin tumours in mice [ref: 1].
Skin application of crude oils from both low- and high-temperature retorting induced skin tumours in mice and rabbits; the high-temperature retorted oils had greater carcinogenic activity. A low-temperature crude oil produced lung tumours in mice after intratracheal instillation [ref: 1].Various fractions of shale-oils were carcinogenic when applied to the skin of mice and rabbits [ref: 1]. Shale-oil distillates, residues, blends, and commercial products of the oil-shale industry were tested in mice by skin application, producing skin tumours. Distillation fractions from less highly refined shale-oils were more carcinogenic than the more highly refined products [ref: 1]." ]

As far as is known there are no plans for pyrolysis (heating or burning of oil shale) of the Kimmeridge oil shale in the Weald, so therefore there is no reason for high temperatures to be involved. Thus there is no particular reason to believe that carcinogens might be formed. There seems to be no report of injury from carcinogens in the case of Kimmeridge oil shale. Further information on this topic with regard to other shale oils can be found on the internet.

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Onlap etc.

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